* Re: ipc/sem.c: change perform_atomic_semop parameters
2014-06-10 14:19 ` ipc/sem.c: change perform_atomic_semop parameters Dave Jones
@ 2014-06-10 17:56 ` Manfred Spraul
0 siblings, 0 replies; 2+ messages in thread
From: Manfred Spraul @ 2014-06-10 17:56 UTC (permalink / raw)
To: Dave Jones, Linux Kernel Mailing List, davidlohr.bueso
Hi Dave,
On 06/10/2014 04:19 PM, Dave Jones wrote:
> On Fri, Jun 06, 2014 at 11:50:06PM +0000, Linux Kernel wrote:
> > Gitweb: http://git.kernel.org/linus/;a=commit;h=d198cd6d6d02d0a335af2deacb60816ebb4719d1
> > Commit: d198cd6d6d02d0a335af2deacb60816ebb4719d1
> > Parent: 2f2ed41dcaec34f2d6f224aa84efcc5a9dd8d5c3
> > Refname: refs/heads/next
> > Author: Manfred Spraul <manfred@colorfullife.com>
> > AuthorDate: Fri Jun 6 14:37:49 2014 -0700
> > Committer: Linus Torvalds <torvalds@linux-foundation.org>
> > CommitDate: Fri Jun 6 16:08:15 2014 -0700
> >
> > ipc/sem.c: change perform_atomic_semop parameters
> >
> > Right now, perform_atomic_semop gets the content of sem_queue as
> > individual fields. Changes that, instead pass a pointer to sem_queue.
> >
> > This is a preparation for the next patch: it uses sem_queue to store the
> > reason why a task must sleep.
> >
> > Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
> > Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
> > Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
>
> After this patch, coverity started complaining about a potential deref
> after null check. (It may have been a problem before, but the code motion
> causes a new 'event')
>
> 33. var_compare_op: Comparing un to null implies that un might be null.
> 1878 if (un && un->semid == -1)
> 1879 goto out_unlock_free;
> 1880
> 1881 queue.sops = sops;
> 1882 queue.nsops = nsops;
Correct, un can be NULL.
But only if sem_flg & SEM_UNDO is false for every operation:
from semtimedop():
>> if (sop->sem_flg & SEM_UNDO)
>> undos = 1;
>> [...]
>> if (undos) {
>> /* On success, find_alloc_undo takes the
>> rcu_read_lock */
>> un = find_alloc_undo(ns, semid);
>> if (IS_ERR(un)) {
>> error = PTR_ERR(un);
>> goto out_free;
>> }
>> } else {
>> un = NULL;
>> rcu_read_lock();
>> }
> 34. alias_transfer: Assigning: queue.undo = un.
> 1883 queue.undo = un;
> 1884 queue.pid = task_tgid_vnr(current);
> 1885 queue.alter = alter;
> 1886
Correct.
>
> CID 1222114 (#1 of 1): Dereference after null check (FORWARD_NULL)
> 35. var_deref_model: Passing &queue to perform_atomic_semop, which dereferences null queue.undo. [show details]
> 1887 error = perform_atomic_semop(sma, &queue);
Passing &queue to perform_atomic_semop (correct),
which dereferences null queue.undo: Incorrect. queue.undo is only
dereferenced if sem_flg&SEM_UNDO is true.
Thus: As far as I can see it's a false alarm.
And - even more simply: queue.un=NULL is the common case.
--
Manfred
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