From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1032834Ab2CPOSw (ORCPT ); Fri, 16 Mar 2012 10:18:52 -0400 Received: from arkanian.console-pimps.org ([212.110.184.194]:33364 "EHLO arkanian.console-pimps.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1032793Ab2CPOSs (ORCPT ); Fri, 16 Mar 2012 10:18:48 -0400 From: Matt Fleming To: "H. Peter Anvin" Cc: Matthew Garret , Matt Fleming , linux-kernel@vger.kernel.org, x86@kernel.org, Randy Dunlap Subject: [PATCH 3/3] x86, efi: Add EFI boot stub documentation Date: Fri, 16 Mar 2012 14:18:37 +0000 Message-Id: <1331907517-3985-4-git-send-email-matt@console-pimps.org> X-Mailer: git-send-email 1.7.4.4 In-Reply-To: <1331907517-3985-1-git-send-email-matt@console-pimps.org> References: <1331907517-3985-1-git-send-email-matt@console-pimps.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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, +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. 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" -- 1.7.4.4