Re: [PATCH v2 2/2] efi: Put Linux specific magic number in the DOS header

[Ard Biesheuvel]

On Thu, 1 Dec 2022 at 15:30, Daniel Kiper <daniel.kiper@oracle.com> 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 <chenhuacai@kernel.org>
> > Cc: Atish Patra <atishp@rivosinc.com>
> > Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> > Cc: Daniel Kiper <daniel.kiper@oracle.com>
> > Cc: Leif Lindholm <quic_llindhol@quicinc.com>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.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
>
> s/MSDOS/MS-DOS/
>
> > + * 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 <daniel.kiper@oracle.com> to your both patches.
>

Thanks

> 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?
>

Yes, I already pushed a version, but I will update the patch to take
your feedback into account.

The patch is queued up for v6.2, so it should be in mainline by the
end of the year.