selftests: mptcp: explicitly trigger the listener diag code-path

The mptcp diag interface already experienced a few locking bugs
that lockdep and appropriate coverage have detected in advance.

Let's add a test-case triggering the relevant code path, to prevent
similar issues in the future.

Signed-off-by: Paolo Abeni <[email protected]>

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

@@ -81,6 +81,15 @@ chk_msk_nr()
 	__chk_msk_nr "grep -c token:" "$@"
 }
 
+chk_listener_nr()
+{
+	local expected=$1
+	local msg="$2"
+
+	__chk_nr "ss -inmlHMON $ns | wc -l" "$expected" "$msg - mptcp" 0
+	__chk_nr "ss -inmlHtON $ns | wc -l" "$expected" "$msg - subflows"
+}
+
 wait_msk_nr()
 {
 	local condition="grep -c token:"
@@ -279,5 +288,20 @@ flush_pids
 chk_msk_inuse 0 "many->0"
 chk_msk_cestab 0 "many->0"
 
+chk_listener_nr 0 "no listener sockets"
+NR_SERVERS=100
+for I in $(seq 1 $NR_SERVERS); do
+	ip netns exec $ns ./mptcp_connect -p $((I + 20001)) -l 0.0.0.0 2>&1 >/dev/null &
+	mptcp_lib_wait_local_port_listen $ns $((I + 20001))
+done
+
+chk_listener_nr $NR_SERVERS "many listener sockets"
+
+# gracefull termination
+for I in $(seq 1 $NR_SERVERS); do
+	echo a | ip netns exec $ns ./mptcp_connect -p $((I + 20001)) 127.0.0.1 2>&1 >/dev/null
+done
+flush_pids
+
 mptcp_lib_result_print_all_tap
 exit $ret