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From: "goldstein.w.n at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug libc/29863] Segmentation fault in memcmp-sse2.S if memory
 contents can concurrently change
Date: Tue, 13 Dec 2022 23:39:33 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29863

--- Comment #15 from Noah Goldstein <goldstein.w.n at gmail dot com> ---
(In reply to Narayanan Iyer from comment #14)
> (In reply to Noah Goldstein from comment #13)
> >=20
> > Can you test the following dif:
> > ```
> > @@ -308,7 +308,7 @@ L(ret_nonzero_vec_end_0):
> >   setg %dl
> >   leal -1(%rdx, %rdx), %eax
> >  #  else
> > - addl %edx, %eax
> > + addq %rdx, %rax
> >   movzbl (VEC_SIZE * -1 + SIZE_OFFSET)(%rsi, %rax), %ecx
> >   movzbl (VEC_SIZE * -1 + SIZE_OFFSET)(%rdi, %rax), %eax
> >   subl %ecx, %eax
> >=20
> > ```
> >=20
> > If that fixes things I'm okay making the change.
> >=20
> > I'm opposed to explicitly supporting it, but am happy
> > to try and make this failure from the unsupported
> > usage less dramatic.
>=20
> This change sounds much better.
>=20
> Is there a url that describes how to build glibc from source and use it in
> my application?

I just built the current/fixed memcmp into a single library and used
LD_PRELOAD.

This fix seems to keep it from segfaulting although I wouldn't be
surprised if there are some edge cases in other impls / paths
around page boundaries where it can still segfault.=20

I'll submit a patch with these changes although I'm not sure
its going to get backported.

--=20
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