xsk: always return ENOBUFS from sendmsg if there is no TX queue

This patch makes sure ENOBUFS is always returned from sendmsg if there
is no TX queue configured. This was not the case for zero-copy
mode. With this patch this error reporting is consistent between copy
mode and zero-copy mode.

Fixes: ac98d8aa ("xsk: wire upp Tx zero-copy functions")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xsk.c

@@ -218,9 +218,6 @@ static int xsk_generic_xmit(struct sock *sk, struct msghdr *m,
 	struct sk_buff *skb;
 	int err = 0;
 
-	if (unlikely(!xs->tx))
-		return -ENOBUFS;
-
 	mutex_lock(&xs->mutex);
 
 	while (xskq_peek_desc(xs->tx, &desc)) {
@@ -296,6 +293,8 @@ static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
 		return -ENXIO;
 	if (unlikely(!(xs->dev->flags & IFF_UP)))
 		return -ENETDOWN;
+	if (unlikely(!xs->tx))
+		return -ENOBUFS;
 	if (need_wait)
 		return -EOPNOTSUPP;