linux-kernel - Re: perf NULL pointer dereference on -rc5

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Date:	Tue, 13 Dec 2011 13:23:49 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: perf NULL pointer dereference on -rc5

On Tue, Dec 13, 2011 at 1:08 PM, Will Deacon <will.deacon@....com> wrote:
> On Tue, Dec 13, 2011 at 07:48:55PM +0000, Peter Zijlstra wrote:
>> On Tue, 2011-12-13 at 15:26 +0000, Will Deacon wrote:
>> >
>> > Commit 10c6db11 ("perf: Fix loss of notification with multi-event") seems to
>> > dereference a NULL event->rb in the wakeup handler during Vince Weaver's perf
>> > tests (specifically corner_cases/overflow_requires_mmap).
>> >
>> > This diff seems to fix the problem, but I'm not sure if it just hides something else:
>>
>> No that is about right.. not so very good of us to have missed that.
>
> Well, at least we caught it in the end.
>
>> Can I add your SoB to this?
>
> Sure:
>
> Signed-off-by: Will Deacon <will.deacon@....com>
>
Thanks for fixing this. Somehow in my testing I never ran into this problem.

> Will
>
>> ---
>> Subject: perf: Fix ring_buffer_wakeup()
>> From: Will Deacon <will.deacon@....com>
>> Date: Tue Dec 13 20:40:45 CET 2011
>>
>> Commit 10c6db11 ("perf: Fix loss of notification with multi-event")
>> seems to unconditionally dereference event->rb in the wakeup handler,
>> this is wrong, there might not be a buffer attached.
>>
>> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>> Link: http://lkml.kernel.org/r/20111213152651.GP20297@mudshark.cambridge.arm.com
>> ---
>>  kernel/events/core.c |    3 +++
>>  1 file changed, 3 insertions(+)
>>
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -3360,9 +3360,12 @@ static void ring_buffer_wakeup(struct pe
>>
>>       rcu_read_lock();
>>       rb = rcu_dereference(event->rb);
>> +     if (!rb)
>> +             goto unlock;
>>       list_for_each_entry_rcu(event, &rb->event_list, rb_entry) {
>>               wake_up_all(&event->waitq);
>>       }
>> +unlock:
>>       rcu_read_unlock();
>>  }
>>
>>
>>
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