[block] question about potential null pointer dereference

["Gustavo A. R. Silva" <garsilva@xxxxxxxxxxxxxx>]

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