Re: 3.9-rc1 NULL pointer crash at find_pid_ns

[Li Zefan]

>>>>>> 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?
> 
> RCU does not prevent the list from changing.  Instead, it prevents anything
> that was in the list from being freed during a given RCU read-side critical
> section.  Here is how it is supposed to happen:
> 
> 	head---->A
> 
> Task 1 picks up the pointer from head to A, and sees that it is non-NULL.
> 
> Task 2 removes A from the list, so that the pointer from head is now NULL:
> 
> 	head     A
> 	  |
> 	  |
> 	  V
>         NULL
> 
> Now task 1 refetches from head, and is fatally disappointed to get a
> NULL pointer.
> 
> Now, had task 1 avoided the refetch, it would be still working with
> a pointer to A.  Since A won't be freed until the end of an RCU grace
> period, all would have been well.  Again, one way to handle this is
> as follows:
> 
> #define hlist_entry_safe(ptr, type, member) \
> 	({ typeof(ptr) ____ptr = (ptr); \
> 	   ____ptr ? hlist_entry(____ptr, type, member) : NULL; \
> 	})
> 
> This way, "ptr" is executed exactly once, and the check and the
> hlist_entry() are both using the same value.
> 

I just played with trinity, and triggered this bug in just a few mins,
and I tried Paul's proposed fix and it works.

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