I hope people don't mind I cc'd linux-block about this userspace code.
Please, let me know if I shouldn't do that.

-- >8 --

The number of sectors read/written is used to verify forward progress of
the request inside the kernel.  If we return 0 here, the kernel
understands that as an IO failure (see first check in ubd_complete_rq),
and will reissue the request, causing an infinite loop of unfullfilled
requests.  This can be reproduced with:

  ubdsrv/ubd add -t null -n0 -q1 -d1
  dd if=/dev/vda of=/dev/ubdb0 count=1 bs=4k

The approach minics nullblk, which returns the total IO size.

Signed-off-by: Gabriel Krisman Bertazi <redacted>
---
 tgt_null.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tgt_null.c b/tgt_null.c
index 85636c405f0c..61850a2cd046 100644
--- a/tgt_null.c
+++ b/tgt_null.c

@@ -20,7 +20,9 @@ static int null_init_tgt(struct ubdsrv_tgt_info *tgt, int type, int argc,
 static int null_handle_io_async(struct ubdsrv_queue *q, struct ubd_io *io,
 		int tag)
 {
-	ubdsrv_mark_io_done(io, 0);
+	const struct ubdsrv_io_desc *iod = ubdsrv_get_iod(q, tag);
+
+	ubdsrv_mark_io_done(io, iod->nr_sectors << 9);