selftests: mptcp: join: properly kill background tasks

The 'run_tests' function is executed in the background, but killing its
associated PID would not kill the children tasks running in the
background.

To properly kill all background tasks, 'kill -- -PID' could be used, but
this requires kill from procps-ng. Instead, all children tasks are
listed using 'ps', and 'kill' is called with all PIDs of this group.

Fixes: 31ee4ad ("selftests: mptcp: join: stop transfer when check is done (part 1)")
Cc: stable@vger.kernel.org
Fixes: 04b57c9 ("selftests: mptcp: join: stop transfer when check is done (part 2)")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251110-net-mptcp-sft-join-unstable-v1-6-a4332c714e10@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -3831,7 +3831,7 @@ userspace_tests()
 		chk_mptcp_info subflows 0 subflows 0
 		chk_subflows_total 1 1
 		kill_events_pids
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 	fi
 
 	# userspace pm create destroy subflow
@@ -3859,7 +3859,7 @@ userspace_tests()
 		chk_mptcp_info subflows 0 subflows 0
 		chk_subflows_total 1 1
 		kill_events_pids
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 	fi
 
 	# userspace pm create id 0 subflow
@@ -3880,7 +3880,7 @@ userspace_tests()
 		chk_mptcp_info subflows 1 subflows 1
 		chk_subflows_total 2 2
 		kill_events_pids
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 	fi
 
 	# userspace pm remove initial subflow
@@ -3904,7 +3904,7 @@ userspace_tests()
 		chk_mptcp_info subflows 1 subflows 1
 		chk_subflows_total 1 1
 		kill_events_pids
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 	fi
 
 	# userspace pm send RM_ADDR for ID 0
@@ -3930,7 +3930,7 @@ userspace_tests()
 		chk_mptcp_info subflows 1 subflows 1
 		chk_subflows_total 1 1
 		kill_events_pids
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 	fi
 }
 
@@ -3960,7 +3960,7 @@ endpoint_tests()
 		pm_nl_add_endpoint $ns2 10.0.2.2 flags signal
 		pm_nl_check_endpoint "modif is allowed" \
 			$ns2 10.0.2.2 id 1 flags signal
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 	fi
 
 	if reset_with_tcp_filter "delete and re-add" ns2 10.0.3.2 REJECT OUTPUT &&
@@ -4015,7 +4015,7 @@ endpoint_tests()
 			chk_mptcp_info subflows 3 subflows 3
 		done
 
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 
 		kill_events_pids
 		chk_evt_nr ns1 MPTCP_LIB_EVENT_LISTENER_CREATED 1
@@ -4089,7 +4089,7 @@ endpoint_tests()
 		wait_mpj $ns2
 		chk_subflow_nr "after re-re-add ID 0" 3
 		chk_mptcp_info subflows 3 subflows 3
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 
 		kill_events_pids
 		chk_evt_nr ns1 MPTCP_LIB_EVENT_LISTENER_CREATED 1
@@ -4137,7 +4137,7 @@ endpoint_tests()
 		wait_mpj $ns2
 		pm_nl_add_endpoint $ns1 10.0.3.1 id 2 flags signal
 		wait_mpj $ns2
-		mptcp_lib_kill_wait $tests_pid
+		mptcp_lib_kill_group_wait $tests_pid
 
 		join_syn_tx=3 join_connect_err=1 \
 			chk_join_nr 2 2 2

tools/testing/selftests/net/mptcp/mptcp_lib.sh

@@ -350,6 +350,27 @@ mptcp_lib_kill_wait() {
 	wait "${1}" 2>/dev/null
 }
 
+# $1: PID
+mptcp_lib_pid_list_children() {
+	local curr="${1}"
+	# evoke 'ps' only once
+	local pids="${2:-"$(ps o pid,ppid)"}"
+
+	echo "${curr}"
+
+	local pid
+	for pid in $(echo "${pids}" | awk "\$2 == ${curr} { print \$1 }"); do
+		mptcp_lib_pid_list_children "${pid}" "${pids}"
+	done
+}
+
+# $1: PID
+mptcp_lib_kill_group_wait() {
+	# Some users might not have procps-ng: cannot use "kill -- -PID"
+	mptcp_lib_pid_list_children "${1}" | xargs -r kill &>/dev/null
+	wait "${1}" 2>/dev/null
+}
+
 # $1: IP address
 mptcp_lib_is_v6() {
 	[ -z "${1##*:*}" ]