commit 81c734dae203 ("ip6_tunnel: use skb_vlan_inet_prepare() in
__ip6_tnl_rcv()") was fine in and of itself, but its backport to 6.12
(and 6.6) broke IPv4-in-IPv6 tunneling, see [1]. This adds a self-test
for basic IPv4-in-IPv6 and IPv6-in-IPv6 functionality.

[1]: https://lore.kernel.org/all/CAA2RiuSnH_2xc+-W6EnFEG00XjS-dszMq61JEvRjcGS31CBw=g@mail.gmail.com/

--
Thanks for the review, Hangbin!
---
 tools/testing/selftests/net/Makefile      |  1 +
 tools/testing/selftests/net/ip6_tunnel.sh | 43 +++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 tools/testing/selftests/net/ip6_tunnel.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 45c4ea381bc36..5037a344ad826 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -43,6 +43,7 @@ TEST_PROGS := \
 	io_uring_zerocopy_tx.sh \
 	ioam6.sh \
 	ip6_gre_headroom.sh \
+	ip6_tunnel.sh \
 	ip_defrag.sh \
 	ip_local_port_range.sh \
 	ipv6_flowlabel.sh \
diff --git a/tools/testing/selftests/net/ip6_tunnel.sh b/tools/testing/selftests/net/ip6_tunnel.sh
new file mode 100644
index 0000000000000..4886618fd2689
--- /dev/null
+++ b/tools/testing/selftests/net/ip6_tunnel.sh
@@ -0,0 +1,43 @@
+#!/bin/bash
+# Test that IPv4-over-IPv6 tunneling works.
+
+source lib.sh
+
+setup_prepare() {
+  ip link add transport1 type veth peer name transport2
+
+  setup_ns ns1
+  ip link set transport1 netns $ns1
+  ip -n $ns1 address add 2001:db8::1/64 dev transport1 nodad
+  ip -n $ns1 address add 2001:db8::3/64 dev transport1 nodad
+  ip -n $ns1 link set transport1 up
+  ip -n $ns1 link add link transport1 name tunnel4 type ip6tnl mode ipip6 local 2001:db8::1 remote 2001:db8::2
+  ip -n $ns1 address add 172.0.0.1/32 peer 172.0.0.2/32 dev tunnel4
+  ip -n $ns1 link set tunnel4 up
+  ip -n $ns1 link add link transport1 name tunnel6 type ip6tnl mode ip6ip6 local 2001:db8::3 remote 2001:db8::4
+  ip -n $ns1 address add 2001:db8:6::1/64 dev tunnel6
+  ip -n $ns1 link set tunnel6 up
+
+  setup_ns ns2
+  ip link set transport2 netns $ns2
+  ip -n $ns2 address add 2001:db8::2/64 dev transport2 nodad
+  ip -n $ns2 address add 2001:db8::4/64 dev transport2 nodad
+  ip -n $ns2 link set transport2 up
+  ip -n $ns2 link add link transport2 name tunnel4 type ip6tnl mode ipip6 local 2001:db8::2 remote 2001:db8::1
+  ip -n $ns2 address add 172.0.0.2/32 peer 172.0.0.1/32 dev tunnel4
+  ip -n $ns2 link set tunnel4 up
+  ip -n $ns2 link add link transport2 name tunnel6 type ip6tnl mode ip6ip6 local 2001:db8::4 remote 2001:db8::3
+  ip -n $ns2 address add 2001:db8:6::2/64 dev tunnel6
+  ip -n $ns2 link set tunnel6 up
+}
+
+cleanup() {
+  cleanup_all_ns
+  # in case the namespaces haven't been set up yet
+  ip link delete transport1 || true
+}
+
+trap cleanup EXIT
+setup_prepare
+ip netns exec $ns1 ping -W1 -c1 172.0.0.2
+ip netns exec $ns1 ping -W1 -c1 2001:db8:6::2
-- 
2.52.0