Re: [PATCH] x86emul: V{,P}{COMPRESS,EXPAND}* can (wrongly) trigger assertion

On 16.07.2026 12:44, Andrew Cooper wrote:
> On 16/07/2026 11:43 am, Jan Beulich wrote:
>> AFL has pointed out that the op_bytes-is-not-0 assertion in common SIMD
>> handling can trigger for these insns. Indeed when the (relevant part of)
>> the controlling mask register is 0, no memory is accessed at all. Leave
>> op_bytes unaltered in this case, to engage the short-circuiting in common
>> SIMD handling when fault_suppression is true and op_bytes is 0.
>>
>> While there also correct a related typo in the test harness.
>>
>> Fixes: 65f82d4ce1ea ("x86emul: support AVX512{F,_VBMI2} compress/expand insns")
>> Signed-off-by: Jan Beulich <[email protected]>
> 
> Acked-by: Andrew Cooper <[email protected]>

Thanks.

> Is this local AFL testing?  I'm slightly surprised that OSS-Fuzz hasn't
> found this.

Yes, I'm trying to remember to run the fuzzer every once in a while. Earlier
runs of mine didn't spot the issue either. (I'm presently re-running things,
to have a limited level of certainty that the issues it found are all sorted
now. The other issues it found were in patches I continue to have pending.)

Jan