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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 20 Mar 2019 14:57:01 +0100
Message-ID: <CACT4Y+axojyHxk5K34YuLUyj+NJ05+FC3n8ozseHC91B1qn5ZQ@mail.gmail.com>
Subject: Re: kernel panic: corrupted stack end in wb_workfn
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        syzbot <syzbot+ec1b7575afef85a0e5ca@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Qian Cai <cai@lca.pw>, David Miller <davem@davemloft.net>,
        guro@fb.com, Johannes Weiner <hannes@cmpxchg.org>,
        Josef Bacik <jbacik@fb.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, linux-sctp@vger.kernel.org,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>,
        netdev <netdev@vger.kernel.org>,
        Neil Horman <nhorman@tuxdriver.com>,
        Shakeel Butt <shakeelb@google.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Vladislav Yasevich <vyasevich@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Xin Long <lucien.xin@gmail.com>
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On Wed, Mar 20, 2019 at 2:33 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>
>
>
> On 3/20/19 1:38 PM, Dmitry Vyukov wrote:
> > On Wed, Mar 20, 2019 at 11:24 AM Tetsuo Handa
> > <penguin-kernel@i-love.sakura.ne.jp> wrote:
> >>
> >> On 2019/03/20 18:59, Dmitry Vyukov wrote:
> >>>> From bisection log:
> >>>>
> >>>>         testing release v4.17
> >>>>         testing commit 29dcea88779c856c7dc92040a0c01233263101d4 with gcc (GCC) 8.1.0
> >>>>         run #0: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         run #1: crashed: kernel panic: corrupted stack end in worker_thread
> >>>>         run #2: crashed: kernel panic: Out of memory and no killable processes...
> >>>>         run #3: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         run #4: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         run #5: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         run #6: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         run #7: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         run #8: crashed: kernel panic: Out of memory and no killable processes...
> >>>>         run #9: crashed: kernel panic: corrupted stack end in wb_workfn
> >>>>         testing release v4.16
> >>>>         testing commit 0adb32858b0bddf4ada5f364a84ed60b196dbcda with gcc (GCC) 8.1.0
> >>>>         run #0: OK
> >>>>         run #1: OK
> >>>>         run #2: OK
> >>>>         run #3: OK
> >>>>         run #4: OK
> >>>>         run #5: crashed: kernel panic: Out of memory and no killable processes...
> >>>>         run #6: OK
> >>>>         run #7: crashed: kernel panic: Out of memory and no killable processes...
> >>>>         run #8: OK
> >>>>         run #9: OK
> >>>>         testing release v4.15
> >>>>         testing commit d8a5b80568a9cb66810e75b182018e9edb68e8ff with gcc (GCC) 8.1.0
> >>>>         all runs: OK
> >>>>         # git bisect start v4.16 v4.15
> >>>>
> >>>> Why bisect started between 4.16 4.15 instead of 4.17 4.16?
> >>>
> >>> Because 4.16 was still crashing and 4.15 was not crashing. 4.15..4.16
> >>> looks like the right range, no?
> >>
> >> No, syzbot should bisect between 4.16 and 4.17 regarding this bug, for
> >> "Stack corruption" can't manifest as "Out of memory and no killable processes".
> >>
> >> "kernel panic: Out of memory and no killable processes..." is completely
> >> unrelated to "kernel panic: corrupted stack end in wb_workfn".
> >
> >
> > Do you think this predicate is possible to code?
>
> Something like bellow probably would work better than current behavior.
>
> For starters, is_duplicates() might just compare 'crash' title with 'target_crash' title and its duplicates titles.

Lots of bugs (half?) manifest differently. On top of this, titles
change as we go back in history. On top of this, if we see a different
bug, it does not mean that the original bug is also not there.
This will sure solve some subset of cases better then the current
logic. But I feel that that subset is smaller then what the current
logic solves.

> syzbot has some knowledge about duplicates with different crash titles when people use "syz dup" command.

This is very limited set of info. And in the end I think we've seen
all bug types being duped on all other bugs types pair-wise, and at
the same time we've seen all bug types being not dups to all other bug
types. So I don't see where this gets us.
And again as we go back in history all these titles change.

> Also it might be worth to experiment with using neural networks to identify duplicates.
>
>
> target_crash = 'kernel panic: corrupted stack end in wb_workfn'
> test commit:
>         bad = false;
>         skip = true;
>         foreach run:
>                 run_started, crashed, crash := run_repro();
>
>                 //kernel built, booted, reproducer launched successfully
>                 if (run_started)
>                         skip = false;
>                 if (crashed && is_duplicates(crash, target_crash))
>                         bad = true;
>
>         if (skip)
>                 git bisect skip;
>         else if (bad)
>                 git bisect bad;
>         else
>                 git bisect good;