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

[Julia Lawall <julia.lawall@lip6.fr>]

On Tue, 8 Sep 2015, Joseph Qi wrote:

> 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.

Your code contains:

list_for_each_entry(lock, tmpq, list)

In the body of the loop, there is an assignment of lock to NULL.  So now
pos will be NULL.

julia

>
> 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
> >>>>
> >>>
> >>> .
> >>>
> >>
> >>
> >>
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> >
> > .
> >
>
>
>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>