The EFI handover protocol permits a bootloader to invoke the kernel as a EFI PE/COFF application, while passing a bootparams struct as a third argument to the entrypoint function call. This has no basis in the UEFI specification, and there are better ways to pass additional data to a UEFI application (UEFI configuration tables, UEFI variables, UEFI protocols) than going around the StartImage() boot service and jumping to a fixed offset in the loaded image, just to call a different function that takes a third parameter. The reason for handling struct bootparams in the bootloader was that the EFI stub could only load initrd images from the EFI system partition, and so passing it via struct bootparams was needed for loaders like GRUB, which pass the initrd in memory, and may load it from anywhere, including from the network. Another motivation was EFI mixed mode, which could not use the initrd loader in the EFI stub at all due to 32/64 bit incompatibilities (which will be fixed shortly [0]), and could not invoke the ordinary PE/COFF entry point either, for the same reasons. Given that loaders such as GRUB already carried the bootparams handling in order to implement non-EFI boot, retaining that code and just passing bootparams to the EFI stub was a reasonable choice (although defining an alternate entrypoint could have been avoided.) However, the GRUB side changes never made it upstream, and are only shipped by some of the distros in their downstream versions. In the meantime, EFI support has been added to other Linux architecture ports, as well as to U-boot and systemd, including arch-agnostic methods for passing initrd images in memory [1], and for doing mixed mode boot [2], none of them requiring anything like the EFI handover protocol. So given that only out-of-tree distro GRUB relies on this, let's permit it to be omitted from the build, in preparation for retiring it completely at a later date. (Note that systemd-boot does have an implementation as well, but only uses it as a fallback for booting images that do not implement the LoadFile2 based initrd loading method, i.e., v5.8 or older) [0] https://lore.kernel.org/all/20220927085842.2860715-1-ardb@xxxxxxxxxx/ [1] ec93fc371f01 ("efi/libstub: Add support for loading the initrd ...") [2] 97aa276579b2 ("efi/x86: Add true mixed mode entry point into ...") Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> --- arch/x86/Kconfig | 17 +++++++++++++++++ arch/x86/boot/compressed/head_64.S | 4 +++- arch/x86/boot/header.S | 2 +- arch/x86/boot/tools/build.c | 2 ++ 4 files changed, 23 insertions(+), 2 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 8c6da5e42d5a6c25..121f1fdca3145fd2 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -1981,6 +1981,23 @@ config EFI_STUB See Documentation/admin-guide/efi-stub.rst for more information. +config EFI_HANDOVER_PROTOCOL + bool "EFI handover protocol (DEPRECATED)" + depends on EFI_STUB + default y + help + Select this in order to include support for the deprecated EFI + handover protocol, which defines alternative entry points into the + EFI stub. This is a practice that has no basis in the UEFI + specification, and requires a priori knowledge on the part of the + bootloader about Linux/x86 specific ways of passing the command line + and initrd, and where in memory those assets may be loaded. + + If in doubt, say Y. Even though he corresponding support is not + present in upstream GRUB or other bootloaders, most distros build + GRUB with numerous downstream patches applied, and may rely on the + handover protocol as as result. + config EFI_MIXED bool "EFI mixed-mode support" depends on EFI_STUB && X86_64 diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index dd18216cff5c37e0..a75712991df3e936 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -294,7 +294,7 @@ SYM_FUNC_START(startup_32) lret SYM_FUNC_END(startup_32) -#ifdef CONFIG_EFI_MIXED +#if IS_ENABLED(CONFIG_EFI_MIXED) && IS_ENABLED(CONFIG_EFI_HANDOVER_PROTOCOL) .org 0x190 SYM_FUNC_START(efi32_stub_entry) add $0x4, %esp /* Discard return address */ @@ -524,7 +524,9 @@ trampoline_return: SYM_CODE_END(startup_64) #ifdef CONFIG_EFI_STUB +#ifdef CONFIG_EFI_HANDOVER_PROTOCOL .org 0x390 +#endif SYM_FUNC_START(efi64_stub_entry) and $~0xf, %rsp /* realign the stack */ movq %rdx, %rbx /* save boot_params pointer */ diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S index f912d777013052ea..d31982509654dcb1 100644 --- a/arch/x86/boot/header.S +++ b/arch/x86/boot/header.S @@ -406,7 +406,7 @@ xloadflags: # define XLF1 0 #endif -#ifdef CONFIG_EFI_STUB +#ifdef CONFIG_EFI_HANDOVER_PROTOCOL # ifdef CONFIG_EFI_MIXED # define XLF23 (XLF_EFI_HANDOVER_32|XLF_EFI_HANDOVER_64) # else diff --git a/arch/x86/boot/tools/build.c b/arch/x86/boot/tools/build.c index a3725ad46c5a0b49..bd247692b70174f0 100644 --- a/arch/x86/boot/tools/build.c +++ b/arch/x86/boot/tools/build.c @@ -290,6 +290,7 @@ static void efi_stub_entry_update(void) { unsigned long addr = efi32_stub_entry; +#ifdef CONFIG_EFI_HANDOVER_PROTOCOL #ifdef CONFIG_X86_64 /* Yes, this is really how we defined it :( */ addr = efi64_stub_entry - 0x200; @@ -298,6 +299,7 @@ static void efi_stub_entry_update(void) #ifdef CONFIG_EFI_MIXED if (efi32_stub_entry != addr) die("32-bit and 64-bit EFI entry points do not match\n"); +#endif #endif put_unaligned_le32(addr, &buf[0x264]); } -- 2.35.1
