linux-kernel - Re: 3.9-rc1 NULL pointer crash at find_pid

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Date:	Thu, 07 Mar 2013 12:50:47 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	ebiederm@...ssion.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Li Zefan <lizefan@...wei.com>, CAI Qian <caiqian@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>
Subject: Re: 3.9-rc1 NULL pointer crash at find_pid_ns

On 03/07/2013 12:46 PM, Eric Dumazet wrote:
> On Thu, 2013-03-07 at 12:36 -0500, Sasha Levin wrote:
> 
>> Looks like the hlist change is probably the issue, though it specifically
>> uses:
>>
>> 	#define hlist_entry_safe(ptr, type, member) \
>>         	(ptr) ? hlist_entry(ptr, type, member) : NULL
>>
>> I'm still looking at the code in question and it's assembly, but I can't
>> figure out what's going wrong. I was also trying to see what's so special
>> about this loop in find_pid_ns as opposed to the rest of the kernel code
>> that uses hlist_for_each_entry_rcu() but couldn't find out why.
>>
>> Is it somehow possible that if we rcu_dereference_raw() the same thing twice
>> inside the same rcu_read_lock() section we'll get different results? That's
>> really the only reason for this crash that comes to mind at the moment, very
>> unlikely - but that's all I have right now.
>>
> 
> Yep
> 
> #define hlist_entry_safe(ptr, type, member) \
> 	(ptr) ? hlist_entry(ptr, type, member) : NULL
> 
> Is not safe, as ptr can be evaluated twice, and thats not good at all...

ptr is being evaluated twice, but in this case this is an
rcu_dereference_raw() value within the same rcu_read_lock() section.

Is it still problematic?


Thanks,
Sasha

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