[PATCH net-next v5 2/2] selftests: net: add FOU multicast encapsulation resubmit test

[Anton Danilov <littlesmilingcloud@gmail.com>]

Add a selftest to verify that FOU-encapsulated packets addressed to a
multicast destination are correctly resubmitted to the inner protocol
handler (GRE) via the UDP multicast delivery path.  Both IPv4 and IPv6
paths are tested.

The test creates two network namespaces connected by a veth pair with
a FOU/GRETAP (IPv4) and FOU/ip6gretap (IPv6) tunnel using multicast
remote addresses (239.0.0.1 and ff0e::1).  Ping is sent through each
tunnel and received packets are counted on the receiver's tunnel
interface.

The veth pair is created directly inside the namespaces to avoid
possible name collisions with devices in the root namespace.

Static neighbor entries are configured on the sender because ARP/ND
replies from the receiver cannot traverse the unidirectional multicast
tunnel back to the sender.

The early demux optimization (net.ipv4.ip_early_demux, which controls
both IPv4 and IPv6) is disabled on the receiver to force packets
through __udp4_lib_mcast_deliver() / __udp6_lib_mcast_deliver(), which
is the code path being tested.

Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Assisted-by: Claude:claude-opus-4-6
---
 tools/testing/selftests/net/Makefile          |   1 +
 .../testing/selftests/net/fou_mcast_encap.sh  | 177 ++++++++++++++++++
 2 files changed, 178 insertions(+)
 create mode 100755 tools/testing/selftests/net/fou_mcast_encap.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 708d960ae07d..7e9ae937cffa 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -39,6 +39,7 @@ TEST_PROGS := \
 	fib_rule_tests.sh \
 	fib_tests.sh \
 	fin_ack_lat.sh \
+	fou_mcast_encap.sh \
 	fq_band_pktlimit.sh \
 	gre_gso.sh \
 	gre_ipv6_lladdr.sh \
diff --git a/tools/testing/selftests/net/fou_mcast_encap.sh b/tools/testing/selftests/net/fou_mcast_encap.sh
new file mode 100755
index 000000000000..728513d55db4
--- /dev/null
+++ b/tools/testing/selftests/net/fou_mcast_encap.sh
@@ -0,0 +1,177 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Test that UDP encapsulation (FOU) correctly handles packet resubmit
+# when packets are delivered via the multicast UDP delivery path.
+#
+# When a FOU-encapsulated packet arrives with a multicast destination IP,
+# __udp4_lib_mcast_deliver() / __udp6_lib_mcast_deliver() must resubmit
+# it to the inner protocol handler (e.g., GRE) rather than consuming it.
+# This test verifies both IPv4 and IPv6 paths by creating a FOU/GRETAP
+# tunnel with a multicast remote address and sending ping through it.
+#
+# The early demux optimization can mask this issue by routing packets via
+# the unicast path (udp[6]_unicast_rcv_skb), so we disable it to force
+# packets through the multicast delivery function.
+
+source lib.sh
+
+NSENDER=""
+NRECV=""
+
+FOU_PORT4=4797
+FOU_PORT6=4798
+MCAST4=239.0.0.1
+MCAST6=ff0e::1
+
+TUN4_S=192.168.99.1
+TUN4_R=192.168.99.2
+TUN6_S=2001:db8:99::1
+TUN6_R=2001:db8:99::2
+
+cleanup() {
+	cleanup_all_ns
+}
+
+trap cleanup EXIT
+
+setup_common() {
+	setup_ns NSENDER NRECV
+
+	# Create veth pair directly inside namespaces to avoid name
+	# collisions with devices in the root namespace.
+	ip link add veth_s netns "$NSENDER" type veth \
+		peer name veth_r netns "$NRECV"
+
+	ip -n "$NSENDER" link set veth_s up
+	ip -n "$NRECV" link set veth_r up
+
+	# Same sysctl controls early demux for both IPv4 and IPv6.
+	ip netns exec "$NRECV" sysctl -wq net.ipv4.ip_early_demux=0
+}
+
+setup_ipv4() {
+	ip -n "$NSENDER" addr add 10.0.0.1/24 dev veth_s
+	ip -n "$NRECV" addr add 10.0.0.2/24 dev veth_r
+
+	# Join multicast group on receiver
+	ip -n "$NRECV" addr add "$MCAST4/32" dev veth_r autojoin
+
+	ip -n "$NSENDER" route add 239.0.0.0/8 dev veth_s
+	ip -n "$NRECV" route add 239.0.0.0/8 dev veth_r
+
+	# Sender: GRETAP with FOU encap (no FOU listener needed on TX side)
+	ip -n "$NSENDER" link add eoudp4 type gretap \
+		remote "$MCAST4" local 10.0.0.1 \
+		encap fou encap-sport "$FOU_PORT4" encap-dport "$FOU_PORT4" \
+		key "$MCAST4"
+	ip -n "$NSENDER" link set eoudp4 up
+	ip -n "$NSENDER" addr add "$TUN4_S/24" dev eoudp4
+
+	# Receiver: FOU listener + GRETAP
+	ip netns exec "$NRECV" ip fou add port "$FOU_PORT4" ipproto 47
+	ip -n "$NRECV" link add eoudp4 type gretap \
+		remote "$MCAST4" local 10.0.0.2 \
+		encap fou encap-sport "$FOU_PORT4" encap-dport "$FOU_PORT4" \
+		key "$MCAST4"
+	ip -n "$NRECV" link set eoudp4 up
+	ip -n "$NRECV" addr add "$TUN4_R/24" dev eoudp4
+
+	# Static neigh on sender: ARP replies cannot traverse the
+	# unidirectional multicast tunnel.
+	local recv_mac
+	recv_mac=$(ip -n "$NRECV" link show eoudp4 | awk '/ether/{print $2}')
+	ip -n "$NSENDER" neigh add "$TUN4_R" lladdr "$recv_mac" dev eoudp4
+}
+
+setup_ipv6() {
+	# Skip cleanly if IPv6 is not available in the running kernel.
+	[ -e /proc/sys/net/ipv6 ] || return "$ksft_skip"
+	modprobe -q fou6 || return "$ksft_skip"
+
+	ip -n "$NSENDER" addr add 2001:db8::1/64 dev veth_s nodad
+	ip -n "$NRECV" addr add 2001:db8::2/64 dev veth_r nodad
+
+	# Join multicast group on receiver
+	ip -n "$NRECV" addr add "$MCAST6/128" dev veth_r autojoin
+
+	ip -n "$NSENDER" -6 route add ff00::/8 dev veth_s
+	ip -n "$NRECV" -6 route add ff00::/8 dev veth_r
+
+	# Sender: ip6gretap with FOU encap
+	ip -n "$NSENDER" link add eoudp6 type ip6gretap \
+		remote "$MCAST6" local 2001:db8::1 \
+		encap fou encap-sport "$FOU_PORT6" encap-dport "$FOU_PORT6" \
+		key 42
+	ip -n "$NSENDER" link set eoudp6 up
+	ip -n "$NSENDER" addr add "$TUN6_S/64" dev eoudp6 nodad
+
+	# Receiver: FOU listener (IPv6) + ip6gretap
+	ip netns exec "$NRECV" ip fou add port "$FOU_PORT6" ipproto 47 -6
+	ip -n "$NRECV" link add eoudp6 type ip6gretap \
+		remote "$MCAST6" local 2001:db8::2 \
+		encap fou encap-sport "$FOU_PORT6" encap-dport "$FOU_PORT6" \
+		key 42
+	ip -n "$NRECV" link set eoudp6 up
+	ip -n "$NRECV" addr add "$TUN6_R/64" dev eoudp6 nodad
+
+	# Static neigh on sender: neighbor discovery cannot traverse the
+	# unidirectional multicast tunnel.
+	local recv_mac
+	recv_mac=$(ip -n "$NRECV" link show eoudp6 | awk '/ether/{print $2}')
+	ip -n "$NSENDER" neigh add "$TUN6_R" lladdr "$recv_mac" dev eoudp6
+}
+
+get_rx_packets() {
+	local dev="$1"
+
+	ip -n "$NRECV" -s link show "$dev" | awk '/RX:/{getline; print $2}'
+}
+
+run_ping_test() {
+	local family="$1"
+	local dev="$2"
+	local dst="$3"
+	local count=100
+	local rx_before rx_after rx_delta
+
+	# Warmup: let any initial broadcast/ND traffic settle
+	ip netns exec "$NSENDER" ping "$family" -c 1 -W 1 "$dst" \
+		>/dev/null 2>&1
+	sleep 1
+
+	rx_before=$(get_rx_packets "$dev")
+	ip netns exec "$NSENDER" ping "$family" -c $count -W 1 "$dst" \
+		>/dev/null 2>&1
+	sleep 1
+	rx_after=$(get_rx_packets "$dev")
+
+	rx_delta=$((rx_after - rx_before))
+
+	if [ "$rx_delta" -ge "$count" ]; then
+		echo "PASS: received $rx_delta/$count packets"
+		return "$ksft_pass"
+	elif [ "$rx_delta" -gt 0 ]; then
+		echo "FAIL: only $rx_delta/$count packets received"
+		return "$ksft_fail"
+	else
+		echo "FAIL: 0/$count packets received"
+		return "$ksft_fail"
+	fi
+}
+
+ret=0
+
+echo "TEST: FOU/GRETAP IPv4 multicast encapsulation resubmit"
+setup_common
+setup_ipv4
+run_ping_test -4 eoudp4 "$TUN4_R" || ret=$?
+
+echo "TEST: FOU/GRETAP IPv6 multicast encapsulation resubmit"
+if setup_ipv6; then
+	run_ping_test -6 eoudp6 "$TUN6_R" || ret=$?
+else
+	echo "SKIP: IPv6 unavailable"
+fi
+
+exit $ret
-- 
2.47.3