On Mon, 2010-06-28 at 21:17 +0800, shenghui wrote:
> Hi,
>
> I'm reading cfs code, and get the following potential bug.
>
> In kernel/sched_fair.c, we can get the following call thread:
>
> 1778static struct task_struct *pick_next_task_fair(struct rq *rq)
> 1779{
> ...
> 1787 do {
> 1788 se = pick_next_entity(cfs_rq);
> 1789 set_next_entity(cfs_rq, se);
> 1790 cfs_rq = group_cfs_rq(se);
> 1791 } while (cfs_rq);
> ...
> 1797}
>
> 925static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
> 926{
> 927 struct sched_entity *se = __pick_next_entity(cfs_rq);
> ...
> 941 return se;
> 942}
>
> 377static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
> 378{
> 379 struct rb_node *left = cfs_rq->rb_leftmost;
> 380
> 381 if (!left)
> 382 return NULL;
> ...
> 385}
>
> To manipulate cfs_rq->rb_leftmost, __dequeue_entity does the following:
>
> 365static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
> 366{
> 367 if (cfs_rq->rb_leftmost == &se->run_node) {
> 368 struct rb_node *next_node;
> 369
> 370 next_node = rb_next(&se->run_node);
> 371 cfs_rq->rb_leftmost = next_node;
> 372 }
> 373
> 374 rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
> 375}
>
> Here, if se->run_node is the root rb_node, next_node will be set NULL
> by rb_next.
> Then __pick_next_entity may get NULL on some call, and set_next_entity
> may deference
> NULL value.
So if ->rb_leftmost is NULL, then the if (!left) check in
__pick_next_entity() would return null.
As to the NULL deref in in pick_next_task_fair()->set_next_entity() that
should never happen because pick_next_task_fair() will bail
on !->nr_running.
Furthermore, you've failed to mention what kernel version you're looking
at.
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