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

[Paolo Abeni <pabeni@redhat.com>]

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.

Thanks,

Paolo