|author||Matt Fleming <firstname.lastname@example.org>||2012-03-16 12:03:13 +0000|
|committer||H. Peter Anvin <email@example.com>||2012-06-01 09:11:41 -0700|
|parent||x86, efi; Add EFI boot stub console support (diff)|
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 <firstname.lastname@example.org> Cc: Randy Dunlap <email@example.com> Signed-off-by: Matt Fleming <firstname.lastname@example.org> Link: http://email@example.com Signed-off-by: H. Peter Anvin <firstname.lastname@example.org>
2 files changed, 67 insertions, 0 deletions
diff --git a/Documentation/x86/efi-stub.txt b/Documentation/x86/efi-stub.txt
new file mode 100644
@@ -0,0 +1,65 @@
+ 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,
+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,
+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.
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d700811785ea..c70684f859e1 100644
@@ -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.
prompt "Enable seccomp to safely compute untrusted bytecode"