selftests: net: add test case for NAT46 looping back dst

Simple test for crash involving multicast loopback and stale dst.
Reuse exising NAT46 program.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250610001245.1981782-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/Makefile

@@ -27,6 +27,7 @@ TEST_PROGS += amt.sh
 TEST_PROGS += unicast_extensions.sh
 TEST_PROGS += udpgro_fwd.sh
 TEST_PROGS += udpgro_frglist.sh
+TEST_PROGS += nat6to4.sh
 TEST_PROGS += veth.sh
 TEST_PROGS += ioam6.sh
 TEST_PROGS += gro.sh

tools/testing/selftests/net/nat6to4.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+NS="ns-peer-$(mktemp -u XXXXXX)"
+
+ip netns add "${NS}"
+ip -netns "${NS}" link set lo up
+ip -netns "${NS}" route add default via 127.0.0.2 dev lo
+
+tc -n "${NS}" qdisc add dev lo ingress
+tc -n "${NS}" filter add dev lo ingress prio 4 protocol ip \
+   bpf object-file nat6to4.bpf.o section schedcls/egress4/snat4 direct-action
+
+ip netns exec "${NS}" \
+   bash -c 'echo 012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789abc | socat - UDP4-DATAGRAM:224.1.0.1:6666,ip-multicast-loop=1'