linux-kernel - Re: [PATCH 1/4] rtmutex: avoid null derefence in WARN

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Date:	Fri, 09 Jul 2010 20:29:48 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	John Kacur <jkacur@...hat.com>, Mike Galbraith <efault@....de>,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 1/4] rtmutex: avoid null derefence in WARN_ON

On Fri, 2010-07-09 at 15:32 -0700, Darren Hart wrote:
> If the pi_blocked_on variable is NULL, the subsequent WARN_ON's
> will cause an OOPS. Only perform the susequent checks if
> pi_blocked_on is valid.
> 
> Signed-off-by: Darren Hart <dvhltc@...ibm.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: John Kacur <jkacur@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mike Galbraith <efault@....de>
> ---
>  kernel/rtmutex.c |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
> index 23dd443..baac7d9 100644
> --- a/kernel/rtmutex.c
> +++ b/kernel/rtmutex.c
> @@ -579,9 +579,10 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
>  
>  	raw_spin_lock(&pendowner->pi_lock);
>  
> -	WARN_ON(!pendowner->pi_blocked_on);
> -	WARN_ON(pendowner->pi_blocked_on != waiter);
> -	WARN_ON(pendowner->pi_blocked_on->lock != lock);
> +	if (!WARN_ON(!pendowner->pi_blocked_on)) {
> +		WARN_ON(pendowner->pi_blocked_on != waiter);

The above actually has no issue if the pi_blocked_on is NULL.

The below, well yeah.

-- Steve

> +		WARN_ON(pendowner->pi_blocked_on->lock != lock);
> +	}
>  
>  	pendowner->pi_blocked_on = NULL;
>  


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