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

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-07-23 12:37, Brian Gerst wrote:
> On Wed, Jul 22, 2026 at 2:53 PM H. Peter Anvin <hpa@zytor.com> wrote:
>>
>> On July 22, 2026 10:03:34 AM PDT, Borislav Petkov <bp@alien8.de> 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.
>>>
>>>> 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.
>>>
>>> Then you can base the rest ontop.
>>>
>>> Thx.
>>>
>>
>> Also, this is silly. Instead of adding a whole separate test, just do "test %3,%3" before the repe to set ZF and let the REPE skip.
> 
> REPE does not check ZF before the first iteration.
> 
> From the Intel SDM:
> "When the REPE/REPZ and REPNE/REPNZ prefixes are used, the ZF flag
> does not require initialization because both
> the CMPS and SCAS instructions affect the ZF flag according to the
> results of the comparisons they make."
> 

The point was to ensure we have a valid result in ZF (which should be ZF=1)
even if len = 0.

	-hpa