linux-kernel - Re: kernel BUG at fs/btrfs/volumes.c:LINE!

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Date:   Tue, 10 Dec 2019 16:11:55 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     dsterba@...e.cz, Johannes Thumshirn <jthumshirn@...e.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Johannes Thumshirn <jth@...nel.org>,
        syzbot <syzbot+5b658d997a83984507a6@...kaller.appspotmail.com>,
        Chris Mason <clm@...com>, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: kernel BUG at fs/btrfs/volumes.c:LINE!

On Thu, Dec 5, 2019 at 1:06 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > > > The correct syntax would be (no dash + colon):
> > > >
> > > > #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/jth/linux.git
> > > > close_fs_devices
> > >
> > > Ah ok, thanks.
> > >
> > > Although syzbot already said it can't test because it has no reproducer.
> > > Anyways good to know for future reports.
> >
> > According to
> >
> > https://syzkaller.appspot.com/bug?id=d50670eeb21302915bde3f25871dfb7ea43db1e4
> >
> > there is a way how to test it, many reports and the last one about a
> > week old. Is there a way to instruct syzbot to run the same tests on a
> > given branch?
> >
> > (The reproducer is basically setting up environment with limited amount
> > of memory available for allocation and this hits the BUG_ON.)
>
> syzkaller does this ("rerun the same tests") for every bug always. If
> it succeeds (kernel crashes again), it results in a reproducer, that
> can later be used for cause/fix bisection and patch testing. In this
> case it does not reproduce, so rerunning the same tests will not lead
> to anything useful (only if to false confirmation that a patch fixes
> the crash).
>
> There is a large number of reasons why a kernel crash may not
> reproduce. It may be global accumulated state, non-hermetic tests,
> poor syzkaller btrfs descriptions (most likely true) and others.
>
> Need to take a closer look, on first sight it looks like something
> that should be reproduced...

Yes, there was a bug around image mount reproduction. Should be fixed
now by https://github.com/google/syzkaller/commit/cb704a294c54aed90281c016a6dc0c40ae295601