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

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Date:   Mon, 27 Feb 2023 11:16:26 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>,
        paulmck@...nel.org, 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, 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.

-- Steve