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

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Message-ID: <Zo2auA2r/hkJWWcs@gondor.apana.org.au>
Date: Wed, 10 Jul 2024 08:16:56 +1200
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Fred Li <dracodingfly@...il.com>, aleksander.lobakin@...el.com,
	andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
	daniel@...earbox.net, davem@...emloft.net, edumazet@...gle.com,
	haoluo@...gle.com, hawk@...nel.org, 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, sashal@...nel.org,
	sdf@...gle.com, song@...nel.org, yonghong.song@...ux.dev
Subject: Re: [PATCH] net: linearizing skb when downgrade gso_size

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.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt