[PATCH net] tun: complete the zerocopy buffer of packets consumed by XDP

[Antonio Quartulli <antonio@mandelbit.com>]

When vhost-net passes a zerocopy buffer that tun_get_user() does not use,
because the packet spans too many pages, the packet is copied and the
buffer completed. If the copy goes through build_skb() and native XDP
consumes the packet (DROP, TX or REDIRECT), tun_get_user() returns
without completing it: vhost-net never gets the descriptor back, and
closing the vhost-net device then blocks forever.

Complete it in that case too, as copied.

Reproducer: vhost-net with experimental_zcopytx=1 on a tap device with a
native XDP_DROP program; send one 806-byte packet as 40 descriptors, each
in its own page. The descriptor is never returned and close() of the
vhost-net fd does not return. With the patch it is returned.

Tested in an x86-64 VM with the reproducer and the tun kselftest.

Found by the Sashiko AI review bot, as a pre-existing issue while
reviewing an unrelated tun patch, and confirmed with the reproducer.

Link: https://sashiko.dev/#/bug/linux-f9997fb6-303f-41d6-93e2-60c392b1856f
Fixes: 761876c857cb ("tap: XDP support")
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
---
Notes:
- Completed as copied (success false), as the copy path does: the packet
  was copied into a page fragment.
- net-next replaced ubuf_info->ops with net_zcopy_complete() in
  ecca9bb2c855; there the call becomes
  net_zcopy_complete(NULL, msg_control, false).

 drivers/net/tun.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 86fd44e10755..927196840725 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1871,8 +1871,15 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		err = PTR_ERR_OR_ZERO(skb);
 		if (err)
 			goto drop;
-		if (!skb)
+		if (!skb) {
+			/* consumed by XDP, after copying */
+			if (msg_control) {
+				struct ubuf_info *uarg = msg_control;
+
+				uarg->ops->complete(NULL, uarg, false);
+			}
 			return total_len;
+		}
 	} else {
 		if (!zerocopy) {
 			copylen = len;

base-commit: ead6fa9a0ab194f02a2f36d634b193e9c5f9a1c6
-- 
2.56.0