[block] question about potential null pointer dereference

From: Gustavo A. R. Silva
Date: Tue May 23 2017 - 17:52:42 EST



Hello everybody,

While looking into Coverity ID 1408828 I ran into the following piece of code at block/bfq-wf2q.c:542:

542static struct rb_node *bfq_find_deepest(struct rb_node *node)
543{
544 struct rb_node *deepest;
545
546 if (!node->rb_right && !node->rb_left)
547 deepest = rb_parent(node);
548 else if (!node->rb_right)
549 deepest = node->rb_left;
550 else if (!node->rb_left)
551 deepest = node->rb_right;
552 else {
553 deepest = rb_next(node);
554 if (deepest->rb_right)
555 deepest = deepest->rb_right;
556 else if (rb_parent(deepest) != node)
557 deepest = rb_parent(deepest);
558 }
559
560 return deepest;
561}

The issue here is that there is a potential NULL pointer dereference at line 554, in case function rb_next() returns NULL.

Maybe a patch like the following could be applied in order to avoid any chance of a NULL pointer dereference:

index 8726ede..28d8b90 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -551,6 +551,8 @@ static struct rb_node *bfq_find_deepest(struct rb_node *node)
deepest = node->rb_right;
else {
deepest = rb_next(node);
+ if (!deepest)
+ return NULL;
if (deepest->rb_right)
deepest = deepest->rb_right;
else if (rb_parent(deepest) != node)

What do you think?

I'd really appreciate any comment on this.

Thank you!
--
Gustavo A. R. Silva