selftests: net: move amt to socat for better compatibility

The test seems to expect that nc will exit after the first
received message. This is not the case with Ncat 7.94.
There are multiple versions of nc out there, switch
to socat for better compatibility.

Tell socat to exit after 128 bytes and pad the message.

Since the test sets -e make sure we don't set exit code
(|| true) and print the pass / fail rather then silently
moving over the test and just setting non-zero exit code
with no output indicating what failed.

Fixes: c08e8ba ("selftests: add amt interface selftest script")
Acked-by: Paolo Abeni<pabeni@redhat.com>
Tested-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20240509161952.3940476-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

tools/testing/selftests/net/amt.sh

@@ -210,8 +210,8 @@ check_features()
 
 test_ipv4_forward()
 {
-	RESULT4=$(ip netns exec "${LISTENER}" nc -w 1 -l -u 239.0.0.1 4000)
-	if [ "$RESULT4" == "172.17.0.2" ]; then
+	RESULT4=$(ip netns exec "${LISTENER}" timeout 15 socat - UDP4-LISTEN:4000,readbytes=128 || true)
+	if echo "$RESULT4" | grep -q "172.17.0.2"; then
 		printf "TEST: %-60s  [ OK ]\n" "IPv4 amt multicast forwarding"
 		exit 0
 	else
@@ -222,8 +222,8 @@ test_ipv4_forward()
 
 test_ipv6_forward()
 {
-	RESULT6=$(ip netns exec "${LISTENER}" nc -w 1 -l -u ff0e::5:6 6000)
-	if [ "$RESULT6" == "2001:db8:3::2" ]; then
+	RESULT6=$(ip netns exec "${LISTENER}" timeout 15 socat - UDP6-LISTEN:6000,readbytes=128 || true)
+	if echo "$RESULT6" | grep -q "2001:db8:3::2"; then
 		printf "TEST: %-60s  [ OK ]\n" "IPv6 amt multicast forwarding"
 		exit 0
 	else
@@ -236,14 +236,14 @@ send_mcast4()
 {
 	sleep 2
 	ip netns exec "${SOURCE}" bash -c \
-		'echo 172.17.0.2 | nc -w 1 -u 239.0.0.1 4000' &
+		'printf "%s %128s" 172.17.0.2 | nc -w 1 -u 239.0.0.1 4000' &
 }
 
 send_mcast6()
 {
 	sleep 2
 	ip netns exec "${SOURCE}" bash -c \
-		'echo 2001:db8:3::2 | nc -w 1 -u ff0e::5:6 6000' &
+		'printf "%s %128s" 2001:db8:3::2 | nc -w 1 -u ff0e::5:6 6000' &
 }
 
 check_features