ovpn: properly deconfigure UDP-tunnel

When deconfiguring a UDP-tunnel from a socket, we cannot
call setup_udp_tunnel_sock() with an empty config, because
this helper is expected to be invoked only during setup.

Get rid of the call to setup_udp_tunnel_sock() and just
revert what it did during socket initialization..

Note that the global udp_encap_needed_key and the GRO state
are left untouched: udp_destroy_socket() will eventually
take care of them.

Cc: Sabrina Dubroca <[e-mail]>
Cc: Oleksandr Natalenko <[e-mail]>
Fixes: ab66abbc769b ("ovpn: implement basic RX path (UDP)")
Reported-by: Paolo Abeni <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]
Signed-off-by: Antonio Quartulli <[e-mail]>

drivers/net/ovpn/udp.c

···
  */
 void ovpn_udp_socket_detach(struct ovpn_socket *ovpn_sock)
 {
-	struct udp_tunnel_sock_cfg cfg = { };
+	struct sock *sk = ovpn_sock->sock->sk;
 
-	setup_udp_tunnel_sock(sock_net(ovpn_sock->sock->sk), ovpn_sock->sock,
-			      &cfg);
+	/* Re-enable multicast loopback */
+	inet_set_bit(MC_LOOP, sk);
+	/* Disable CHECKSUM_UNNECESSARY to CHECKSUM_COMPLETE conversion */
+	inet_dec_convert_csum(sk);
+
+	WRITE_ONCE(udp_sk(sk)->encap_type, 0);
+	WRITE_ONCE(udp_sk(sk)->encap_rcv, NULL);
+	WRITE_ONCE(udp_sk(sk)->encap_destroy, NULL);
+
+	rcu_assign_sk_user_data(sk, NULL);
 }