selftests: net: add test for variable PMTU in broadcast routes

Added a test for variable PMTU in broadcast routes.

This test uses iputils' ping and attempts to send a ping between
two peers, which should result in a regular echo reply.

This test will fail when the receiving peer does not receive the echo
request due to a lack of packet fragmentation.

Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
Link: https://patch.msgid.link/20250710142714.12986-2-oscmaes92@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: oscar-maes

tools/testing/selftests/net/Makefile

@@ -115,6 +115,7 @@ TEST_PROGS += skf_net_off.sh
 TEST_GEN_FILES += skf_net_off
 TEST_GEN_FILES += tfo
 TEST_PROGS += tfo_passive.sh
+TEST_PROGS += broadcast_pmtu.sh
 
 # YNL files, must be before "include ..lib.mk"
 YNL_GEN_FILES := busy_poller netlink-dumps

tools/testing/selftests/net/broadcast_pmtu.sh

@@ -0,0 +1,47 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Ensures broadcast route MTU is respected
+
+CLIENT_NS=$(mktemp -u client-XXXXXXXX)
+CLIENT_IP4="192.168.0.1/24"
+CLIENT_BROADCAST_ADDRESS="192.168.0.255"
+
+SERVER_NS=$(mktemp -u server-XXXXXXXX)
+SERVER_IP4="192.168.0.2/24"
+
+setup() {
+	ip netns add "${CLIENT_NS}"
+	ip netns add "${SERVER_NS}"
+
+	ip -net "${SERVER_NS}" link add link1 type veth peer name link0 netns "${CLIENT_NS}"
+
+	ip -net "${CLIENT_NS}" link set link0 up
+	ip -net "${CLIENT_NS}" link set link0 mtu 9000
+	ip -net "${CLIENT_NS}" addr add "${CLIENT_IP4}" dev link0
+
+	ip -net "${SERVER_NS}" link set link1 up
+	ip -net "${SERVER_NS}" link set link1 mtu 1500
+	ip -net "${SERVER_NS}" addr add "${SERVER_IP4}" dev link1
+
+	read -r -a CLIENT_BROADCAST_ENTRY <<< "$(ip -net "${CLIENT_NS}" route show table local type broadcast)"
+	ip -net "${CLIENT_NS}" route del "${CLIENT_BROADCAST_ENTRY[@]}"
+	ip -net "${CLIENT_NS}" route add "${CLIENT_BROADCAST_ENTRY[@]}" mtu 1500
+
+	ip net exec "${SERVER_NS}" sysctl -wq net.ipv4.icmp_echo_ignore_broadcasts=0
+}
+
+cleanup() {
+	ip -net "${SERVER_NS}" link del link1
+	ip netns del "${CLIENT_NS}"
+	ip netns del "${SERVER_NS}"
+}
+
+trap cleanup EXIT
+
+setup &&
+	echo "Testing for broadcast route MTU" &&
+	ip net exec "${CLIENT_NS}" ping -f -M want -q -c 1 -s 8000 -w 1 -b "${CLIENT_BROADCAST_ADDRESS}" > /dev/null 2>&1
+
+exit $?
+