udp: add encap_destroy callback

Users of udp encapsulation currently have an encap_rcv callback which they can
use to hook into the udp receive path.

In situations where a encapsulation user allocates resources associated with a
udp encap socket, it may be convenient to be able to also hook the proto
.destroy operation.  For example, if an encap user holds a reference to the
udp socket, the destroy hook might be used to relinquish this reference.

This patch adds a socket destroy hook into udp, which is set and enabled
in the same way as the existing encap_rcv hook.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/udp.h

@@ -68,6 +68,7 @@ struct udp_sock {
 	 * For encapsulation sockets.
 	 */
 	int (*encap_rcv)(struct sock *sk, struct sk_buff *skb);
+	void (*encap_destroy)(struct sock *sk);
 };
 
 static inline struct udp_sock *udp_sk(const struct sock *sk)

net/ipv4/udp.c

@@ -1762,9 +1762,16 @@ int udp_rcv(struct sk_buff *skb)
 
 void udp_destroy_sock(struct sock *sk)
 {
+	struct udp_sock *up = udp_sk(sk);
 	bool slow = lock_sock_fast(sk);
 	udp_flush_pending_frames(sk);
 	unlock_sock_fast(sk, slow);
+	if (static_key_false(&udp_encap_needed) && up->encap_type) {
+		void (*encap_destroy)(struct sock *sk);
+		encap_destroy = ACCESS_ONCE(up->encap_destroy);
+		if (encap_destroy)
+			encap_destroy(sk);
+	}
 }
 
 /*

net/ipv6/udp.c

@@ -1285,10 +1285,18 @@ int udpv6_sendmsg(struct kiocb *iocb, struct sock *sk,
 
 void udpv6_destroy_sock(struct sock *sk)
 {
+	struct udp_sock *up = udp_sk(sk);
 	lock_sock(sk);
 	udp_v6_flush_pending_frames(sk);
 	release_sock(sk);
 
+	if (static_key_false(&udpv6_encap_needed) && up->encap_type) {
+		void (*encap_destroy)(struct sock *sk);
+		encap_destroy = ACCESS_ONCE(up->encap_destroy);
+		if (encap_destroy)
+			encap_destroy(sk);
+	}
+
 	inet6_destroy_sock(sk);
 }