Re: 3.9-rc1 NULL pointer crash at find_pid_ns

Sasha Levin
2013-03-07T18:14:33Z

On 03/07/2013 01:05 PM, ebiederm@xmission.com wrote:
> Sasha Levin <sasha.levin@oracle.com> 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.

Okay, I'm even more confused now.

The expression in question is:

hlist_entry_safe(rcu_dereference_bh(hlist_first_rcu(head)))

You're saying that "rcu_dereference_bh(hlist_first_rcu(head))" can change between
the two evaluations we do. That would mean that 'head' has changed in between, right?

In that case, the list itself has changed - which means that RCU has changed the
list underneath us.

hlist_first_rcu() doesn't have any side-effects, it doesn't modify the list whatsoever,
so the only thing that can change is 'head'. Why is it allowed to change if the list
is protected by RCU?

Thanks,
Sasha