GRUB currently relies on the magic number in the image header of ARM and arm64 EFI kernel images to decide whether or not the image in question is a bootable kernel. However, the purpose of the magic number is to identify the image as one that implements the bare metal boot protocol, and so GRUB, which only does EFI boot, can only boot images that could potentially be booted in a non-EFI manner as well. This is problematic for the new zboot decompressor image format, as it can only boot in EFI mode, and must therefore not use the bare metal boot magic number in its header. For this reason, the strict magic number was dropped from GRUB, to permit essentially any kind of EFI executable to be booted via the 'linux' command, blurring the line between the linux loader and the chainloader. So let's use the same field in the DOS header that RISC-V and arm64 already use for their 'bare metal' magic numbers to store a 'generic Linux kernel' magic number, which can be used to identify bootable kernel images in PE format which don't necessarily implement a bare metal boot protocol in the same binary. Note that, in the context of EFI, the MSDOS header is only described in terms of the fields that it shares with the hybrid PE/COFF image format, (i.e., the magic number at offset #0 and the PE header offset at byte offset #0x3c). Since we aim for compatibility with EFI only, and not with MS-DOS or MS-Windows, we can use the remaining space in the MS-DOS header however we want. Let's set the generic magic number for x86 images as well: existing bootloaders already have their own methods to identify x86 Linux images that can be booted in a non-EFI manner, and having the magic number in place there will ease any future transitions in loader implementations to merge the x86 and non-x86 EFI boot paths. Note that 32-bit ARM already uses the same location in the header for a different purpose, but the ARM support is already widely implemented and the EFI zboot decompressor is not available on ARM anyway, so we just disregard it here. Cc: Huacai Chen <chenhuacai@xxxxxxxxxx> Cc: Atish Patra <atishp@xxxxxxxxxxxx> Cc: Heinrich Schuchardt <heinrich.schuchardt@xxxxxxxxxxxxx> Cc: Daniel Kiper <daniel.kiper@xxxxxxxxxx> Cc: Leif Lindholm <quic_llindhol@xxxxxxxxxxx> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> --- arch/loongarch/kernel/head.S | 3 ++- arch/x86/boot/header.S | 3 ++- drivers/firmware/efi/libstub/zboot-header.S | 3 ++- include/linux/pe.h | 7 +++++++ 4 files changed, 13 insertions(+), 3 deletions(-) diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S index 84970e2666588963..caa74439700eee93 100644 --- a/arch/loongarch/kernel/head.S +++ b/arch/loongarch/kernel/head.S @@ -25,7 +25,8 @@ _head: .dword kernel_entry /* Kernel entry point */ .dword _end - _text /* Kernel image effective size */ .quad 0 /* Kernel image load offset from start of RAM */ - .org 0x3c /* 0x20 ~ 0x3b reserved */ + .org 0x38 /* 0x20 ~ 0x38 reserved */ + .long LINUX_PE_MAGIC .long pe_header - _head /* Offset to the PE header */ pe_header: diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S index f912d777013052ea..be8f78a7ee325475 100644 --- a/arch/x86/boot/header.S +++ b/arch/x86/boot/header.S @@ -80,10 +80,11 @@ bs_die: ljmp $0xf000,$0xfff0 #ifdef CONFIG_EFI_STUB - .org 0x3c + .org 0x38 # # Offset to the PE header. # + .long LINUX_PE_MAGIC .long pe_header #endif /* CONFIG_EFI_STUB */ diff --git a/drivers/firmware/efi/libstub/zboot-header.S b/drivers/firmware/efi/libstub/zboot-header.S index bc2d7750d7f14174..ec4525d40e0cf6d6 100644 --- a/drivers/firmware/efi/libstub/zboot-header.S +++ b/drivers/firmware/efi/libstub/zboot-header.S @@ -20,7 +20,8 @@ __efistub_efi_zboot_header: .long __efistub__gzdata_size - 12 // payload size .long 0, 0 // reserved .asciz COMP_TYPE // compression type - .org .Ldoshdr + 0x3c + .org .Ldoshdr + 0x38 + .long LINUX_PE_MAGIC .long .Lpehdr - .Ldoshdr // PE header offset .Lpehdr: diff --git a/include/linux/pe.h b/include/linux/pe.h index 056a1762de904fc1..1db4c944efd78f51 100644 --- a/include/linux/pe.h +++ b/include/linux/pe.h @@ -31,6 +31,13 @@ #define LINUX_EFISTUB_MAJOR_VERSION 0x1 #define LINUX_EFISTUB_MINOR_VERSION 0x1 +/* + * LINUX_PE_MAGIC appears at offset 0x38 into the MSDOS header of EFI bootable + * Linux kernel images that target the architecture as specified by the PE/COFF + * header machine type field. + */ +#define LINUX_PE_MAGIC 0x818223cd + #define MZ_MAGIC 0x5a4d /* "MZ" */ #define PE_MAGIC 0x00004550 /* "PE\0\0" */ -- 2.35.1
