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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/06/2025 8:22 am, Jan Beulich wrote:
> On 05.06.2025 19:01, Andrew Cooper wrote:
>> On 05/06/2025 2:24 pm, Jan Beulich wrote:
>>> 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:
>>>> 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).
>> It's all hacks, but no.
>>
>> I'm amazed MISRA hasn't spotted that we've got a global `struct efi
>> efi;` and a label named efi, creating an alias for the object with it
>> out of bounds in the compiled image.  But even then, it's based on
>> XEN_BUILD_EFI not XEN_BUILD_PE and does not distinguish the property
>> that matters.
> The use of XEN_BUILD_EFI in the linker script should have been switched
> to XEN_BUILD_PE when the split was introduced.

That doesn't build.  As I already explained, the stubs aren't split in a
way that allows that.
>> But the argument I'm going to make this this:  Why do you want a check,
>> even if you can find a correct one (and as said before, I cannot)?
>>
>> This function is run exactly once.  We've excluded "nothing given by the
>> toolchain", and excluded "what the toolchain gave us was not the
>> expected ELF note".  The only thing left (modulo toolchain bugs) is the
>> CodeView region, and if it's not a valid CodeView region then we've
>> wasted a handful of cycles.
> Two reasons: Having code which cannot possibly do anything useful isn't
> good. Misra calls the latest the body of the inner if() "unreachable code"
> and objects to the presence of such in a build.

It's not unreachable code, not even theoretically.

*If* there was a suitable check, I'd be using it, but everything you've
proposed has been buggy or doesn't even compile.

> And then, based on your reasoning above, why don't you also drop the
> #ifdef CONFIG_X86?

Because that's the one non-buggy way of excluding an impossible case.

x86 is the only architecture possibly linking with pep emulation, and
therefore the only architecture to possibly have a CodeView record.

~Andrew