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

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: 702c2f6 ("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>

Author: matttbe

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);
 			}

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