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

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Message-Id: <DGAS4219YAVM.1FO5C2440EBYZ@suse.com>
Date: Mon, 09 Feb 2026 19:19:53 -0300
From: Ricardo B. Marlière <rbm@...e.com>
To: Ricardo B. Marlière <rbm@...e.com>, "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 Mon Feb 9, 2026 at 6:32 PM -03, Ricardo B. Marlière wrote:
> 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

oops, that would be 9990ddf47d41 ("net: tunnel: make skb_vlan_inet_prepare() return drop reasons")

> 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