From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org>,
Narayanan Iyer <nars@yottadb.com>
Subject: Re: Bug 29863 - Segmentation fault in memcmp-sse2.S if memory contents can concurrently change
Date: Tue, 13 Dec 2022 14:59:32 -0800 [thread overview]
Message-ID: <CAFUsyfJNFR4cAr8QEpckRvrUtxBHHHwf3bN_zXZbD7-x7qZnXg@mail.gmail.com> (raw)
In-Reply-To: <871qp380hp.fsf@oldenburg.str.redhat.com>
On Tue, Dec 13, 2022 at 1:20 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Noah Goldstein via Libc-alpha:
>
> > Is this something we have to support? I believe other functions /
> > implementations of memcmp will suffer from a similar bug.
>
> Of course the crash is by no means deterministic, so I'm not sure how
> useful it is to detect application bugs. Maybe papering over the
> application bug is the right approach here.
>
> On the other hand, I really don't see how such a racing memcmp call
> could deliver any useful information whatsoever. The result will always
> be arbitrary in practice. So I hope such application bugs are really
> rare.
The usecase in the bugzilla is optimistic reads in concurrent databases.
>
> > 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
> > ```
>
> How costly is this change? I would have thought about ANDing the offset
> so that it is always in range (but maybe it will stil result in a page
> crossing, I don't really know how this works).
Its a bit expensive b.c it misaligned a lot of hot paths.
A better fix (untested) is probably:
```
@@ -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
```
The issue is the 32-bit negative number becomes a very large
unsigned 64-bit number. If we just use 64-bit addition the
issue goes away (I think at least).
I'm okay with this change going in (assuming it works).
This fix can be a "happy accident" that this unsupported
usage no longer causes a sig-11, but I think it's a mistake to
explicitly support this use case as anything other than UB.
>
> Thanks,
> Florian
>
next prev parent reply other threads:[~2022-12-13 22:59 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-13 18:20 Narayanan Iyer
2022-12-13 18:31 ` Andrew Pinski
2022-12-13 18:39 ` Narayanan Iyer
2022-12-13 18:39 ` Cristian Rodríguez
2022-12-13 19:08 ` Noah Goldstein
2022-12-13 19:13 ` Narayanan Iyer
2022-12-13 19:25 ` Noah Goldstein
2022-12-13 20:56 ` Zack Weinberg
2022-12-13 23:29 ` Carlos O'Donell
2022-12-14 2:28 ` Zack Weinberg
2022-12-14 4:16 ` Carlos O'Donell
2022-12-14 14:16 ` Zack Weinberg
2022-12-14 17:36 ` Paolo Bonzini
2022-12-29 7:09 ` Zack Weinberg
2022-12-29 19:32 ` “Undefined behavior” considered harmful (was Re: Bug 29863 - Segmentation fault in memcmp-sse2.S…) Zack Weinberg
2022-12-29 22:20 ` Andreas Schwab
2022-12-30 13:28 ` Florian Weimer
2022-12-30 15:09 ` Florian Weimer
2022-12-13 22:52 ` Bug 29863 - Segmentation fault vs invalid results, memory models, and control/data dependencies Carlos O'Donell
2022-12-14 12:03 ` Florian Weimer
2022-12-13 21:20 ` Bug 29863 - Segmentation fault in memcmp-sse2.S if memory contents can concurrently change Florian Weimer
2022-12-13 22:59 ` Noah Goldstein [this message]
2022-12-14 12:06 ` Florian Weimer
[not found] <PAWPR08MB89825887E12FF900540365F483E09@PAWPR08MB8982.eurprd08.prod.outlook.com>
[not found] ` <PAWPR08MB898260DA844D695EA70ED3E483E09@PAWPR08MB8982.eurprd08.prod.outlook.com>
2022-12-14 21:56 ` Wilco Dijkstra
2022-12-29 7:21 ` Zack Weinberg
2022-12-29 20:02 ` Alejandro Colomar
2022-12-30 18:02 ` Joseph Myers
2023-03-20 15:40 ` Zack Weinberg
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAFUsyfJNFR4cAr8QEpckRvrUtxBHHHwf3bN_zXZbD7-x7qZnXg@mail.gmail.com \
--to=goldstein.w.n@gmail.com \
--cc=fweimer@redhat.com \
--cc=libc-alpha@sourceware.org \
--cc=nars@yottadb.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).