net: l3mdev: Allow send on enslaved interface

Allow udp and raw sockets to send by oif that is an enslaved interface
versus the l3mdev/VRF device. For example, this allows BFD to use ifindex
from IP_PKTINFO on a receive to send a response without the need to
convert to the VRF index. It also allows ping and ping6 to work when
specifying an enslaved interface (e.g., ping -I swp1 <ip>) which is
a natural use case.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l3mdev/l3mdev.c b/net/l3mdev/l3mdev.c
index 898d01e..6651a78 100644
--- a/net/l3mdev/l3mdev.c
+++ b/net/l3mdev/l3mdev.c
@@ -112,12 +112,18 @@
 	struct dst_entry *dst = NULL;
 	struct net_device *dev;
 
-	dev = dev_get_by_index(net, fl6->flowi6_oif);
-	if (dev) {
-		if (netif_is_l3_master(dev) &&
+	if (fl6->flowi6_oif) {
+		rcu_read_lock();
+
+		dev = dev_get_by_index_rcu(net, fl6->flowi6_oif);
+		if (dev && netif_is_l3_slave(dev))
+			dev = netdev_master_upper_dev_get_rcu(dev);
+
+		if (dev && netif_is_l3_master(dev) &&
 		    dev->l3mdev_ops->l3mdev_get_rt6_dst)
 			dst = dev->l3mdev_ops->l3mdev_get_rt6_dst(dev, fl6);
-		dev_put(dev);
+
+		rcu_read_unlock();
 	}
 
 	return dst;
@@ -141,6 +147,9 @@
 		rcu_read_lock();
 
 		dev = dev_get_by_index_rcu(net, ifindex);
+		if (dev && netif_is_l3_slave(dev))
+			dev = netdev_master_upper_dev_get_rcu(dev);
+
 		if (dev && netif_is_l3_master(dev) &&
 		    dev->l3mdev_ops->l3mdev_get_saddr)
 			rc = dev->l3mdev_ops->l3mdev_get_saddr(dev, fl4);