85 lines
3.2 KiB
Plaintext
85 lines
3.2 KiB
Plaintext
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The EFI Boot Stub
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---------------------------
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On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
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as a PE/COFF image, thereby convincing EFI firmware loaders to load
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it as an EFI executable. The code that modifies the bzImage header,
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along with the EFI-specific entry point that the firmware loader
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jumps to are collectively known as the "EFI boot stub", and live in
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arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
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respectively. For ARM the EFI stub is implemented in
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arch/arm/boot/compressed/efi-header.S and
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arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared
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between architectures is in drivers/firmware/efi/libstub.
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For arm64, there is no compressed kernel support, so the Image itself
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masquerades as a PE/COFF image and the EFI stub is linked into the
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kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
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and drivers/firmware/efi/libstub/arm64-stub.c.
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By using the EFI boot stub it's possible to boot a Linux kernel
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without the use of a conventional EFI boot loader, such as grub or
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elilo. Since the EFI boot stub performs the jobs of a boot loader, in
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a certain sense it *IS* the boot loader.
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The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
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**** How to install bzImage.efi
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The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
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System Partition (ESP) and renamed with the extension ".efi". Without
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the extension the EFI firmware loader will refuse to execute it. It's
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not possible to execute bzImage.efi from the usual Linux file systems
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because EFI firmware doesn't have support for them. For ARM the
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arch/arm/boot/zImage should be copied to the system partition, and it
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may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
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should be copied but not necessarily renamed.
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**** Passing kernel parameters from the EFI shell
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Arguments to the kernel can be passed after bzImage.efi, e.g.
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fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
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**** The "initrd=" option
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Like most boot loaders, the EFI stub allows the user to specify
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multiple initrd files using the "initrd=" option. This is the only EFI
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stub-specific command line parameter, everything else is passed to the
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kernel when it boots.
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The path to the initrd file must be an absolute path from the
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beginning of the ESP, relative path names do not work. Also, the path
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is an EFI-style path and directory elements must be separated with
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backslashes (\). For example, given the following directory layout,
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fs0:>
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Kernels\
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bzImage.efi
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initrd-large.img
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Ramdisks\
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initrd-small.img
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initrd-medium.img
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to boot with the initrd-large.img file if the current working
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directory is fs0:\Kernels, the following command must be used,
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fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
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Notice how bzImage.efi can be specified with a relative path. That's
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because the image we're executing is interpreted by the EFI shell,
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which understands relative paths, whereas the rest of the command line
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is passed to bzImage.efi.
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**** The "dtb=" option
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For the ARM and arm64 architectures, we also need to be able to provide a
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device tree to the kernel. This is done with the "dtb=" command line option,
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and is processed in the same manner as the "initrd=" option that is
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described above.
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