net/mptcp: don't overwrite sock_ops in mptcp_is_tcpsk()

Eric suggests:

 > The fact that mptcp_is_tcpsk() was able to write over sock->ops was a
 > bit strange to me.
 > mptcp_is_tcpsk() should answer a question, with a read-only argument.

re-factor code to avoid overwriting sock_ops inside that function. Also,
change the helper name to reflect the semantic, and to disambiguate from
its dual sk_is_mptcp().

Link: multipath-tcp/mptcp_net-next#432
Suggested-by: Eric Dumazet <[email protected]>
Signed-off-by: Davide Caratti <[email protected]>

net/mptcp/protocol.c

@@ -55,28 +55,15 @@ u64 mptcp_wnd_end(const struct mptcp_sock *msk)
 	return READ_ONCE(msk->wnd_end);
 }
 
-static bool mptcp_is_tcpsk(struct sock *sk)
+static const struct proto_ops *mptcp_get_fallback_tcp_ops(const struct sock *sk)
 {
-	struct socket *sock = sk->sk_socket;
-
-	if (unlikely(sk->sk_prot == &tcp_prot)) {
-		/* we are being invoked after mptcp_accept() has
-		 * accepted a non-mp-capable flow: sk is a tcp_sk,
-		 * not an mptcp one.
-		 *
-		 * Hand the socket over to tcp so all further socket ops
-		 * bypass mptcp.
-		 */
-		WRITE_ONCE(sock->ops, &inet_stream_ops);
-		return true;
+	if (unlikely(sk->sk_prot == &tcp_prot))
+		return &inet_stream_ops;
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	} else if (unlikely(sk->sk_prot == &tcpv6_prot)) {
-		WRITE_ONCE(sock->ops, &inet6_stream_ops);
-		return true;
+	else if (unlikely(sk->sk_prot == &tcpv6_prot))
+		return &inet6_stream_ops;
 #endif
-	}
-
-	return false;
+	return NULL;
 }
 
 static int __mptcp_socket_create(struct mptcp_sock *msk)
@@ -3832,6 +3819,7 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 			       int flags, bool kern)
 {
 	struct mptcp_sock *msk = mptcp_sk(sock->sk);
+	const struct proto_ops *fallback_ops;
 	struct sock *ssk, *newsk;
 	int err;
 
@@ -3851,7 +3839,8 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 	lock_sock(newsk);
 
 	__inet_accept(sock, newsock, newsk);
-	if (!mptcp_is_tcpsk(newsock->sk)) {
+	fallback_ops = mptcp_get_fallback_tcp_ops(newsock->sk);
+	if (!fallback_ops) {
 		struct mptcp_sock *msk = mptcp_sk(newsk);
 		struct mptcp_subflow_context *subflow;
 
@@ -3877,6 +3866,15 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 			if (unlikely(list_is_singular(&msk->conn_list)))
 				inet_sk_state_store(newsk, TCP_CLOSE);
 		}
+	} else {
+		/* we are being invoked after mptcp_accept() has
+		 * accepted a non-mp-capable flow: sk is a tcp_sk,
+		 * not an mptcp one.
+		 *
+		 * Hand the socket over to tcp so all further socket ops
+		 * bypass mptcp.
+		 */
+		WRITE_ONCE(newsock->sk->sk_socket->ops, fallback_ops);
 	}
 	release_sock(newsk);