Re: 3.9-rc1 NULL pointer crash at find_pid_ns

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Mar 07, 2013 at 12:50:47PM -0500, Sasha Levin wrote:
> 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?

Sigh!  Eric is right -- if "ptr" involves side effects, evaluating
it twice is completely broken.

							Thanx, Paul