netdev - Re: [PATCH net-next] sunvnet: don't change gso data on clones

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Message-ID: <54DB92CD.1070309@oracle.com>
Date:	Wed, 11 Feb 2015 12:35:09 -0500
From:	David L Stevens <david.stevens@...cle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] sunvnet: don't change gso data on clones

On 02/11/2015 08:56 AM, Eric Dumazet wrote:
> On Wed, 2015-02-11 at 08:20 -0500, David L Stevens wrote:
>> This patch unclones an skb for the case where the sunvnet driver needs to
>> change the segmentation size so that it doesn't interfere with TCP SACK's
>> use of them.
>>
>> Signed-off-by: David L Stevens <david.stevens@...cle.com>
>> ---
> 
> Hmm... this would point to a TCP bug ?
> 
> What happens if not GSO is needed, TCP corrupts data currently
> read/processed by NIC/driver ?

I don't think I understand your concern. This problem can result in a
panic using sunvnet because the sunvnet driver is changing the original
skb, which is always, or at least almost always, a clone. TCP uses gso_segs
to track packet counts, so changing it in the driver can result in bad math--
TCP assumes its copy of the clone's data shouldn't change (of course).

A driver that doesn't change the segmentation or original data doesn't
need to care whether it's a clone or not-- it'll free it and drop a
reference. Since sunvnet is changing the gso_size and gso_segs, it needs
to unclone first.

						+-DLS

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