linux-kernel - Re: [PATCH net] selftests: net: csum: Fix checksums for packets with non-zero padding

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Message-ID: <66df9a6d42871_81fd3294e8@willemb.c.googlers.com.notmuch>
Date: Mon, 09 Sep 2024 21:01:33 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jakub Kicinski <kuba@...nel.org>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>, 
 Sean Anderson <sean.anderson@...ux.dev>, 
 "David S . Miller" <davem@...emloft.net>, 
 Paolo Abeni <pabeni@...hat.com>, 
 netdev@...r.kernel.org, 
 Willem de Bruijn <willemb@...gle.com>, 
 linux-kernel@...r.kernel.org, 
 Shuah Khan <shuah@...nel.org>, 
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net] selftests: net: csum: Fix checksums for packets with
 non-zero padding

Jakub Kicinski wrote:
> On Mon, 09 Sep 2024 13:26:42 -0400 Willem de Bruijn wrote:
> > > This seems to be a bug in the driver.
> > > 
> > > A call to skb_put_padto(skb, ETH_ZLEN) should be added.  
> > 
> > In which case this test detecting it may be nice to have, for lack of
> > a more targeted test.
> 
> IIUC we're basically saying that we don't need to trim because pad
> should be 0? In that case maybe let's keep the patch but add a check 
> on top which scans the pad for non-zero bytes, and print an informative
> warning?

Data arriving with padding probably deserves a separate test.

We can use this csum test as stand-in, I suppose.

Is it safe to assume that all padding is wrong on ingress, not just
non-zero padding. The ip stack itself treats it as benign and trims
the trailing bytes silently.

I do know of legitimate cases of trailer data lifting along.