Re: [PATCH net] net: fix segmentation after TCP/UDP fraglist GRO

[Paolo Abeni <pabeni@redhat.com>]

On 7/6/25 3:45 PM, Willem de Bruijn wrote:
> Felix Fietkau wrote:
>> Since "net: gro: use cb instead of skb->network_header", the skb network
>> header is no longer set in the GRO path.
>> This breaks fraglist segmentation, which relies on ip_hdr()/tcp_hdr()
> 
> Only ip_hdr is in scope.
> 
> Reviewing TCP and UDP GSO, tcp_hdr/transport header is used also
> outside segment list. Non segment list GSO also uses ip_hdr in case
> pseudo checksum needs to be set.
> 
> The GSO code is called with skb->data at the relevant header, so L4
> helpers are not strictly needed. The main issue is that data will be
> at the L4 header, and some GSO code also needs to see the IP header
> (e.g., for aforementioned pseudo checksum calculation).
> 
>> to check for address/port changes.
> 
> If in GSO, then the headers are probably more correctly set at the end
> of GRO, in gro_complete.

+1 on setting the headers at GSO time.

> The blamed commit was added to support tunneling. It's not obvious
> that unconditionally setting network header again, instead of inner
> network header, will break that.

I think this actually breaks tunneled use-case, when the aggregated
packet is forwarded to an output device before traversing the relevant
tunnel.

/P