From: Eric Dumazet <edumazet@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
Marco Elver <elver@google.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Andrea Parri <parri.andrea@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Mon, 11 Nov 2019 10:44:41 -0800 [thread overview]
Message-ID: <CANn89iJiuOkKc2AVmccM8z9e_d4zbV61K-3z49ao1UwRDdFiHw@mail.gmail.com> (raw)
In-Reply-To: <CANn89i+hRhweL2N=r1chMpWKU2ue8fiQO=dLxGs9sgLFbgHEWQ@mail.gmail.com>
On Mon, Nov 11, 2019 at 10:31 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Mon, Nov 11, 2019 at 10:05 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Mon, Nov 11, 2019 at 9:52 AM Eric Dumazet <edumazet@google.com> wrote:
> > >
> > > Now I wonder what to do with the ~400 KCSAN reports sitting in
> > > pre-moderation queue.
> >
> > So regular KASAN reports are fairly easy to deal with: they report
> > actual bugs. They may be hard to hit, but generally there's no
> > question about something like a use-after-free or whatever.
> >
> > The problem with KCSAN is that it's not clear how many of the reports
> > have been actual real honest-to-goodness bugs that could cause
> > problems, and how many of them are "this isn't actually a bug, but an
> > annotation will shut up KCSAN".
> >
> > My gut feeling would be that it would be best to ignore the ones that
> > are "an annotation will shut up KCSAN", and look at the ones that are
> > real bugs.
> >
> > Is there a pattern to those real bugs? Is there perhaps a way to make
> > KCSAN notice _that_ pattern in particular, and suppress the ones that
> > are "we can shut these up with annotations that don't really change
> > the code"?
> >
> > I think it would be much better for the kernel - and much better for
> > KCSAN - if the problem reports KCSAN reports are real problems that
> > can actually be triggered as problems, and that it behaves much more
> > like KASAN in that respect.
> >
> > Yes, yes, then once the *real* problems have been handled, maybe we
> > can expand the search to be "stylistic issues" and "in theory, this
> > could cause problems with a compiler that did X" issues.
> >
> > But I think the "just annotate" thing makes people more likely to
> > dismiss KCSAN issues, and I don't think it's healthy.
> >
>
> Problem is that KASAN/KCSAN stops as soon as one issue is hit,
> regardless of it being a false positive or not.
>
> (Same happens with LOCKDEP seeing only one issue, then disabling itself)
>
> If we do not annotate the false positive, the real issues might be
> hidden for years.
>
> There is no pattern really, only a lot of noise (small ' bugs' that
> have no real impact)
An interesting case is the race in ksys_write()
if (ppos) {
pos = *ppos; // data-race
ppos = &pos;
}
ret = vfs_write(f.file, buf, count, ppos);
if (ret >= 0 && ppos)
f.file->f_pos = pos; // data-race
BUG: KCSAN: data-race in ksys_write / ksys_write
write to 0xffff8880a9c29568 of 8 bytes by task 27477 on cpu 1:
ksys_write+0x101/0x1b0 fs/read_write.c:613
__do_sys_write fs/read_write.c:623 [inline]
__se_sys_write fs/read_write.c:620 [inline]
__x64_sys_write+0x4c/0x60 fs/read_write.c:620
do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x44/0xa9
next prev parent reply other threads:[~2019-11-11 18:45 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAHk-=wjB61GNmqpX0BLA5tpL4tsjWV7akaTc2Roth7uGgax+mw@mail.gmail.com>
2019-11-10 16:09 ` KCSAN: data-race in __alloc_file / __alloc_file Alan Stern
2019-11-10 19:10 ` Marco Elver
2019-11-11 15:51 ` Alan Stern
2019-11-11 16:51 ` Linus Torvalds
2019-11-11 17:52 ` Eric Dumazet
2019-11-11 18:04 ` Linus Torvalds
2019-11-11 18:31 ` Eric Dumazet
2019-11-11 18:44 ` Eric Dumazet [this message]
2019-11-11 19:00 ` Linus Torvalds
2019-11-11 19:13 ` Eric Dumazet
2019-11-11 20:43 ` Linus Torvalds
2019-11-11 20:46 ` Linus Torvalds
2019-11-11 21:53 ` Eric Dumazet
2019-11-11 23:51 ` Linus Torvalds
2019-11-12 16:50 ` Kirill Smelkov
2019-11-12 17:23 ` Linus Torvalds
2019-11-12 17:36 ` Linus Torvalds
2019-11-17 18:56 ` Kirill Smelkov
2019-11-17 19:20 ` Linus Torvalds
2019-11-11 18:50 ` Linus Torvalds
2019-11-11 18:59 ` Marco Elver
2019-11-11 18:59 ` Eric Dumazet
2019-11-10 19:12 ` Linus Torvalds
2019-11-10 19:20 ` Linus Torvalds
2019-11-10 20:44 ` Paul E. McKenney
2019-11-10 21:10 ` Linus Torvalds
2019-11-10 21:31 ` Paul E. McKenney
2019-11-11 14:17 ` Marco Elver
2019-11-11 14:31 ` Paul E. McKenney
2019-11-11 15:10 ` Marco Elver
2019-11-13 0:25 ` Paul E. McKenney
2019-11-12 19:14 ` Alan Stern
2019-11-12 19:47 ` Linus Torvalds
2019-11-12 20:29 ` Alan Stern
2019-11-12 20:58 ` Linus Torvalds
2019-11-12 21:13 ` Linus Torvalds
2019-11-12 22:05 ` Marco Elver
2019-11-12 21:48 ` Alan Stern
2019-11-12 22:07 ` Eric Dumazet
2019-11-12 22:44 ` Alexei Starovoitov
2019-11-12 23:17 ` Eric Dumazet
2019-11-12 23:40 ` Linus Torvalds
2019-11-13 15:00 ` Marco Elver
2019-11-13 16:57 ` Linus Torvalds
2019-11-13 21:33 ` Marco Elver
2019-11-13 21:50 ` Alan Stern
2019-11-13 22:48 ` Marco Elver
2019-11-08 13:16 syzbot
2019-11-08 13:28 ` Eric Dumazet
2019-11-08 17:01 ` Linus Torvalds
2019-11-08 17:22 ` Eric Dumazet
2019-11-08 17:38 ` Linus Torvalds
2019-11-08 17:53 ` Eric Dumazet
2019-11-08 17:55 ` Eric Dumazet
2019-11-08 18:02 ` Eric Dumazet
2019-11-08 18:12 ` Linus Torvalds
2019-11-08 20:30 ` Linus Torvalds
2019-11-08 20:53 ` Eric Dumazet
2019-11-08 21:36 ` Linus Torvalds
2019-11-08 18:05 ` Linus Torvalds
2019-11-08 18:15 ` Marco Elver
2019-11-08 18:40 ` Linus Torvalds
2019-11-08 19:48 ` Marco Elver
2019-11-08 20:26 ` Linus Torvalds
2019-11-08 21:57 ` Alan Stern
2019-11-08 22:06 ` Linus Torvalds
2019-11-09 23:08 ` Alan Stern
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