linux-kernel - Re: [PATCH] net: linearizing skb when downgrade gso

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Message-ID: <668dabd7e7066_1ce27f29435@willemb.c.googlers.com.notmuch>
Date: Tue, 09 Jul 2024 17:29:59 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Herbert Xu <herbert@...dor.apana.org.au>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Fred Li <dracodingfly@...il.com>, 
 aleksander.lobakin@...el.com, 
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 ast@...nel.org, 
 bpf@...r.kernel.org, 
 daniel@...earbox.net, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 haoluo@...gle.com, 
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 john.fastabend@...il.com, 
 jolsa@...nel.org, 
 kpsingh@...nel.org, 
 kuba@...nel.org, 
 linux-kernel@...r.kernel.org, 
 linux@...ssschuh.net, 
 martin.lau@...ux.dev, 
 mkhalfella@...estorage.com, 
 nbd@....name, 
 netdev@...r.kernel.org, 
 pabeni@...hat.com, 
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 song@...nel.org, 
 yonghong.song@...ux.dev
Subject: Re: [PATCH] net: linearizing skb when downgrade gso_size

Herbert Xu wrote:
> On Tue, Jul 09, 2024 at 11:53:21AM -0400, Willem de Bruijn wrote:
> >
> > > +		/* Due to header grow, MSS needs to be downgraded.
> > > +		 * There is BUG_ON When segment the frag_list with
> > > +		 * head_frag true so linearize skb after downgrade
> > > +		 * the MSS.
> > > +		 */
> 
> This sounds completely wrong.  You should never grow the TCP header
> by changing gso_size.  What is the usage-scenario for this?
> 
> Think about it, if a router forwards a TCP packet, and ends up
> growing its TCP header and then splits the packet into two, then
> this router is brain-dead.

This is an unfortunate feature, but already exists.

It decreases gso_size to account for tunnel headers.

For USO, we added BPF_F_ADJ_ROOM_FIXED_GSO to avoid this in better,
newer users.