blk-mq: fix undefined behaviour in order_to_size()

When this_order variable in blk_mq_init_rq_map() becomes zero
the code incorrectly decrements the variable and passes the result
to order_to_size() helper causing undefined behaviour:

 UBSAN: Undefined behaviour in block/blk-mq.c:1459:27
 shift exponent 4294967295 is too large for 32-bit type 'unsigned int'
 CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.6.0-rc6-00072-g33656a1 #22

Fix the code by checking this_order variable for not having the zero
value first.

Reported-by: Meelis Roos <mroos@linux.ee>
Fixes: 320ae51feed5 ("blk-mq: new multi-queue block IO queueing mechanism")
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0c2ed831c926e89983a1a04c7462b1a1033e07bb..7df9c9263b2125a86bceb05c1d357ded38051c70 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1495,7 +1495,7 @@ static struct blk_mq_tags *blk_mq_init_rq_map(struct blk_mq_tag_set *set,
                int to_do;
                void *p;
 
-               while (left < order_to_size(this_order - 1) && this_order)
+               while (this_order && left < order_to_size(this_order - 1))
                        this_order--;
 
                do {