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

[H. Peter Anvin]

On July 23, 2026 5:49:13 PM PDT, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>On Thu, Jul 23, 2026 at 7:13 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>>
>> On 2026-07-23 12:37, Brian Gerst wrote:
>> > On Wed, Jul 22, 2026 at 2:53 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> >>
>> >> On July 22, 2026 10:03:34 AM PDT, Borislav Petkov <bp@xxxxxxxxx> 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@xxxxxxxxxx>
>> >>>> ---
>> >>>>  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.
>
>I had originally thought that the test for RCX==0 was also at the end
>of the loop, after the decrement.  But a closer look at the
>pseudo-code in the SDM does show that the test is at the start of the
>loop.  Starting with RCX==0 does indeed skip the comparison and does
>not touch ZF.  Your suggestion is good, but should have a comment
>explaining why it's there to someone reading the code in the future.
>

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