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

[Ido Schimmel <idosch@nvidia.com>]

On Thu, Aug 06, 2026 at 06:28:00PM +0200, Paolo Abeni wrote:
> On 8/6/26 11:06 AM, zhilin zou wrote:
> > On Thu, Aug 6, 2026 at 4:52 PM Ido Schimmel <idosch@nvidia.com> wrote:
> >>
> >> 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.
> > 
> > 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.