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Date:   Tue, 28 Apr 2020 23:36:27 +0200
From:   Jiri Olsa <jolsa@redhat.com>
To:     Masami Hiramatsu <mhiramat@kernel.org>, Ingo Molnar <mingo@elte.hu>
Cc:     Jiri Olsa <jolsa@kernel.org>,
        "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Peter Zijlstra <peterz@infradead.org>,
        lkml <linux-kernel@vger.kernel.org>,
        "bibo,mao" <bibo.mao@intel.com>,
        "Ziqian SUN (Zamir)" <zsun@redhat.com>, stable@vger.kernel.org
Subject: Re: [PATCHv2] kretprobe: Prevent triggering kretprobe from within
 kprobe_flush_task
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On Fri, Apr 17, 2020 at 04:38:10PM +0900, Masami Hiramatsu wrote:

SNIP

> > 
> > The code within the kretprobe handler checks for probe reentrancy,
> > so we won't trigger any _raw_spin_lock_irqsave probe in there.
> > 
> > The problem is in outside kprobe_flush_task, where we call:
> > 
> >   kprobe_flush_task
> >     kretprobe_table_lock
> >       raw_spin_lock_irqsave
> >         _raw_spin_lock_irqsave
> > 
> > where _raw_spin_lock_irqsave triggers the kretprobe and installs
> > kretprobe_trampoline handler on _raw_spin_lock_irqsave return.
> > 
> > The kretprobe_trampoline handler is then executed with already
> > locked kretprobe_table_locks, and first thing it does is to
> > lock kretprobe_table_locks ;-) the whole lockup path like:
> > 
> >   kprobe_flush_task
> >     kretprobe_table_lock
> >       raw_spin_lock_irqsave
> >         _raw_spin_lock_irqsave ---> probe triggered, kretprobe_trampoline installed
> > 
> >         ---> kretprobe_table_locks locked
> > 
> >         kretprobe_trampoline
> >           trampoline_handler
> >             kretprobe_hash_lock(current, &head, &flags);  <--- deadlock
> > 
> > Adding kprobe_busy_begin/end helpers that mark code with fake
> > probe installed to prevent triggering of another kprobe within
> > this code.
> > 
> > Using these helpers in kprobe_flush_task, so the probe recursion
> > protection check is hit and the probe is never set to prevent
> > above lockup.
> > 
> 
> Thanks Jiri!
> 
> Ingo, could you pick this up?

Ingo, any chance you could take this one?

thanks,
jirka