[PATCH v3 1/2] mptcp: add per-event MIB counters for MPTCP_RST_EMPTCP resets

[Shardul Bankar <shardulsb08@gmail.com>]

MPTCP_RST_EMPTCP (reset reason 1) is used as a catch-all for several
distinct error conditions across subflow setup, authentication, and
data-path validation.  The existing MPRstTx/MPRstRx counters only
track aggregate reset volume, making it difficult to diagnose which
code path is triggering subflow resets in production.

Add per-event MIB counters covering each MPTCP_RST_EMPTCP use site
that is not already covered by an existing counter, named after the
underlying event or condition rather than the reset action:

  MD5SigReset           MD5SIG enabled on listener (incompatible)
  MPJoinSynAckNoMPJoin  SYN/ACK missing MP_JOIN option
  MPJoinAckNoMPJoin     server-side ACK missing MP_JOIN option
                          (fallback path, MPJoin required)
  MPJoinAckNoCtx        server-side ACK with no subflow context
  DssReset              data mapping invalid (also fires on
                          MAPPING_NODSS / EMIDDLEBOX path)
  MPJoinNotEstablished  JOIN attempted on a not-fully-established msk

The aggregate MPRstTx/MPRstRx counters are unchanged.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/511
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
v3:
  - Move new SNMP_MIB_ITEM entries and enum values next to their
    semantic neighbors (e.g. MD5SigReset after MD5SigFallback,
    MPJoinSynAckNoMPJoin after the MPJoinSynAck* group) instead of
    appending them in a single block after MPRst*.  No functional
    change.
v2: https://lore.kernel.org/all/20260509183335.969018-1-shardul.b@mpiricsoftware.com/
v1: https://lore.kernel.org/all/20260421095646.3741956-1-shardul.b@mpiricsoftware.com/

 net/mptcp/mib.c      | 6 ++++++
 net/mptcp/mib.h      | 6 ++++++
 net/mptcp/protocol.c | 1 +
 net/mptcp/subflow.c  | 9 +++++++++
 4 files changed, 22 insertions(+)

diff --git a/net/mptcp/mib.c b/net/mptcp/mib.c
index f23fda0c55a72..ca1da14d30b08 100644
--- a/net/mptcp/mib.c
+++ b/net/mptcp/mib.c
@@ -26,9 +26,13 @@ static const struct snmp_mib mptcp_snmp_list[] = {
 	SNMP_MIB_ITEM("MPJoinSynAckRx", MPTCP_MIB_JOINSYNACKRX),
 	SNMP_MIB_ITEM("MPJoinSynAckBackupRx", MPTCP_MIB_JOINSYNACKBACKUPRX),
 	SNMP_MIB_ITEM("MPJoinSynAckHMacFailure", MPTCP_MIB_JOINSYNACKMAC),
+	SNMP_MIB_ITEM("MPJoinSynAckNoMPJoin", MPTCP_MIB_MPJOINSYNACKNOMPJOIN),
 	SNMP_MIB_ITEM("MPJoinAckRx", MPTCP_MIB_JOINACKRX),
 	SNMP_MIB_ITEM("MPJoinAckHMacFailure", MPTCP_MIB_JOINACKMAC),
+	SNMP_MIB_ITEM("MPJoinAckNoMPJoin", MPTCP_MIB_MPJOINACKNOMPJOIN),
+	SNMP_MIB_ITEM("MPJoinAckNoCtx", MPTCP_MIB_MPJOINACKNOCTX),
 	SNMP_MIB_ITEM("MPJoinRejected", MPTCP_MIB_JOINREJECTED),
+	SNMP_MIB_ITEM("MPJoinNotEstablished", MPTCP_MIB_MPJOINNOTESTABLISHED),
 	SNMP_MIB_ITEM("MPJoinSynTx", MPTCP_MIB_JOINSYNTX),
 	SNMP_MIB_ITEM("MPJoinSynTxCreatSkErr", MPTCP_MIB_JOINSYNTXCREATSKERR),
 	SNMP_MIB_ITEM("MPJoinSynTxBindErr", MPTCP_MIB_JOINSYNTXBINDERR),
@@ -81,7 +85,9 @@ static const struct snmp_mib mptcp_snmp_list[] = {
 	SNMP_MIB_ITEM("Blackhole", MPTCP_MIB_BLACKHOLE),
 	SNMP_MIB_ITEM("MPCapableDataFallback", MPTCP_MIB_MPCAPABLEDATAFALLBACK),
 	SNMP_MIB_ITEM("MD5SigFallback", MPTCP_MIB_MD5SIGFALLBACK),
+	SNMP_MIB_ITEM("MD5SigReset", MPTCP_MIB_MD5SIGRESET),
 	SNMP_MIB_ITEM("DssFallback", MPTCP_MIB_DSSFALLBACK),
+	SNMP_MIB_ITEM("DssReset", MPTCP_MIB_DSSRESET),
 	SNMP_MIB_ITEM("SimultConnectFallback", MPTCP_MIB_SIMULTCONNFALLBACK),
 	SNMP_MIB_ITEM("FallbackFailed", MPTCP_MIB_FALLBACKFAILED),
 	SNMP_MIB_ITEM("WinProbe", MPTCP_MIB_WINPROBE),
diff --git a/net/mptcp/mib.h b/net/mptcp/mib.h
index 812218b5ed2bf..27fc53367d18f 100644
--- a/net/mptcp/mib.h
+++ b/net/mptcp/mib.h
@@ -21,9 +21,13 @@ enum linux_mptcp_mib_field {
 	MPTCP_MIB_JOINSYNACKRX,		/* Received a SYN/ACK + MP_JOIN */
 	MPTCP_MIB_JOINSYNACKBACKUPRX,	/* Received a SYN/ACK + MP_JOIN + backup flag */
 	MPTCP_MIB_JOINSYNACKMAC,	/* HMAC was wrong on SYN/ACK + MP_JOIN */
+	MPTCP_MIB_MPJOINSYNACKNOMPJOIN,	/* MP_RST: missing MP_JOIN in SYN/ACK */
 	MPTCP_MIB_JOINACKRX,		/* Received an ACK + MP_JOIN */
 	MPTCP_MIB_JOINACKMAC,		/* HMAC was wrong on ACK + MP_JOIN */
+	MPTCP_MIB_MPJOINACKNOMPJOIN,	/* MP_RST: missing MP_JOIN in ACK */
+	MPTCP_MIB_MPJOINACKNOCTX,	/* MP_RST: no subflow context on ACK */
 	MPTCP_MIB_JOINREJECTED,		/* The PM rejected the JOIN request */
+	MPTCP_MIB_MPJOINNOTESTABLISHED,	/* MP_RST: JOIN on not-fully-established msk */
 	MPTCP_MIB_JOINSYNTX,		/* 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 */
@@ -84,7 +88,9 @@ enum linux_mptcp_mib_field {
 					 * established packet
 					 */
 	MPTCP_MIB_MD5SIGFALLBACK,	/* Conflicting TCP option enabled */
+	MPTCP_MIB_MD5SIGRESET,		/* MP_RST: MD5SIG enabled on listener */
 	MPTCP_MIB_DSSFALLBACK,		/* Bad or missing DSS */
+	MPTCP_MIB_DSSRESET,		/* MP_RST: bad data mapping */
 	MPTCP_MIB_SIMULTCONNFALLBACK,	/* Simultaneous connect */
 	MPTCP_MIB_FALLBACKFAILED,	/* Can't fallback due to msk status */
 	MPTCP_MIB_WINPROBE,		/* MPTCP-level zero window probe */
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index ce8372fb3c6af..3b5041fb2588a 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3851,6 +3851,7 @@ bool mptcp_finish_join(struct sock *ssk)
 
 	/* mptcp socket already closing? */
 	if (!mptcp_is_fully_established(parent)) {
+		MPTCP_INC_STATS(sock_net(parent), MPTCP_MIB_MPJOINNOTESTABLISHED);
 		subflow->reset_reason = MPTCP_RST_EMPTCP;
 		return false;
 	}
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index d562e149606f6..6fce3f431667c 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -160,6 +160,7 @@ static int subflow_check_req(struct request_sock *req,
 	 * TCP option space.
 	 */
 	if (rcu_access_pointer(tcp_sk(sk_listener)->md5sig_info)) {
+		MPTCP_INC_STATS(sock_net(sk_listener), MPTCP_MIB_MD5SIGRESET);
 		subflow_add_reset_reason(skb, MPTCP_RST_EMPTCP);
 		return -EINVAL;
 	}
@@ -568,6 +569,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 		u8 hmac[SHA256_DIGEST_SIZE];
 
 		if (!(mp_opt.suboptions & OPTION_MPTCP_MPJ_SYNACK)) {
+			MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPJOINSYNACKNOMPJOIN);
 			subflow->reset_reason = MPTCP_RST_EMPTCP;
 			goto do_reset;
 		}
@@ -870,6 +872,12 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 		 */
 		if (!ctx || fallback) {
 			if (fallback_is_fatal) {
+				if (!ctx)
+					MPTCP_INC_STATS(sock_net(sk),
+							MPTCP_MIB_MPJOINACKNOCTX);
+				else
+					MPTCP_INC_STATS(sock_net(sk),
+							MPTCP_MIB_MPJOINACKNOMPJOIN);
 				subflow_add_reset_reason(skb, MPTCP_RST_EMPTCP);
 				goto dispose_child;
 			}
@@ -1421,6 +1429,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			 * subflow_error_report() will introduce the appropriate barriers
 			 */
 			subflow->reset_transient = 0;
+			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_DSSRESET);
 			subflow->reset_reason = status == MAPPING_NODSS ?
 						MPTCP_RST_EMIDDLEBOX :
 						MPTCP_RST_EMPTCP;
-- 
2.34.1