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Re: [BUG] x86emul/test: avoid assertion in emul_test_read_xcr() when XSAVE is absent
- To: Andrew Precious <andrewprecious388@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 20 Aug 2026 16:12:53 +0200
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- Cc: andrew.cooper3@xxxxxxxxxx, roger@xxxxxxxxxxxxxx, teddy.astie@xxxxxxxxxx, anthony.perard@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 20 Aug 2026 14:12:58 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 20.08.2026 14:30, Andrew Precious wrote:
> I just realized that I attached the binary result without the fuzzer error.
> To generate the previous assertion error I would have to rerun the fuzzer
> again which took many hours(~18hrs). Though that was the only error I found
> after that long.
>
> VM machine that I performed the fuzzing on:
> - A VM running Debian GNU/Linux 13 (trixie)
> - x86_64,QEMU Virtual CPU version 2.5
What does that mean CPUID-wise, seeing that ...
> On Thu, Aug 20, 2026 at 11:45 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 19.08.2026 20:07, Andrew Mbugua wrote:
>>> While running the x86_instruction_emulator fuzzer via AFL, I encountered
>> an assertion failure in the emul_test_read_xcr() function.
>>
>> Thanks for the report.
>>
>>> The fuzzer is able to generate a CPU state where cpu_has_xsave is false.
>>
>> I'm having trouble here: cpu_policy isn't populated from fuzzing input, and
>>
>> /* Intentionally checking OSXSAVE here. */
>> #define cpu_has_xsave (cpu_policy.basic.raw[1].c & (1u << 27))
>>
>> would mean that upon filling cpu_policy (emul_test_init() ->
>> x86_cpu_policy_fill_native()) the OSXSAVE bit would be clear. Are you
>> suggesting you did the fuzzing on some really old hardware?
... there was this aspect that I couldn't understand?
Jan
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