udp: fix FOU/GUE over multicast

When a UDP encapsulation socket (e.g., FOU) receives a multicast packet, __udp4_lib_mcast_deliver() and __udp6_lib_mcast_deliver() call consume_skb() when udp_queue_rcv_skb() returns a positive value. A positive return value from udp_queue_rcv_skb() indicates that the encap_rcv handler (e.g., fou_udp_recv) has consumed the UDP header and wants the packet to be resubmitted to the IP protocol handler for further processing (e.g., as a GRE packet).

The unicast paths handle this correctly by propagating the return value up to ip_protocol_deliver_rcu() / ip6_protocol_deliver_rcu() for resubmission. However, the multicast paths destroy the packet via consume_skb() instead of resubmitting it, causing silent packet loss.

This affects any UDP encapsulation (FOU, GUE) combined with multicast destination addresses.

Fix this by returning the value from udp_queue_rcv_skb() when it is positive, matching the behavior of the corresponding unicast paths. Note the sign difference between IPv4 and IPv6:

- IPv4: udp_unicast_rcv_skb() returns -ret, and ip_protocol_deliver_rcu() resubmits when ret < 0 (using -ret as the protocol number).
- IPv6: udp6_unicast_rcv_skb() returns ret, and ip6_protocol_deliver_rcu() resubmits when ret > 0 (using ret as the nexthdr).

Both mcast paths now follow the same convention as their respective unicast paths.

Suggested-by Willem de Bruijn <[email protected]>

Suggested-by Kuniyuki Iwashima <[email protected]>

Signed-off-by Anton Danilov <[email protected]>

Assisted-by Claude:claude-opus-4-6

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 <[email protected]>

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

tools/testing/selftests/net/Makefile +1 -0

index 708d960ae07d..7e9ae937cffa 100644
@@ -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 \

tools/testing/selftests/net/fou_mcast_encap.sh new file +80 -0

new file mode 100755
index 000000000000..728513d55db4
@@ -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

tools/testing/selftests/net/fou_mcast_encap.sh new file +22 -0

+
+	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 \

tools/testing/selftests/net/fou_mcast_encap.sh new file +13 -0

+}
+
+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

tools/testing/selftests/net/fou_mcast_encap.sh new file +19 -0

+	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
+

tools/testing/selftests/net/fou_mcast_encap.sh new file +3 -0

+fi
+
+exit $ret

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