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Re: [PATCH v2 2/2] efi: Put Linux specific magic number in the DOS heade
Re: [PATCH v2 2/2] efi: Put Linux specific magic number in the DOS header
Tue, 6 Dec 2022 01:52:04 -0800
On Thu, Dec 1, 2022 at 6:30 AM Daniel Kiper <firstname.lastname@example.org> wrote:
> On Tue, Nov 29, 2022 at 06:56:16PM +0100, Ard Biesheuvel wrote:
> > 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
> s/MSDOS/MS-DOS/ to be consistent with other places in the patch...
> > 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
> s/the magic number at offset #0/MS-DOS EXE magic number at offset #0/?
> "the magic number at offset #0" itself is confusing in the context of
> number of "magic number" phrases in the patch... :-)
> > 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 <email@example.com>
> > Cc: Atish Patra <firstname.lastname@example.org>
> > Cc: Heinrich Schuchardt <email@example.com>
> > Cc: Daniel Kiper <firstname.lastname@example.org>
> > Cc: Leif Lindholm <email@example.com>
> > Signed-off-by: Ard Biesheuvel <firstname.lastname@example.org>
> > ---
> > 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" */
> If you take into account these minor things mentioned above you can add
> Reviewed-by: Daniel Kiper <email@example.com> to your both patches.
> Atish, could you respin your GRUB "Unify ARM64 & RISC-V Linux Loader"
> patch set when both Ard's patches are in at least "efi" Linux kernel
> branch? Hmmm... I am not sure it will go through "efi" branch. Ard?
Sure. We need to remove the arch specific header files in the next
version as well.
I am planning to revise the series before/during the holidays. Hope
that's fine w.r.t Grub release.
Otherwise, please let me know.