drbd: only clone bio if we have a backing device

Commit c347a787 (drbd: set ->bi_bdev in drbd_req_new) moved a
bio_set_dev call (which has since been removed) to "earlier", from
drbd_request_prepare to drbd_req_new.

The problem is that this accesses device->ldev->backing_bdev, which is
not NULL-checked at this point. When we don't have an ldev (i.e. when
the DRBD device is diskless), this leads to a null pointer deref.

So, only allocate the private_bio if we actually have a disk. This is
also a small optimization, since we don't clone the bio to only to
immediately free it again in the diskless case.

Fixes: c347a787 ("drbd: set ->bi_bdev in drbd_req_new")
Co-developed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Co-developed-by: Joel Colledge <joel.colledge@linbit.com>
Signed-off-by: Joel Colledge <joel.colledge@linbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221020085205.129090-1-christoph.boehmwalder@linbit.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_req.c

@@ -30,11 +30,6 @@ static struct drbd_request *drbd_req_new(struct drbd_device *device, struct bio
 		return NULL;
 	memset(req, 0, sizeof(*req));
 
-	req->private_bio = bio_alloc_clone(device->ldev->backing_bdev, bio_src,
-					   GFP_NOIO, &drbd_io_bio_set);
-	req->private_bio->bi_private = req;
-	req->private_bio->bi_end_io = drbd_request_endio;
-
 	req->rq_state = (bio_data_dir(bio_src) == WRITE ? RQ_WRITE : 0)
 		      | (bio_op(bio_src) == REQ_OP_WRITE_ZEROES ? RQ_ZEROES : 0)
 		      | (bio_op(bio_src) == REQ_OP_DISCARD ? RQ_UNMAP : 0);
@@ -1219,9 +1214,12 @@ drbd_request_prepare(struct drbd_device *device, struct bio *bio)
 	/* Update disk stats */
 	req->start_jif = bio_start_io_acct(req->master_bio);
 
-	if (!get_ldev(device)) {
-		bio_put(req->private_bio);
-		req->private_bio = NULL;
+	if (get_ldev(device)) {
+		req->private_bio = bio_alloc_clone(device->ldev->backing_bdev,
+						   bio, GFP_NOIO,
+						   &drbd_io_bio_set);
+		req->private_bio->bi_private = req;
+		req->private_bio->bi_end_io = drbd_request_endio;
 	}
 
 	/* process discards always from our submitter thread */