ip6gre: avoid tx_error when sending MLD/DAD on external tunnels

similarly to what has been done with commit 9d14904 ("geneve: change
from tx_error to tx_dropped on missing metadata"), avoid reporting errors
to userspace in case the kernel doesn't find any tunnel information for a
skb that is going to be transmitted: an increase of tx_dropped is enough.

tested with the following script:

 # for t in ip6gre ip6gretap ip6erspan; do
 > ip link add dev gre6-test0 type $t external
 > ip address add dev gre6-test0 2001:db8::1/64
 > ip link set dev gre6-test0 up
 > sleep 30
 > ip -s -j link show dev gre6-test0 | jq \
 > '.[0].stats64.tx | {"errors": .errors, "dropped": .dropped}'
 > ip link del dev gre6-test0
 > done

Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -707,6 +707,17 @@ static int prepare_ip6gre_xmit_ipv6(struct sk_buff *skb,
 	return 0;
 }
 
+static struct ip_tunnel_info *skb_tunnel_info_txcheck(struct sk_buff *skb)
+{
+	struct ip_tunnel_info *tun_info;
+
+	tun_info = skb_tunnel_info(skb);
+	if (unlikely(!tun_info || !(tun_info->mode & IP_TUNNEL_INFO_TX)))
+		return ERR_PTR(-EINVAL);
+
+	return tun_info;
+}
+
 static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 			       struct net_device *dev, __u8 dsfield,
 			       struct flowi6 *fl6, int encap_limit,
@@ -734,10 +745,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 		const struct ip_tunnel_key *key;
 		__be16 flags;
 
-		tun_info = skb_tunnel_info(skb);
-		if (unlikely(!tun_info ||
-			     !(tun_info->mode & IP_TUNNEL_INFO_TX) ||
-			     ip_tunnel_info_af(tun_info) != AF_INET6))
+		tun_info = skb_tunnel_info_txcheck(skb);
+		if (IS_ERR(tun_info) ||
+		    unlikely(ip_tunnel_info_af(tun_info) != AF_INET6))
 			return -EINVAL;
 
 		key = &tun_info->key;
@@ -908,7 +918,8 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
 	return NETDEV_TX_OK;
 
 tx_err:
-	stats->tx_errors++;
+	if (!t->parms.collect_md || !IS_ERR(skb_tunnel_info_txcheck(skb)))
+		stats->tx_errors++;
 	stats->tx_dropped++;
 	kfree_skb(skb);
 	return NETDEV_TX_OK;
@@ -917,6 +928,7 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
 static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 					 struct net_device *dev)
 {
+	struct ip_tunnel_info *tun_info = NULL;
 	struct ip6_tnl *t = netdev_priv(dev);
 	struct dst_entry *dst = skb_dst(skb);
 	struct net_device_stats *stats;
@@ -964,15 +976,13 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	 * for native mode, call prepare_ip6gre_xmit_{ipv4,ipv6}.
 	 */
 	if (t->parms.collect_md) {
-		struct ip_tunnel_info *tun_info;
 		const struct ip_tunnel_key *key;
 		struct erspan_metadata *md;
 		__be32 tun_id;
 
-		tun_info = skb_tunnel_info(skb);
-		if (unlikely(!tun_info ||
-			     !(tun_info->mode & IP_TUNNEL_INFO_TX) ||
-			     ip_tunnel_info_af(tun_info) != AF_INET6))
+		tun_info = skb_tunnel_info_txcheck(skb);
+		if (IS_ERR(tun_info) ||
+		    unlikely(ip_tunnel_info_af(tun_info) != AF_INET6))
 			goto tx_err;
 
 		key = &tun_info->key;
@@ -1065,7 +1075,8 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 
 tx_err:
 	stats = &t->dev->stats;
-	stats->tx_errors++;
+	if (!IS_ERR(tun_info))
+		stats->tx_errors++;
 	stats->tx_dropped++;
 	kfree_skb(skb);
 	return NETDEV_TX_OK;