Recently, a few issues have been discovered around the creation of
additional subflows. Without these counters, it was difficult to point
out the reason why some subflows were not created as expected.

These counters should have been added earlier, because there is no other
simple ways to extract such information from the kernel, and understand
why subflows have not been created.

While at it, some pr_debug() have been added, just in case the errno
needs to be printed.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/509
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
  - v2:
    - Add "ERR" suffix in variable names. (Geliang)
---
 net/mptcp/mib.c     |  5 +++++
 net/mptcp/mib.h     |  5 +++++
 net/mptcp/subflow.c | 24 ++++++++++++++++++++----
 3 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/mib.c b/net/mptcp/mib.c
index 7884217f33eb..36edef1e6861 100644
--- a/net/mptcp/mib.c
+++ b/net/mptcp/mib.c
@@ -25,6 +25,11 @@ static const struct snmp_mib mptcp_snmp_list[] = {
 	SNMP_MIB_ITEM("MPJoinSynAckHMacFailure", MPTCP_MIB_JOINSYNACKMAC),
 	SNMP_MIB_ITEM("MPJoinAckRx", MPTCP_MIB_JOINACKRX),
 	SNMP_MIB_ITEM("MPJoinAckHMacFailure", MPTCP_MIB_JOINACKMAC),
+	SNMP_MIB_ITEM("MPJoinSynTx", MPTCP_MIB_JOINSYNTX),
+	SNMP_MIB_ITEM("MPJoinSynTxFEstabErr", MPTCP_MIB_JOINSYNTXFESTABERR),
+	SNMP_MIB_ITEM("MPJoinSynTxCreatSkErr", MPTCP_MIB_JOINSYNTXCREATSKERR),
+	SNMP_MIB_ITEM("MPJoinSynTxBindErr", MPTCP_MIB_JOINSYNTXBINDERR),
+	SNMP_MIB_ITEM("MPJoinSynTxConnectErr", MPTCP_MIB_JOINSYNTXCONNECTERR),
 	SNMP_MIB_ITEM("DSSNotMatching", MPTCP_MIB_DSSNOMATCH),
 	SNMP_MIB_ITEM("InfiniteMapTx", MPTCP_MIB_INFINITEMAPTX),
 	SNMP_MIB_ITEM("InfiniteMapRx", MPTCP_MIB_INFINITEMAPRX),
diff --git a/net/mptcp/mib.h b/net/mptcp/mib.h
index 66aa67f49d03..6f2e1ef1dce9 100644
--- a/net/mptcp/mib.h
+++ b/net/mptcp/mib.h
@@ -20,6 +20,11 @@ enum linux_mptcp_mib_field {
 	MPTCP_MIB_JOINSYNACKMAC,	/* HMAC was wrong on SYN/ACK + MP_JOIN */
 	MPTCP_MIB_JOINACKRX,		/* Received an ACK + MP_JOIN */
 	MPTCP_MIB_JOINACKMAC,		/* HMAC was wrong on ACK + MP_JOIN */
+	MPTCP_MIB_JOINSYNTX,		/* Sending a SYN + MP_JOIN */
+	MPTCP_MIB_JOINSYNTXFESTABERR,	/* Not in fully established when sending a SYN + MP_JOIN */
+	MPTCP_MIB_JOINSYNTXCREATSKERR,	/* Not able to create a socket when sending a SYN + MP_JOIN */
+	MPTCP_MIB_JOINSYNTXBINDERR,	/* Not able to bind() the address when sending a SYN + MP_JOIN */
+	MPTCP_MIB_JOINSYNTXCONNECTERR,	/* Not able to connect() when sending a SYN + MP_JOIN */
 	MPTCP_MIB_DSSNOMATCH,		/* Received a new mapping that did not match the previous one */
 	MPTCP_MIB_INFINITEMAPTX,	/* Sent an infinite mapping */
 	MPTCP_MIB_INFINITEMAPRX,	/* Received an infinite mapping */
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index a21c712350c3..69568d928b35 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1577,12 +1577,18 @@ int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
 	int ifindex;
 	u8 flags;
 
-	if (!mptcp_is_fully_established(sk))
+	if (!mptcp_is_fully_established(sk)) {
+		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNTXFESTABERR);
 		goto err_out;
+	}
 
 	err = mptcp_subflow_create_socket(sk, loc->family, &sf);
-	if (err)
+	if (err) {
+		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNTXCREATSKERR);
+		pr_debug("msk=%p local=%d remote:%d create sock error: %d\n",
+			 msk, local_id, remote_id, err);
 		goto err_out;
+	}
 
 	ssk = sf->sk;
 	subflow = mptcp_subflow_ctx(ssk);
@@ -1608,8 +1614,12 @@ int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
 #endif
 	ssk->sk_bound_dev_if = ifindex;
 	err = kernel_bind(sf, (struct sockaddr *)&addr, addrlen);
-	if (err)
+	if (err) {
+		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNTXBINDERR);
+		pr_debug("msk=%p local=%d remote:%d bind error: %d\n",
+			 msk, local_id, remote_id, err);
 		goto failed;
+	}
 
 	mptcp_crypto_key_sha(subflow->remote_key, &remote_token, NULL);
 	pr_debug("msk=%p remote_token=%u local_id=%d remote_id=%d", msk,
@@ -1624,8 +1634,14 @@ int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
 	sock_hold(ssk);
 	list_add_tail(&subflow->node, &msk->conn_list);
 	err = kernel_connect(sf, (struct sockaddr *)&addr, addrlen, O_NONBLOCK);
-	if (err && err != -EINPROGRESS)
+	if (err && err != -EINPROGRESS) {
+		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNTXCONNECTERR);
+		pr_debug("msk=%p local=%d remote:%d connect error: %d\n",
+			 msk, local_id, remote_id, err);
 		goto failed_unlink;
+	}
+
+	MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNTX);
 
 	/* discard the subflow socket */
 	mptcp_sock_graft(ssk, sk->sk_socket);

-- 
2.45.2