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

[Daniel Kiper]

On Thu, Dec 01, 2022 at 03:48:12PM +0100, Ard Biesheuvel wrote:
> 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 */

Sadly one more thing... :-(

s/0x20 ~ 0x38 reserved/0x20 ~ 0x37 reserved/

Daniel