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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 20:09:51 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29863

--- Comment #9 from Noah Goldstein <goldstein.w.n at gmail dot com> ---
(In reply to K.S. Bhaskar from comment #8)
> (In reply to Andrew Pinski from comment #6)
> > (In reply to Narayanan Iyer from comment #5)=20
> > > Seems to me you are saying that `memcmp()` can only be called on memo=
ry that
> > > is guaranteed to be never changing. And that it should never be calle=
d on a
> > > shared memory buffer whose contents could be concurrently changing as=
 it
> > > goes into undefined behavior territory. That does not sound right to =
me as
> > > we have been using `memcmp()` with shared memory (where multiple proc=
esses
> > > write to that memory buffer all the time) for the past 25+ years on a
> > > variety of architectures and operating systems and have never once se=
en a
> > > SIG-11 in memcmp().
> >=20
> > That does not mean it is correctly well defined code.
> > memcmp cannot be used on memory which is going to be changed under its =
back.
> > since it is not atomic.
>=20
> Since memcmp() is not atomic, it is of course appropriate for the the
> results of the comparison to have a race condition. However, since the
> addresses by which a process has that memory mapped don't change, a SIG-11
> should never occur. Even if all other processes which have that memory
> mapped terminate while the memcmp() is running in a process, that process
> will not unmap the memory and it will remain valid (assuming it was valid=
 to
> start with).

memcmp() is incorrect if the values change from under it. How that
incorrectness will manifest is completely undefined.

--=20
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