Re: fs/ocfs2/dlm/dlmrecovery.c:1824:4-23: iterator with update on line 1827

[Joseph Qi <joseph.qi@huawei.com>]

On 2015/9/8 15:22, Julia Lawall wrote:
> On Tue, 8 Sep 2015, Joseph Qi wrote:
> 
>> Hi Julia,
>>
>> On 2015/9/7 22:01, Julia Lawall wrote:
>>> It looks like a serious problem, because the loop update does a
>>> dereference of the first argument of list_for_each via list_entry.
>>>
>> Could you give more details about this? IMO, it doesn't make any
>> difference in functional logic.
> 
> Do you expect that setting lock to NULL will cause a break out of the
> loop?  Because it does not.  The expansion of list_for_each_entry is:
> 
> #define list_for_each_entry(pos, head, member)                          \
> 	for (pos = list_first_entry(head, typeof(*pos), member);        \
>              &pos->member != (head);                                    \
>              pos = list_next_entry(pos, member))
> 
> Since pos is NULL, &pos->member != (head), so we will take the loop
> update. List_next_entry is:
> 
My understanding is, pos is an address rather than NULL, which is
calculated from head's address with an offset.
In case of an empty head, &pos->member will compensate the offset again
and makes it evaluated the same head. Then it breaks out the loop.

Joseph

> #define list_next_entry(pos, member) \
>         list_entry((pos)->member.next, typeof(*(pos)), member)
> 
> list_entry is container_of, which does
> 
> const typeof( ((type *)0)->member ) *__mptr = (ptr);
> 
> This causes (pos)->member.next to be evaluated, which since pos is NULL
> will crash.
> 
> julia
> 
>>
>>> julia
>>>
>>> On Mon, 7 Sep 2015, kbuild test robot wrote:
>>>
>>>> TO: Joseph Qi <joseph.qi@huawei.com>
>>>> CC: kbuild-all@01.org
>>>> CC: Andrew Morton <akpm@linux-foundation.org>
>>>> CC: Linux Memory Management List <linux-mm@kvack.org>
>>>>
>>>> tree:   git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>>>> head:   7d9071a095023cd1db8fa18fa0d648dc1a5210e0
>>>> commit: f83c7b5e9fd633fe91128af116e6472a8c4d29a5 ocfs2/dlm: use list_for_each_entry instead of list_for_each
>>>> date:   3 days ago
>>>> :::::: branch date: 33 hours ago
>>>> :::::: commit date: 3 days ago
>>>>
>>>>>> fs/ocfs2/dlm/dlmrecovery.c:1824:4-23: iterator with update on line 1827
>>>>
>>>> git remote add linus git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>>> git remote update linus
>>>> git checkout f83c7b5e9fd633fe91128af116e6472a8c4d29a5
>>>> vim +1824 fs/ocfs2/dlm/dlmrecovery.c
>>>>
>>>> 6714d8e8 Kurt Hackel 2005-12-15  1818  			BUG_ON(!(mres->flags & DLM_MRES_MIGRATION));
>>>> 6714d8e8 Kurt Hackel 2005-12-15  1819
>>>> 34aa8dac Junxiao Bi  2014-04-03  1820  			lock = NULL;
>>>> 6714d8e8 Kurt Hackel 2005-12-15  1821  			spin_lock(&res->spinlock);
>>>> e17e75ec Kurt Hackel 2007-01-05  1822  			for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
>>>> e17e75ec Kurt Hackel 2007-01-05  1823  				tmpq = dlm_list_idx_to_ptr(res, j);
>>>> f83c7b5e Joseph Qi   2015-09-04 @1824  				list_for_each_entry(lock, tmpq, list) {
>>>> 34aa8dac Junxiao Bi  2014-04-03  1825  					if (lock->ml.cookie == ml->cookie)
>>>> 6714d8e8 Kurt Hackel 2005-12-15  1826  						break;
>>>> 34aa8dac Junxiao Bi  2014-04-03 @1827  					lock = NULL;
>>>> 6714d8e8 Kurt Hackel 2005-12-15  1828  				}
>>>> e17e75ec Kurt Hackel 2007-01-05  1829  				if (lock)
>>>> e17e75ec Kurt Hackel 2007-01-05  1830  					break;
>>>>
>>>> ---
>>>> 0-DAY kernel test infrastructure                Open Source Technology Center
>>>> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation
>>>>
>>>
>>> .
>>>
>>
>>
>>
> 
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