Re: 3.9-rc1 NULL pointer crash at find_pid_ns

[Paul E. McKenney]

On Thu, Mar 07, 2013 at 10:05:34AM -0800, Eric W. Biederman wrote:
> Sasha Levin <sasha.levin@xxxxxxxxxx> writes:
> 
> > 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?
> 
> Definitely.
> 
> Head in this instance the expression: &pid_hash[pid_hashfn(nr, ns)]
> 
> And the crash clearly shows that when hilst_entry is being evaluated the
> HEAD is NULL.
> 
> Perhaps this shows up in proc because the hash chains are short and
> frequently NULL?

So it should be possible to do something like the following:

#define hlist_entry_safe(ptr, type, member) \
	({ typeof(ptr) ____ptr = ACCESS_ONCE(ptr); \
	   ____ptr ? hlist_entry(____ptr, type, member) : NULL; \
	})

Does that help?

							Thanx, Paul

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