netdev - Re: [PATCH] selftests/net: add test for IPv4-in-IPv6 tunneling

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Message-Id: <DGAR3PO1D33S.1TI7W7U4SPTOG@suse.com>
Date: Mon, 09 Feb 2026 18:32:25 -0300
From: Ricardo B. Marlière <rbm@...e.com>
To: "Linus Heckemann" <git@...alerite.org>, <edumazet@...gle.com>
Cc: <davem@...emloft.net>, <eric.dumazet@...il.com>, <horms@...nel.org>,
 <kuba@...nel.org>, <morikw2@...il.com>, <netdev@...r.kernel.org>,
 <pabeni@...hat.com>,
 <syzbot+d4dda070f833dc5dc89a@...kaller.appspotmail.com>
Subject: Re: [PATCH] selftests/net: add test for IPv4-in-IPv6 tunneling

On Sun Feb 8, 2026 at 11:46 AM -03, Linus Heckemann wrote:
> 81c734dae203757fb3c9eee6f9896386940776bd 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 functionality.
>
> [1]: https://lore.kernel.org/all/CAA2RiuSnH_2xc+-W6EnFEG00XjS-dszMq61JEvRjcGS31CBw=g@mail.gmail.com/

I used this script to test the regression in a clean v6.12 tree with
commit 81c734dae203 ("ip6_tunnel: use skb_vlan_inet_prepare() in
__ip6_tnl_rcv()") applied on top:


# ../ip6_tunnel.sh
PING 172.0.0.2 (172.0.0.2) 56(84) bytes of data.

--- 172.0.0.2 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms


dropwatch shows:

1 drops at __ip6_tnl_rcv+74 (0xffffffffc05c7064) [software]


Now, when also applying commit 4b406f814817 ("net: tunnel: make
skb_vlan_inet_prepare() return drop reasons") on top of that, the issue
goes away. Is there a way to trigger stable to pick this up?

> ---
>  tools/testing/selftests/net/Makefile      |  1 +
>  tools/testing/selftests/net/ip6_tunnel.sh | 41 +++++++++++++++++++++++
>  2 files changed, 42 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..366f4c06cd6a3
> --- /dev/null
> +++ b/tools/testing/selftests/net/ip6_tunnel.sh
> @@ -0,0 +1,41 @@
> +#!/bin/bash
> +# Test that IPv4-over-IPv6 tunneling works.
> +
> +set -e
> +
> +setup_prepare() {
> +  ip link add transport1 type veth peer name transport2
> +
> +  ip netns add ns1
> +  ip link set transport1 netns ns1
> +  ip netns exec ns1 bash <<EOF
> +  set -e
> +  ip address add 2001:db8::1/64 dev transport1 nodad
> +  ip link set transport1 up
> +  ip link add link transport1 name tunnel1 type ip6tnl mode ipip6 local 2001:db8::1 remote 2001:db8::2
> +  ip address add 172.0.0.1/32 peer 172.0.0.2/32 dev tunnel1
> +  ip link set tunnel1 up
> +EOF
> +
> +  ip netns add ns2
> +  ip link set transport2 netns ns2
> +  ip netns exec ns2 bash <<EOF
> +  set -e
> +  ip address add 2001:db8::2/64 dev transport2 nodad
> +  ip link set transport2 up
> +  ip link add link transport2 name tunnel2 type ip6tnl mode ipip6 local 2001:db8::2 remote 2001:db8::1
> +  ip address add 172.0.0.2/32 peer 172.0.0.1/32 dev tunnel2
> +  ip link set tunnel2 up
> +EOF
> +}
> +
> +cleanup() {
> +  ip netns delete ns1
> +  ip netns delete ns2
> +  # in case the namespaces haven't been set up yet
> +  ip link delete transport1
> +}
> +
> +trap cleanup EXIT
> +setup_prepare
> +ip netns exec ns1 ping -W1 -c1 172.0.0.2