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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.06.2025 14:14, Andrew Cooper wrote:
> On 05/06/2025 1:02 pm, 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.)
> 
> MB2+EFI came long before any buildid support.  If you want a fixes tag,
> it's eee5909e9d1

That commit talks of an earlier hack, though. And no, MB2 work came later,
albeit still in the 4.9 dev window (commit 9180f53655245).

>>> --- 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.
> 
> No - that's differently buggy.  I suppose the commit message wasn't
> clear enough?
> 
> We'd still have a CodeView record if we booted xen.efi using it's MB2
> entrypoint without the EFI extensions.

Hmm, if that's a possible mode of operation (as said in reply to Marek, I
wasn't aware of that) - yes.

> This really is a property of being a PE32+ binary, and nothing to do
> with EFI.

Which still can be checked for without having this code path being taken
for xen.gz, too: You could e.g. check for &efi > &_end. That's firmly an
image property (yet I expect you're going to sigh about yet another hack).

Jan