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    Subject[PATCH 3/3] x86, efi: Add EFI boot stub documentation
    From: Matt Fleming <>

    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: H. Peter Anvin <>
    Cc: Matthew Garrett <>
    Cc: Randy Dunlap <>
    Signed-off-by: Matt Fleming <>
    Documentation/x86/efi-stub.txt | 65 ++++++++++++++++++++++++++++++++++++++++
    arch/x86/Kconfig | 2 +
    2 files changed, 67 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/x86/efi-stub.txt

    diff --git a/Documentation/x86/efi-stub.txt b/Documentation/x86/efi-stub.txt
    new file mode 100644
    index 0000000..44e6bb6
    --- /dev/null
    +++ b/Documentation/x86/efi-stub.txt
    @@ -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,
    + 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.
    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index 5bed94e..591c1a5 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -1521,6 +1521,8 @@ 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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