Re: Bug#1037264: cksum crashes intermittently with "Illegal instruction" on some Xen DomU

[Pádraig Brady]

On 09/06/2023 18:40, Axel Beckert wrote:
> Package: coreutils
> Version: 9.1-1
> Severity: important
> X-Debbugs-Cc: abe@debian.org
> Control: affects -1 aptitude-robot
>
> On a Xen DomU running Debian 12, cksum intermittently crashes as
> follows:
>
> # while :; do dd if=/dev/urandom count=1 2> /dev/null | cksum ; done
> 1758277878 512
> 2101634611 512
> Illegal instruction

> So to summarise
>
> * Debian 12 in Xen DomU exihibits this behaviour.
>
> * Debian 11 in Xen DomU on same Dom0 does not exihibit this behaviour.
>
> * The Xen Dom0 (Debian 11 though) itself does not exihibit this
>    behaviour.
>
> * A Debian 12 installation on bare metal with the same CPU ("AMD EPYC
>    7313P 16-Core Processor") as the Dom0 does not exhibit this
>    behaviour.

> Hence some more details about the system:
>
> * cksum --debug says: "cksum: using pclmul hardware support"
>
> * amd64-microcode on the Dom0 is at 3.20191218.1

cksum since v9.0 checks at runtime whether pclmul is supported.
It seems that check is not working appropriately on a Xen DomU.
The routine in question is pclmul_supported() at:
https://github.com/coreutils/coreutils/blob/b841f111/src/cksum.c#L160-L191

That either suggests xen is incorrectly setting PCLMUL and AVX bits,
or perhaps these two bits are not sufficient.
Hmm I wonder do we also need to explicitly check for SSSE3 support?

I.e. I wonder does cksum built with the following help?

BTW it would be worth checking if ssse3 is mentioned in /proc/cpuinfo also.
If it was NOT, then there would be more of a chance of that change helping.

diff --git a/src/cksum.c b/src/cksum.c
index 85afab0ac..98733dadf 100644
--- a/src/cksum.c
+++ b/src/cksum.c
@@ -172,7 +172,7 @@ pclmul_supported (void)
       return false;
     }

-  if (! (ecx & bit_PCLMUL) || ! (ecx & bit_AVX))
+  if (! (ecx & bit_PCLMUL) || ! (ecx & bit_AVX) || ! (ecx & bit_SSSE3))
     {
       if (cksum_debug)
         error (0, 0, "%s", _("pclmul support not detected"));