net: mptcp: improve fallback to TCP

Keep using MPTCP sockets and a use "dummy mapping" in case of fallback
to regular TCP. When fallback is triggered, skip addition of the MPTCP
option on send.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/11
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/22
Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 31 insertions, 16 deletions

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 102db8c88e97..cb8a42ff4646 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -216,7 +216,6 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct mptcp_options_received mp_opt;
 	struct sock *parent = subflow->conn;
-	struct tcp_sock *tp = tcp_sk(sk);
 
 	subflow->icsk_af_ops->sk_rx_dst_set(sk, skb);
 
@@ -230,6 +229,8 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 		return;
 
 	subflow->conn_finished = 1;
+	subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
+	pr_debug("subflow=%p synack seq=%x", subflow, subflow->ssn_offset);
 
 	mptcp_get_options(skb, &mp_opt);
 	if (subflow->request_mptcp && mp_opt.mp_capable) {
@@ -245,21 +246,20 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 		pr_debug("subflow=%p, thmac=%llu, remote_nonce=%u", subflow,
 			 subflow->thmac, subflow->remote_nonce);
 	} else {
-		tp->is_mptcp = 0;
+		if (subflow->request_mptcp)
+			MPTCP_INC_STATS(sock_net(sk),
+					MPTCP_MIB_MPCAPABLEACTIVEFALLBACK);
+		mptcp_do_fallback(sk);
+		pr_fallback(mptcp_sk(subflow->conn));
 	}
 
-	if (!tp->is_mptcp)
+	if (mptcp_check_fallback(sk))
 		return;
 
 	if (subflow->mp_capable) {
 		pr_debug("subflow=%p, remote_key=%llu", mptcp_subflow_ctx(sk),
 			 subflow->remote_key);
 		mptcp_finish_connect(sk);
-
-		if (skb) {
-			pr_debug("synack seq=%u", TCP_SKB_CB(skb)->seq);
-			subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
-		}
 	} else if (subflow->mp_join) {
 		u8 hmac[SHA256_DIGEST_SIZE];
 
@@ -279,9 +279,6 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 
 		memcpy(subflow->hmac, hmac, MPTCPOPT_HMAC_LEN);
 
-		if (skb)
-			subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
-
 		if (!mptcp_finish_join(sk))
 			goto do_reset;
 
@@ -557,7 +554,8 @@ enum mapping_status {
 	MAPPING_OK,
 	MAPPING_INVALID,
 	MAPPING_EMPTY,
-	MAPPING_DATA_FIN
+	MAPPING_DATA_FIN,
+	MAPPING_DUMMY
 };
 
 static u64 expand_seq(u64 old_seq, u16 old_data_len, u64 seq)
@@ -621,6 +619,9 @@ static enum mapping_status get_mapping_status(struct sock *ssk)
 	if (!skb)
 		return MAPPING_EMPTY;
 
+	if (mptcp_check_fallback(ssk))
+		return MAPPING_DUMMY;
+
 	mpext = mptcp_get_ext(skb);
 	if (!mpext || !mpext->use_map) {
 		if (!subflow->map_valid && !skb->len) {
@@ -762,6 +763,16 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			ssk->sk_err = EBADMSG;
 			goto fatal;
 		}
+		if (status == MAPPING_DUMMY) {
+			__mptcp_do_fallback(msk);
+			skb = skb_peek(&ssk->sk_receive_queue);
+			subflow->map_valid = 1;
+			subflow->map_seq = READ_ONCE(msk->ack_seq);
+			subflow->map_data_len = skb->len;
+			subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq -
+						   subflow->ssn_offset;
+			return true;
+		}
 
 		if (status != MAPPING_OK)
 			return false;
@@ -885,14 +896,18 @@ static void subflow_data_ready(struct sock *sk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct sock *parent = subflow->conn;
+	struct mptcp_sock *msk;
 
-	if (!subflow->mp_capable && !subflow->mp_join) {
-		subflow->tcp_data_ready(sk);
-
+	msk = mptcp_sk(parent);
+	if (inet_sk_state_load(sk) == TCP_LISTEN) {
+		set_bit(MPTCP_DATA_READY, &msk->flags);
 		parent->sk_data_ready(parent);
 		return;
 	}
 
+	WARN_ON_ONCE(!__mptcp_check_fallback(msk) && !subflow->mp_capable &&
+		     !subflow->mp_join);
+
 	if (mptcp_subflow_data_available(sk))
 		mptcp_data_ready(parent, sk);
 }
@@ -1117,7 +1132,7 @@ static void subflow_state_change(struct sock *sk)
 	 * a fin packet carrying a DSS can be unnoticed if we don't trigger
 	 * the data available machinery here.
 	 */
-	if (subflow->mp_capable && mptcp_subflow_data_available(sk))
+	if (mptcp_subflow_data_available(sk))
 		mptcp_data_ready(parent, sk);
 
 	if (!(parent->sk_shutdown & RCV_SHUTDOWN) &&