[PATCH 6.1.y 0/2] mptcp: fix recent failed backports (20250919)

[PATCH 6.1.y 0/2] mptcp: fix recent failed backports (20250919)

[Matthieu Baerts (NGI0)]

The following patches could not be applied without conflicts in this
tree:

 - 2293c57484ae ("mptcp: pm: nl: announce deny-join-id0 flag")
 - 24733e193a0d ("selftests: mptcp: userspace pm: validate deny-join-id0 flag")

Conflicts have been resolved, and documented in each patch.

Matthieu Baerts (NGI0) (2):
  mptcp: pm: nl: announce deny-join-id0 flag
  selftests: mptcp: userspace pm: validate deny-join-id0 flag

 include/uapi/linux/mptcp.h                        |  6 ++++--
 net/mptcp/pm_netlink.c                            |  7 +++++++
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c     |  7 +++++++
 tools/testing/selftests/net/mptcp/userspace_pm.sh | 14 ++++++++++++--
 4 files changed, 30 insertions(+), 4 deletions(-)

-- 
2.51.0

[PATCH 6.1.y 1/2] mptcp: pm: nl: announce deny-join-id0 flag

[Matthieu Baerts (NGI0)]

commit 2293c57484ae64c9a3c847c8807db8c26a3a4d41 upstream.

During the connection establishment, a peer can tell the other one that
it cannot establish new subflows to the initial IP address and port by
setting the 'C' flag [1]. Doing so makes sense when the sender is behind
a strict NAT, operating behind a legacy Layer 4 load balancer, or using
anycast IP address for example.

When this 'C' flag is set, the path-managers must then not try to
establish new subflows to the other peer's initial IP address and port.
The in-kernel PM has access to this info, but the userspace PM didn't.

The RFC8684 [1] is strict about that:

  (...) therefore the receiver MUST NOT try to open any additional
  subflows toward this address and port.

So it is important to tell the userspace about that as it is responsible
for the respect of this flag.

When a new connection is created and established, the Netlink events
now contain the existing but not currently used 'flags' attribute. When
MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 is set, it means no other subflows
to the initial IP address and port -- info that are also part of the
event -- can be established.

Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.1-20.6 [1]
Fixes: 702c2f646d42 ("mptcp: netlink: allow userspace-driven subflow establishment")
Reported-by: Marek Majkowski <marek@cloudflare.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/532
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-2-40171884ade8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in mptcp_pm.yaml, and mptcp_pm.h, because these files have
  been added later by commit bc8aeb2045e2 ("Documentation: netlink: add
  a YAML spec for mptcp"), and commit 9d1ed17f93ce ("uapi: mptcp: use
  header file generated from YAML spec"), which are not in this version.
  Applying the same modifications, but only in mptcp.h.
  Conflict in pm_netlink.c, because of a difference in the context,
  introduced by commit b9f4554356f6 ("mptcp: annotate lockless access
  for token"), which is not in this version. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 include/uapi/linux/mptcp.h | 6 ++++--
 net/mptcp/pm_netlink.c     | 7 +++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/mptcp.h b/include/uapi/linux/mptcp.h
index dfe19bf13f4c..dacc7af2ea1e 100644
--- a/include/uapi/linux/mptcp.h
+++ b/include/uapi/linux/mptcp.h
@@ -81,6 +81,8 @@ enum {
 
 #define MPTCP_PM_ADDR_ATTR_MAX (__MPTCP_PM_ADDR_ATTR_MAX - 1)
 
+#define MPTCP_PM_EV_FLAG_DENY_JOIN_ID0		_BITUL(0)
+
 #define MPTCP_PM_ADDR_FLAG_SIGNAL			(1 << 0)
 #define MPTCP_PM_ADDR_FLAG_SUBFLOW			(1 << 1)
 #define MPTCP_PM_ADDR_FLAG_BACKUP			(1 << 2)
@@ -127,13 +129,13 @@ struct mptcp_info {
 
 /*
  * MPTCP_EVENT_CREATED: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *                      sport, dport
+ *                      sport, dport, server-side, [flags]
  * A new MPTCP connection has been created. It is the good time to allocate
  * memory and send ADD_ADDR if needed. Depending on the traffic-patterns
  * it can take a long time until the MPTCP_EVENT_ESTABLISHED is sent.
  *
  * MPTCP_EVENT_ESTABLISHED: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *			    sport, dport
+ *			    sport, dport, server-side, [flags]
  * A MPTCP connection is established (can start new subflows).
  *
  * MPTCP_EVENT_CLOSED: token
diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index cf9244a3644f..7e72862a6b54 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -2242,6 +2242,7 @@ static int mptcp_event_created(struct sk_buff *skb,
 			       const struct sock *ssk)
 {
 	int err = nla_put_u32(skb, MPTCP_ATTR_TOKEN, msk->token);
+	u16 flags = 0;
 
 	if (err)
 		return err;
@@ -2249,6 +2250,12 @@ static int mptcp_event_created(struct sk_buff *skb,
 	if (nla_put_u8(skb, MPTCP_ATTR_SERVER_SIDE, READ_ONCE(msk->pm.server_side)))
 		return -EMSGSIZE;
 
+	if (READ_ONCE(msk->pm.remote_deny_join_id0))
+		flags |= MPTCP_PM_EV_FLAG_DENY_JOIN_ID0;
+
+	if (flags && nla_put_u16(skb, MPTCP_ATTR_FLAGS, flags))
+		return -EMSGSIZE;
+
 	return mptcp_event_add_subflow(skb, ssk);
 }
 
-- 
2.51.0

[PATCH 6.1.y 2/2] selftests: mptcp: userspace pm: validate deny-join-id0 flag

[Matthieu Baerts (NGI0)]

commit 24733e193a0d68f20d220e86da0362460c9aa812 upstream.

The previous commit adds the MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 flag. Make
sure it is correctly announced by the other peer when it has been
received.

pm_nl_ctl will now display 'deny_join_id0:1' when monitoring the events,
and when this flag was set by the other peer.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 702c2f646d42 ("mptcp: netlink: allow userspace-driven subflow establishment")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-3-40171884ade8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflict in userspace_pm.sh, because of a difference in the context,
  introduced by commit c66fb480a330 ("selftests: userspace pm: avoid
  relaunching pm events"), which is not in this version. The same lines
  can still be added at the same place.
  Conflicts in userspace_pm.sh, because of different refactoring, like
  with commit ae1fa39da991 ("selftests: mptcp: add evts_get_info
  helper"), and commit e198ad759273 ("selftests: mptcp: userspace_pm:
  uniform results printing"). The modifications have been adapted to the
  old version, without the new helpers. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c     |  7 +++++++
 tools/testing/selftests/net/mptcp/userspace_pm.sh | 14 ++++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
index 1887bd61bd9a..17e5b7ec53b6 100644
--- a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
+++ b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
@@ -188,6 +188,13 @@ static int capture_events(int fd, int event_group)
 					fprintf(stderr, ",error:%u", *(__u8 *)RTA_DATA(attrs));
 				else if (attrs->rta_type == MPTCP_ATTR_SERVER_SIDE)
 					fprintf(stderr, ",server_side:%u", *(__u8 *)RTA_DATA(attrs));
+				else if (attrs->rta_type == MPTCP_ATTR_FLAGS) {
+					__u16 flags = *(__u16 *)RTA_DATA(attrs);
+
+					/* only print when present, easier */
+					if (flags & MPTCP_PM_EV_FLAG_DENY_JOIN_ID0)
+						fprintf(stderr, ",deny_join_id0:1");
+				}
 
 				attrs = RTA_NEXT(attrs, msg_len);
 			}
diff --git a/tools/testing/selftests/net/mptcp/userspace_pm.sh b/tools/testing/selftests/net/mptcp/userspace_pm.sh
index cb6c28d40129..a82ac12e816d 100755
--- a/tools/testing/selftests/net/mptcp/userspace_pm.sh
+++ b/tools/testing/selftests/net/mptcp/userspace_pm.sh
@@ -139,6 +139,9 @@ make_connection()
 		is_v6="v4"
 	fi
 
+	# set this on the client side only: will not affect the rest
+	ip netns exec "$ns2" sysctl -q net.mptcp.allow_join_initial_addr_port=0
+
 	# Capture netlink events over the two network namespaces running
 	# the MPTCP client and server
 	local client_evts
@@ -173,21 +176,28 @@ make_connection()
 	local client_token
 	local client_port
 	local client_serverside
+	local client_nojoin
 	local server_token
 	local server_serverside
+	local server_nojoin
 
 	client_token=$(sed --unbuffered -n 's/.*\(token:\)\([[:digit:]]*\).*$/\2/p;q' "$client_evts")
 	client_port=$(sed --unbuffered -n 's/.*\(sport:\)\([[:digit:]]*\).*$/\2/p;q' "$client_evts")
 	client_serverside=$(sed --unbuffered -n 's/.*\(server_side:\)\([[:digit:]]*\).*$/\2/p;q'\
 				      "$client_evts")
+	client_nojoin=$(sed --unbuffered -n 's/.*\(deny_join_id0:\)\([[:digit:]]*\).*$/\2/p;q'\
+				      "$client_evts")
 	kill_wait $server_evts_pid
 	server_token=$(sed --unbuffered -n 's/.*\(token:\)\([[:digit:]]*\).*$/\2/p;q' "$server_evts")
 	server_serverside=$(sed --unbuffered -n 's/.*\(server_side:\)\([[:digit:]]*\).*$/\2/p;q'\
 				      "$server_evts")
+	server_nojoin=$(sed --unbuffered -n 's/.*\(deny_join_id0:\)\([[:digit:]]*\).*$/\2/p;q'\
+				      "$server_evts")
 	rm -f "$client_evts" "$server_evts" "$file"
 
-	if [ "$client_token" != "" ] && [ "$server_token" != "" ] && [ "$client_serverside" = 0 ] &&
-		   [ "$server_serverside" = 1 ]
+	if [ "${client_token}" != "" ] && [ "${server_token}" != "" ] &&
+	   [ "${client_serverside}" = 0 ] && [ "${server_serverside}" = 1 ] &&
+	   [ "${client_nojoin:-0}" = 0 ] && [ "${server_nojoin:-0}" = 1 ]
 	then
 		stdbuf -o0 -e0 printf "Established IP%s MPTCP Connection ns2 => ns1    \t\t[OK]\n" $is_v6
 	else
-- 
2.51.0

Patch "mptcp: pm: nl: announce deny-join-id0 flag" has been added to the 6.1-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    mptcp: pm: nl: announce deny-join-id0 flag

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mptcp-pm-nl-announce-deny-join-id0-flag.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From stable+bounces-180715-greg=kroah.com@vger.kernel.org Sat Sep 20 00:51:43 2025
From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>
Date: Sat, 20 Sep 2025 00:51:20 +0200
Subject: mptcp: pm: nl: announce deny-join-id0 flag
To: mptcp@lists.linux.dev, stable@vger.kernel.org, gregkh@linuxfoundation.org
Cc: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>, sashal@kernel.org, Marek Majkowski <marek@cloudflare.com>, Mat Martineau <martineau@kernel.org>, Jakub Kicinski <kuba@kernel.org>
Message-ID: <20250919225118.3781035-5-matttbe@kernel.org>

From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>

commit 2293c57484ae64c9a3c847c8807db8c26a3a4d41 upstream.

During the connection establishment, a peer can tell the other one that
it cannot establish new subflows to the initial IP address and port by
setting the 'C' flag [1]. Doing so makes sense when the sender is behind
a strict NAT, operating behind a legacy Layer 4 load balancer, or using
anycast IP address for example.

When this 'C' flag is set, the path-managers must then not try to
establish new subflows to the other peer's initial IP address and port.
The in-kernel PM has access to this info, but the userspace PM didn't.

The RFC8684 [1] is strict about that:

  (...) therefore the receiver MUST NOT try to open any additional
  subflows toward this address and port.

So it is important to tell the userspace about that as it is responsible
for the respect of this flag.

When a new connection is created and established, the Netlink events
now contain the existing but not currently used 'flags' attribute. When
MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 is set, it means no other subflows
to the initial IP address and port -- info that are also part of the
event -- can be established.

Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.1-20.6 [1]
Fixes: 702c2f646d42 ("mptcp: netlink: allow userspace-driven subflow establishment")
Reported-by: Marek Majkowski <marek@cloudflare.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/532
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-2-40171884ade8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in mptcp_pm.yaml, and mptcp_pm.h, because these files have
  been added later by commit bc8aeb2045e2 ("Documentation: netlink: add
  a YAML spec for mptcp"), and commit 9d1ed17f93ce ("uapi: mptcp: use
  header file generated from YAML spec"), which are not in this version.
  Applying the same modifications, but only in mptcp.h.
  Conflict in pm_netlink.c, because of a difference in the context,
  introduced by commit b9f4554356f6 ("mptcp: annotate lockless access
  for token"), which is not in this version. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 include/uapi/linux/mptcp.h |    6 ++++--
 net/mptcp/pm_netlink.c     |    7 +++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

--- a/include/uapi/linux/mptcp.h
+++ b/include/uapi/linux/mptcp.h
@@ -81,6 +81,8 @@ enum {
 
 #define MPTCP_PM_ADDR_ATTR_MAX (__MPTCP_PM_ADDR_ATTR_MAX - 1)
 
+#define MPTCP_PM_EV_FLAG_DENY_JOIN_ID0		_BITUL(0)
+
 #define MPTCP_PM_ADDR_FLAG_SIGNAL			(1 << 0)
 #define MPTCP_PM_ADDR_FLAG_SUBFLOW			(1 << 1)
 #define MPTCP_PM_ADDR_FLAG_BACKUP			(1 << 2)
@@ -127,13 +129,13 @@ struct mptcp_info {
 
 /*
  * MPTCP_EVENT_CREATED: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *                      sport, dport
+ *                      sport, dport, server-side, [flags]
  * A new MPTCP connection has been created. It is the good time to allocate
  * memory and send ADD_ADDR if needed. Depending on the traffic-patterns
  * it can take a long time until the MPTCP_EVENT_ESTABLISHED is sent.
  *
  * MPTCP_EVENT_ESTABLISHED: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *			    sport, dport
+ *			    sport, dport, server-side, [flags]
  * A MPTCP connection is established (can start new subflows).
  *
  * MPTCP_EVENT_CLOSED: token
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -2242,6 +2242,7 @@ static int mptcp_event_created(struct sk
 			       const struct sock *ssk)
 {
 	int err = nla_put_u32(skb, MPTCP_ATTR_TOKEN, msk->token);
+	u16 flags = 0;
 
 	if (err)
 		return err;
@@ -2249,6 +2250,12 @@ static int mptcp_event_created(struct sk
 	if (nla_put_u8(skb, MPTCP_ATTR_SERVER_SIDE, READ_ONCE(msk->pm.server_side)))
 		return -EMSGSIZE;
 
+	if (READ_ONCE(msk->pm.remote_deny_join_id0))
+		flags |= MPTCP_PM_EV_FLAG_DENY_JOIN_ID0;
+
+	if (flags && nla_put_u16(skb, MPTCP_ATTR_FLAGS, flags))
+		return -EMSGSIZE;
+
 	return mptcp_event_add_subflow(skb, ssk);
 }
 


Patches currently in stable-queue which might be from matttbe@kernel.org are

queue-6.1/mptcp-pm-nl-announce-deny-join-id0-flag.patch
queue-6.1/selftests-mptcp-avoid-spurious-errors-on-tcp-disconnect.patch
queue-6.1/mptcp-set-remote_deny_join_id0-on-syn-recv.patch
queue-6.1/selftests-mptcp-userspace-pm-validate-deny-join-id0-flag.patch

Patch "selftests: mptcp: userspace pm: validate deny-join-id0 flag" has been added to the 6.1-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    selftests: mptcp: userspace pm: validate deny-join-id0 flag

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     selftests-mptcp-userspace-pm-validate-deny-join-id0-flag.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From stable+bounces-180716-greg=kroah.com@vger.kernel.org Sat Sep 20 00:52:39 2025
From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>
Date: Sat, 20 Sep 2025 00:51:21 +0200
Subject: selftests: mptcp: userspace pm: validate deny-join-id0 flag
To: mptcp@lists.linux.dev, stable@vger.kernel.org, gregkh@linuxfoundation.org
Cc: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>, sashal@kernel.org, Mat Martineau <martineau@kernel.org>, Jakub Kicinski <kuba@kernel.org>
Message-ID: <20250919225118.3781035-6-matttbe@kernel.org>

From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>

commit 24733e193a0d68f20d220e86da0362460c9aa812 upstream.

The previous commit adds the MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 flag. Make
sure it is correctly announced by the other peer when it has been
received.

pm_nl_ctl will now display 'deny_join_id0:1' when monitoring the events,
and when this flag was set by the other peer.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 702c2f646d42 ("mptcp: netlink: allow userspace-driven subflow establishment")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-3-40171884ade8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflict in userspace_pm.sh, because of a difference in the context,
  introduced by commit c66fb480a330 ("selftests: userspace pm: avoid
  relaunching pm events"), which is not in this version. The same lines
  can still be added at the same place.
  Conflicts in userspace_pm.sh, because of different refactoring, like
  with commit ae1fa39da991 ("selftests: mptcp: add evts_get_info
  helper"), and commit e198ad759273 ("selftests: mptcp: userspace_pm:
  uniform results printing"). The modifications have been adapted to the
  old version, without the new helpers. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c     |    7 +++++++
 tools/testing/selftests/net/mptcp/userspace_pm.sh |   14 ++++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

--- a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
+++ b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
@@ -188,6 +188,13 @@ static int capture_events(int fd, int ev
 					fprintf(stderr, ",error:%u", *(__u8 *)RTA_DATA(attrs));
 				else if (attrs->rta_type == MPTCP_ATTR_SERVER_SIDE)
 					fprintf(stderr, ",server_side:%u", *(__u8 *)RTA_DATA(attrs));
+				else if (attrs->rta_type == MPTCP_ATTR_FLAGS) {
+					__u16 flags = *(__u16 *)RTA_DATA(attrs);
+
+					/* only print when present, easier */
+					if (flags & MPTCP_PM_EV_FLAG_DENY_JOIN_ID0)
+						fprintf(stderr, ",deny_join_id0:1");
+				}
 
 				attrs = RTA_NEXT(attrs, msg_len);
 			}
--- a/tools/testing/selftests/net/mptcp/userspace_pm.sh
+++ b/tools/testing/selftests/net/mptcp/userspace_pm.sh
@@ -139,6 +139,9 @@ make_connection()
 		is_v6="v4"
 	fi
 
+	# set this on the client side only: will not affect the rest
+	ip netns exec "$ns2" sysctl -q net.mptcp.allow_join_initial_addr_port=0
+
 	# Capture netlink events over the two network namespaces running
 	# the MPTCP client and server
 	local client_evts
@@ -173,21 +176,28 @@ make_connection()
 	local client_token
 	local client_port
 	local client_serverside
+	local client_nojoin
 	local server_token
 	local server_serverside
+	local server_nojoin
 
 	client_token=$(sed --unbuffered -n 's/.*\(token:\)\([[:digit:]]*\).*$/\2/p;q' "$client_evts")
 	client_port=$(sed --unbuffered -n 's/.*\(sport:\)\([[:digit:]]*\).*$/\2/p;q' "$client_evts")
 	client_serverside=$(sed --unbuffered -n 's/.*\(server_side:\)\([[:digit:]]*\).*$/\2/p;q'\
 				      "$client_evts")
+	client_nojoin=$(sed --unbuffered -n 's/.*\(deny_join_id0:\)\([[:digit:]]*\).*$/\2/p;q'\
+				      "$client_evts")
 	kill_wait $server_evts_pid
 	server_token=$(sed --unbuffered -n 's/.*\(token:\)\([[:digit:]]*\).*$/\2/p;q' "$server_evts")
 	server_serverside=$(sed --unbuffered -n 's/.*\(server_side:\)\([[:digit:]]*\).*$/\2/p;q'\
 				      "$server_evts")
+	server_nojoin=$(sed --unbuffered -n 's/.*\(deny_join_id0:\)\([[:digit:]]*\).*$/\2/p;q'\
+				      "$server_evts")
 	rm -f "$client_evts" "$server_evts" "$file"
 
-	if [ "$client_token" != "" ] && [ "$server_token" != "" ] && [ "$client_serverside" = 0 ] &&
-		   [ "$server_serverside" = 1 ]
+	if [ "${client_token}" != "" ] && [ "${server_token}" != "" ] &&
+	   [ "${client_serverside}" = 0 ] && [ "${server_serverside}" = 1 ] &&
+	   [ "${client_nojoin:-0}" = 0 ] && [ "${server_nojoin:-0}" = 1 ]
 	then
 		stdbuf -o0 -e0 printf "Established IP%s MPTCP Connection ns2 => ns1    \t\t[OK]\n" $is_v6
 	else


Patches currently in stable-queue which might be from matttbe@kernel.org are

queue-6.1/mptcp-pm-nl-announce-deny-join-id0-flag.patch
queue-6.1/selftests-mptcp-avoid-spurious-errors-on-tcp-disconnect.patch
queue-6.1/mptcp-set-remote_deny_join_id0-on-syn-recv.patch
queue-6.1/selftests-mptcp-userspace-pm-validate-deny-join-id0-flag.patch