net: netdevice: Add operation ndo_sk_get_lower_dev

ndo_sk_get_lower_dev returns the lower netdev that corresponds to
a given socket.
Additionally, we implement a helper netdev_sk_get_lowest_dev() to get
the lowest one in chain.
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Boris Pismenny <borisp@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -1398,6 +1398,8 @@ struct net_device_ops {
 	struct net_device*	(*ndo_get_xmit_slave)(struct net_device *dev,
 						      struct sk_buff *skb,
 						      bool all_slaves);
+	struct net_device*	(*ndo_sk_get_lower_dev)(struct net_device *dev,
+							struct sock *sk);
 	netdev_features_t	(*ndo_fix_features)(struct net_device *dev,
 						    netdev_features_t features);
 	int			(*ndo_set_features)(struct net_device *dev,
@@ -2858,6 +2860,8 @@ int init_dummy_netdev(struct net_device *dev);
 struct net_device *netdev_get_xmit_slave(struct net_device *dev,
 					 struct sk_buff *skb,
 					 bool all_slaves);
+struct net_device *netdev_sk_get_lowest_dev(struct net_device *dev,
+					    struct sock *sk);
 struct net_device *dev_get_by_index(struct net *net, int ifindex);
 struct net_device *__dev_get_by_index(struct net *net, int ifindex);
 struct net_device *dev_get_by_index_rcu(struct net *net, int ifindex);

net/core/dev.c

@@ -8105,6 +8105,39 @@ struct net_device *netdev_get_xmit_slave(struct net_device *dev,
 }
 EXPORT_SYMBOL(netdev_get_xmit_slave);
 
+static struct net_device *netdev_sk_get_lower_dev(struct net_device *dev,
+						  struct sock *sk)
+{
+	const struct net_device_ops *ops = dev->netdev_ops;
+
+	if (!ops->ndo_sk_get_lower_dev)
+		return NULL;
+	return ops->ndo_sk_get_lower_dev(dev, sk);
+}
+
+/**
+ * netdev_sk_get_lowest_dev - Get the lowest device in chain given device and socket
+ * @dev: device
+ * @sk: the socket
+ *
+ * %NULL is returned if no lower device is found.
+ */
+
+struct net_device *netdev_sk_get_lowest_dev(struct net_device *dev,
+					    struct sock *sk)
+{
+	struct net_device *lower;
+
+	lower = netdev_sk_get_lower_dev(dev, sk);
+	while (lower) {
+		dev = lower;
+		lower = netdev_sk_get_lower_dev(dev, sk);
+	}
+
+	return dev;
+}
+EXPORT_SYMBOL(netdev_sk_get_lowest_dev);
+
 static void netdev_adjacent_add_links(struct net_device *dev)
 {
 	struct netdev_adjacent *iter;