linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference in rcu

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Date:   Tue, 28 Feb 2023 15:58:20 +0800
From:   Zhouyi Zhou <zhouzhouyi@...il.com>
To:     paulmck@...nel.org
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>,
        frederic@...nel.org, quic_neeraju@...cinc.com,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller@...glegroups.com,
        contact@...zz.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in rcu_core

On Tue, Feb 28, 2023 at 12:33 AM Paul E. McKenney <paulmck@...nel.org> 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@...il.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 before 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 in
> > 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