Re: [PATCH v7 2/5] x86/asm: add volatile, clobbers and zero-length check in inline memcmp

[Mauricio Faria de Oliveira <mfo@igalia.com>]

On 2026-07-22 14:03, Borislav Petkov wrote:
> hpa in To:.
> 
> On Tue, Jul 21, 2026 at 12:56:43PM -0300, Mauricio Faria de Oliveira wrote:
>> Add the volatile qualifier and clobbers parameter to prevent bugs with
>> instruction reordering and optimization.
>> 
>> Also check the zero-length case, as the 'repe' prefix does not run the
>> 'cmpsb' instruction if the 'count' register is zero, which doesn't set
> 
> Please use capital letters for insns: REPE, CMPSB and you don't need to put
> words in '' - it reads fine without them.

Ok.

> 
>> the condition-code/zero flag, so the result is based on a stale flag.
>> 
>> Those are pre-existing issues found by Sashiko.
>> 
>> Link: https://sashiko.dev/#/patchset/20260701-pvh-kasan-inline-v6-0-ba99045dfa9f%40igalia.com
>> Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
>> ---
>>  arch/x86/include/asm/shared/string.h | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/shared/string.h b/arch/x86/include/asm/shared/string.h
>> index 02b92927553f7b8e1c87e6122bbaa70439e57ea7..166274e44f3cb49e3dccab3cdac281d67aef5d44 100644
>> --- a/arch/x86/include/asm/shared/string.h
>> +++ b/arch/x86/include/asm/shared/string.h
>> @@ -11,8 +11,12 @@ static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_
>>  {
>>  	bool diff;
>>  
>> -	asm("repe cmpsb"
>> -	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
>> +	if (len == 0)
>> +		return 0;
>> +
>> +	asm volatile("repe cmpsb"
>> +		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
>> +		     : : "cc", "memory");
>>  
>>  	return diff;
>>  }
> 
> So this is a fix which should probably go to stable, I think. Going back into
> git history, it points to
> 
>   62bd0337d0c4 ("Top header file for new x86 setup code")
> 
> from 2007. And we have carried it this way through the years and who knows
> what hit this or not. So please make this the first patch in your set - you
> can even send it separately so that I can get route it through stable.

Sure:
https://lore.kernel.org/all/20260723-x86-memcmp-asm-v2-1-d93ecb43797f@igalia.com/

> Then you can base the rest ontop.

Done:
https://lore.kernel.org/all/20260723-pvh-kasan-inline-v8-0-c1f62c156f52@igalia.com/

Thanks,

> 
> Thx.

-- 
Mauricio