Commit 0c759662 authored by Matt Fleming's avatar Matt Fleming Committed by H. Peter Anvin
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x86, efi: Add EFI boot stub documentation 



Since we can't expect every user to read the EFI boot stub code it
seems prudent to have a couple of paragraphs explaining what it is and
how it works.

The "initrd=" option in particular is tricky because it only
understands absolute EFI-style paths (backslashes as directory
separators), and until now this hasn't been documented anywhere. This
has tripped up a couple of users.

Cc: Matthew Garrett <mjg@redhat.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: default avatarMatt Fleming <matt.fleming@intel.com>
Link: http://lkml.kernel.org/r/1331907517-3985-4-git-send-email-matt@console-pimps.org


Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent 9fa7deda

Documentation/x86/efi-stub.txt

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			  The EFI Boot Stub

		     ---------------------------



On the x86 platform, a bzImage can masquerade as a PE/COFF image,

thereby convincing EFI firmware loaders to load it as an EFI

executable. The code that modifies the bzImage header, along with the

EFI-specific entry point that the firmware loader jumps to are

collectively known as the "EFI boot stub", and live in

arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,

respectively.



By using the EFI boot stub it's possible to boot a Linux kernel

without the use of a conventional EFI boot loader, such as grub or

elilo. Since the EFI boot stub performs the jobs of a boot loader, in

a certain sense it *IS* the boot loader.



The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.





**** How to install bzImage.efi



The bzImage located in arch/x86/boot/bzImage must be copied to the EFI

System Partiion (ESP) and renamed with the extension ".efi". Without

the extension the EFI firmware loader will refuse to execute it. It's

not possible to execute bzImage.efi from the usual Linux file systems

because EFI firmware doesn't have support for them.





**** Passing kernel parameters from the EFI shell



Arguments to the kernel can be passed after bzImage.efi, e.g.



	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4





**** The "initrd=" option



Like most boot loaders, the EFI stub allows the user to specify

multiple initrd files using the "initrd=" option. This is the only EFI

stub-specific command line parameter, everything else is passed to the

kernel when it boots.



The path to the initrd file must be an absolute path from the

beginning of the ESP, relative path names do not work. Also, the path

is an EFI-style path and directory elements must be separated with

backslashes (\). For example, given the following directory layout,



fs0:>

	Kernels\

			bzImage.efi

			initrd-large.img



	Ramdisks\

			initrd-small.img

			initrd-medium.img



to boot with the initrd-large.img file if the current working

directory is fs0:\Kernels, the following command must be used,



	fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img



Notice how bzImage.efi can be specified with a relative path. That's

because the image we're executing is interpreted by the EFI shell,

which understands relative paths, whereas the rest of the command line

is passed to bzImage.efi.

arch/x86/Kconfig

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@@ -1506,6 +1506,8 @@ config EFI_STUB 
          This kernel feature allows a bzImage to be loaded directly

	  by EFI firmware without the use of a bootloader.



	  See Documentation/x86/efi-stub.txt for more information.



config SECCOMP

	def_bool y

	prompt "Enable seccomp to safely compute untrusted bytecode"

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