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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
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

  reply index

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 ` 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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