Re: [PATCH 1/3] x86/EFI: Fix detection of buildid

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Jun 05, 2025 at 03:08:07PM +0200, Jan Beulich wrote:
> On 05.06.2025 14:20, Marek Marczykowski-Górecki wrote:
> > On Thu, Jun 05, 2025 at 02:02:21PM +0200, Jan Beulich wrote:
> >> On 05.06.2025 13:16, Andrew Cooper wrote:
> >>> The format of the buildid is a property of the binary, not a property of 
> >>> how
> >>> it was loaded.  This fixes buildid recognition when starting xen.efi from 
> >>> it's
> >>> MB2 entrypoint.
> >>>
> >>> Suggested-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> >>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >>
> >> I'll pick this up without a Fixes: tag, but I think it ought to have one. 
> >> (I
> >> didn't check whether MB2 or build-id support came later, hence introducing 
> >> the
> >> issue.)
> >>
> >>> --- a/xen/common/version.c
> >>> +++ b/xen/common/version.c
> >>> @@ -203,8 +203,11 @@ void __init xen_build_init(void)
> >>>      rc = xen_build_id_check(n, sz, &build_id_p, &build_id_len);
> >>>  
> >>>  #ifdef CONFIG_X86
> >>> -    /* Alternatively we may have a CodeView record from an EFI build. */
> >>> -    if ( rc && efi_enabled(EFI_LOADER) )
> >>> +    /*
> >>> +     * xen.efi built with a new enough toolchain will have a CodeView 
> >>> record,
> >>> +     * not an ELF note.
> >>> +     */
> >>> +    if ( rc )
> >>
> >> Instead of dropping the efi_enabled(), I think you want to replace 
> >> EFI_LOADER
> >> by EFI_BOOT.
> > 
> > Part of the motivation for MB2 entry point in xen.efi is using the
> > same binary in all boot modes, including legacy boot - in which case
> > none of EFI_* checks would be appropriate here. The grub series adds
> > support for loading PE binaries, and IIUC it isn't tied to booting via
> > EFI.
> 
> "The grub series" being which one? 

https://lists.gnu.org/archive/html/grub-devel/2024-03/msg00080.html
| This patch series implements support for loading and verifying a signed
| xen binary. This would allow the same xen binary to be used for BIOS
| boot, UEFI boot, and UEFI boot with Secure Boot verification.
| There is an accompanying Xen patch series.

> I didn't know xen.efi's non-EFI MB2 entry
> point could be used; instead I was under the impression that someone was
> having (half?) a patch eliminating the MB data from xen.efi, as being dead
> code.

See also
https://lore.kernel.org/xen-devel/CAG7k0EroeA=cRRDWnJqzH8esoaSmtg8-xjTwc-01og5R9JwPzg@xxxxxxxxxxxxxx/
for some extra context.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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