[PATCH v2] udp_vu: Discard datagrams when RX virtqueue is not usable

[Laurent Vivier <lvivier@redhat.com>]

During vhost-user device initialization, UDP datagrams may arrive on
listening sockets before the guest has enabled the RX virtqueue.

When this happens, udp_vu_sock_recv() returns 0 without consuming
the datagram from the socket. The caller, udp_sock_fwd(), uses a
while loop with udp_peek_addr() to process pending datagrams. Since
the datagram remains in the socket buffer, udp_peek_addr() keeps
returning data available, causing a busy loop with 100% CPU usage.

To avoid that, we need to discard the data when the virtqueue is not
ready. udp_buf_sock_to_tap() actually does the same as it reads data
with udp_sock_recv() and if fd_tap is not initialized tap_send_frames()
drops them.

Fixes: 28997fcb29b5 ("vhost-user: add vhost-user")
Link: https://bugs.passt.top/show_bug.cgi?id=185
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---

Notes:
    v2:
      - move recvmsg() from udp_vu_sock_to_tap() to udp_vu_sock_recv()

 udp_vu.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/udp_vu.c b/udp_vu.c
index c30dcf97698f..3774d538a2d0 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -65,7 +65,8 @@ static size_t udp_vu_hdrlen(bool v6)
  * @v6:		Set for IPv6 connections
  * @dlen:	Size of received data (output)
  *
- * Return: number of iov entries used to store the datagram
+ * Return: number of iov entries used to store the datagram, 0 if the datagram
+ *         was discarded because the virtqueue is not ready, -1 on error
  */
 static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 			    bool v6, ssize_t *dlen)
@@ -77,6 +78,15 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 
 	ASSERT(!c->no_udp);
 
+	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
+		debug("Got UDP packet, but RX virtqueue not usable yet");
+
+		if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
+			debug_perror("Failed to discard datagram");
+
+		return 0;
+	}
+
 	/* compute L2 header length */
 	hdrlen = udp_vu_hdrlen(v6);
 
@@ -87,7 +97,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 			     sizeof(struct virtio_net_hdr_mrg_rxbuf),
 			     NULL);
 	if (iov_cnt == 0)
-		return 0;
+		return -1;
 
 	/* reserve space for the headers */
 	ASSERT(iov_vu[0].iov_len >= MAX(hdrlen, ETH_ZLEN));
@@ -101,7 +111,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 	*dlen = recvmsg(s, &msg, 0);
 	if (*dlen < 0) {
 		vu_queue_rewind(vq, iov_cnt);
-		return 0;
+		return -1;
 	}
 
 	/* restore the pointer to the headers address */
@@ -216,15 +226,17 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
 		int iov_used;
 
 		iov_used = udp_vu_sock_recv(c, vq, s, v6, &dlen);
-		if (iov_used <= 0)
+		if (iov_used < 0)
 			break;
 
-		udp_vu_prepare(c, toside, dlen);
-		if (*c->pcap) {
-			udp_vu_csum(toside, iov_used);
-			pcap_iov(iov_vu, iov_used,
-				 sizeof(struct virtio_net_hdr_mrg_rxbuf));
+		if (iov_used > 0) {
+			udp_vu_prepare(c, toside, dlen);
+			if (*c->pcap) {
+				udp_vu_csum(toside, iov_used);
+				pcap_iov(iov_vu, iov_used,
+					sizeof(struct virtio_net_hdr_mrg_rxbuf));
+			}
+			vu_flush(vdev, vq, elem, iov_used);
 		}
-		vu_flush(vdev, vq, elem, iov_used);
 	}
 }
-- 
2.52.0