block: avoid to READ fields of null bio

rq->bio can be NULL sometimes, such as flush request, so don't
read bio->bi_seg_front_size until this 'bio' is checked as valid.

Cc: Bart Van Assche <bvanassche@acm.org>
Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: dcebd755926b0f39dd1e ("block: use bio_for_each_bvec() to compute multi-page bvec count")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 1 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index bed065904677..066b66430523 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -363,13 +363,15 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
         struct bio_vec bv, bvprv = { NULL };
         int prev = 0;
         unsigned int seg_size, nr_phys_segs;
-        unsigned front_seg_size = bio->bi_seg_front_size;
+        unsigned front_seg_size;
         struct bio *fbio, *bbio;
         struct bvec_iter iter;
 
         if (!bio)
                 return 0;
 
+        front_seg_size = bio->bi_seg_front_size;
+
         switch (bio_op(bio)) {
         case REQ_OP_DISCARD:
         case REQ_OP_SECURE_ERASE: