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

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Message-ID: <47e7fb60-9e61-419f-ba22-8f3c5337627b@redhat.com>
Date: Thu, 10 Jul 2025 10:58:24 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Felix Fietkau <nbd@....name>, netdev@...r.kernel.org,
 Eric Dumazet <edumazet@...gle.com>, Neal Cardwell <ncardwell@...gle.com>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, "David S. Miller"
 <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
 Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
 Willem de Bruijn <willemb@...gle.com>,
 Richard Gobert <richardbgobert@...il.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: fix segmentation after TCP/UDP fraglist GRO

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