Bug 29863 - Segmentation fault in memcmp-sse2.S if memory contents can concurrently change

[Narayanan Iyer]

Noah,

Thank you for acknowledging that it is a bug.

In case it helps, I found in our testing cluster, that a system which uses sysdeps/x86_64/multiarch/memcmp-sse2.S has the bug whereas a system that uses sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S does not have the bug.

So it is possible, the issue is only in the sse2 version.

Thanks,
Narayanan.

-----Original Message-----
From: Noah Goldstein [mailto:goldstein.w.n@gmail.com] 
Sent: Tuesday, December 13, 2022 2:08 PM
To: Narayanan Iyer <nars@yottadb.com>
Cc: libc-alpha@sourceware.org
Subject: Re: Bug 29863 - Segmentation fault in memcmp-sse2.S if memory contents can concurrently change

On Tue, Dec 13, 2022 at 10:20 AM Narayanan Iyer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Hi,
>
> I had created a glibc bug report at https://sourceware.org/bugzilla/show_bug.cgi?id=29863
>
> And have included the title of that report in the email subject as well.
>
> It has been a week since I created the issue and I did not hear anything back. I believe this is a regression in glibc 2.36 and have provided enough information in the bug report (along with the commit that caused the regression).
>
> Given the upcoming glibc 2.37 release, I was asked (by Carlos O'Donell <carlos@redhat.com>) to raise it on this mailing list to get the attention of the glibc developers.
>
> I am hoping one of you could take a quick look at the bug report and see if it is worth fixing it in time for glibc 2.37.
>
> Thanks,
> Narayanan.
>
>

The fix:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_64/multiarch/memcmp-sse2.S;h=afd450d0206d6633da9fbc4607a7fa6aeb4e137c;hb=HEAD#l46
```
-#   define SIZE_OFFSET (CHAR_PER_VEC * 2)
+#   define SIZE_OFFSET 0
```

Is this something we have to support? I believe other functions /
implementations
of memcmp will suffer from a similar bug.