# \[PATCH v2\] vringh: Use wiov-\>used to check for read/write desc order

- Subsystems: VIRTIO HOST (VHOST), THE REST
- Maintainers: "Michael S. Tsirkin", Jason Wang, Eugenio Pérez, Linus Torvalds
- Status: LANDED · 2 reviews, 2 by maintainers (e74cfa91f42c · 2021-08-11 10:44 M Jason Wang M Stefano Garzarella)
- Landed in mainline as e74cfa91f42c on 2021-08-11.
- 2 review trailers.
- List: netdev
- 1 message, 1 author
- Page: https://ratatoskr.run/netdev/2021/06/1425521/t

## \[PATCH v2\] vringh: Use wiov-\>used to check for read/write desc order

- From: Neeraj Upadhyay \<hidden\>
- Date: 2021-06-25 03:25:12 
- Message: https://ratatoskr.run/netdev/2021/06/1425521.md

```text
As __vringh_iov() traverses a descriptor chain, it populates
each descriptor entry into either read or write vring iov
and increments that iov's ->used member. So, as we iterate
over a descriptor chain, at any point, (riov/wriov)->used
value gives the number of descriptor enteries available,
which are to be read or written by the device. As all read
iovs must precede the write iovs, wiov->used should be zero
when we are traversing a read descriptor. Current code checks
for wiov->i, to figure out whether any previous entry in the
current descriptor chain was a write descriptor. However,
iov->i is only incremented, when these vring iovs are consumed,
at a later point, and remain 0 in __vringh_iov(). So, correct
the check for read and write descriptor order, to use
wiov->used.

Acked-by: Jason Wang <redacted>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Neeraj Upadhyay <redacted>
---
Changes in v2:
 Commit message updated to clarify why wiov->i cannot be used.

 drivers/vhost/vringh.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index 4af8fa2..14e2043 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -359,7 +359,7 @@ __vringh_iov(struct vringh *vrh, u16 i,
 			iov = wiov;
 		else {
 			iov = riov;
-			if (unlikely(wiov && wiov->i)) {
+			if (unlikely(wiov && wiov->used)) {
 				vringh_bad("Readable desc %p after writable",
 					   &descs[i]);
 				err = -EINVAL;
-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, 
hosted by The Linux Foundation

```