Subject: Re: [Bug][perf hw-breakpoints] Null pointer exception when using
 register_user_hw_breakpoint with inherit flag
From: Peter Zijlstra <peterz@infradead.org>
To: prasad@linux.vnet.ibm.com
Cc: Frederic Weisbecker <fweisbec@gmail.com>, "mingo@elte.hu" <mingo@elte.hu>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <20091217170309.GA5457@in.ibm.com>
References: <20091217170309.GA5457@in.ibm.com>
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Date: Thu, 17 Dec 2009 18:13:09 +0100
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On Thu, 2009-12-17 at 22:33 +0530, K.Prasad wrote:
> Hi,
>   While testing out two new interfaces (to be sent separately) that
> in-turn uses register_user_hw_breakpoint() and unregister_hw_breakpoint(),
> I noticed the following error.
> 
> The bug is seen only when 'inherit' flag of 'perf_event_attr' is set
> (not otherwise) before the call to register_user_hw_breakpoint(),
> and when the user-space process tries to spawn a new thread.
> 
> Please find the screen logs taken from an x86 box, running -tip kernel 
> commit 7818b3d0fc68f5c2a85fed86d9fa37131c5a3068.
> 
> Digging further, the causative line of code appears to be
> 
> 	atomic_long_inc(&parent_event->filp->f_count);
> 
> in inherit_event() function, wherein f_count is NULL (and hence the
> error). I haven't analysed further to understand why/if the said
> pointer is/should be NULL.
> 
> Thought might bring it to the community's notice for wider
> notice/further investigation.

Ooh, cute, it seems the inherit code assumes we have a file structure
around, which isn't true for events that get created through the kernel
interface.

I think its a simple matter of testing to see if event->filp is set, but
we'd better audit the code for it.

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