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

On Thu, Aug 06, 2026 at 06:28:00PM +0200, Paolo Abeni wrote:

On 8/6/26 11:06 AM, zhilin zou wrote:

quoted

On Thu, Aug 6, 2026 at 4:52 PM Ido Schimmel [off-list ref] wrote:

quoted

On Wed, Aug 05, 2026 at 05:29:02PM -0700, Jakub Kicinski wrote:

quoted

On Wed, 5 Aug 2026 12:07:21 +0300 Ido Schimmel wrote:

quoted

__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.

Thanks, Ido. I'll add a note to the commit message explaining that
ip6gretap and ip6erspan clear IFF_TX_SKB_SHARING and therefore do not
expect shared skbs, and send a v2 with your Reviewed-by tag retained.

I'm not sure if it's too late, but since a v2 is required, I think it
would be better to keep the skb_shared check: that will avoid a later
patch for the pktgen/shared issue that inevitably will land, possibly
via the security channel.

What do you mean by "keep the skb_shared check"? Return an error if the
skb is shared before calling skb_cow_head()? How would that avoid "a
later patch for the pktgen/shared issue" when other tunnels are already
calling skb_cow_head() without an skb_shared() check (they clear
IFF_TX_SKB_SHARING)?

Not saying that there isn't a possible problem with pktgen, but it
should be fixed there and not papered over in various drivers.