linux-kernel - Re: lockdep: Warning & NULL ptr deref

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Message-ID: <CA+1xoqfMad+s4-E-Nn29m3=bVASEkH38ZNvWg2vY99ZizdAT3Q@mail.gmail.com>
Date:	Tue, 28 Feb 2012 16:19:30 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Dave Jones <davej@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: lockdep: Warning & NULL ptr deref

On Tue, Feb 28, 2012 at 3:29 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2012-02-28 at 10:24 -0500, Sasha Levin wrote:
>>
>> I'll send some patches to prevent the NULL deref itself which happens
>> since a chunk of the code in lockdep.c assumed hlock_class() can't
>> return NULL.
>>
> There's tons of code actually assuming that..
>
>> This won't fix the actual problem though, I'm not sure exactly how a
>> held lock wouldn't have a class initialized to it.
>
> static inline struct lock_class *hlock_class(struct held_lock *hlock)
> {
>        if (!hlock->class_idx) {
>                /*
>                 * Someone passed in garbage, we give up.
>                 */
>                DEBUG_LOCKS_WARN_ON(1);
>                return NULL;
>        }
>        return lock_classes + hlock->class_idx - 1;
> }
>
> Typically that translates to severe memory corruption.

I can also reproduce it easily by triggering sysrq once or twice after
trinity has stopped.
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