linux-kernel - [PATCH net v2 2/2] selftests: net: add test for variable PMTU in broadcast routes

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Message-Id: <20250703152838.2993-2-oscmaes92@gmail.com>
Date: Thu,  3 Jul 2025 17:28:38 +0200
From: Oscar Maes <oscmaes92@...il.com>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	stable@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Oscar Maes <oscmaes92@...il.com>
Subject: [PATCH net v2 2/2] 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@...il.com>
---
 tools/testing/selftests/net/Makefile          |  1 +
 tools/testing/selftests/net/broadcast_pmtu.sh | 47 +++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100755 tools/testing/selftests/net/broadcast_pmtu.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 332f38761..f4aa94588 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -112,6 +112,7 @@ 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
diff --git a/tools/testing/selftests/net/broadcast_pmtu.sh b/tools/testing/selftests/net/broadcast_pmtu.sh
new file mode 100755
index 000000000..726eb5d25
--- /dev/null
+++ b/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 $?
+
-- 
2.39.5