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

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Date:	Tue, 13 Dec 2011 20:48:55 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Will Deacon <will.deacon@....com>
Cc:	eranian@...gle.com, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: perf NULL pointer dereference on -rc5

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.

Can I add your SoB to this? 

---
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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