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From:   Zhouyi Zhou <zhouzhouyi@gmail.com>
Date:   Tue, 28 Feb 2023 15:58:20 +0800
Message-ID: <CAABZP2yqc0r8auf-m_tBTq2TbE1yr3Sn3cZ2zF8NPp3axJH+tw@mail.gmail.com>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in rcu_core
To:     paulmck@kernel.org
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Sanan Hasanov <sanan.hasanov@knights.ucf.edu>,
        frederic@kernel.org, quic_neeraju@quicinc.com,
        josh@joshtriplett.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller@googlegroups.com,
        contact@pgazz.com
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On Tue, Feb 28, 2023 at 12:33=E2=80=AFAM Paul E. McKenney <paulmck@kernel.o=
rg> wrote:
>
> On Mon, Feb 27, 2023 at 11:16:26AM -0500, Steven Rostedt wrote:
> > On Tue, 28 Feb 2023 00:11:51 +0800
> > Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
> >
> > > > Yes certainly, the rcu_head is allocated on the caller side so it
> > > > could have been trampled while the callback was still in flight.
> > > Thank you all for your guidance, I learned a lot during this process
> > > >
> > > > > OR it could be a bug with RCU if the synchronize_rcu() ended befo=
re the
> > > > > grace periods have finished.
> > > Thanks again.
> > >
> > > By the way, the syzkaller on my local machine has been running for 8
> > > hours, only three bugs reported[1][2][3], but they don't seem to be
> > > related to Sanan's original report.
> > > Maybe there are some configuration mismatches between us.The test
> > > continues,  I will report to you once I have any new discovery.
> >
> > Note, the above races (either bug, the one that tramples on something i=
n
> > RCU flight, or a synchronize_sched() returning early) may be extremely =
hard
> > to hit. It could have been the planets were lined up just right to hit =
the
> > bug, and won't happen for another 27,000 years.
>
> Which turns into once per week or two across a million-system fleet.  ;-)
>
> Not that I know of any fleets running syzkaller...
My syzkaller has been running for 24 hours, the bug can't be reproduced.
Yes, the above races are extremely hard to hit.
I learned a lot during the process ;-)

Please inform me if there are any more clues.

Thank you all for your guidance ;-)
Zhouyi
>
>                                                         Thanx, Paul