4590 lines
117 KiB
Plaintext
4590 lines
117 KiB
Plaintext
User Mode Linux HOWTO
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User Mode Linux Core Team
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Mon Nov 18 14:16:16 EST 2002
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This document describes the use and abuse of Jeff Dike's User Mode
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Linux: a port of the Linux kernel as a normal Intel Linux process.
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______________________________________________________________________
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Table of Contents
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1. Introduction
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1.1 How is User Mode Linux Different?
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1.2 Why Would I Want User Mode Linux?
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2. Compiling the kernel and modules
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2.1 Compiling the kernel
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2.2 Compiling and installing kernel modules
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2.3 Compiling and installing uml_utilities
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3. Running UML and logging in
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3.1 Running UML
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3.2 Logging in
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3.3 Examples
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4. UML on 2G/2G hosts
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4.1 Introduction
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4.2 The problem
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4.3 The solution
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5. Setting up serial lines and consoles
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5.1 Specifying the device
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5.2 Specifying the channel
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5.3 Examples
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6. Setting up the network
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6.1 General setup
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6.2 Userspace daemons
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6.3 Specifying ethernet addresses
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6.4 UML interface setup
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6.5 Multicast
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6.6 TUN/TAP with the uml_net helper
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6.7 TUN/TAP with a preconfigured tap device
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6.8 Ethertap
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6.9 The switch daemon
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6.10 Slip
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6.11 Slirp
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6.12 pcap
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6.13 Setting up the host yourself
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7. Sharing Filesystems between Virtual Machines
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7.1 A warning
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7.2 Using layered block devices
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7.3 Note!
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7.4 Another warning
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7.5 uml_moo : Merging a COW file with its backing file
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8. Creating filesystems
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8.1 Create the filesystem file
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8.2 Assign the file to a UML device
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8.3 Creating and mounting the filesystem
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9. Host file access
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9.1 Using hostfs
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9.2 hostfs as the root filesystem
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9.3 Building hostfs
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10. The Management Console
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10.1 version
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10.2 halt and reboot
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10.3 config
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10.4 remove
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10.5 sysrq
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10.6 help
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10.7 cad
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10.8 stop
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10.9 go
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11. Kernel debugging
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11.1 Starting the kernel under gdb
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11.2 Examining sleeping processes
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11.3 Running ddd on UML
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11.4 Debugging modules
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11.5 Attaching gdb to the kernel
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11.6 Using alternate debuggers
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12. Kernel debugging examples
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12.1 The case of the hung fsck
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12.2 Episode 2: The case of the hung fsck
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13. What to do when UML doesn't work
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13.1 Strange compilation errors when you build from source
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13.2 (obsolete)
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13.3 A variety of panics and hangs with /tmp on a reiserfs filesystem
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13.4 The compile fails with errors about conflicting types for 'open', 'dup', and 'waitpid'
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13.5 UML doesn't work when /tmp is an NFS filesystem
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13.6 UML hangs on boot when compiled with gprof support
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13.7 syslogd dies with a SIGTERM on startup
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13.8 TUN/TAP networking doesn't work on a 2.4 host
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13.9 You can network to the host but not to other machines on the net
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13.10 I have no root and I want to scream
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13.11 UML build conflict between ptrace.h and ucontext.h
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13.12 The UML BogoMips is exactly half the host's BogoMips
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13.13 When you run UML, it immediately segfaults
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13.14 xterms appear, then immediately disappear
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13.15 Any other panic, hang, or strange behavior
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14. Diagnosing Problems
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14.1 Case 1 : Normal kernel panics
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14.2 Case 2 : Tracing thread panics
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14.3 Case 3 : Tracing thread panics caused by other threads
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14.4 Case 4 : Hangs
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15. Thanks
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15.1 Code and Documentation
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15.2 Flushing out bugs
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15.3 Buglets and clean-ups
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15.4 Case Studies
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15.5 Other contributions
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______________________________________________________________________
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1. Introduction
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Welcome to User Mode Linux. It's going to be fun.
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1.1. How is User Mode Linux Different?
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Normally, the Linux Kernel talks straight to your hardware (video
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card, keyboard, hard drives, etc), and any programs which run ask the
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kernel to operate the hardware, like so:
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+-----------+-----------+----+
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| Process 1 | Process 2 | ...|
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+-----------+-----------+----+
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| Linux Kernel |
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+----------------------------+
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| Hardware |
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+----------------------------+
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The User Mode Linux Kernel is different; instead of talking to the
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hardware, it talks to a `real' Linux kernel (called the `host kernel'
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from now on), like any other program. Programs can then run inside
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User-Mode Linux as if they were running under a normal kernel, like
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so:
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+----------------+
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| Process 2 | ...|
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+-----------+----------------+
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| Process 1 | User-Mode Linux|
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+----------------------------+
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| Linux Kernel |
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+----------------------------+
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| Hardware |
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+----------------------------+
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1.2. Why Would I Want User Mode Linux?
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1. If User Mode Linux crashes, your host kernel is still fine.
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2. You can run a usermode kernel as a non-root user.
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3. You can debug the User Mode Linux like any normal process.
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4. You can run gprof (profiling) and gcov (coverage testing).
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5. You can play with your kernel without breaking things.
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6. You can use it as a sandbox for testing new apps.
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7. You can try new development kernels safely.
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8. You can run different distributions simultaneously.
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9. It's extremely fun.
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2. Compiling the kernel and modules
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2.1. Compiling the kernel
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Compiling the user mode kernel is just like compiling any other
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kernel. Let's go through the steps, using 2.4.0-prerelease (current
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as of this writing) as an example:
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1. Download the latest UML patch from
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the download page <http://user-mode-linux.sourceforge.net/
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In this example, the file is uml-patch-2.4.0-prerelease.bz2.
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2. Download the matching kernel from your favourite kernel mirror,
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such as:
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ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2
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<ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2>
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.
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3. Make a directory and unpack the kernel into it.
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host%
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mkdir ~/uml
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host%
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cd ~/uml
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host%
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tar -xzvf linux-2.4.0-prerelease.tar.bz2
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4. Apply the patch using
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host%
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cd ~/uml/linux
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host%
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bzcat uml-patch-2.4.0-prerelease.bz2 | patch -p1
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5. Run your favorite config; `make xconfig ARCH=um' is the most
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convenient. `make config ARCH=um' and 'make menuconfig ARCH=um'
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will work as well. The defaults will give you a useful kernel. If
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you want to change something, go ahead, it probably won't hurt
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anything.
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Note: If the host is configured with a 2G/2G address space split
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rather than the usual 3G/1G split, then the packaged UML binaries
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will not run. They will immediately segfault. See ``UML on 2G/2G
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hosts'' for the scoop on running UML on your system.
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6. Finish with `make linux ARCH=um': the result is a file called
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`linux' in the top directory of your source tree.
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Make sure that you don't build this kernel in /usr/src/linux. On some
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distributions, /usr/include/asm is a link into this pool. The user-
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mode build changes the other end of that link, and things that include
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<asm/anything.h> stop compiling.
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The sources are also available from cvs at the project's cvs page,
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which has directions on getting the sources. You can also browse the
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CVS pool from there.
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If you get the CVS sources, you will have to check them out into an
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empty directory. You will then have to copy each file into the
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corresponding directory in the appropriate kernel pool.
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If you don't have the latest kernel pool, you can get the
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corresponding user-mode sources with
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host% cvs co -r v_2_3_x linux
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where 'x' is the version in your pool. Note that you will not get the
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bug fixes and enhancements that have gone into subsequent releases.
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2.2. Compiling and installing kernel modules
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UML modules are built in the same way as the native kernel (with the
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exception of the 'ARCH=um' that you always need for UML):
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host% make modules ARCH=um
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Any modules that you want to load into this kernel need to be built in
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the user-mode pool. Modules from the native kernel won't work.
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You can install them by using ftp or something to copy them into the
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virtual machine and dropping them into /lib/modules/`uname -r`.
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You can also get the kernel build process to install them as follows:
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1. with the kernel not booted, mount the root filesystem in the top
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level of the kernel pool:
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host% mount root_fs mnt -o loop
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2. run
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host%
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make modules_install INSTALL_MOD_PATH=`pwd`/mnt ARCH=um
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3. unmount the filesystem
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host% umount mnt
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4. boot the kernel on it
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When the system is booted, you can use insmod as usual to get the
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modules into the kernel. A number of things have been loaded into UML
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as modules, especially filesystems and network protocols and filters,
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so most symbols which need to be exported probably already are.
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However, if you do find symbols that need exporting, let us
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<http://user-mode-linux.sourceforge.net/> know, and
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they'll be "taken care of".
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2.3. Compiling and installing uml_utilities
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Many features of the UML kernel require a user-space helper program,
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so a uml_utilities package is distributed separately from the kernel
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patch which provides these helpers. Included within this is:
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o port-helper - Used by consoles which connect to xterms or ports
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o tunctl - Configuration tool to create and delete tap devices
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o uml_net - Setuid binary for automatic tap device configuration
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o uml_switch - User-space virtual switch required for daemon
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transport
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The uml_utilities tree is compiled with:
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host#
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make && make install
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Note that UML kernel patches may require a specific version of the
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uml_utilities distribution. If you don't keep up with the mailing
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lists, ensure that you have the latest release of uml_utilities if you
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are experiencing problems with your UML kernel, particularly when
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dealing with consoles or command-line switches to the helper programs
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3. Running UML and logging in
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3.1. Running UML
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It runs on 2.2.15 or later, and all 2.4 kernels.
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Booting UML is straightforward. Simply run 'linux': it will try to
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mount the file `root_fs' in the current directory. You do not need to
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run it as root. If your root filesystem is not named `root_fs', then
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you need to put a `ubd0=root_fs_whatever' switch on the linux command
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line.
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You will need a filesystem to boot UML from. There are a number
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available for download from here <http://user-mode-
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linux.sourceforge.net/> . There are also several tools
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<http://user-mode-linux.sourceforge.net/> which can be
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used to generate UML-compatible filesystem images from media.
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The kernel will boot up and present you with a login prompt.
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Note: If the host is configured with a 2G/2G address space split
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rather than the usual 3G/1G split, then the packaged UML binaries will
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not run. They will immediately segfault. See ``UML on 2G/2G hosts''
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for the scoop on running UML on your system.
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3.2. Logging in
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The prepackaged filesystems have a root account with password 'root'
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and a user account with password 'user'. The login banner will
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generally tell you how to log in. So, you log in and you will find
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yourself inside a little virtual machine. Our filesystems have a
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variety of commands and utilities installed (and it is fairly easy to
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add more), so you will have a lot of tools with which to poke around
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the system.
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There are a couple of other ways to log in:
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o On a virtual console
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Each virtual console that is configured (i.e. the device exists in
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/dev and /etc/inittab runs a getty on it) will come up in its own
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xterm. If you get tired of the xterms, read ``Setting up serial
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lines and consoles'' to see how to attach the consoles to
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something else, like host ptys.
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o Over the serial line
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In the boot output, find a line that looks like:
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serial line 0 assigned pty /dev/ptyp1
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Attach your favorite terminal program to the corresponding tty. I.e.
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for minicom, the command would be
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host% minicom -o -p /dev/ttyp1
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o Over the net
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If the network is running, then you can telnet to the virtual
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machine and log in to it. See ``Setting up the network'' to learn
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about setting up a virtual network.
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When you're done using it, run halt, and the kernel will bring itself
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down and the process will exit.
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3.3. Examples
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Here are some examples of UML in action:
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o A login session <http://user-mode-linux.sourceforge.net/login.html>
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o A virtual network <http://user-mode-linux.sourceforge.net/net.html>
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4. UML on 2G/2G hosts
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4.1. Introduction
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Most Linux machines are configured so that the kernel occupies the
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upper 1G (0xc0000000 - 0xffffffff) of the 4G address space and
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processes use the lower 3G (0x00000000 - 0xbfffffff). However, some
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machine are configured with a 2G/2G split, with the kernel occupying
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the upper 2G (0x80000000 - 0xffffffff) and processes using the lower
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2G (0x00000000 - 0x7fffffff).
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4.2. The problem
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The prebuilt UML binaries on this site will not run on 2G/2G hosts
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because UML occupies the upper .5G of the 3G process address space
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(0xa0000000 - 0xbfffffff). Obviously, on 2G/2G hosts, this is right
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in the middle of the kernel address space, so UML won't even load - it
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will immediately segfault.
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4.3. The solution
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The fix for this is to rebuild UML from source after enabling
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CONFIG_HOST_2G_2G (under 'General Setup'). This will cause UML to
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load itself in the top .5G of that smaller process address space,
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where it will run fine. See ``Compiling the kernel and modules'' if
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you need help building UML from source.
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5. Setting up serial lines and consoles
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It is possible to attach UML serial lines and consoles to many types
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of host I/O channels by specifying them on the command line.
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You can attach them to host ptys, ttys, file descriptors, and ports.
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This allows you to do things like
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o have a UML console appear on an unused host console,
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o hook two virtual machines together by having one attach to a pty
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and having the other attach to the corresponding tty
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o make a virtual machine accessible from the net by attaching a
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console to a port on the host.
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The general format of the command line option is device=channel.
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5.1. Specifying the device
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Devices are specified with "con" or "ssl" (console or serial line,
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respectively), optionally with a device number if you are talking
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about a specific device.
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Using just "con" or "ssl" describes all of the consoles or serial
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lines. If you want to talk about console #3 or serial line #10, they
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would be "con3" and "ssl10", respectively.
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A specific device name will override a less general "con=" or "ssl=".
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So, for example, you can assign a pty to each of the serial lines
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except for the first two like this:
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ssl=pty ssl0=tty:/dev/tty0 ssl1=tty:/dev/tty1
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The specificity of the device name is all that matters; order on the
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command line is irrelevant.
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5.2. Specifying the channel
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There are a number of different types of channels to attach a UML
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device to, each with a different way of specifying exactly what to
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attach to.
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o pseudo-terminals - device=pty pts terminals - device=pts
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This will cause UML to allocate a free host pseudo-terminal for the
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device. The terminal that it got will be announced in the boot
|
|
log. You access it by attaching a terminal program to the
|
|
corresponding tty:
|
|
|
|
o screen /dev/pts/n
|
|
|
|
o screen /dev/ttyxx
|
|
|
|
o minicom -o -p /dev/ttyxx - minicom seems not able to handle pts
|
|
devices
|
|
|
|
o kermit - start it up, 'open' the device, then 'connect'
|
|
|
|
|
|
|
|
|
|
|
|
o terminals - device=tty:tty device file
|
|
|
|
|
|
This will make UML attach the device to the specified tty (i.e
|
|
|
|
|
|
con1=tty:/dev/tty3
|
|
|
|
|
|
|
|
|
|
will attach UML's console 1 to the host's /dev/tty3). If the tty that
|
|
you specify is the slave end of a tty/pty pair, something else must
|
|
have already opened the corresponding pty in order for this to work.
|
|
|
|
|
|
|
|
|
|
|
|
o xterms - device=xterm
|
|
|
|
|
|
UML will run an xterm and the device will be attached to it.
|
|
|
|
|
|
|
|
|
|
|
|
o Port - device=port:port number
|
|
|
|
|
|
This will attach the UML devices to the specified host port.
|
|
Attaching console 1 to the host's port 9000 would be done like
|
|
this:
|
|
|
|
|
|
con1=port:9000
|
|
|
|
|
|
|
|
|
|
Attaching all the serial lines to that port would be done similarly:
|
|
|
|
|
|
ssl=port:9000
|
|
|
|
|
|
|
|
|
|
You access these devices by telnetting to that port. Each active tel-
|
|
net session gets a different device. If there are more telnets to a
|
|
port than UML devices attached to it, then the extra telnet sessions
|
|
will block until an existing telnet detaches, or until another device
|
|
becomes active (i.e. by being activated in /etc/inittab).
|
|
|
|
This channel has the advantage that you can both attach multiple UML
|
|
devices to it and know how to access them without reading the UML boot
|
|
log. It is also unique in allowing access to a UML from remote
|
|
machines without requiring that the UML be networked. This could be
|
|
useful in allowing public access to UMLs because they would be
|
|
accessible from the net, but wouldn't need any kind of network
|
|
filtering or access control because they would have no network access.
|
|
|
|
|
|
If you attach the main console to a portal, then the UML boot will
|
|
appear to hang. In reality, it's waiting for a telnet to connect, at
|
|
which point the boot will proceed.
|
|
|
|
|
|
|
|
|
|
|
|
o already-existing file descriptors - device=file descriptor
|
|
|
|
|
|
If you set up a file descriptor on the UML command line, you can
|
|
attach a UML device to it. This is most commonly used to put the
|
|
main console back on stdin and stdout after assigning all the other
|
|
consoles to something else:
|
|
|
|
|
|
con0=fd:0,fd:1 con=pts
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
o Nothing - device=null
|
|
|
|
|
|
This allows the device to be opened, in contrast to 'none', but
|
|
reads will block, and writes will succeed and the data will be
|
|
thrown out.
|
|
|
|
|
|
|
|
|
|
|
|
o None - device=none
|
|
|
|
|
|
This causes the device to disappear.
|
|
|
|
|
|
|
|
You can also specify different input and output channels for a device
|
|
by putting a comma between them:
|
|
|
|
|
|
ssl3=tty:/dev/tty2,xterm
|
|
|
|
|
|
|
|
|
|
will cause serial line 3 to accept input on the host's /dev/tty2 and
|
|
display output on an xterm. That's a silly example - the most common
|
|
use of this syntax is to reattach the main console to stdin and stdout
|
|
as shown above.
|
|
|
|
|
|
If you decide to move the main console away from stdin/stdout, the
|
|
initial boot output will appear in the terminal that you're running
|
|
UML in. However, once the console driver has been officially
|
|
initialized, then the boot output will start appearing wherever you
|
|
specified that console 0 should be. That device will receive all
|
|
subsequent output.
|
|
|
|
|
|
|
|
5.3. Examples
|
|
|
|
There are a number of interesting things you can do with this
|
|
capability.
|
|
|
|
|
|
First, this is how you get rid of those bleeding console xterms by
|
|
attaching them to host ptys:
|
|
|
|
|
|
con=pty con0=fd:0,fd:1
|
|
|
|
|
|
|
|
|
|
This will make a UML console take over an unused host virtual console,
|
|
so that when you switch to it, you will see the UML login prompt
|
|
rather than the host login prompt:
|
|
|
|
|
|
con1=tty:/dev/tty6
|
|
|
|
|
|
|
|
|
|
You can attach two virtual machines together with what amounts to a
|
|
serial line as follows:
|
|
|
|
Run one UML with a serial line attached to a pty -
|
|
|
|
|
|
ssl1=pty
|
|
|
|
|
|
|
|
|
|
Look at the boot log to see what pty it got (this example will assume
|
|
that it got /dev/ptyp1).
|
|
|
|
Boot the other UML with a serial line attached to the corresponding
|
|
tty -
|
|
|
|
|
|
ssl1=tty:/dev/ttyp1
|
|
|
|
|
|
|
|
|
|
Log in, make sure that it has no getty on that serial line, attach a
|
|
terminal program like minicom to it, and you should see the login
|
|
prompt of the other virtual machine.
|
|
|
|
|
|
6. Setting up the network
|
|
|
|
|
|
|
|
This page describes how to set up the various transports and to
|
|
provide a UML instance with network access to the host, other machines
|
|
on the local net, and the rest of the net.
|
|
|
|
|
|
As of 2.4.5, UML networking has been completely redone to make it much
|
|
easier to set up, fix bugs, and add new features.
|
|
|
|
|
|
There is a new helper, uml_net, which does the host setup that
|
|
requires root privileges.
|
|
|
|
|
|
There are currently five transport types available for a UML virtual
|
|
machine to exchange packets with other hosts:
|
|
|
|
o ethertap
|
|
|
|
o TUN/TAP
|
|
|
|
o Multicast
|
|
|
|
o a switch daemon
|
|
|
|
o slip
|
|
|
|
o slirp
|
|
|
|
o pcap
|
|
|
|
The TUN/TAP, ethertap, slip, and slirp transports allow a UML
|
|
instance to exchange packets with the host. They may be directed
|
|
to the host or the host may just act as a router to provide access
|
|
to other physical or virtual machines.
|
|
|
|
|
|
The pcap transport is a synthetic read-only interface, using the
|
|
libpcap binary to collect packets from interfaces on the host and
|
|
filter them. This is useful for building preconfigured traffic
|
|
monitors or sniffers.
|
|
|
|
|
|
The daemon and multicast transports provide a completely virtual
|
|
network to other virtual machines. This network is completely
|
|
disconnected from the physical network unless one of the virtual
|
|
machines on it is acting as a gateway.
|
|
|
|
|
|
With so many host transports, which one should you use? Here's when
|
|
you should use each one:
|
|
|
|
o ethertap - if you want access to the host networking and it is
|
|
running 2.2
|
|
|
|
o TUN/TAP - if you want access to the host networking and it is
|
|
running 2.4. Also, the TUN/TAP transport is able to use a
|
|
preconfigured device, allowing it to avoid using the setuid uml_net
|
|
helper, which is a security advantage.
|
|
|
|
o Multicast - if you want a purely virtual network and you don't want
|
|
to set up anything but the UML
|
|
|
|
o a switch daemon - if you want a purely virtual network and you
|
|
don't mind running the daemon in order to get somewhat better
|
|
performance
|
|
|
|
o slip - there is no particular reason to run the slip backend unless
|
|
ethertap and TUN/TAP are just not available for some reason
|
|
|
|
o slirp - if you don't have root access on the host to setup
|
|
networking, or if you don't want to allocate an IP to your UML
|
|
|
|
o pcap - not much use for actual network connectivity, but great for
|
|
monitoring traffic on the host
|
|
|
|
Ethertap is available on 2.4 and works fine. TUN/TAP is preferred
|
|
to it because it has better performance and ethertap is officially
|
|
considered obsolete in 2.4. Also, the root helper only needs to
|
|
run occasionally for TUN/TAP, rather than handling every packet, as
|
|
it does with ethertap. This is a slight security advantage since
|
|
it provides fewer opportunities for a nasty UML user to somehow
|
|
exploit the helper's root privileges.
|
|
|
|
|
|
6.1. General setup
|
|
|
|
First, you must have the virtual network enabled in your UML. If are
|
|
running a prebuilt kernel from this site, everything is already
|
|
enabled. If you build the kernel yourself, under the "Network device
|
|
support" menu, enable "Network device support", and then the three
|
|
transports.
|
|
|
|
|
|
The next step is to provide a network device to the virtual machine.
|
|
This is done by describing it on the kernel command line.
|
|
|
|
The general format is
|
|
|
|
|
|
eth <n> = <transport> , <transport args>
|
|
|
|
|
|
|
|
|
|
For example, a virtual ethernet device may be attached to a host
|
|
ethertap device as follows:
|
|
|
|
|
|
eth0=ethertap,tap0,fe:fd:0:0:0:1,192.168.0.254
|
|
|
|
|
|
|
|
|
|
This sets up eth0 inside the virtual machine to attach itself to the
|
|
host /dev/tap0, assigns it an ethernet address, and assigns the host
|
|
tap0 interface an IP address.
|
|
|
|
|
|
|
|
Note that the IP address you assign to the host end of the tap device
|
|
must be different than the IP you assign to the eth device inside UML.
|
|
If you are short on IPs and don't want to consume two per UML, then
|
|
you can reuse the host's eth IP address for the host ends of the tap
|
|
devices. Internally, the UMLs must still get unique IPs for their eth
|
|
devices. You can also give the UMLs non-routable IPs (192.168.x.x or
|
|
10.x.x.x) and have the host masquerade them. This will let outgoing
|
|
connections work, but incoming connections won't without more work,
|
|
such as port forwarding from the host.
|
|
Also note that when you configure the host side of an interface, it is
|
|
only acting as a gateway. It will respond to pings sent to it
|
|
locally, but is not useful to do that since it's a host interface.
|
|
You are not talking to the UML when you ping that interface and get a
|
|
response.
|
|
|
|
|
|
You can also add devices to a UML and remove them at runtime. See the
|
|
``The Management Console'' page for details.
|
|
|
|
|
|
The sections below describe this in more detail.
|
|
|
|
|
|
Once you've decided how you're going to set up the devices, you boot
|
|
UML, log in, configure the UML side of the devices, and set up routes
|
|
to the outside world. At that point, you will be able to talk to any
|
|
other machines, physical or virtual, on the net.
|
|
|
|
|
|
If ifconfig inside UML fails and the network refuses to come up, run
|
|
tell you what went wrong.
|
|
|
|
|
|
|
|
6.2. Userspace daemons
|
|
|
|
You will likely need the setuid helper, or the switch daemon, or both.
|
|
They are both installed with the RPM and deb, so if you've installed
|
|
either, you can skip the rest of this section.
|
|
|
|
|
|
If not, then you need to check them out of CVS, build them, and
|
|
install them. The helper is uml_net, in CVS /tools/uml_net, and the
|
|
daemon is uml_switch, in CVS /tools/uml_router. They are both built
|
|
with a plain 'make'. Both need to be installed in a directory that's
|
|
in your path - /usr/bin is recommend. On top of that, uml_net needs
|
|
to be setuid root.
|
|
|
|
|
|
|
|
6.3. Specifying ethernet addresses
|
|
|
|
Below, you will see that the TUN/TAP, ethertap, and daemon interfaces
|
|
allow you to specify hardware addresses for the virtual ethernet
|
|
devices. This is generally not necessary. If you don't have a
|
|
specific reason to do it, you probably shouldn't. If one is not
|
|
specified on the command line, the driver will assign one based on the
|
|
device IP address. It will provide the address fe:fd:nn:nn:nn:nn
|
|
where nn.nn.nn.nn is the device IP address. This is nearly always
|
|
sufficient to guarantee a unique hardware address for the device. A
|
|
couple of exceptions are:
|
|
|
|
o Another set of virtual ethernet devices are on the same network and
|
|
they are assigned hardware addresses using a different scheme which
|
|
may conflict with the UML IP address-based scheme
|
|
|
|
o You aren't going to use the device for IP networking, so you don't
|
|
assign the device an IP address
|
|
|
|
If you let the driver provide the hardware address, you should make
|
|
sure that the device IP address is known before the interface is
|
|
brought up. So, inside UML, this will guarantee that:
|
|
|
|
|
|
|
|
UML#
|
|
ifconfig eth0 192.168.0.250 up
|
|
|
|
|
|
|
|
|
|
If you decide to assign the hardware address yourself, make sure that
|
|
the first byte of the address is even. Addresses with an odd first
|
|
byte are broadcast addresses, which you don't want assigned to a
|
|
device.
|
|
|
|
|
|
|
|
6.4. UML interface setup
|
|
|
|
Once the network devices have been described on the command line, you
|
|
should boot UML and log in.
|
|
|
|
|
|
The first thing to do is bring the interface up:
|
|
|
|
|
|
UML# ifconfig ethn ip-address up
|
|
|
|
|
|
|
|
|
|
You should be able to ping the host at this point.
|
|
|
|
|
|
To reach the rest of the world, you should set a default route to the
|
|
host:
|
|
|
|
|
|
UML# route add default gw host ip
|
|
|
|
|
|
|
|
|
|
Again, with host ip of 192.168.0.4:
|
|
|
|
|
|
UML# route add default gw 192.168.0.4
|
|
|
|
|
|
|
|
|
|
This page used to recommend setting a network route to your local net.
|
|
This is wrong, because it will cause UML to try to figure out hardware
|
|
addresses of the local machines by arping on the interface to the
|
|
host. Since that interface is basically a single strand of ethernet
|
|
with two nodes on it (UML and the host) and arp requests don't cross
|
|
networks, they will fail to elicit any responses. So, what you want
|
|
is for UML to just blindly throw all packets at the host and let it
|
|
figure out what to do with them, which is what leaving out the network
|
|
route and adding the default route does.
|
|
|
|
|
|
Note: If you can't communicate with other hosts on your physical
|
|
ethernet, it's probably because of a network route that's
|
|
automatically set up. If you run 'route -n' and see a route that
|
|
looks like this:
|
|
|
|
|
|
|
|
|
|
Destination Gateway Genmask Flags Metric Ref Use Iface
|
|
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
|
|
|
|
|
|
|
|
|
|
with a mask that's not 255.255.255.255, then replace it with a route
|
|
to your host:
|
|
|
|
|
|
UML#
|
|
route del -net 192.168.0.0 dev eth0 netmask 255.255.255.0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
UML#
|
|
route add -host 192.168.0.4 dev eth0
|
|
|
|
|
|
|
|
|
|
This, plus the default route to the host, will allow UML to exchange
|
|
packets with any machine on your ethernet.
|
|
|
|
|
|
|
|
6.5. Multicast
|
|
|
|
The simplest way to set up a virtual network between multiple UMLs is
|
|
to use the mcast transport. This was written by Harald Welte and is
|
|
present in UML version 2.4.5-5um and later. Your system must have
|
|
multicast enabled in the kernel and there must be a multicast-capable
|
|
network device on the host. Normally, this is eth0, but if there is
|
|
no ethernet card on the host, then you will likely get strange error
|
|
messages when you bring the device up inside UML.
|
|
|
|
|
|
To use it, run two UMLs with
|
|
|
|
|
|
eth0=mcast
|
|
|
|
|
|
|
|
|
|
on their command lines. Log in, configure the ethernet device in each
|
|
machine with different IP addresses:
|
|
|
|
|
|
UML1# ifconfig eth0 192.168.0.254
|
|
|
|
|
|
|
|
|
|
|
|
|
|
UML2# ifconfig eth0 192.168.0.253
|
|
|
|
|
|
|
|
|
|
and they should be able to talk to each other.
|
|
|
|
The full set of command line options for this transport are
|
|
|
|
|
|
|
|
ethn=mcast,ethernet address,multicast
|
|
address,multicast port,ttl
|
|
|
|
|
|
|
|
|
|
Harald's original README is here <http://user-mode-linux.source-
|
|
forge.net/> and explains these in detail, as well as
|
|
some other issues.
|
|
|
|
There is also a related point-to-point only "ucast" transport.
|
|
This is useful when your network does not support multicast, and
|
|
all network connections are simple point to point links.
|
|
|
|
The full set of command line options for this transport are
|
|
|
|
|
|
ethn=ucast,ethernet address,remote address,listen port,remote port
|
|
|
|
|
|
|
|
|
|
6.6. TUN/TAP with the uml_net helper
|
|
|
|
TUN/TAP is the preferred mechanism on 2.4 to exchange packets with the
|
|
host. The TUN/TAP backend has been in UML since 2.4.9-3um.
|
|
|
|
|
|
The easiest way to get up and running is to let the setuid uml_net
|
|
helper do the host setup for you. This involves insmod-ing the tun.o
|
|
module if necessary, configuring the device, and setting up IP
|
|
forwarding, routing, and proxy arp. If you are new to UML networking,
|
|
do this first. If you're concerned about the security implications of
|
|
the setuid helper, use it to get up and running, then read the next
|
|
section to see how to have UML use a preconfigured tap device, which
|
|
avoids the use of uml_net.
|
|
|
|
|
|
If you specify an IP address for the host side of the device, the
|
|
uml_net helper will do all necessary setup on the host - the only
|
|
requirement is that TUN/TAP be available, either built in to the host
|
|
kernel or as the tun.o module.
|
|
|
|
The format of the command line switch to attach a device to a TUN/TAP
|
|
device is
|
|
|
|
|
|
eth <n> =tuntap,,, <IP address>
|
|
|
|
|
|
|
|
|
|
For example, this argument will attach the UML's eth0 to the next
|
|
available tap device and assign an ethernet address to it based on its
|
|
IP address
|
|
|
|
|
|
eth0=tuntap,,,192.168.0.254
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Note that the IP address that must be used for the eth device inside
|
|
UML is fixed by the routing and proxy arp that is set up on the
|
|
TUN/TAP device on the host. You can use a different one, but it won't
|
|
work because reply packets won't reach the UML. This is a feature.
|
|
It prevents a nasty UML user from doing things like setting the UML IP
|
|
to the same as the network's nameserver or mail server.
|
|
|
|
|
|
There are a couple potential problems with running the TUN/TAP
|
|
transport on a 2.4 host kernel
|
|
|
|
o TUN/TAP seems not to work on 2.4.3 and earlier. Upgrade the host
|
|
kernel or use the ethertap transport.
|
|
|
|
o With an upgraded kernel, TUN/TAP may fail with
|
|
|
|
|
|
File descriptor in bad state
|
|
|
|
|
|
|
|
|
|
This is due to a header mismatch between the upgraded kernel and the
|
|
kernel that was originally installed on the machine. The fix is to
|
|
make sure that /usr/src/linux points to the headers for the running
|
|
kernel.
|
|
|
|
These were pointed out by Tim Robinson <timro at trkr dot net> in
|
|
<http://www.geocrawler.com/> name="this uml-
|
|
user post"> .
|
|
|
|
|
|
|
|
6.7. TUN/TAP with a preconfigured tap device
|
|
|
|
If you prefer not to have UML use uml_net (which is somewhat
|
|
insecure), with UML 2.4.17-11, you can set up a TUN/TAP device
|
|
beforehand. The setup needs to be done as root, but once that's done,
|
|
there is no need for root assistance. Setting up the device is done
|
|
as follows:
|
|
|
|
o Create the device with tunctl (available from the UML utilities
|
|
tarball)
|
|
|
|
|
|
|
|
|
|
host# tunctl -u uid
|
|
|
|
|
|
|
|
|
|
where uid is the user id or username that UML will be run as. This
|
|
will tell you what device was created.
|
|
|
|
o Configure the device IP (change IP addresses and device name to
|
|
suit)
|
|
|
|
|
|
|
|
|
|
host# ifconfig tap0 192.168.0.254 up
|
|
|
|
|
|
|
|
|
|
|
|
o Set up routing and arping if desired - this is my recipe, there are
|
|
other ways of doing the same thing
|
|
|
|
|
|
host#
|
|
bash -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'
|
|
|
|
host#
|
|
route add -host 192.168.0.253 dev tap0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
bash -c 'echo 1 > /proc/sys/net/ipv4/conf/tap0/proxy_arp'
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
arp -Ds 192.168.0.253 eth0 pub
|
|
|
|
|
|
|
|
|
|
Note that this must be done every time the host boots - this configu-
|
|
ration is not stored across host reboots. So, it's probably a good
|
|
idea to stick it in an rc file. An even better idea would be a little
|
|
utility which reads the information from a config file and sets up
|
|
devices at boot time.
|
|
|
|
o Rather than using up two IPs and ARPing for one of them, you can
|
|
also provide direct access to your LAN by the UML by using a
|
|
bridge.
|
|
|
|
|
|
host#
|
|
brctl addbr br0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
ifconfig eth0 0.0.0.0 promisc up
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
ifconfig tap0 0.0.0.0 promisc up
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
ifconfig br0 192.168.0.1 netmask 255.255.255.0 up
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
brctl stp br0 off
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
brctl setfd br0 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
brctl sethello br0 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
brctl addif br0 eth0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
brctl addif br0 tap0
|
|
|
|
|
|
|
|
|
|
Note that 'br0' should be setup using ifconfig with the existing IP
|
|
address of eth0, as eth0 no longer has its own IP.
|
|
|
|
o
|
|
|
|
|
|
Also, the /dev/net/tun device must be writable by the user running
|
|
UML in order for the UML to use the device that's been configured
|
|
for it. The simplest thing to do is
|
|
|
|
|
|
host# chmod 666 /dev/net/tun
|
|
|
|
|
|
|
|
|
|
Making it world-writable looks bad, but it seems not to be
|
|
exploitable as a security hole. However, it does allow anyone to cre-
|
|
ate useless tap devices (useless because they can't configure them),
|
|
which is a DOS attack. A somewhat more secure alternative would to be
|
|
to create a group containing all the users who have preconfigured tap
|
|
devices and chgrp /dev/net/tun to that group with mode 664 or 660.
|
|
|
|
|
|
o Once the device is set up, run UML with 'eth0=tuntap,device name'
|
|
(i.e. 'eth0=tuntap,tap0') on the command line (or do it with the
|
|
mconsole config command).
|
|
|
|
o Bring the eth device up in UML and you're in business.
|
|
|
|
If you don't want that tap device any more, you can make it non-
|
|
persistent with
|
|
|
|
|
|
host# tunctl -d tap device
|
|
|
|
|
|
|
|
|
|
Finally, tunctl has a -b (for brief mode) switch which causes it to
|
|
output only the name of the tap device it created. This makes it
|
|
suitable for capture by a script:
|
|
|
|
|
|
host# TAP=`tunctl -u 1000 -b`
|
|
|
|
|
|
|
|
|
|
|
|
|
|
6.8. Ethertap
|
|
|
|
Ethertap is the general mechanism on 2.2 for userspace processes to
|
|
exchange packets with the kernel.
|
|
|
|
|
|
|
|
To use this transport, you need to describe the virtual network device
|
|
on the UML command line. The general format for this is
|
|
|
|
|
|
eth <n> =ethertap, <device> , <ethernet address> , <tap IP address>
|
|
|
|
|
|
|
|
|
|
So, the previous example
|
|
|
|
|
|
eth0=ethertap,tap0,fe:fd:0:0:0:1,192.168.0.254
|
|
|
|
|
|
|
|
|
|
attaches the UML eth0 device to the host /dev/tap0, assigns it the
|
|
ethernet address fe:fd:0:0:0:1, and assigns the IP address
|
|
192.168.0.254 to the tap device.
|
|
|
|
|
|
|
|
The tap device is mandatory, but the others are optional. If the
|
|
ethernet address is omitted, one will be assigned to it.
|
|
|
|
|
|
The presence of the tap IP address will cause the helper to run and do
|
|
whatever host setup is needed to allow the virtual machine to
|
|
communicate with the outside world. If you're not sure you know what
|
|
you're doing, this is the way to go.
|
|
|
|
|
|
If it is absent, then you must configure the tap device and whatever
|
|
arping and routing you will need on the host. However, even in this
|
|
case, the uml_net helper still needs to be in your path and it must be
|
|
setuid root if you're not running UML as root. This is because the
|
|
tap device doesn't support SIGIO, which UML needs in order to use
|
|
something as a source of input. So, the helper is used as a
|
|
convenient asynchronous IO thread.
|
|
|
|
If you're using the uml_net helper, you can ignore the following host
|
|
setup - uml_net will do it for you. You just need to make sure you
|
|
have ethertap available, either built in to the host kernel or
|
|
available as a module.
|
|
|
|
|
|
If you want to set things up yourself, you need to make sure that the
|
|
appropriate /dev entry exists. If it doesn't, become root and create
|
|
it as follows:
|
|
|
|
|
|
mknod /dev/tap <minor> c 36 <minor> + 16
|
|
|
|
|
|
|
|
|
|
For example, this is how to create /dev/tap0:
|
|
|
|
|
|
mknod /dev/tap0 c 36 0 + 16
|
|
|
|
|
|
|
|
|
|
You also need to make sure that the host kernel has ethertap support.
|
|
If ethertap is enabled as a module, you apparently need to insmod
|
|
ethertap once for each ethertap device you want to enable. So,
|
|
|
|
|
|
host#
|
|
insmod ethertap
|
|
|
|
|
|
|
|
|
|
will give you the tap0 interface. To get the tap1 interface, you need
|
|
to run
|
|
|
|
|
|
host#
|
|
insmod ethertap unit=1 -o ethertap1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
6.9. The switch daemon
|
|
|
|
Note: This is the daemon formerly known as uml_router, but which was
|
|
renamed so the network weenies of the world would stop growling at me.
|
|
|
|
|
|
The switch daemon, uml_switch, provides a mechanism for creating a
|
|
totally virtual network. By default, it provides no connection to the
|
|
host network (but see -tap, below).
|
|
|
|
|
|
The first thing you need to do is run the daemon. Running it with no
|
|
arguments will make it listen on a default pair of unix domain
|
|
sockets.
|
|
|
|
|
|
If you want it to listen on a different pair of sockets, use
|
|
|
|
|
|
-unix control socket data socket
|
|
|
|
|
|
|
|
|
|
|
|
If you want it to act as a hub rather than a switch, use
|
|
|
|
|
|
-hub
|
|
|
|
|
|
|
|
|
|
|
|
If you want the switch to be connected to host networking (allowing
|
|
the umls to get access to the outside world through the host), use
|
|
|
|
|
|
-tap tap0
|
|
|
|
|
|
|
|
|
|
|
|
Note that the tap device must be preconfigured (see "TUN/TAP with a
|
|
preconfigured tap device", above). If you're using a different tap
|
|
device than tap0, specify that instead of tap0.
|
|
|
|
|
|
uml_switch can be backgrounded as follows
|
|
|
|
|
|
host%
|
|
uml_switch [ options ] < /dev/null > /dev/null
|
|
|
|
|
|
|
|
|
|
The reason it doesn't background by default is that it listens to
|
|
stdin for EOF. When it sees that, it exits.
|
|
|
|
|
|
The general format of the kernel command line switch is
|
|
|
|
|
|
|
|
ethn=daemon,ethernet address,socket
|
|
type,control socket,data socket
|
|
|
|
|
|
|
|
|
|
You can leave off everything except the 'daemon'. You only need to
|
|
specify the ethernet address if the one that will be assigned to it
|
|
isn't acceptable for some reason. The rest of the arguments describe
|
|
how to communicate with the daemon. You should only specify them if
|
|
you told the daemon to use different sockets than the default. So, if
|
|
you ran the daemon with no arguments, running the UML on the same
|
|
machine with
|
|
eth0=daemon
|
|
|
|
|
|
|
|
|
|
will cause the eth0 driver to attach itself to the daemon correctly.
|
|
|
|
|
|
|
|
6.10. Slip
|
|
|
|
Slip is another, less general, mechanism for a process to communicate
|
|
with the host networking. In contrast to the ethertap interface,
|
|
which exchanges ethernet frames with the host and can be used to
|
|
transport any higher-level protocol, it can only be used to transport
|
|
IP.
|
|
|
|
|
|
The general format of the command line switch is
|
|
|
|
|
|
|
|
ethn=slip,slip IP
|
|
|
|
|
|
|
|
|
|
The slip IP argument is the IP address that will be assigned to the
|
|
host end of the slip device. If it is specified, the helper will run
|
|
and will set up the host so that the virtual machine can reach it and
|
|
the rest of the network.
|
|
|
|
|
|
There are some oddities with this interface that you should be aware
|
|
of. You should only specify one slip device on a given virtual
|
|
machine, and its name inside UML will be 'umn', not 'eth0' or whatever
|
|
you specified on the command line. These problems will be fixed at
|
|
some point.
|
|
|
|
|
|
|
|
6.11. Slirp
|
|
|
|
slirp uses an external program, usually /usr/bin/slirp, to provide IP
|
|
only networking connectivity through the host. This is similar to IP
|
|
masquerading with a firewall, although the translation is performed in
|
|
user-space, rather than by the kernel. As slirp does not set up any
|
|
interfaces on the host, or changes routing, slirp does not require
|
|
root access or setuid binaries on the host.
|
|
|
|
|
|
The general format of the command line switch for slirp is:
|
|
|
|
|
|
|
|
ethn=slirp,ethernet address,slirp path
|
|
|
|
|
|
|
|
|
|
The ethernet address is optional, as UML will set up the interface
|
|
with an ethernet address based upon the initial IP address of the
|
|
interface. The slirp path is generally /usr/bin/slirp, although it
|
|
will depend on distribution.
|
|
|
|
|
|
The slirp program can have a number of options passed to the command
|
|
line and we can't add them to the UML command line, as they will be
|
|
parsed incorrectly. Instead, a wrapper shell script can be written or
|
|
the options inserted into the /.slirprc file. More information on
|
|
all of the slirp options can be found in its man pages.
|
|
|
|
|
|
The eth0 interface on UML should be set up with the IP 10.2.0.15,
|
|
although you can use anything as long as it is not used by a network
|
|
you will be connecting to. The default route on UML should be set to
|
|
use
|
|
|
|
|
|
UML#
|
|
route add default dev eth0
|
|
|
|
|
|
|
|
|
|
slirp provides a number of useful IP addresses which can be used by
|
|
UML, such as 10.0.2.3 which is an alias for the DNS server specified
|
|
in /etc/resolv.conf on the host or the IP given in the 'dns' option
|
|
for slirp.
|
|
|
|
|
|
Even with a baudrate setting higher than 115200, the slirp connection
|
|
is limited to 115200. If you need it to go faster, the slirp binary
|
|
needs to be compiled with FULL_BOLT defined in config.h.
|
|
|
|
|
|
|
|
6.12. pcap
|
|
|
|
The pcap transport is attached to a UML ethernet device on the command
|
|
line or with uml_mconsole with the following syntax:
|
|
|
|
|
|
|
|
ethn=pcap,host interface,filter
|
|
expression,option1,option2
|
|
|
|
|
|
|
|
|
|
The expression and options are optional.
|
|
|
|
|
|
The interface is whatever network device on the host you want to
|
|
sniff. The expression is a pcap filter expression, which is also what
|
|
tcpdump uses, so if you know how to specify tcpdump filters, you will
|
|
use the same expressions here. The options are up to two of
|
|
'promisc', control whether pcap puts the host interface into
|
|
promiscuous mode. 'optimize' and 'nooptimize' control whether the pcap
|
|
expression optimizer is used.
|
|
|
|
|
|
Example:
|
|
|
|
|
|
|
|
eth0=pcap,eth0,tcp
|
|
|
|
eth1=pcap,eth0,!tcp
|
|
|
|
|
|
|
|
will cause the UML eth0 to emit all tcp packets on the host eth0 and
|
|
the UML eth1 to emit all non-tcp packets on the host eth0.
|
|
|
|
|
|
|
|
6.13. Setting up the host yourself
|
|
|
|
If you don't specify an address for the host side of the ethertap or
|
|
slip device, UML won't do any setup on the host. So this is what is
|
|
needed to get things working (the examples use a host-side IP of
|
|
192.168.0.251 and a UML-side IP of 192.168.0.250 - adjust to suit your
|
|
own network):
|
|
|
|
o The device needs to be configured with its IP address. Tap devices
|
|
are also configured with an mtu of 1484. Slip devices are
|
|
configured with a point-to-point address pointing at the UML ip
|
|
address.
|
|
|
|
|
|
host# ifconfig tap0 arp mtu 1484 192.168.0.251 up
|
|
|
|
|
|
|
|
|
|
|
|
|
|
host#
|
|
ifconfig sl0 192.168.0.251 pointopoint 192.168.0.250 up
|
|
|
|
|
|
|
|
|
|
|
|
o If a tap device is being set up, a route is set to the UML IP.
|
|
|
|
|
|
UML# route add -host 192.168.0.250 gw 192.168.0.251
|
|
|
|
|
|
|
|
|
|
|
|
o To allow other hosts on your network to see the virtual machine,
|
|
proxy arp is set up for it.
|
|
|
|
|
|
host# arp -Ds 192.168.0.250 eth0 pub
|
|
|
|
|
|
|
|
|
|
|
|
o Finally, the host is set up to route packets.
|
|
|
|
|
|
host# echo 1 > /proc/sys/net/ipv4/ip_forward
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
7. Sharing Filesystems between Virtual Machines
|
|
|
|
|
|
|
|
|
|
7.1. A warning
|
|
|
|
Don't attempt to share filesystems simply by booting two UMLs from the
|
|
same file. That's the same thing as booting two physical machines
|
|
from a shared disk. It will result in filesystem corruption.
|
|
|
|
|
|
|
|
7.2. Using layered block devices
|
|
|
|
The way to share a filesystem between two virtual machines is to use
|
|
the copy-on-write (COW) layering capability of the ubd block driver.
|
|
As of 2.4.6-2um, the driver supports layering a read-write private
|
|
device over a read-only shared device. A machine's writes are stored
|
|
in the private device, while reads come from either device - the
|
|
private one if the requested block is valid in it, the shared one if
|
|
not. Using this scheme, the majority of data which is unchanged is
|
|
shared between an arbitrary number of virtual machines, each of which
|
|
has a much smaller file containing the changes that it has made. With
|
|
a large number of UMLs booting from a large root filesystem, this
|
|
leads to a huge disk space saving. It will also help performance,
|
|
since the host will be able to cache the shared data using a much
|
|
smaller amount of memory, so UML disk requests will be served from the
|
|
host's memory rather than its disks.
|
|
|
|
|
|
|
|
|
|
To add a copy-on-write layer to an existing block device file, simply
|
|
add the name of the COW file to the appropriate ubd switch:
|
|
|
|
|
|
ubd0=root_fs_cow,root_fs_debian_22
|
|
|
|
|
|
|
|
|
|
where 'root_fs_cow' is the private COW file and 'root_fs_debian_22' is
|
|
the existing shared filesystem. The COW file need not exist. If it
|
|
doesn't, the driver will create and initialize it. Once the COW file
|
|
has been initialized, it can be used on its own on the command line:
|
|
|
|
|
|
ubd0=root_fs_cow
|
|
|
|
|
|
|
|
|
|
The name of the backing file is stored in the COW file header, so it
|
|
would be redundant to continue specifying it on the command line.
|
|
|
|
|
|
|
|
7.3. Note!
|
|
|
|
When checking the size of the COW file in order to see the gobs of
|
|
space that you're saving, make sure you use 'ls -ls' to see the actual
|
|
disk consumption rather than the length of the file. The COW file is
|
|
sparse, so the length will be very different from the disk usage.
|
|
Here is a 'ls -l' of a COW file and backing file from one boot and
|
|
shutdown:
|
|
host% ls -l cow.debian debian2.2
|
|
-rw-r--r-- 1 jdike jdike 492504064 Aug 6 21:16 cow.debian
|
|
-rwxrw-rw- 1 jdike jdike 537919488 Aug 6 20:42 debian2.2
|
|
|
|
|
|
|
|
|
|
Doesn't look like much saved space, does it? Well, here's 'ls -ls':
|
|
|
|
|
|
host% ls -ls cow.debian debian2.2
|
|
880 -rw-r--r-- 1 jdike jdike 492504064 Aug 6 21:16 cow.debian
|
|
525832 -rwxrw-rw- 1 jdike jdike 537919488 Aug 6 20:42 debian2.2
|
|
|
|
|
|
|
|
|
|
Now, you can see that the COW file has less than a meg of disk, rather
|
|
than 492 meg.
|
|
|
|
|
|
|
|
7.4. Another warning
|
|
|
|
Once a filesystem is being used as a readonly backing file for a COW
|
|
file, do not boot directly from it or modify it in any way. Doing so
|
|
will invalidate any COW files that are using it. The mtime and size
|
|
of the backing file are stored in the COW file header at its creation,
|
|
and they must continue to match. If they don't, the driver will
|
|
refuse to use the COW file.
|
|
|
|
|
|
|
|
|
|
If you attempt to evade this restriction by changing either the
|
|
backing file or the COW header by hand, you will get a corrupted
|
|
filesystem.
|
|
|
|
|
|
|
|
|
|
Among other things, this means that upgrading the distribution in a
|
|
backing file and expecting that all of the COW files using it will see
|
|
the upgrade will not work.
|
|
|
|
|
|
|
|
|
|
7.5. uml_moo : Merging a COW file with its backing file
|
|
|
|
Depending on how you use UML and COW devices, it may be advisable to
|
|
merge the changes in the COW file into the backing file every once in
|
|
a while.
|
|
|
|
|
|
|
|
|
|
The utility that does this is uml_moo. Its usage is
|
|
|
|
|
|
host% uml_moo COW file new backing file
|
|
|
|
|
|
|
|
|
|
There's no need to specify the backing file since that information is
|
|
already in the COW file header. If you're paranoid, boot the new
|
|
merged file, and if you're happy with it, move it over the old backing
|
|
file.
|
|
|
|
|
|
|
|
|
|
uml_moo creates a new backing file by default as a safety measure. It
|
|
also has a destructive merge option which will merge the COW file
|
|
directly into its current backing file. This is really only usable
|
|
when the backing file only has one COW file associated with it. If
|
|
there are multiple COWs associated with a backing file, a -d merge of
|
|
one of them will invalidate all of the others. However, it is
|
|
convenient if you're short of disk space, and it should also be
|
|
noticeably faster than a non-destructive merge.
|
|
|
|
|
|
|
|
|
|
uml_moo is installed with the UML deb and RPM. If you didn't install
|
|
UML from one of those packages, you can also get it from the UML
|
|
utilities <http://user-mode-linux.sourceforge.net/
|
|
utilities> tar file in tools/moo.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
8. Creating filesystems
|
|
|
|
|
|
You may want to create and mount new UML filesystems, either because
|
|
your root filesystem isn't large enough or because you want to use a
|
|
filesystem other than ext2.
|
|
|
|
|
|
This was written on the occasion of reiserfs being included in the
|
|
2.4.1 kernel pool, and therefore the 2.4.1 UML, so the examples will
|
|
talk about reiserfs. This information is generic, and the examples
|
|
should be easy to translate to the filesystem of your choice.
|
|
|
|
|
|
8.1. Create the filesystem file
|
|
|
|
dd is your friend. All you need to do is tell dd to create an empty
|
|
file of the appropriate size. I usually make it sparse to save time
|
|
and to avoid allocating disk space until it's actually used. For
|
|
example, the following command will create a sparse 100 meg file full
|
|
of zeroes.
|
|
|
|
|
|
host%
|
|
dd if=/dev/zero of=new_filesystem seek=100 count=1 bs=1M
|
|
|
|
|
|
|
|
|
|
|
|
|
|
8.2. Assign the file to a UML device
|
|
|
|
Add an argument like the following to the UML command line:
|
|
|
|
ubd4=new_filesystem
|
|
|
|
|
|
|
|
|
|
making sure that you use an unassigned ubd device number.
|
|
|
|
|
|
|
|
8.3. Creating and mounting the filesystem
|
|
|
|
Make sure that the filesystem is available, either by being built into
|
|
the kernel, or available as a module, then boot up UML and log in. If
|
|
the root filesystem doesn't have the filesystem utilities (mkfs, fsck,
|
|
etc), then get them into UML by way of the net or hostfs.
|
|
|
|
|
|
Make the new filesystem on the device assigned to the new file:
|
|
|
|
|
|
host# mkreiserfs /dev/ubd/4
|
|
|
|
|
|
<----------- MKREISERFSv2 ----------->
|
|
|
|
ReiserFS version 3.6.25
|
|
Block size 4096 bytes
|
|
Block count 25856
|
|
Used blocks 8212
|
|
Journal - 8192 blocks (18-8209), journal header is in block 8210
|
|
Bitmaps: 17
|
|
Root block 8211
|
|
Hash function "r5"
|
|
ATTENTION: ALL DATA WILL BE LOST ON '/dev/ubd/4'! (y/n)y
|
|
journal size 8192 (from 18)
|
|
Initializing journal - 0%....20%....40%....60%....80%....100%
|
|
Syncing..done.
|
|
|
|
|
|
|
|
|
|
Now, mount it:
|
|
|
|
|
|
UML#
|
|
mount /dev/ubd/4 /mnt
|
|
|
|
|
|
|
|
|
|
and you're in business.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
9. Host file access
|
|
|
|
|
|
If you want to access files on the host machine from inside UML, you
|
|
can treat it as a separate machine and either nfs mount directories
|
|
from the host or copy files into the virtual machine with scp or rcp.
|
|
However, since UML is running on the host, it can access those
|
|
files just like any other process and make them available inside the
|
|
virtual machine without needing to use the network.
|
|
|
|
|
|
This is now possible with the hostfs virtual filesystem. With it, you
|
|
can mount a host directory into the UML filesystem and access the
|
|
files contained in it just as you would on the host.
|
|
|
|
|
|
9.1. Using hostfs
|
|
|
|
To begin with, make sure that hostfs is available inside the virtual
|
|
machine with
|
|
|
|
|
|
UML# cat /proc/filesystems
|
|
|
|
|
|
|
|
. hostfs should be listed. If it's not, either rebuild the kernel
|
|
with hostfs configured into it or make sure that hostfs is built as a
|
|
module and available inside the virtual machine, and insmod it.
|
|
|
|
|
|
Now all you need to do is run mount:
|
|
|
|
|
|
UML# mount none /mnt/host -t hostfs
|
|
|
|
|
|
|
|
|
|
will mount the host's / on the virtual machine's /mnt/host.
|
|
|
|
|
|
If you don't want to mount the host root directory, then you can
|
|
specify a subdirectory to mount with the -o switch to mount:
|
|
|
|
|
|
UML# mount none /mnt/home -t hostfs -o /home
|
|
|
|
|
|
|
|
|
|
will mount the hosts's /home on the virtual machine's /mnt/home.
|
|
|
|
|
|
|
|
9.2. hostfs as the root filesystem
|
|
|
|
It's possible to boot from a directory hierarchy on the host using
|
|
hostfs rather than using the standard filesystem in a file.
|
|
|
|
To start, you need that hierarchy. The easiest way is to loop mount
|
|
an existing root_fs file:
|
|
|
|
|
|
host# mount root_fs uml_root_dir -o loop
|
|
|
|
|
|
|
|
|
|
You need to change the filesystem type of / in etc/fstab to be
|
|
'hostfs', so that line looks like this:
|
|
|
|
/dev/ubd/0 / hostfs defaults 1 1
|
|
|
|
|
|
|
|
|
|
Then you need to chown to yourself all the files in that directory
|
|
that are owned by root. This worked for me:
|
|
|
|
|
|
host# find . -uid 0 -exec chown jdike {} \;
|
|
|
|
|
|
|
|
|
|
Next, make sure that your UML kernel has hostfs compiled in, not as a
|
|
module. Then run UML with the boot device pointing at that directory:
|
|
|
|
|
|
ubd0=/path/to/uml/root/directory
|
|
|
|
|
|
|
|
|
|
UML should then boot as it does normally.
|
|
|
|
|
|
9.3. Building hostfs
|
|
|
|
If you need to build hostfs because it's not in your kernel, you have
|
|
two choices:
|
|
|
|
|
|
|
|
o Compiling hostfs into the kernel:
|
|
|
|
|
|
Reconfigure the kernel and set the 'Host filesystem' option under
|
|
|
|
|
|
o Compiling hostfs as a module:
|
|
|
|
|
|
Reconfigure the kernel and set the 'Host filesystem' option under
|
|
be in arch/um/fs/hostfs/hostfs.o. Install that in
|
|
/lib/modules/`uname -r`/fs in the virtual machine, boot it up, and
|
|
|
|
|
|
UML# insmod hostfs
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
10. The Management Console
|
|
|
|
|
|
|
|
The UML management console is a low-level interface to the kernel,
|
|
somewhat like the i386 SysRq interface. Since there is a full-blown
|
|
operating system under UML, there is much greater flexibility possible
|
|
than with the SysRq mechanism.
|
|
|
|
|
|
There are a number of things you can do with the mconsole interface:
|
|
|
|
o get the kernel version
|
|
|
|
o add and remove devices
|
|
|
|
o halt or reboot the machine
|
|
|
|
o Send SysRq commands
|
|
|
|
o Pause and resume the UML
|
|
|
|
|
|
You need the mconsole client (uml_mconsole) which is present in CVS
|
|
(/tools/mconsole) in 2.4.5-9um and later, and will be in the RPM in
|
|
2.4.6.
|
|
|
|
|
|
You also need CONFIG_MCONSOLE (under 'General Setup') enabled in UML.
|
|
When you boot UML, you'll see a line like:
|
|
|
|
|
|
mconsole initialized on /home/jdike/.uml/umlNJ32yL/mconsole
|
|
|
|
|
|
|
|
|
|
If you specify a unique machine id one the UML command line, i.e.
|
|
|
|
|
|
umid=debian
|
|
|
|
|
|
|
|
|
|
you'll see this
|
|
|
|
|
|
mconsole initialized on /home/jdike/.uml/debian/mconsole
|
|
|
|
|
|
|
|
|
|
That file is the socket that uml_mconsole will use to communicate with
|
|
UML. Run it with either the umid or the full path as its argument:
|
|
|
|
|
|
host% uml_mconsole debian
|
|
|
|
|
|
|
|
|
|
or
|
|
|
|
|
|
host% uml_mconsole /home/jdike/.uml/debian/mconsole
|
|
|
|
|
|
|
|
|
|
You'll get a prompt, at which you can run one of these commands:
|
|
|
|
o version
|
|
|
|
o halt
|
|
|
|
o reboot
|
|
|
|
o config
|
|
|
|
o remove
|
|
|
|
o sysrq
|
|
|
|
o help
|
|
|
|
o cad
|
|
|
|
o stop
|
|
|
|
o go
|
|
|
|
|
|
10.1. version
|
|
|
|
This takes no arguments. It prints the UML version.
|
|
|
|
|
|
(mconsole) version
|
|
OK Linux usermode 2.4.5-9um #1 Wed Jun 20 22:47:08 EDT 2001 i686
|
|
|
|
|
|
|
|
|
|
There are a couple actual uses for this. It's a simple no-op which
|
|
can be used to check that a UML is running. It's also a way of
|
|
sending an interrupt to the UML. This is sometimes useful on SMP
|
|
hosts, where there's a bug which causes signals to UML to be lost,
|
|
often causing it to appear to hang. Sending such a UML the mconsole
|
|
version command is a good way to 'wake it up' before networking has
|
|
been enabled, as it does not do anything to the function of the UML.
|
|
|
|
|
|
|
|
10.2. halt and reboot
|
|
|
|
These take no arguments. They shut the machine down immediately, with
|
|
no syncing of disks and no clean shutdown of userspace. So, they are
|
|
pretty close to crashing the machine.
|
|
|
|
|
|
(mconsole) halt
|
|
OK
|
|
|
|
|
|
|
|
|
|
|
|
|
|
10.3. config
|
|
|
|
"config" adds a new device to the virtual machine. Currently the ubd
|
|
and network drivers support this. It takes one argument, which is the
|
|
device to add, with the same syntax as the kernel command line.
|
|
|
|
|
|
|
|
|
|
(mconsole)
|
|
config ubd3=/home/jdike/incoming/roots/root_fs_debian22
|
|
|
|
OK
|
|
(mconsole) config eth1=mcast
|
|
OK
|
|
|
|
|
|
|
|
|
|
|
|
|
|
10.4. remove
|
|
|
|
"remove" deletes a device from the system. Its argument is just the
|
|
name of the device to be removed. The device must be idle in whatever
|
|
sense the driver considers necessary. In the case of the ubd driver,
|
|
the removed block device must not be mounted, swapped on, or otherwise
|
|
open, and in the case of the network driver, the device must be down.
|
|
|
|
|
|
(mconsole) remove ubd3
|
|
OK
|
|
(mconsole) remove eth1
|
|
OK
|
|
|
|
|
|
|
|
|
|
|
|
|
|
10.5. sysrq
|
|
|
|
This takes one argument, which is a single letter. It calls the
|
|
generic kernel's SysRq driver, which does whatever is called for by
|
|
that argument. See the SysRq documentation in Documentation/sysrq.txt
|
|
in your favorite kernel tree to see what letters are valid and what
|
|
they do.
|
|
|
|
|
|
|
|
10.6. help
|
|
|
|
"help" returns a string listing the valid commands and what each one
|
|
does.
|
|
|
|
|
|
|
|
10.7. cad
|
|
|
|
This invokes the Ctl-Alt-Del action on init. What exactly this ends
|
|
up doing is up to /etc/inittab. Normally, it reboots the machine.
|
|
With UML, this is usually not desired, so if a halt would be better,
|
|
then find the section of inittab that looks like this
|
|
|
|
|
|
# What to do when CTRL-ALT-DEL is pressed.
|
|
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
|
|
|
|
|
|
|
|
|
|
and change the command to halt.
|
|
|
|
|
|
|
|
10.8. stop
|
|
|
|
This puts the UML in a loop reading mconsole requests until a 'go'
|
|
mconsole command is received. This is very useful for making backups
|
|
of UML filesystems, as the UML can be stopped, then synced via 'sysrq
|
|
s', so that everything is written to the filesystem. You can then copy
|
|
the filesystem and then send the UML 'go' via mconsole.
|
|
|
|
|
|
Note that a UML running with more than one CPU will have problems
|
|
after you send the 'stop' command, as only one CPU will be held in a
|
|
mconsole loop and all others will continue as normal. This is a bug,
|
|
and will be fixed.
|
|
|
|
|
|
|
|
10.9. go
|
|
|
|
This resumes a UML after being paused by a 'stop' command. Note that
|
|
when the UML has resumed, TCP connections may have timed out and if
|
|
the UML is paused for a long period of time, crond might go a little
|
|
crazy, running all the jobs it didn't do earlier.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
11. Kernel debugging
|
|
|
|
|
|
Note: The interface that makes debugging, as described here, possible
|
|
is present in 2.4.0-test6 kernels and later.
|
|
|
|
|
|
Since the user-mode kernel runs as a normal Linux process, it is
|
|
possible to debug it with gdb almost like any other process. It is
|
|
slightly different because the kernel's threads are already being
|
|
ptraced for system call interception, so gdb can't ptrace them.
|
|
However, a mechanism has been added to work around that problem.
|
|
|
|
|
|
In order to debug the kernel, you need build it from source. See
|
|
``Compiling the kernel and modules'' for information on doing that.
|
|
Make sure that you enable CONFIG_DEBUGSYM and CONFIG_PT_PROXY during
|
|
the config. These will compile the kernel with -g, and enable the
|
|
ptrace proxy so that gdb works with UML, respectively.
|
|
|
|
|
|
|
|
|
|
11.1. Starting the kernel under gdb
|
|
|
|
You can have the kernel running under the control of gdb from the
|
|
beginning by putting 'debug' on the command line. You will get an
|
|
xterm with gdb running inside it. The kernel will send some commands
|
|
to gdb which will leave it stopped at the beginning of start_kernel.
|
|
At this point, you can get things going with 'next', 'step', or
|
|
'cont'.
|
|
|
|
|
|
There is a transcript of a debugging session here <debug-
|
|
session.html> , with breakpoints being set in the scheduler and in an
|
|
interrupt handler.
|
|
11.2. Examining sleeping processes
|
|
|
|
Not every bug is evident in the currently running process. Sometimes,
|
|
processes hang in the kernel when they shouldn't because they've
|
|
deadlocked on a semaphore or something similar. In this case, when
|
|
you ^C gdb and get a backtrace, you will see the idle thread, which
|
|
isn't very relevant.
|
|
|
|
|
|
What you want is the stack of whatever process is sleeping when it
|
|
shouldn't be. You need to figure out which process that is, which is
|
|
generally fairly easy. Then you need to get its host process id,
|
|
which you can do either by looking at ps on the host or at
|
|
task.thread.extern_pid in gdb.
|
|
|
|
|
|
Now what you do is this:
|
|
|
|
o detach from the current thread
|
|
|
|
|
|
(UML gdb) det
|
|
|
|
|
|
|
|
|
|
|
|
o attach to the thread you are interested in
|
|
|
|
|
|
(UML gdb) att <host pid>
|
|
|
|
|
|
|
|
|
|
|
|
o look at its stack and anything else of interest
|
|
|
|
|
|
(UML gdb) bt
|
|
|
|
|
|
|
|
|
|
Note that you can't do anything at this point that requires that a
|
|
process execute, e.g. calling a function
|
|
|
|
o when you're done looking at that process, reattach to the current
|
|
thread and continue it
|
|
|
|
|
|
(UML gdb)
|
|
att 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(UML gdb)
|
|
c
|
|
|
|
|
|
|
|
|
|
Here, specifying any pid which is not the process id of a UML thread
|
|
will cause gdb to reattach to the current thread. I commonly use 1,
|
|
but any other invalid pid would work.
|
|
|
|
|
|
|
|
11.3. Running ddd on UML
|
|
|
|
ddd works on UML, but requires a special kludge. The process goes
|
|
like this:
|
|
|
|
o Start ddd
|
|
|
|
|
|
host% ddd linux
|
|
|
|
|
|
|
|
|
|
|
|
o With ps, get the pid of the gdb that ddd started. You can ask the
|
|
gdb to tell you, but for some reason that confuses things and
|
|
causes a hang.
|
|
|
|
o run UML with 'debug=parent gdb-pid=<pid>' added to the command line
|
|
- it will just sit there after you hit return
|
|
|
|
o type 'att 1' to the ddd gdb and you will see something like
|
|
|
|
|
|
0xa013dc51 in __kill ()
|
|
|
|
|
|
(gdb)
|
|
|
|
|
|
|
|
|
|
|
|
o At this point, type 'c', UML will boot up, and you can use ddd just
|
|
as you do on any other process.
|
|
|
|
|
|
|
|
11.4. Debugging modules
|
|
|
|
gdb has support for debugging code which is dynamically loaded into
|
|
the process. This support is what is needed to debug kernel modules
|
|
under UML.
|
|
|
|
|
|
Using that support is somewhat complicated. You have to tell gdb what
|
|
object file you just loaded into UML and where in memory it is. Then,
|
|
it can read the symbol table, and figure out where all the symbols are
|
|
from the load address that you provided. It gets more interesting
|
|
when you load the module again (i.e. after an rmmod). You have to
|
|
tell gdb to forget about all its symbols, including the main UML ones
|
|
for some reason, then load then all back in again.
|
|
|
|
|
|
There's an easy way and a hard way to do this. The easy way is to use
|
|
the umlgdb expect script written by Chandan Kudige. It basically
|
|
automates the process for you.
|
|
|
|
|
|
First, you must tell it where your modules are. There is a list in
|
|
the script that looks like this:
|
|
set MODULE_PATHS {
|
|
"fat" "/usr/src/uml/linux-2.4.18/fs/fat/fat.o"
|
|
"isofs" "/usr/src/uml/linux-2.4.18/fs/isofs/isofs.o"
|
|
"minix" "/usr/src/uml/linux-2.4.18/fs/minix/minix.o"
|
|
}
|
|
|
|
|
|
|
|
|
|
You change that to list the names and paths of the modules that you
|
|
are going to debug. Then you run it from the toplevel directory of
|
|
your UML pool and it basically tells you what to do:
|
|
|
|
|
|
|
|
|
|
******** GDB pid is 21903 ********
|
|
Start UML as: ./linux <kernel switches> debug gdb-pid=21903
|
|
|
|
|
|
|
|
GNU gdb 5.0rh-5 Red Hat Linux 7.1
|
|
Copyright 2001 Free Software Foundation, Inc.
|
|
GDB is free software, covered by the GNU General Public License, and you are
|
|
welcome to change it and/or distribute copies of it under certain conditions.
|
|
Type "show copying" to see the conditions.
|
|
There is absolutely no warranty for GDB. Type "show warranty" for details.
|
|
This GDB was configured as "i386-redhat-linux"...
|
|
(gdb) b sys_init_module
|
|
Breakpoint 1 at 0xa0011923: file module.c, line 349.
|
|
(gdb) att 1
|
|
|
|
|
|
|
|
|
|
After you run UML and it sits there doing nothing, you hit return at
|
|
the 'att 1' and continue it:
|
|
|
|
|
|
Attaching to program: /home/jdike/linux/2.4/um/./linux, process 1
|
|
0xa00f4221 in __kill ()
|
|
(UML gdb) c
|
|
Continuing.
|
|
|
|
|
|
|
|
|
|
At this point, you debug normally. When you insmod something, the
|
|
expect magic will kick in and you'll see something like:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
*** Module hostfs loaded ***
|
|
Breakpoint 1, sys_init_module (name_user=0x805abb0 "hostfs",
|
|
mod_user=0x8070e00) at module.c:349
|
|
349 char *name, *n_name, *name_tmp = NULL;
|
|
(UML gdb) finish
|
|
Run till exit from #0 sys_init_module (name_user=0x805abb0 "hostfs",
|
|
mod_user=0x8070e00) at module.c:349
|
|
0xa00e2e23 in execute_syscall (r=0xa8140284) at syscall_kern.c:411
|
|
411 else res = EXECUTE_SYSCALL(syscall, regs);
|
|
Value returned is $1 = 0
|
|
(UML gdb)
|
|
p/x (int)module_list + module_list->size_of_struct
|
|
|
|
$2 = 0xa9021054
|
|
(UML gdb) symbol-file ./linux
|
|
Load new symbol table from "./linux"? (y or n) y
|
|
Reading symbols from ./linux...
|
|
done.
|
|
(UML gdb)
|
|
add-symbol-file /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o 0xa9021054
|
|
|
|
add symbol table from file "/home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o" at
|
|
.text_addr = 0xa9021054
|
|
(y or n) y
|
|
|
|
Reading symbols from /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o...
|
|
done.
|
|
(UML gdb) p *module_list
|
|
$1 = {size_of_struct = 84, next = 0xa0178720, name = 0xa9022de0 "hostfs",
|
|
size = 9016, uc = {usecount = {counter = 0}, pad = 0}, flags = 1,
|
|
nsyms = 57, ndeps = 0, syms = 0xa9023170, deps = 0x0, refs = 0x0,
|
|
init = 0xa90221f0 <init_hostfs>, cleanup = 0xa902222c <exit_hostfs>,
|
|
ex_table_start = 0x0, ex_table_end = 0x0, persist_start = 0x0,
|
|
persist_end = 0x0, can_unload = 0, runsize = 0, kallsyms_start = 0x0,
|
|
kallsyms_end = 0x0,
|
|
archdata_start = 0x1b855 <Address 0x1b855 out of bounds>,
|
|
archdata_end = 0xe5890000 <Address 0xe5890000 out of bounds>,
|
|
kernel_data = 0xf689c35d <Address 0xf689c35d out of bounds>}
|
|
>> Finished loading symbols for hostfs ...
|
|
|
|
|
|
|
|
|
|
That's the easy way. It's highly recommended. The hard way is
|
|
described below in case you're interested in what's going on.
|
|
|
|
|
|
Boot the kernel under the debugger and load the module with insmod or
|
|
modprobe. With gdb, do:
|
|
|
|
|
|
(UML gdb) p module_list
|
|
|
|
|
|
|
|
|
|
This is a list of modules that have been loaded into the kernel, with
|
|
the most recently loaded module first. Normally, the module you want
|
|
is at module_list. If it's not, walk down the next links, looking at
|
|
the name fields until find the module you want to debug. Take the
|
|
address of that structure, and add module.size_of_struct (which in
|
|
2.4.10 kernels is 96 (0x60)) to it. Gdb can make this hard addition
|
|
for you :-):
|
|
|
|
|
|
|
|
(UML gdb)
|
|
printf "%#x\n", (int)module_list module_list->size_of_struct
|
|
|
|
|
|
|
|
|
|
The offset from the module start occasionally changes (before 2.4.0,
|
|
it was module.size_of_struct + 4), so it's a good idea to check the
|
|
init and cleanup addresses once in a while, as describe below. Now
|
|
do:
|
|
|
|
|
|
(UML gdb)
|
|
add-symbol-file /path/to/module/on/host that_address
|
|
|
|
|
|
|
|
|
|
Tell gdb you really want to do it, and you're in business.
|
|
|
|
|
|
If there's any doubt that you got the offset right, like breakpoints
|
|
appear not to work, or they're appearing in the wrong place, you can
|
|
check it by looking at the module structure. The init and cleanup
|
|
fields should look like:
|
|
|
|
|
|
init = 0x588066b0 <init_hostfs>, cleanup = 0x588066c0 <exit_hostfs>
|
|
|
|
|
|
|
|
|
|
with no offsets on the symbol names. If the names are right, but they
|
|
are offset, then the offset tells you how much you need to add to the
|
|
address you gave to add-symbol-file.
|
|
|
|
|
|
When you want to load in a new version of the module, you need to get
|
|
gdb to forget about the old one. The only way I've found to do that
|
|
is to tell gdb to forget about all symbols that it knows about:
|
|
|
|
|
|
(UML gdb) symbol-file
|
|
|
|
|
|
|
|
|
|
Then reload the symbols from the kernel binary:
|
|
|
|
|
|
(UML gdb) symbol-file /path/to/kernel
|
|
|
|
|
|
|
|
|
|
and repeat the process above. You'll also need to re-enable break-
|
|
points. They were disabled when you dumped all the symbols because
|
|
gdb couldn't figure out where they should go.
|
|
|
|
|
|
|
|
11.5. Attaching gdb to the kernel
|
|
|
|
If you don't have the kernel running under gdb, you can attach gdb to
|
|
it later by sending the tracing thread a SIGUSR1. The first line of
|
|
the console output identifies its pid:
|
|
tracing thread pid = 20093
|
|
|
|
|
|
|
|
|
|
When you send it the signal:
|
|
|
|
|
|
host% kill -USR1 20093
|
|
|
|
|
|
|
|
|
|
you will get an xterm with gdb running in it.
|
|
|
|
|
|
If you have the mconsole compiled into UML, then the mconsole client
|
|
can be used to start gdb:
|
|
|
|
|
|
(mconsole) (mconsole) config gdb=xterm
|
|
|
|
|
|
|
|
|
|
will fire up an xterm with gdb running in it.
|
|
|
|
|
|
|
|
11.6. Using alternate debuggers
|
|
|
|
UML has support for attaching to an already running debugger rather
|
|
than starting gdb itself. This is present in CVS as of 17 Apr 2001.
|
|
I sent it to Alan for inclusion in the ac tree, and it will be in my
|
|
2.4.4 release.
|
|
|
|
|
|
This is useful when gdb is a subprocess of some UI, such as emacs or
|
|
ddd. It can also be used to run debuggers other than gdb on UML.
|
|
Below is an example of using strace as an alternate debugger.
|
|
|
|
|
|
To do this, you need to get the pid of the debugger and pass it in
|
|
with the
|
|
|
|
|
|
If you are using gdb under some UI, then tell it to 'att 1', and
|
|
you'll find yourself attached to UML.
|
|
|
|
|
|
If you are using something other than gdb as your debugger, then
|
|
you'll need to get it to do the equivalent of 'att 1' if it doesn't do
|
|
it automatically.
|
|
|
|
|
|
An example of an alternate debugger is strace. You can strace the
|
|
actual kernel as follows:
|
|
|
|
o Run the following in a shell
|
|
|
|
|
|
host%
|
|
sh -c 'echo pid=$$; echo -n hit return; read x; exec strace -p 1 -o strace.out'
|
|
|
|
|
|
|
|
o Run UML with 'debug' and 'gdb-pid=<pid>' with the pid printed out
|
|
by the previous command
|
|
|
|
o Hit return in the shell, and UML will start running, and strace
|
|
output will start accumulating in the output file.
|
|
|
|
Note that this is different from running
|
|
|
|
|
|
host% strace ./linux
|
|
|
|
|
|
|
|
|
|
That will strace only the main UML thread, the tracing thread, which
|
|
doesn't do any of the actual kernel work. It just oversees the vir-
|
|
tual machine. In contrast, using strace as described above will show
|
|
you the low-level activity of the virtual machine.
|
|
|
|
|
|
|
|
|
|
|
|
12. Kernel debugging examples
|
|
|
|
12.1. The case of the hung fsck
|
|
|
|
When booting up the kernel, fsck failed, and dropped me into a shell
|
|
to fix things up. I ran fsck -y, which hung:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Setting hostname uml [ OK ]
|
|
Checking root filesystem
|
|
/dev/fhd0 was not cleanly unmounted, check forced.
|
|
Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.
|
|
|
|
/dev/fhd0: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
|
|
(i.e., without -a or -p options)
|
|
[ FAILED ]
|
|
|
|
*** An error occurred during the file system check.
|
|
*** Dropping you to a shell; the system will reboot
|
|
*** when you leave the shell.
|
|
Give root password for maintenance
|
|
(or type Control-D for normal startup):
|
|
|
|
[root@uml /root]# fsck -y /dev/fhd0
|
|
fsck -y /dev/fhd0
|
|
Parallelizing fsck version 1.14 (9-Jan-1999)
|
|
e2fsck 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09
|
|
/dev/fhd0 contains a file system with errors, check forced.
|
|
Pass 1: Checking inodes, blocks, and sizes
|
|
Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780. Ignore error? yes
|
|
|
|
Inode 19780, i_blocks is 1548, should be 540. Fix? yes
|
|
|
|
Pass 2: Checking directory structure
|
|
Error reading block 49405 (Attempt to read block from filesystem resulted in short read). Ignore error? yes
|
|
|
|
Directory inode 11858, block 0, offset 0: directory corrupted
|
|
Salvage? yes
|
|
|
|
Missing '.' in directory inode 11858.
|
|
Fix? yes
|
|
|
|
Missing '..' in directory inode 11858.
|
|
Fix? yes
|
|
|
|
|
|
|
|
|
|
|
|
The standard drill in this sort of situation is to fire up gdb on the
|
|
signal thread, which, in this case, was pid 1935. In another window,
|
|
I run gdb and attach pid 1935.
|
|
|
|
|
|
|
|
|
|
~/linux/2.3.26/um 1016: gdb linux
|
|
GNU gdb 4.17.0.11 with Linux support
|
|
Copyright 1998 Free Software Foundation, Inc.
|
|
GDB is free software, covered by the GNU General Public License, and you are
|
|
welcome to change it and/or distribute copies of it under certain conditions.
|
|
Type "show copying" to see the conditions.
|
|
There is absolutely no warranty for GDB. Type "show warranty" for details.
|
|
This GDB was configured as "i386-redhat-linux"...
|
|
|
|
(gdb) att 1935
|
|
Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 1935
|
|
0x100756d9 in __wait4 ()
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Let's see what's currently running:
|
|
|
|
|
|
|
|
(gdb) p current_task.pid
|
|
$1 = 0
|
|
|
|
|
|
|
|
|
|
|
|
It's the idle thread, which means that fsck went to sleep for some
|
|
reason and never woke up.
|
|
|
|
|
|
Let's guess that the last process in the process list is fsck:
|
|
|
|
|
|
|
|
(gdb) p current_task.prev_task.comm
|
|
$13 = "fsck.ext2\000\000\000\000\000\000"
|
|
|
|
|
|
|
|
|
|
|
|
It is, so let's see what it thinks it's up to:
|
|
|
|
|
|
|
|
(gdb) p current_task.prev_task.thread
|
|
$14 = {extern_pid = 1980, tracing = 0, want_tracing = 0, forking = 0,
|
|
kernel_stack_page = 0, signal_stack = 1342627840, syscall = {id = 4, args = {
|
|
3, 134973440, 1024, 0, 1024}, have_result = 0, result = 50590720},
|
|
request = {op = 2, u = {exec = {ip = 1350467584, sp = 2952789424}, fork = {
|
|
regs = {1350467584, 2952789424, 0 <repeats 15 times>}, sigstack = 0,
|
|
pid = 0}, switch_to = 0x507e8000, thread = {proc = 0x507e8000,
|
|
arg = 0xaffffdb0, flags = 0, new_pid = 0}, input_request = {
|
|
op = 1350467584, fd = -1342177872, proc = 0, pid = 0}}}}
|
|
|
|
|
|
|
|
|
|
|
|
The interesting things here are the fact that its .thread.syscall.id
|
|
is __NR_write (see the big switch in arch/um/kernel/syscall_kern.c or
|
|
the defines in include/asm-um/arch/unistd.h), and that it never
|
|
returned. Also, its .request.op is OP_SWITCH (see
|
|
arch/um/include/user_util.h). These mean that it went into a write,
|
|
and, for some reason, called schedule().
|
|
|
|
|
|
The fact that it never returned from write means that its stack should
|
|
be fairly interesting. Its pid is 1980 (.thread.extern_pid). That
|
|
process is being ptraced by the signal thread, so it must be detached
|
|
before gdb can attach it:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(gdb) call detach(1980)
|
|
|
|
Program received signal SIGSEGV, Segmentation fault.
|
|
<function called from gdb>
|
|
The program being debugged stopped while in a function called from GDB.
|
|
When the function (detach) is done executing, GDB will silently
|
|
stop (instead of continuing to evaluate the expression containing
|
|
the function call).
|
|
(gdb) call detach(1980)
|
|
$15 = 0
|
|
|
|
|
|
|
|
|
|
|
|
The first detach segfaults for some reason, and the second one
|
|
succeeds.
|
|
|
|
|
|
Now I detach from the signal thread, attach to the fsck thread, and
|
|
look at its stack:
|
|
|
|
|
|
(gdb) det
|
|
Detaching from program: /home/dike/linux/2.3.26/um/linux Pid 1935
|
|
(gdb) att 1980
|
|
Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 1980
|
|
0x10070451 in __kill ()
|
|
(gdb) bt
|
|
#0 0x10070451 in __kill ()
|
|
#1 0x10068ccd in usr1_pid (pid=1980) at process.c:30
|
|
#2 0x1006a03f in _switch_to (prev=0x50072000, next=0x507e8000)
|
|
at process_kern.c:156
|
|
#3 0x1006a052 in switch_to (prev=0x50072000, next=0x507e8000, last=0x50072000)
|
|
at process_kern.c:161
|
|
#4 0x10001d12 in schedule () at core.c:777
|
|
#5 0x1006a744 in __down (sem=0x507d241c) at semaphore.c:71
|
|
#6 0x1006aa10 in __down_failed () at semaphore.c:157
|
|
#7 0x1006c5d8 in segv_handler (sc=0x5006e940) at trap_user.c:174
|
|
#8 0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
|
|
#9 <signal handler called>
|
|
#10 0x10155404 in errno ()
|
|
#11 0x1006c0aa in segv (address=1342179328, is_write=2) at trap_kern.c:50
|
|
#12 0x1006c5d8 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
|
|
#13 0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
|
|
#14 <signal handler called>
|
|
#15 0xc0fd in ?? ()
|
|
#16 0x10016647 in sys_write (fd=3,
|
|
buf=0x80b8800 <Address 0x80b8800 out of bounds>, count=1024)
|
|
at read_write.c:159
|
|
#17 0x1006d5b3 in execute_syscall (syscall=4, args=0x5006ef08)
|
|
at syscall_kern.c:254
|
|
#18 0x1006af87 in really_do_syscall (sig=12) at syscall_user.c:35
|
|
#19 <signal handler called>
|
|
#20 0x400dc8b0 in ?? ()
|
|
|
|
|
|
|
|
|
|
|
|
The interesting things here are :
|
|
|
|
o There are two segfaults on this stack (frames 9 and 14)
|
|
|
|
o The first faulting address (frame 11) is 0x50000800
|
|
|
|
(gdb) p (void *)1342179328
|
|
$16 = (void *) 0x50000800
|
|
|
|
|
|
|
|
|
|
|
|
The initial faulting address is interesting because it is on the idle
|
|
thread's stack. I had been seeing the idle thread segfault for no
|
|
apparent reason, and the cause looked like stack corruption. In hopes
|
|
of catching the culprit in the act, I had turned off all protections
|
|
to that stack while the idle thread wasn't running. This apparently
|
|
tripped that trap.
|
|
|
|
|
|
However, the more immediate problem is that second segfault and I'm
|
|
going to concentrate on that. First, I want to see where the fault
|
|
happened, so I have to go look at the sigcontent struct in frame 8:
|
|
|
|
|
|
|
|
(gdb) up
|
|
#1 0x10068ccd in usr1_pid (pid=1980) at process.c:30
|
|
30 kill(pid, SIGUSR1);
|
|
(gdb)
|
|
#2 0x1006a03f in _switch_to (prev=0x50072000, next=0x507e8000)
|
|
at process_kern.c:156
|
|
156 usr1_pid(getpid());
|
|
(gdb)
|
|
#3 0x1006a052 in switch_to (prev=0x50072000, next=0x507e8000, last=0x50072000)
|
|
at process_kern.c:161
|
|
161 _switch_to(prev, next);
|
|
(gdb)
|
|
#4 0x10001d12 in schedule () at core.c:777
|
|
777 switch_to(prev, next, prev);
|
|
(gdb)
|
|
#5 0x1006a744 in __down (sem=0x507d241c) at semaphore.c:71
|
|
71 schedule();
|
|
(gdb)
|
|
#6 0x1006aa10 in __down_failed () at semaphore.c:157
|
|
157 }
|
|
(gdb)
|
|
#7 0x1006c5d8 in segv_handler (sc=0x5006e940) at trap_user.c:174
|
|
174 segv(sc->cr2, sc->err & 2);
|
|
(gdb)
|
|
#8 0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
|
|
182 segv_handler(sc);
|
|
(gdb) p *sc
|
|
Cannot access memory at address 0x0.
|
|
|
|
|
|
|
|
|
|
That's not very useful, so I'll try a more manual method:
|
|
|
|
|
|
(gdb) p *((struct sigcontext *) (&sig + 1))
|
|
$19 = {gs = 0, __gsh = 0, fs = 0, __fsh = 0, es = 43, __esh = 0, ds = 43,
|
|
__dsh = 0, edi = 1342179328, esi = 1350378548, ebp = 1342630440,
|
|
esp = 1342630420, ebx = 1348150624, edx = 1280, ecx = 0, eax = 0,
|
|
trapno = 14, err = 4, eip = 268480945, cs = 35, __csh = 0, eflags = 66118,
|
|
esp_at_signal = 1342630420, ss = 43, __ssh = 0, fpstate = 0x0, oldmask = 0,
|
|
cr2 = 1280}
|
|
|
|
|
|
|
|
The ip is in handle_mm_fault:
|
|
|
|
|
|
(gdb) p (void *)268480945
|
|
$20 = (void *) 0x1000b1b1
|
|
(gdb) i sym $20
|
|
handle_mm_fault + 57 in section .text
|
|
|
|
|
|
|
|
|
|
|
|
Specifically, it's in pte_alloc:
|
|
|
|
|
|
(gdb) i line *$20
|
|
Line 124 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b1b1 <handle_mm_fault+57>
|
|
and ends at 0x1000b1b7 <handle_mm_fault+63>.
|
|
|
|
|
|
|
|
|
|
|
|
To find where in handle_mm_fault this is, I'll jump forward in the
|
|
code until I see an address in that procedure:
|
|
|
|
|
|
|
|
(gdb) i line *0x1000b1c0
|
|
Line 126 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b1b7 <handle_mm_fault+63>
|
|
and ends at 0x1000b1c3 <handle_mm_fault+75>.
|
|
(gdb) i line *0x1000b1d0
|
|
Line 131 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b1d0 <handle_mm_fault+88>
|
|
and ends at 0x1000b1da <handle_mm_fault+98>.
|
|
(gdb) i line *0x1000b1e0
|
|
Line 61 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b1da <handle_mm_fault+98>
|
|
and ends at 0x1000b1e1 <handle_mm_fault+105>.
|
|
(gdb) i line *0x1000b1f0
|
|
Line 134 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b1f0 <handle_mm_fault+120>
|
|
and ends at 0x1000b200 <handle_mm_fault+136>.
|
|
(gdb) i line *0x1000b200
|
|
Line 135 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b200 <handle_mm_fault+136>
|
|
and ends at 0x1000b208 <handle_mm_fault+144>.
|
|
(gdb) i line *0x1000b210
|
|
Line 139 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
|
|
starts at address 0x1000b210 <handle_mm_fault+152>
|
|
and ends at 0x1000b219 <handle_mm_fault+161>.
|
|
(gdb) i line *0x1000b220
|
|
Line 1168 of "memory.c" starts at address 0x1000b21e <handle_mm_fault+166>
|
|
and ends at 0x1000b222 <handle_mm_fault+170>.
|
|
|
|
|
|
|
|
|
|
|
|
Something is apparently wrong with the page tables or vma_structs, so
|
|
lets go back to frame 11 and have a look at them:
|
|
|
|
|
|
|
|
#11 0x1006c0aa in segv (address=1342179328, is_write=2) at trap_kern.c:50
|
|
50 handle_mm_fault(current, vma, address, is_write);
|
|
(gdb) call pgd_offset_proc(vma->vm_mm, address)
|
|
$22 = (pgd_t *) 0x80a548c
|
|
|
|
|
|
|
|
|
|
|
|
That's pretty bogus. Page tables aren't supposed to be in process
|
|
text or data areas. Let's see what's in the vma:
|
|
|
|
|
|
(gdb) p *vma
|
|
$23 = {vm_mm = 0x507d2434, vm_start = 0, vm_end = 134512640,
|
|
vm_next = 0x80a4f8c, vm_page_prot = {pgprot = 0}, vm_flags = 31200,
|
|
vm_avl_height = 2058, vm_avl_left = 0x80a8c94, vm_avl_right = 0x80d1000,
|
|
vm_next_share = 0xaffffdb0, vm_pprev_share = 0xaffffe63,
|
|
vm_ops = 0xaffffe7a, vm_pgoff = 2952789626, vm_file = 0xafffffec,
|
|
vm_private_data = 0x62}
|
|
(gdb) p *vma.vm_mm
|
|
$24 = {mmap = 0x507d2434, mmap_avl = 0x0, mmap_cache = 0x8048000,
|
|
pgd = 0x80a4f8c, mm_users = {counter = 0}, mm_count = {counter = 134904288},
|
|
map_count = 134909076, mmap_sem = {count = {counter = 135073792},
|
|
sleepers = -1342177872, wait = {lock = <optimized out or zero length>,
|
|
task_list = {next = 0xaffffe63, prev = 0xaffffe7a},
|
|
__magic = -1342177670, __creator = -1342177300}, __magic = 98},
|
|
page_table_lock = {}, context = 138, start_code = 0, end_code = 0,
|
|
start_data = 0, end_data = 0, start_brk = 0, brk = 0, start_stack = 0,
|
|
arg_start = 0, arg_end = 0, env_start = 0, env_end = 0, rss = 1350381536,
|
|
total_vm = 0, locked_vm = 0, def_flags = 0, cpu_vm_mask = 0, swap_cnt = 0,
|
|
swap_address = 0, segments = 0x0}
|
|
|
|
|
|
|
|
|
|
|
|
This also pretty bogus. With all of the 0x80xxxxx and 0xaffffxxx
|
|
addresses, this is looking like a stack was plonked down on top of
|
|
these structures. Maybe it's a stack overflow from the next page:
|
|
|
|
|
|
|
|
(gdb) p vma
|
|
$25 = (struct vm_area_struct *) 0x507d2434
|
|
|
|
|
|
|
|
|
|
|
|
That's towards the lower quarter of the page, so that would have to
|
|
have been pretty heavy stack overflow:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(gdb) x/100x $25
|
|
0x507d2434: 0x507d2434 0x00000000 0x08048000 0x080a4f8c
|
|
0x507d2444: 0x00000000 0x080a79e0 0x080a8c94 0x080d1000
|
|
0x507d2454: 0xaffffdb0 0xaffffe63 0xaffffe7a 0xaffffe7a
|
|
0x507d2464: 0xafffffec 0x00000062 0x0000008a 0x00000000
|
|
0x507d2474: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2484: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2494: 0x00000000 0x00000000 0x507d2fe0 0x00000000
|
|
0x507d24a4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d24b4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d24c4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d24d4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d24e4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d24f4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2504: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2514: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2524: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2534: 0x00000000 0x00000000 0x507d25dc 0x00000000
|
|
0x507d2544: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2554: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2564: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2574: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2584: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d2594: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d25a4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
0x507d25b4: 0x00000000 0x00000000 0x00000000 0x00000000
|
|
|
|
|
|
|
|
|
|
|
|
It's not stack overflow. The only "stack-like" piece of this data is
|
|
the vma_struct itself.
|
|
|
|
|
|
At this point, I don't see any avenues to pursue, so I just have to
|
|
admit that I have no idea what's going on. What I will do, though, is
|
|
stick a trap on the segfault handler which will stop if it sees any
|
|
writes to the idle thread's stack. That was the thing that happened
|
|
first, and it may be that if I can catch it immediately, what's going
|
|
on will be somewhat clearer.
|
|
|
|
|
|
12.2. Episode 2: The case of the hung fsck
|
|
|
|
After setting a trap in the SEGV handler for accesses to the signal
|
|
thread's stack, I reran the kernel.
|
|
|
|
|
|
fsck hung again, this time by hitting the trap:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Setting hostname uml [ OK ]
|
|
Checking root filesystem
|
|
/dev/fhd0 contains a file system with errors, check forced.
|
|
Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.
|
|
|
|
/dev/fhd0: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
|
|
(i.e., without -a or -p options)
|
|
[ FAILED ]
|
|
|
|
*** An error occurred during the file system check.
|
|
*** Dropping you to a shell; the system will reboot
|
|
*** when you leave the shell.
|
|
Give root password for maintenance
|
|
(or type Control-D for normal startup):
|
|
|
|
[root@uml /root]# fsck -y /dev/fhd0
|
|
fsck -y /dev/fhd0
|
|
Parallelizing fsck version 1.14 (9-Jan-1999)
|
|
e2fsck 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09
|
|
/dev/fhd0 contains a file system with errors, check forced.
|
|
Pass 1: Checking inodes, blocks, and sizes
|
|
Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780. Ignore error? yes
|
|
|
|
Pass 2: Checking directory structure
|
|
Error reading block 49405 (Attempt to read block from filesystem resulted in short read). Ignore error? yes
|
|
|
|
Directory inode 11858, block 0, offset 0: directory corrupted
|
|
Salvage? yes
|
|
|
|
Missing '.' in directory inode 11858.
|
|
Fix? yes
|
|
|
|
Missing '..' in directory inode 11858.
|
|
Fix? yes
|
|
|
|
Untested (4127) [100fe44c]: trap_kern.c line 31
|
|
|
|
|
|
|
|
|
|
|
|
I need to get the signal thread to detach from pid 4127 so that I can
|
|
attach to it with gdb. This is done by sending it a SIGUSR1, which is
|
|
caught by the signal thread, which detaches the process:
|
|
|
|
|
|
kill -USR1 4127
|
|
|
|
|
|
|
|
|
|
|
|
Now I can run gdb on it:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
~/linux/2.3.26/um 1034: gdb linux
|
|
GNU gdb 4.17.0.11 with Linux support
|
|
Copyright 1998 Free Software Foundation, Inc.
|
|
GDB is free software, covered by the GNU General Public License, and you are
|
|
welcome to change it and/or distribute copies of it under certain conditions.
|
|
Type "show copying" to see the conditions.
|
|
There is absolutely no warranty for GDB. Type "show warranty" for details.
|
|
This GDB was configured as "i386-redhat-linux"...
|
|
(gdb) att 4127
|
|
Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 4127
|
|
0x10075891 in __libc_nanosleep ()
|
|
|
|
|
|
|
|
|
|
|
|
The backtrace shows that it was in a write and that the fault address
|
|
(address in frame 3) is 0x50000800, which is right in the middle of
|
|
the signal thread's stack page:
|
|
|
|
|
|
(gdb) bt
|
|
#0 0x10075891 in __libc_nanosleep ()
|
|
#1 0x1007584d in __sleep (seconds=1000000)
|
|
at ../sysdeps/unix/sysv/linux/sleep.c:78
|
|
#2 0x1006ce9a in stop () at user_util.c:191
|
|
#3 0x1006bf88 in segv (address=1342179328, is_write=2) at trap_kern.c:31
|
|
#4 0x1006c628 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
|
|
#5 0x1006c63c in kern_segv_handler (sig=11) at trap_user.c:182
|
|
#6 <signal handler called>
|
|
#7 0xc0fd in ?? ()
|
|
#8 0x10016647 in sys_write (fd=3, buf=0x80b8800 "R.", count=1024)
|
|
at read_write.c:159
|
|
#9 0x1006d603 in execute_syscall (syscall=4, args=0x5006ef08)
|
|
at syscall_kern.c:254
|
|
#10 0x1006af87 in really_do_syscall (sig=12) at syscall_user.c:35
|
|
#11 <signal handler called>
|
|
#12 0x400dc8b0 in ?? ()
|
|
#13 <signal handler called>
|
|
#14 0x400dc8b0 in ?? ()
|
|
#15 0x80545fd in ?? ()
|
|
#16 0x804daae in ?? ()
|
|
#17 0x8054334 in ?? ()
|
|
#18 0x804d23e in ?? ()
|
|
#19 0x8049632 in ?? ()
|
|
#20 0x80491d2 in ?? ()
|
|
#21 0x80596b5 in ?? ()
|
|
(gdb) p (void *)1342179328
|
|
$3 = (void *) 0x50000800
|
|
|
|
|
|
|
|
|
|
|
|
Going up the stack to the segv_handler frame and looking at where in
|
|
the code the access happened shows that it happened near line 110 of
|
|
block_dev.c:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(gdb) up
|
|
#1 0x1007584d in __sleep (seconds=1000000)
|
|
at ../sysdeps/unix/sysv/linux/sleep.c:78
|
|
../sysdeps/unix/sysv/linux/sleep.c:78: No such file or directory.
|
|
(gdb)
|
|
#2 0x1006ce9a in stop () at user_util.c:191
|
|
191 while(1) sleep(1000000);
|
|
(gdb)
|
|
#3 0x1006bf88 in segv (address=1342179328, is_write=2) at trap_kern.c:31
|
|
31 KERN_UNTESTED();
|
|
(gdb)
|
|
#4 0x1006c628 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
|
|
174 segv(sc->cr2, sc->err & 2);
|
|
(gdb) p *sc
|
|
$1 = {gs = 0, __gsh = 0, fs = 0, __fsh = 0, es = 43, __esh = 0, ds = 43,
|
|
__dsh = 0, edi = 1342179328, esi = 134973440, ebp = 1342631484,
|
|
esp = 1342630864, ebx = 256, edx = 0, ecx = 256, eax = 1024, trapno = 14,
|
|
err = 6, eip = 268550834, cs = 35, __csh = 0, eflags = 66070,
|
|
esp_at_signal = 1342630864, ss = 43, __ssh = 0, fpstate = 0x0, oldmask = 0,
|
|
cr2 = 1342179328}
|
|
(gdb) p (void *)268550834
|
|
$2 = (void *) 0x1001c2b2
|
|
(gdb) i sym $2
|
|
block_write + 1090 in section .text
|
|
(gdb) i line *$2
|
|
Line 209 of "/home/dike/linux/2.3.26/um/include/asm/arch/string.h"
|
|
starts at address 0x1001c2a1 <block_write+1073>
|
|
and ends at 0x1001c2bf <block_write+1103>.
|
|
(gdb) i line *0x1001c2c0
|
|
Line 110 of "block_dev.c" starts at address 0x1001c2bf <block_write+1103>
|
|
and ends at 0x1001c2e3 <block_write+1139>.
|
|
|
|
|
|
|
|
|
|
|
|
Looking at the source shows that the fault happened during a call to
|
|
copy_from_user to copy the data into the kernel:
|
|
|
|
|
|
107 count -= chars;
|
|
108 copy_from_user(p,buf,chars);
|
|
109 p += chars;
|
|
110 buf += chars;
|
|
|
|
|
|
|
|
|
|
|
|
p is the pointer which must contain 0x50000800, since buf contains
|
|
0x80b8800 (frame 8 above). It is defined as:
|
|
|
|
|
|
p = offset + bh->b_data;
|
|
|
|
|
|
|
|
|
|
|
|
I need to figure out what bh is, and it just so happens that bh is
|
|
passed as an argument to mark_buffer_uptodate and mark_buffer_dirty a
|
|
few lines later, so I do a little disassembly:
|
|
|
|
|
|
|
|
|
|
(gdb) disas 0x1001c2bf 0x1001c2e0
|
|
Dump of assembler code from 0x1001c2bf to 0x1001c2d0:
|
|
0x1001c2bf <block_write+1103>: addl %eax,0xc(%ebp)
|
|
0x1001c2c2 <block_write+1106>: movl 0xfffffdd4(%ebp),%edx
|
|
0x1001c2c8 <block_write+1112>: btsl $0x0,0x18(%edx)
|
|
0x1001c2cd <block_write+1117>: btsl $0x1,0x18(%edx)
|
|
0x1001c2d2 <block_write+1122>: sbbl %ecx,%ecx
|
|
0x1001c2d4 <block_write+1124>: testl %ecx,%ecx
|
|
0x1001c2d6 <block_write+1126>: jne 0x1001c2e3 <block_write+1139>
|
|
0x1001c2d8 <block_write+1128>: pushl $0x0
|
|
0x1001c2da <block_write+1130>: pushl %edx
|
|
0x1001c2db <block_write+1131>: call 0x1001819c <__mark_buffer_dirty>
|
|
End of assembler dump.
|
|
|
|
|
|
|
|
|
|
|
|
At that point, bh is in %edx (address 0x1001c2da), which is calculated
|
|
at 0x1001c2c2 as %ebp + 0xfffffdd4, so I figure exactly what that is,
|
|
taking %ebp from the sigcontext_struct above:
|
|
|
|
|
|
(gdb) p (void *)1342631484
|
|
$5 = (void *) 0x5006ee3c
|
|
(gdb) p 0x5006ee3c+0xfffffdd4
|
|
$6 = 1342630928
|
|
(gdb) p (void *)$6
|
|
$7 = (void *) 0x5006ec10
|
|
(gdb) p *((void **)$7)
|
|
$8 = (void *) 0x50100200
|
|
|
|
|
|
|
|
|
|
|
|
Now, I look at the structure to see what's in it, and particularly,
|
|
what its b_data field contains:
|
|
|
|
|
|
(gdb) p *((struct buffer_head *)0x50100200)
|
|
$13 = {b_next = 0x50289380, b_blocknr = 49405, b_size = 1024, b_list = 0,
|
|
b_dev = 15872, b_count = {counter = 1}, b_rdev = 15872, b_state = 24,
|
|
b_flushtime = 0, b_next_free = 0x501001a0, b_prev_free = 0x50100260,
|
|
b_this_page = 0x501001a0, b_reqnext = 0x0, b_pprev = 0x507fcf58,
|
|
b_data = 0x50000800 "", b_page = 0x50004000,
|
|
b_end_io = 0x10017f60 <end_buffer_io_sync>, b_dev_id = 0x0,
|
|
b_rsector = 98810, b_wait = {lock = <optimized out or zero length>,
|
|
task_list = {next = 0x50100248, prev = 0x50100248}, __magic = 1343226448,
|
|
__creator = 0}, b_kiobuf = 0x0}
|
|
|
|
|
|
|
|
|
|
|
|
The b_data field is indeed 0x50000800, so the question becomes how
|
|
that happened. The rest of the structure looks fine, so this probably
|
|
is not a case of data corruption. It happened on purpose somehow.
|
|
|
|
|
|
The b_page field is a pointer to the page_struct representing the
|
|
0x50000000 page. Looking at it shows the kernel's idea of the state
|
|
of that page:
|
|
|
|
|
|
|
|
(gdb) p *$13.b_page
|
|
$17 = {list = {next = 0x50004a5c, prev = 0x100c5174}, mapping = 0x0,
|
|
index = 0, next_hash = 0x0, count = {counter = 1}, flags = 132, lru = {
|
|
next = 0x50008460, prev = 0x50019350}, wait = {
|
|
lock = <optimized out or zero length>, task_list = {next = 0x50004024,
|
|
prev = 0x50004024}, __magic = 1342193708, __creator = 0},
|
|
pprev_hash = 0x0, buffers = 0x501002c0, virtual = 1342177280,
|
|
zone = 0x100c5160}
|
|
|
|
|
|
|
|
|
|
|
|
Some sanity-checking: the virtual field shows the "virtual" address of
|
|
this page, which in this kernel is the same as its "physical" address,
|
|
and the page_struct itself should be mem_map[0], since it represents
|
|
the first page of memory:
|
|
|
|
|
|
|
|
(gdb) p (void *)1342177280
|
|
$18 = (void *) 0x50000000
|
|
(gdb) p mem_map
|
|
$19 = (mem_map_t *) 0x50004000
|
|
|
|
|
|
|
|
|
|
|
|
These check out fine.
|
|
|
|
|
|
Now to check out the page_struct itself. In particular, the flags
|
|
field shows whether the page is considered free or not:
|
|
|
|
|
|
(gdb) p (void *)132
|
|
$21 = (void *) 0x84
|
|
|
|
|
|
|
|
|
|
|
|
The "reserved" bit is the high bit, which is definitely not set, so
|
|
the kernel considers the signal stack page to be free and available to
|
|
be used.
|
|
|
|
|
|
At this point, I jump to conclusions and start looking at my early
|
|
boot code, because that's where that page is supposed to be reserved.
|
|
|
|
|
|
In my setup_arch procedure, I have the following code which looks just
|
|
fine:
|
|
|
|
|
|
|
|
bootmap_size = init_bootmem(start_pfn, end_pfn - start_pfn);
|
|
free_bootmem(__pa(low_physmem) + bootmap_size, high_physmem - low_physmem);
|
|
|
|
|
|
|
|
|
|
|
|
Two stack pages have already been allocated, and low_physmem points to
|
|
the third page, which is the beginning of free memory.
|
|
The init_bootmem call declares the entire memory to the boot memory
|
|
manager, which marks it all reserved. The free_bootmem call frees up
|
|
all of it, except for the first two pages. This looks correct to me.
|
|
|
|
|
|
So, I decide to see init_bootmem run and make sure that it is marking
|
|
those first two pages as reserved. I never get that far.
|
|
|
|
|
|
Stepping into init_bootmem, and looking at bootmem_map before looking
|
|
at what it contains shows the following:
|
|
|
|
|
|
|
|
(gdb) p bootmem_map
|
|
$3 = (void *) 0x50000000
|
|
|
|
|
|
|
|
|
|
|
|
Aha! The light dawns. That first page is doing double duty as a
|
|
stack and as the boot memory map. The last thing that the boot memory
|
|
manager does is to free the pages used by its memory map, so this page
|
|
is getting freed even its marked as reserved.
|
|
|
|
|
|
The fix was to initialize the boot memory manager before allocating
|
|
those two stack pages, and then allocate them through the boot memory
|
|
manager. After doing this, and fixing a couple of subsequent buglets,
|
|
the stack corruption problem disappeared.
|
|
|
|
|
|
|
|
|
|
|
|
13. What to do when UML doesn't work
|
|
|
|
|
|
|
|
|
|
13.1. Strange compilation errors when you build from source
|
|
|
|
As of test11, it is necessary to have "ARCH=um" in the environment or
|
|
on the make command line for all steps in building UML, including
|
|
clean, distclean, or mrproper, config, menuconfig, or xconfig, dep,
|
|
and linux. If you forget for any of them, the i386 build seems to
|
|
contaminate the UML build. If this happens, start from scratch with
|
|
|
|
|
|
host%
|
|
make mrproper ARCH=um
|
|
|
|
|
|
|
|
|
|
and repeat the build process with ARCH=um on all the steps.
|
|
|
|
|
|
See ``Compiling the kernel and modules'' for more details.
|
|
|
|
|
|
Another cause of strange compilation errors is building UML in
|
|
/usr/src/linux. If you do this, the first thing you need to do is
|
|
clean up the mess you made. The /usr/src/linux/asm link will now
|
|
point to /usr/src/linux/asm-um. Make it point back to
|
|
/usr/src/linux/asm-i386. Then, move your UML pool someplace else and
|
|
build it there. Also see below, where a more specific set of symptoms
|
|
is described.
|
|
|
|
|
|
|
|
13.3. A variety of panics and hangs with /tmp on a reiserfs filesys-
|
|
tem
|
|
|
|
I saw this on reiserfs 3.5.21 and it seems to be fixed in 3.5.27.
|
|
Panics preceded by
|
|
|
|
|
|
Detaching pid nnnn
|
|
|
|
|
|
|
|
are diagnostic of this problem. This is a reiserfs bug which causes a
|
|
thread to occasionally read stale data from a mmapped page shared with
|
|
another thread. The fix is to upgrade the filesystem or to have /tmp
|
|
be an ext2 filesystem.
|
|
|
|
|
|
|
|
13.4. The compile fails with errors about conflicting types for
|
|
'open', 'dup', and 'waitpid'
|
|
|
|
This happens when you build in /usr/src/linux. The UML build makes
|
|
the include/asm link point to include/asm-um. /usr/include/asm points
|
|
to /usr/src/linux/include/asm, so when that link gets moved, files
|
|
which need to include the asm-i386 versions of headers get the
|
|
incompatible asm-um versions. The fix is to move the include/asm link
|
|
back to include/asm-i386 and to do UML builds someplace else.
|
|
|
|
|
|
|
|
13.5. UML doesn't work when /tmp is an NFS filesystem
|
|
|
|
This seems to be a similar situation with the ReiserFS problem above.
|
|
Some versions of NFS seems not to handle mmap correctly, which UML
|
|
depends on. The workaround is have /tmp be a non-NFS directory.
|
|
|
|
|
|
13.6. UML hangs on boot when compiled with gprof support
|
|
|
|
If you build UML with gprof support and, early in the boot, it does
|
|
this
|
|
|
|
|
|
kernel BUG at page_alloc.c:100!
|
|
|
|
|
|
|
|
|
|
you have a buggy gcc. You can work around the problem by removing
|
|
UM_FASTCALL from CFLAGS in arch/um/Makefile-i386. This will open up
|
|
another bug, but that one is fairly hard to reproduce.
|
|
|
|
|
|
|
|
13.7. syslogd dies with a SIGTERM on startup
|
|
|
|
The exact boot error depends on the distribution that you're booting,
|
|
but Debian produces this:
|
|
|
|
|
|
/etc/rc2.d/S10sysklogd: line 49: 93 Terminated
|
|
start-stop-daemon --start --quiet --exec /sbin/syslogd -- $SYSLOGD
|
|
|
|
|
|
|
|
|
|
This is a syslogd bug. There's a race between a parent process
|
|
installing a signal handler and its child sending the signal. See
|
|
this uml-devel post <http://www.geocrawler.com/lists/3/Source-
|
|
Forge/709/0/6612801> for the details.
|
|
|
|
|
|
|
|
13.8. TUN/TAP networking doesn't work on a 2.4 host
|
|
|
|
There are a couple of problems which were
|
|
<http://www.geocrawler.com/lists/3/SourceForge/597/0/> name="pointed
|
|
out"> by Tim Robinson <timro at trkr dot net>
|
|
|
|
o It doesn't work on hosts running 2.4.7 (or thereabouts) or earlier.
|
|
The fix is to upgrade to something more recent and then read the
|
|
next item.
|
|
|
|
o If you see
|
|
|
|
|
|
File descriptor in bad state
|
|
|
|
|
|
|
|
when you bring up the device inside UML, you have a header mismatch
|
|
between the original kernel and the upgraded one. Make /usr/src/linux
|
|
point at the new headers. This will only be a problem if you build
|
|
uml_net yourself.
|
|
|
|
|
|
|
|
13.9. You can network to the host but not to other machines on the
|
|
net
|
|
|
|
If you can connect to the host, and the host can connect to UML, but
|
|
you cannot connect to any other machines, then you may need to enable
|
|
IP Masquerading on the host. Usually this is only experienced when
|
|
using private IP addresses (192.168.x.x or 10.x.x.x) for host/UML
|
|
networking, rather than the public address space that your host is
|
|
connected to. UML does not enable IP Masquerading, so you will need
|
|
to create a static rule to enable it:
|
|
|
|
|
|
host%
|
|
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
|
|
|
|
|
|
|
|
|
|
Replace eth0 with the interface that you use to talk to the rest of
|
|
the world.
|
|
|
|
|
|
Documentation on IP Masquerading, and SNAT, can be found at
|
|
www.netfilter.org <http://www.netfilter.org> .
|
|
|
|
|
|
If you can reach the local net, but not the outside Internet, then
|
|
that is usually a routing problem. The UML needs a default route:
|
|
|
|
|
|
UML#
|
|
route add default gw gateway IP
|
|
|
|
|
|
|
|
|
|
The gateway IP can be any machine on the local net that knows how to
|
|
reach the outside world. Usually, this is the host or the local net-
|
|
work's gateway.
|
|
|
|
|
|
Occasionally, we hear from someone who can reach some machines, but
|
|
not others on the same net, or who can reach some ports on other
|
|
machines, but not others. These are usually caused by strange
|
|
firewalling somewhere between the UML and the other box. You track
|
|
this down by running tcpdump on every interface the packets travel
|
|
over and see where they disappear. When you find a machine that takes
|
|
the packets in, but does not send them onward, that's the culprit.
|
|
|
|
|
|
|
|
13.10. I have no root and I want to scream
|
|
|
|
Thanks to Birgit Wahlich for telling me about this strange one. It
|
|
turns out that there's a limit of six environment variables on the
|
|
kernel command line. When that limit is reached or exceeded, argument
|
|
processing stops, which means that the 'root=' argument that UML
|
|
usually adds is not seen. So, the filesystem has no idea what the
|
|
root device is, so it panics.
|
|
|
|
|
|
The fix is to put less stuff on the command line. Glomming all your
|
|
setup variables into one is probably the best way to go.
|
|
|
|
|
|
|
|
13.11. UML build conflict between ptrace.h and ucontext.h
|
|
|
|
On some older systems, /usr/include/asm/ptrace.h and
|
|
/usr/include/sys/ucontext.h define the same names. So, when they're
|
|
included together, the defines from one completely mess up the parsing
|
|
of the other, producing errors like:
|
|
/usr/include/sys/ucontext.h:47: parse error before
|
|
`10'
|
|
|
|
|
|
|
|
|
|
plus a pile of warnings.
|
|
|
|
|
|
This is a libc botch, which has since been fixed, and I don't see any
|
|
way around it besides upgrading.
|
|
|
|
|
|
|
|
13.12. The UML BogoMips is exactly half the host's BogoMips
|
|
|
|
On i386 kernels, there are two ways of running the loop that is used
|
|
to calculate the BogoMips rating, using the TSC if it's there or using
|
|
a one-instruction loop. The TSC produces twice the BogoMips as the
|
|
loop. UML uses the loop, since it has nothing resembling a TSC, and
|
|
will get almost exactly the same BogoMips as a host using the loop.
|
|
However, on a host with a TSC, its BogoMips will be double the loop
|
|
BogoMips, and therefore double the UML BogoMips.
|
|
|
|
|
|
|
|
13.13. When you run UML, it immediately segfaults
|
|
|
|
If the host is configured with the 2G/2G address space split, that's
|
|
why. See ``UML on 2G/2G hosts'' for the details on getting UML to
|
|
run on your host.
|
|
|
|
|
|
|
|
13.14. xterms appear, then immediately disappear
|
|
|
|
If you're running an up to date kernel with an old release of
|
|
uml_utilities, the port-helper program will not work properly, so
|
|
xterms will exit straight after they appear. The solution is to
|
|
upgrade to the latest release of uml_utilities. Usually this problem
|
|
occurs when you have installed a packaged release of UML then compiled
|
|
your own development kernel without upgrading the uml_utilities from
|
|
the source distribution.
|
|
|
|
|
|
|
|
13.15. Any other panic, hang, or strange behavior
|
|
|
|
If you're seeing truly strange behavior, such as hangs or panics that
|
|
happen in random places, or you try running the debugger to see what's
|
|
happening and it acts strangely, then it could be a problem in the
|
|
host kernel. If you're not running a stock Linus or -ac kernel, then
|
|
try that. An early version of the preemption patch and a 2.4.10 SuSE
|
|
kernel have caused very strange problems in UML.
|
|
|
|
|
|
Otherwise, let me know about it. Send a message to one of the UML
|
|
mailing lists - either the developer list - user-mode-linux-devel at
|
|
lists dot sourceforge dot net (subscription info) or the user list -
|
|
user-mode-linux-user at lists dot sourceforge do net (subscription
|
|
info), whichever you prefer. Don't assume that everyone knows about
|
|
it and that a fix is imminent.
|
|
|
|
|
|
If you want to be super-helpful, read ``Diagnosing Problems'' and
|
|
follow the instructions contained therein.
|
|
14. Diagnosing Problems
|
|
|
|
|
|
If you get UML to crash, hang, or otherwise misbehave, you should
|
|
report this on one of the project mailing lists, either the developer
|
|
list - user-mode-linux-devel at lists dot sourceforge dot net
|
|
(subscription info) or the user list - user-mode-linux-user at lists
|
|
dot sourceforge dot net (subscription info). When you do, it is
|
|
likely that I will want more information. So, it would be helpful to
|
|
read the stuff below, do whatever is applicable in your case, and
|
|
report the results to the list.
|
|
|
|
|
|
For any diagnosis, you're going to need to build a debugging kernel.
|
|
The binaries from this site aren't debuggable. If you haven't done
|
|
this before, read about ``Compiling the kernel and modules'' and
|
|
``Kernel debugging'' UML first.
|
|
|
|
|
|
14.1. Case 1 : Normal kernel panics
|
|
|
|
The most common case is for a normal thread to panic. To debug this,
|
|
you will need to run it under the debugger (add 'debug' to the command
|
|
line). An xterm will start up with gdb running inside it. Continue
|
|
it when it stops in start_kernel and make it crash. Now ^C gdb and
|
|
|
|
|
|
If the panic was a "Kernel mode fault", then there will be a segv
|
|
frame on the stack and I'm going to want some more information. The
|
|
stack might look something like this:
|
|
|
|
|
|
(UML gdb) backtrace
|
|
#0 0x1009bf76 in __sigprocmask (how=1, set=0x5f347940, oset=0x0)
|
|
at ../sysdeps/unix/sysv/linux/sigprocmask.c:49
|
|
#1 0x10091411 in change_sig (signal=10, on=1) at process.c:218
|
|
#2 0x10094785 in timer_handler (sig=26) at time_kern.c:32
|
|
#3 0x1009bf38 in __restore ()
|
|
at ../sysdeps/unix/sysv/linux/i386/sigaction.c:125
|
|
#4 0x1009534c in segv (address=8, ip=268849158, is_write=2, is_user=0)
|
|
at trap_kern.c:66
|
|
#5 0x10095c04 in segv_handler (sig=11) at trap_user.c:285
|
|
#6 0x1009bf38 in __restore ()
|
|
|
|
|
|
|
|
|
|
I'm going to want to see the symbol and line information for the value
|
|
of ip in the segv frame. In this case, you would do the following:
|
|
|
|
|
|
(UML gdb) i sym 268849158
|
|
|
|
|
|
|
|
|
|
and
|
|
|
|
|
|
(UML gdb) i line *268849158
|
|
|
|
|
|
|
|
|
|
The reason for this is the __restore frame right above the segv_han-
|
|
dler frame is hiding the frame that actually segfaulted. So, I have
|
|
to get that information from the faulting ip.
|
|
|
|
|
|
14.2. Case 2 : Tracing thread panics
|
|
|
|
The less common and more painful case is when the tracing thread
|
|
panics. In this case, the kernel debugger will be useless because it
|
|
needs a healthy tracing thread in order to work. The first thing to
|
|
do is get a backtrace from the tracing thread. This is done by
|
|
figuring out what its pid is, firing up gdb, and attaching it to that
|
|
pid. You can figure out the tracing thread pid by looking at the
|
|
first line of the console output, which will look like this:
|
|
|
|
|
|
tracing thread pid = 15851
|
|
|
|
|
|
|
|
|
|
or by running ps on the host and finding the line that looks like
|
|
this:
|
|
|
|
|
|
jdike 15851 4.5 0.4 132568 1104 pts/0 S 21:34 0:05 ./linux [(tracing thread)]
|
|
|
|
|
|
|
|
|
|
If the panic was 'segfault in signals', then follow the instructions
|
|
above for collecting information about the location of the seg fault.
|
|
|
|
|
|
If the tracing thread flaked out all by itself, then send that
|
|
backtrace in and wait for our crack debugging team to fix the problem.
|
|
|
|
|
|
14.3. Case 3 : Tracing thread panics caused by other threads
|
|
|
|
However, there are cases where the misbehavior of another thread
|
|
caused the problem. The most common panic of this type is:
|
|
|
|
|
|
wait_for_stop failed to wait for <pid> to stop with <signal number>
|
|
|
|
|
|
|
|
|
|
In this case, you'll need to get a backtrace from the process men-
|
|
tioned in the panic, which is complicated by the fact that the kernel
|
|
debugger is defunct and without some fancy footwork, another gdb can't
|
|
attach to it. So, this is how the fancy footwork goes:
|
|
|
|
In a shell:
|
|
|
|
|
|
host% kill -STOP pid
|
|
|
|
|
|
|
|
|
|
Run gdb on the tracing thread as described in case 2 and do:
|
|
|
|
|
|
(host gdb) call detach(pid)
|
|
|
|
|
|
If you get a segfault, do it again. It always works the second time.
|
|
|
|
Detach from the tracing thread and attach to that other thread:
|
|
|
|
|
|
(host gdb) detach
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(host gdb) attach pid
|
|
|
|
|
|
|
|
|
|
If gdb hangs when attaching to that process, go back to a shell and
|
|
do:
|
|
|
|
|
|
host%
|
|
kill -CONT pid
|
|
|
|
|
|
|
|
|
|
And then get the backtrace:
|
|
|
|
|
|
(host gdb) backtrace
|
|
|
|
|
|
|
|
|
|
|
|
14.4. Case 4 : Hangs
|
|
|
|
Hangs seem to be fairly rare, but they sometimes happen. When a hang
|
|
happens, we need a backtrace from the offending process. Run the
|
|
kernel debugger as described in case 1 and get a backtrace. If the
|
|
current process is not the idle thread, then send in the backtrace.
|
|
You can tell that it's the idle thread if the stack looks like this:
|
|
|
|
|
|
#0 0x100b1401 in __libc_nanosleep ()
|
|
#1 0x100a2885 in idle_sleep (secs=10) at time.c:122
|
|
#2 0x100a546f in do_idle () at process_kern.c:445
|
|
#3 0x100a5508 in cpu_idle () at process_kern.c:471
|
|
#4 0x100ec18f in start_kernel () at init/main.c:592
|
|
#5 0x100a3e10 in start_kernel_proc (unused=0x0) at um_arch.c:71
|
|
#6 0x100a383f in signal_tramp (arg=0x100a3dd8) at trap_user.c:50
|
|
|
|
|
|
|
|
|
|
If this is the case, then some other process is at fault, and went to
|
|
sleep when it shouldn't have. Run ps on the host and figure out which
|
|
process should not have gone to sleep and stayed asleep. Then attach
|
|
to it with gdb and get a backtrace as described in case 3.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
15. Thanks
|
|
|
|
|
|
A number of people have helped this project in various ways, and this
|
|
page gives recognition where recognition is due.
|
|
|
|
|
|
If you're listed here and you would prefer a real link on your name,
|
|
or no link at all, instead of the despammed email address pseudo-link,
|
|
let me know.
|
|
|
|
|
|
If you're not listed here and you think maybe you should be, please
|
|
let me know that as well. I try to get everyone, but sometimes my
|
|
bookkeeping lapses and I forget about contributions.
|
|
|
|
|
|
15.1. Code and Documentation
|
|
|
|
Rusty Russell <rusty at linuxcare.com.au> -
|
|
|
|
o wrote the HOWTO <http://user-mode-
|
|
linux.sourceforge.net/UserModeLinux-HOWTO.html>
|
|
|
|
o prodded me into making this project official and putting it on
|
|
SourceForge
|
|
|
|
o came up with the way cool UML logo <http://user-mode-
|
|
linux.sourceforge.net/uml-small.png>
|
|
|
|
o redid the config process
|
|
|
|
|
|
Peter Moulder <reiter at netspace.net.au> - Fixed my config and build
|
|
processes, and added some useful code to the block driver
|
|
|
|
|
|
Bill Stearns <wstearns at pobox.com> -
|
|
|
|
o HOWTO updates
|
|
|
|
o lots of bug reports
|
|
|
|
o lots of testing
|
|
|
|
o dedicated a box (uml.ists.dartmouth.edu) to support UML development
|
|
|
|
o wrote the mkrootfs script, which allows bootable filesystems of
|
|
RPM-based distributions to be cranked out
|
|
|
|
o cranked out a large number of filesystems with said script
|
|
|
|
|
|
Jim Leu <jleu at mindspring.com> - Wrote the virtual ethernet driver
|
|
and associated usermode tools
|
|
|
|
Lars Brinkhoff <http://lars.nocrew.org/> - Contributed the ptrace
|
|
proxy from his own project <http://a386.nocrew.org/> to allow easier
|
|
kernel debugging
|
|
|
|
|
|
Andrea Arcangeli <andrea at suse.de> - Redid some of the early boot
|
|
code so that it would work on machines with Large File Support
|
|
|
|
|
|
Chris Emerson <http://www.chiark.greenend.org.uk/~cemerson/> - Did
|
|
the first UML port to Linux/ppc
|
|
|
|
|
|
Harald Welte <laforge at gnumonks.org> - Wrote the multicast
|
|
transport for the network driver
|
|
|
|
|
|
Jorgen Cederlof - Added special file support to hostfs
|
|
|
|
|
|
Greg Lonnon <glonnon at ridgerun dot com> - Changed the ubd driver
|
|
to allow it to layer a COW file on a shared read-only filesystem and
|
|
wrote the iomem emulation support
|
|
|
|
|
|
Henrik Nordstrom <http://hem.passagen.se/hno/> - Provided a variety
|
|
of patches, fixes, and clues
|
|
|
|
|
|
Lennert Buytenhek - Contributed various patches, a rewrite of the
|
|
network driver, the first implementation of the mconsole driver, and
|
|
did the bulk of the work needed to get SMP working again.
|
|
|
|
|
|
Yon Uriarte - Fixed the TUN/TAP network backend while I slept.
|
|
|
|
|
|
Adam Heath - Made a bunch of nice cleanups to the initialization code,
|
|
plus various other small patches.
|
|
|
|
|
|
Matt Zimmerman - Matt volunteered to be the UML Debian maintainer and
|
|
is doing a real nice job of it. He also noticed and fixed a number of
|
|
actually and potentially exploitable security holes in uml_net. Plus
|
|
the occasional patch. I like patches.
|
|
|
|
|
|
James McMechan - James seems to have taken over maintenance of the ubd
|
|
driver and is doing a nice job of it.
|
|
|
|
|
|
Chandan Kudige - wrote the umlgdb script which automates the reloading
|
|
of module symbols.
|
|
|
|
|
|
Steve Schmidtke - wrote the UML slirp transport and hostaudio drivers,
|
|
enabling UML processes to access audio devices on the host. He also
|
|
submitted patches for the slip transport and lots of other things.
|
|
|
|
|
|
David Coulson <http://davidcoulson.net> -
|
|
|
|
o Set up the usermodelinux.org <http://usermodelinux.org> site,
|
|
which is a great way of keeping the UML user community on top of
|
|
UML goings-on.
|
|
|
|
o Site documentation and updates
|
|
|
|
o Nifty little UML management daemon UMLd
|
|
<http://uml.openconsultancy.com/umld/>
|
|
|
|
o Lots of testing and bug reports
|
|
|
|
|
|
|
|
|
|
15.2. Flushing out bugs
|
|
|
|
|
|
|
|
o Yuri Pudgorodsky
|
|
|
|
o Gerald Britton
|
|
|
|
o Ian Wehrman
|
|
|
|
o Gord Lamb
|
|
|
|
o Eugene Koontz
|
|
|
|
o John H. Hartman
|
|
|
|
o Anders Karlsson
|
|
|
|
o Daniel Phillips
|
|
|
|
o John Fremlin
|
|
|
|
o Rainer Burgstaller
|
|
|
|
o James Stevenson
|
|
|
|
o Matt Clay
|
|
|
|
o Cliff Jefferies
|
|
|
|
o Geoff Hoff
|
|
|
|
o Lennert Buytenhek
|
|
|
|
o Al Viro
|
|
|
|
o Frank Klingenhoefer
|
|
|
|
o Livio Baldini Soares
|
|
|
|
o Jon Burgess
|
|
|
|
o Petru Paler
|
|
|
|
o Paul
|
|
|
|
o Chris Reahard
|
|
|
|
o Sverker Nilsson
|
|
|
|
o Gong Su
|
|
|
|
o johan verrept
|
|
|
|
o Bjorn Eriksson
|
|
|
|
o Lorenzo Allegrucci
|
|
|
|
o Muli Ben-Yehuda
|
|
|
|
o David Mansfield
|
|
|
|
o Howard Goff
|
|
|
|
o Mike Anderson
|
|
|
|
o John Byrne
|
|
|
|
o Sapan J. Batia
|
|
|
|
o Iris Huang
|
|
|
|
o Jan Hudec
|
|
|
|
o Voluspa
|
|
|
|
|
|
|
|
|
|
15.3. Buglets and clean-ups
|
|
|
|
|
|
|
|
o Dave Zarzycki
|
|
|
|
o Adam Lazur
|
|
|
|
o Boria Feigin
|
|
|
|
o Brian J. Murrell
|
|
|
|
o JS
|
|
|
|
o Roman Zippel
|
|
|
|
o Wil Cooley
|
|
|
|
o Ayelet Shemesh
|
|
|
|
o Will Dyson
|
|
|
|
o Sverker Nilsson
|
|
|
|
o dvorak
|
|
|
|
o v.naga srinivas
|
|
|
|
o Shlomi Fish
|
|
|
|
o Roger Binns
|
|
|
|
o johan verrept
|
|
|
|
o MrChuoi
|
|
|
|
o Peter Cleve
|
|
|
|
o Vincent Guffens
|
|
|
|
o Nathan Scott
|
|
|
|
o Patrick Caulfield
|
|
|
|
o jbearce
|
|
|
|
o Catalin Marinas
|
|
|
|
o Shane Spencer
|
|
|
|
o Zou Min
|
|
|
|
|
|
o Ryan Boder
|
|
|
|
o Lorenzo Colitti
|
|
|
|
o Gwendal Grignou
|
|
|
|
o Andre' Breiler
|
|
|
|
o Tsutomu Yasuda
|
|
|
|
|
|
|
|
15.4. Case Studies
|
|
|
|
|
|
o Jon Wright
|
|
|
|
o William McEwan
|
|
|
|
o Michael Richardson
|
|
|
|
|
|
|
|
15.5. Other contributions
|
|
|
|
|
|
Bill Carr <Bill.Carr at compaq.com> made the Red Hat mkrootfs script
|
|
work with RH 6.2.
|
|
|
|
Michael Jennings <mikejen at hevanet.com> sent in some material which
|
|
is now gracing the top of the index page <http://user-mode-
|
|
linux.sourceforge.net/> of this site.
|
|
|
|
SGI <http://www.sgi.com> (and more specifically Ralf Baechle <ralf at
|
|
uni-koblenz.de> ) gave me an account on oss.sgi.com
|
|
<http://www.oss.sgi.com> . The bandwidth there made it possible to
|
|
produce most of the filesystems available on the project download
|
|
page.
|
|
|
|
Laurent Bonnaud <Laurent.Bonnaud at inpg.fr> took the old grotty
|
|
Debian filesystem that I've been distributing and updated it to 2.2.
|
|
It is now available by itself here.
|
|
|
|
Rik van Riel gave me some ftp space on ftp.nl.linux.org so I can make
|
|
releases even when Sourceforge is broken.
|
|
|
|
Rodrigo de Castro looked at my broken pte code and told me what was
|
|
wrong with it, letting me fix a long-standing (several weeks) and
|
|
serious set of bugs.
|
|
|
|
Chris Reahard built a specialized root filesystem for running a DNS
|
|
server jailed inside UML. It's available from the download
|
|
<http://user-mode-linux.sourceforge.net/dl-sf.html> page in the Jail
|
|
Filesystems section.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|