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

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Date:	Thu, 07 Mar 2013 09:46:11 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sasha Levin <sasha.levin@...cle.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 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...


> Is this bug reproducible easily on your setup? I've managed to reproduce it
> exactly 3 times in the past month or so, twice when I reported it and only
> once since then - at some point I thought that it was a freak compiler issue
> that went away when the code changed but since you're reporting it again
> I guess that it isn't the case.
> 
> 
> Paul, any chance you can give hlist_for_each_entry_rcu() a second look please?
> I know you've already acked it before, but is it possible I missed a subtle
> detail with RCU that causes this?
> 


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