[PATCH net 0/5] mptcp: pm: nl: announce deny-join-id0 flag

[PATCH net 0/5] mptcp: pm: nl: announce deny-join-id0 flag

[Matthieu Baerts (NGI0)]

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,
not letting the userspace daemon able to respect the RFC8684.

Here are a few fixes related to this 'C' flag (aka 'deny-join-id0'):

- Patch 1: add remote_deny_join_id0 info on passive connections. A fix
  for v5.14.

- Patch 2: let the userspace PM daemon know about the deny_join_id0
  attribute, so when set, it can avoid creating new subflows to the
  initial IP address and port. A fix for v5.19.

- Patch 3: a validation for the previous commit.

- Patch 4: record the deny_join_id0 info when TFO is used. A fix for
  v6.2.

- Patch 5: not related to deny-join-id0, but it fixes errors messages in
  the sockopt selftests, not to create confusions. A fix for v6.5.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Geliang Tang (1):
      selftests: mptcp: sockopt: fix error messages

Matthieu Baerts (NGI0) (4):
      mptcp: set remote_deny_join_id0 on SYN recv
      mptcp: pm: nl: announce deny-join-id0 flag
      selftests: mptcp: userspace pm: validate deny-join-id0 flag
      mptcp: tfo: record 'deny join id0' info

 Documentation/netlink/specs/mptcp_pm.yaml         |  4 ++--
 include/uapi/linux/mptcp.h                        |  2 ++
 include/uapi/linux/mptcp_pm.h                     |  4 ++--
 net/mptcp/options.c                               |  6 +++---
 net/mptcp/pm_netlink.c                            |  7 +++++++
 net/mptcp/subflow.c                               |  4 ++++
 tools/testing/selftests/net/mptcp/mptcp_sockopt.c | 16 ++++++++++------
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c     |  7 +++++++
 tools/testing/selftests/net/mptcp/userspace_pm.sh | 14 +++++++++++---
 9 files changed, 48 insertions(+), 16 deletions(-)
---
base-commit: 2690cb089502b80b905f2abdafd1bf2d54e1abef
change-id: 20250912-net-mptcp-pm-uspace-deny_join_id0-b6111e4e7e69

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>

[PATCH net 1/5] mptcp: set remote_deny_join_id0 on SYN recv

[Matthieu Baerts (NGI0)]

When a SYN containing the 'C' flag (deny join id0) was received, this
piece of information was not propagated to the path-manager.

Even if this flag is mainly set on the server side, a client can also
tell the server it cannot try to establish new subflows to the client's
initial IP address and port. The server's PM should then record such
info when received, and before sending events about the new connection.

Fixes: df377be38725 ("mptcp: add deny_join_id0 in mptcp_options_received")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/subflow.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 3f1b62a9fe889ab1ac07f2a210b9de050436b37e..f31a3a79531a2e160598111d3b0765eee2e599fb 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -883,6 +883,10 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 
 			ctx->subflow_id = 1;
 			owner = mptcp_sk(ctx->conn);
+
+			if (mp_opt.deny_join_id0)
+				WRITE_ONCE(owner->pm.remote_deny_join_id0, true);
+
 			mptcp_pm_new_connection(owner, child, 1);
 
 			/* with OoO packets we can reach here without ingress

-- 
2.51.0

[PATCH net 2/5] mptcp: pm: nl: announce deny-join-id0 flag

[Matthieu Baerts (NGI0)]

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>
---
 Documentation/netlink/specs/mptcp_pm.yaml | 4 ++--
 include/uapi/linux/mptcp.h                | 2 ++
 include/uapi/linux/mptcp_pm.h             | 4 ++--
 net/mptcp/pm_netlink.c                    | 7 +++++++
 4 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/Documentation/netlink/specs/mptcp_pm.yaml b/Documentation/netlink/specs/mptcp_pm.yaml
index d15335684ec3d6256505f2b3887ce5818eb57462..d1b4829b580ad09baf4afd73b67abd7b4ef6883a 100644
--- a/Documentation/netlink/specs/mptcp_pm.yaml
+++ b/Documentation/netlink/specs/mptcp_pm.yaml
@@ -28,13 +28,13 @@ definitions:
           traffic-patterns it can take a long time until the
           MPTCP_EVENT_ESTABLISHED is sent.
           Attributes: token, family, saddr4 | saddr6, daddr4 | daddr6, sport,
-          dport, server-side.
+          dport, server-side, [flags].
       -
         name: established
         doc: >-
           A MPTCP connection is established (can start new subflows).
           Attributes: token, family, saddr4 | saddr6, daddr4 | daddr6, sport,
-          dport, server-side.
+          dport, server-side, [flags].
       -
         name: closed
         doc: >-
diff --git a/include/uapi/linux/mptcp.h b/include/uapi/linux/mptcp.h
index 67d015df8893cc1390a483bda034191525b8fef0..5fd5b4cf75ca1e0099e0effa351507d3ab002f1e 100644
--- a/include/uapi/linux/mptcp.h
+++ b/include/uapi/linux/mptcp.h
@@ -31,6 +31,8 @@
 #define MPTCP_INFO_FLAG_FALLBACK		_BITUL(0)
 #define MPTCP_INFO_FLAG_REMOTE_KEY_RECEIVED	_BITUL(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)
diff --git a/include/uapi/linux/mptcp_pm.h b/include/uapi/linux/mptcp_pm.h
index 6ac84b2f636ca22935c191c645449fb62b673899..7359d34da446b94be148b363079120db03ba8549 100644
--- a/include/uapi/linux/mptcp_pm.h
+++ b/include/uapi/linux/mptcp_pm.h
@@ -16,10 +16,10 @@
  *   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. Attributes: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *   sport, dport, server-side.
+ *   sport, dport, server-side, [flags].
  * @MPTCP_EVENT_ESTABLISHED: A MPTCP connection is established (can start new
  *   subflows). Attributes: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *   sport, dport, server-side.
+ *   sport, dport, server-side, [flags].
  * @MPTCP_EVENT_CLOSED: A MPTCP connection has stopped. Attribute: token.
  * @MPTCP_EVENT_ANNOUNCED: A new address has been announced by the peer.
  *   Attributes: token, rem_id, family, daddr4 | daddr6 [, dport].
diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index 50aaf259959aeaf36e7ab954c6f7957eaf2bc390..ce7d42d3bd007b13f214fab699bc69b5e35c25e3 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -408,6 +408,7 @@ static int mptcp_event_created(struct sk_buff *skb,
 			       const struct sock *ssk)
 {
 	int err = nla_put_u32(skb, MPTCP_ATTR_TOKEN, READ_ONCE(msk->token));
+	u16 flags = 0;
 
 	if (err)
 		return err;
@@ -415,6 +416,12 @@ 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 net 3/5] selftests: mptcp: userspace pm: validate deny-join-id0 flag

[Matthieu Baerts (NGI0)]

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>
---
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c     |  7 +++++++
 tools/testing/selftests/net/mptcp/userspace_pm.sh | 14 +++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
index 994a556f46c15163bae6cb517c371801f0cd6e3b..93fea3442216c8fef43731a99c1d5710f234b150 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 970c329735ff14f87f0048ba0030dc7edaaa86bc..3d45991f24ede919264185e3b5c2a3b95c1dcc85 100755
--- a/tools/testing/selftests/net/mptcp/userspace_pm.sh
+++ b/tools/testing/selftests/net/mptcp/userspace_pm.sh
@@ -201,6 +201,9 @@ make_connection()
 		is_v6="v4"
 	fi
 
+	# set this on the client side only: will not affect the rest
+	ip netns exec "$ns2" sysctl -q net.mptcp.allow_join_initial_addr_port=0
+
 	:>"$client_evts"
 	:>"$server_evts"
 
@@ -223,23 +226,28 @@ 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=$(mptcp_lib_evts_get_info token "$client_evts")
 	client_port=$(mptcp_lib_evts_get_info sport "$client_evts")
 	client_serverside=$(mptcp_lib_evts_get_info server_side "$client_evts")
+	client_nojoin=$(mptcp_lib_evts_get_info deny_join_id0 "$client_evts")
 	server_token=$(mptcp_lib_evts_get_info token "$server_evts")
 	server_serverside=$(mptcp_lib_evts_get_info server_side "$server_evts")
+	server_nojoin=$(mptcp_lib_evts_get_info deny_join_id0 "$server_evts")
 
 	print_test "Established IP${is_v6} MPTCP Connection ns2 => ns1"
-	if [ "$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
 		test_pass
 		print_title "Connection info: ${client_addr}:${client_port} -> ${connect_addr}:${app_port}"
 	else
-		test_fail "Expected tokens (c:${client_token} - s:${server_token}) and server (c:${client_serverside} - s:${server_serverside})"
+		test_fail "Expected tokens (c:${client_token} - s:${server_token}), server (c:${client_serverside} - s:${server_serverside}), nojoin (c:${client_nojoin} - s:${server_nojoin})"
 		mptcp_lib_result_print_all_tap
 		exit ${KSFT_FAIL}
 	fi

-- 
2.51.0

[PATCH net 4/5] mptcp: tfo: record 'deny join id0' info

[Matthieu Baerts (NGI0)]

When TFO is used, the check to see if the 'C' flag (deny join id0) was
set was bypassed.

This flag can be set when TFO is used, so the check should also be done
when TFO is used.

Note that the set_fully_established label is also used when a 4th ACK is
received. In this case, deny_join_id0 will not be set.

Fixes: dfc8d0603033 ("mptcp: implement delayed seq generation for passive fastopen")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/options.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 2a8ea28442b271675411d190604073f779bee7fa..1103b3341a70560f563bfb21ca143ce2f2c60321 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -985,13 +985,13 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 		return false;
 	}
 
-	if (mp_opt->deny_join_id0)
-		WRITE_ONCE(msk->pm.remote_deny_join_id0, true);
-
 	if (unlikely(!READ_ONCE(msk->pm.server_side)))
 		pr_warn_once("bogus mpc option on established client sk");
 
 set_fully_established:
+	if (mp_opt->deny_join_id0)
+		WRITE_ONCE(msk->pm.remote_deny_join_id0, true);
+
 	mptcp_data_lock((struct sock *)msk);
 	__mptcp_subflow_fully_established(msk, subflow, mp_opt);
 	mptcp_data_unlock((struct sock *)msk);

-- 
2.51.0

[PATCH net 5/5] selftests: mptcp: sockopt: fix error messages

[Matthieu Baerts (NGI0)]

From: Geliang Tang <tanggeliang@kylinos.cn>

This patch fixes several issues in the error reporting of the MPTCP sockopt
selftest:

1. Fix diff not printed: The error messages for counter mismatches had
   the actual difference ('diff') as argument, but it was missing in the
   format string. Displaying it makes the debugging easier.

2. Fix variable usage: The error check for 'mptcpi_bytes_acked' incorrectly
   used 'ret2' (sent bytes) for both the expected value and the difference
   calculation. It now correctly uses 'ret' (received bytes), which is the
   expected value for bytes_acked.

3. Fix off-by-one in diff: The calculation for the 'mptcpi_rcv_delta' diff
   was 's.mptcpi_rcv_delta - ret', which is off-by-one. It has been
   corrected to 's.mptcpi_rcv_delta - (ret + 1)' to match the expected
   value in the condition above it.

Fixes: 5dcff89e1455 ("selftests: mptcp: explicitly tests aggregate counters")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 tools/testing/selftests/net/mptcp/mptcp_sockopt.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index e934dd26a59d9b50445d61e8b8013ce3c8d2a8a0..112c07c4c37a3c827290a0b06d6307abca98c849 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -667,22 +667,26 @@ static void process_one_client(int fd, int pipefd)
 
 	do_getsockopts(&s, fd, ret, ret2);
 	if (s.mptcpi_rcv_delta != (uint64_t)ret + 1)
-		xerror("mptcpi_rcv_delta %" PRIu64 ", expect %" PRIu64, s.mptcpi_rcv_delta, ret + 1, s.mptcpi_rcv_delta - ret);
+		xerror("mptcpi_rcv_delta %" PRIu64 ", expect %" PRIu64 ", diff %" PRId64,
+		       s.mptcpi_rcv_delta, ret + 1, s.mptcpi_rcv_delta - (ret + 1));
 
 	/* be nice when running on top of older kernel */
 	if (s.pkt_stats_avail) {
 		if (s.last_sample.mptcpi_bytes_sent != ret2)
-			xerror("mptcpi_bytes_sent %" PRIu64 ", expect %" PRIu64,
+			xerror("mptcpi_bytes_sent %" PRIu64 ", expect %" PRIu64
+			       ", diff %" PRId64,
 			       s.last_sample.mptcpi_bytes_sent, ret2,
 			       s.last_sample.mptcpi_bytes_sent - ret2);
 		if (s.last_sample.mptcpi_bytes_received != ret)
-			xerror("mptcpi_bytes_received %" PRIu64 ", expect %" PRIu64,
+			xerror("mptcpi_bytes_received %" PRIu64 ", expect %" PRIu64
+			       ", diff %" PRId64,
 			       s.last_sample.mptcpi_bytes_received, ret,
 			       s.last_sample.mptcpi_bytes_received - ret);
 		if (s.last_sample.mptcpi_bytes_acked != ret)
-			xerror("mptcpi_bytes_acked %" PRIu64 ", expect %" PRIu64,
-			       s.last_sample.mptcpi_bytes_acked, ret2,
-			       s.last_sample.mptcpi_bytes_acked - ret2);
+			xerror("mptcpi_bytes_acked %" PRIu64 ", expect %" PRIu64
+			       ", diff %" PRId64,
+			       s.last_sample.mptcpi_bytes_acked, ret,
+			       s.last_sample.mptcpi_bytes_acked - ret);
 	}
 
 	close(fd);

-- 
2.51.0

Re: [PATCH net 0/5] mptcp: pm: nl: announce deny-join-id0 flag

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Fri, 12 Sep 2025 14:52:19 +0200 you wrote:
> 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,
> not letting the userspace daemon able to respect the RFC8684.
> 
> [...]

Here is the summary with links:
  - [net,1/5] mptcp: set remote_deny_join_id0 on SYN recv
    https://git.kernel.org/netdev/net/c/96939cec9940
  - [net,2/5] mptcp: pm: nl: announce deny-join-id0 flag
    https://git.kernel.org/netdev/net/c/2293c57484ae
  - [net,3/5] selftests: mptcp: userspace pm: validate deny-join-id0 flag
    https://git.kernel.org/netdev/net/c/24733e193a0d
  - [net,4/5] mptcp: tfo: record 'deny join id0' info
    https://git.kernel.org/netdev/net/c/92da495cb657
  - [net,5/5] selftests: mptcp: sockopt: fix error messages
    https://git.kernel.org/netdev/net/c/b86418beade1

You are awesome, thank you!
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