Re: [PATCH net 1/1] ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

On Wed, Aug 05, 2026 at 05:29:02PM -0700, Jakub Kicinski wrote:
> On Wed, 5 Aug 2026 12:07:21 +0300 Ido Schimmel wrote:
> > __gre6_xmit() and ip6erspan_tunnel_xmit() already call skb_cow_head()
> > before calling ip6_tnl_xmit().
> 
> Probably just me but can't they all be buggy?
> PKTGEN likes to send shared skbs around in funny ways.
> Can we get a good explanation in the commit msg or maybe let's
> keep the check?

ip6_tnl_xmit() is accessible via two Ethernet devices (pktgen doesn't
support other types) and they both clear IFF_TX_SKB_SHARING, so if
pktgen sends them shared skbs, I would say that it's a pktgen bug and
not a reason to block this patch. Note that pktgen is not available to
unprivileged users, so it's a less severe bug.

The patch also makes ip6_tnl_xmit() consistent with its IPv4 counterpart
(ip_tunnel_xmit()) which is already using skb_cow_head().

Zhiling, please add a note in the commit message that ip6gretap and
ip6erspan do not expect to be handed shared skbs given that they clear
IFF_TX_SKB_SHARING.