Subject: Re: [PATCH net-next 1/2] l2tp: account for IP version in SKB headroom

From: Jakub Kicinski <[email-masked]>
Date: Sat, 28 Feb 2026 07:35:28 -0800

On Fri, 27 Feb 2026 23:07:37 +0100 David Bauer wrote:
> Account for the IP version of the tunnel when accounting skb headroom on
> xmit. This avoids having to potentially copy the skb a second time down
> the stack due to allocating not enough space for IPv6 headers in case
> the tunnel uses IPv6.

This appears to crash immediately in the net/l2tp.sh selftest:

# selftests: net: l2tp.sh
# 0.27 [+0.27] ./l2tp.sh: line 138: 22310 Killed                     ip -netns $host_1 l2tp add session name l2tp4 tunnel_id 1041 session_id 1041 peer_session_id 1042
not ok 1 selftests: net: l2tp.sh # exit=137

[ 1585.224860] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 1585.224891] RIP: 0010:l2tp_eth_create+0xf2/0x300 [l2tp_eth]
[ 1585.225362]  <TASK>
[ 1585.225377]  l2tp_nl_cmd_session_create+0x204/0x470 [l2tp_netlink]
-- 
pw-bot: cr