Re: [PATCH net v2 1/1] ip6_tunnel: snapshot encap in xmit

On Sat, Aug 08, 2026 at 07:38:52PM +0000, Kuniyuki Iwashima wrote:

From: Ren Wei <redacted>
Date: Sat,  8 Aug 2026 16:40:49 +0800

quoted

From: Zixuan Chai <redacted>

ip6_tnl_changelink() can update encapsulation parameters while the
netdevice is transmitting packets. ip6_tnl_xmit() can calculate packet
headroom with t->encap_hlen and later build an encapsulation header from
the live t->encap. A concurrent update can change the encapsulation
header between these accesses and make skb_push() underflow the skb head.

Take a local snapshot of t->encap before calculating the encapsulation
header length.

This intorduce per-skb cost in the fast path for unlikely changelink.

Assuming the common case where the tunnel doesn't use fou / gue
encapsulation, the added cost is one compare and a copy of 8 bytes.

Right approach is to convert it to RCU pointer (and remove
synchronize_net() there).

0ba269933f73 geneve: convert config to RCU-protected pointer
777434f53e77 geneve: pass geneve_config pointer to helper functions

It's on my TODO list since last week, but I don't have the time to work
on it right now. It's a very large change (see the geneve change) that
is needed across all the IP tunnels, not something that I consider
suitable for net.

What are you proposing for net?