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* [PATCH v2] vringh: Use wiov->used to check for read/write desc order
@ 2021-06-25  3:25 Neeraj Upadhyay
  0 siblings, 0 replies; only message in thread
From: Neeraj Upadhyay @ 2021-06-25  3:25 UTC (permalink / raw)
  To: mst, jasowang
  Cc: virtualization, netdev, linux-kernel, sgarzare, Neeraj Upadhyay

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

Acked-by: Jason Wang <>
Reviewed-by: Stefano Garzarella <>
Signed-off-by: Neeraj Upadhyay <>
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",
 				err = -EINVAL;
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2021-06-25  3:25 [PATCH v2] vringh: Use wiov->used to check for read/write desc order Neeraj Upadhyay

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