Re: [Openvpn-devel] [PATCH ovpn-net-next] ovpn: reset MAC header before passing skb up

Antonio Quartulli
2026-04-28T09:36:36Z

Hi,

On 28/04/2026 04:08, Qingfang Deng wrote:
> Hi,
>
> On 2026/4/27 17:45, Antonio Quartulli wrote:
>> Hi Qingfang,
>>
>> thanks for the patch!
>>
>> On 27/04/2026 06:00, Qingfang Deng wrote:
>>> After decapsulating a packet, the skb->mac_header still points to the
>>> outer transport header. Call skb_reset_mac_header() in
>>> ovpn_netdev_write() to ensure the MAC header points to the beginning of
>>> the inner IP packet.
>>
>> May you elaborate on what this is exactly fixing?
>> Did you encounter a bug triggered by this missing line?
>>
>> I am asking because I wonder what is "expected" as MAC header for a
>> packet not having one at all (packets delivered to the ovpn interface
>> are L3 only, as per the interface type itself).
>
> For L3-only devices, the net core expects skb->mac_header == skb-
> >network_header.
>
> For example, in __netif_receive_skb_core(), skb_reset_mac_len() sets
> skb->mac_len to (skb->network_header - skb->mac_header).
> If skb->mac_header still has a stale value, this will incorrectly assign
> a non-zero value to skb->mac_len.
>
> Also, if generic XDP or SOCK_PACKET is used, either will do
>   skb_push(skb, skb->data - skb_mac_header(skb));

Thanks a lot! This makes sense!

I'm applying the patch to my tree.

Regards,

--
Antonio Quartulli