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>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index bed0659046779868211fd28575cdd61c54a8f65f..066b66430523d64f9eee7888e24c1f65084b0497 100644 (file)
--- 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: