netdev - Re: [PATCH] net: bridge: check the length of skb after nf_bridge_maybe_copy

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Message-Id: <20110103.092214.193706896.davem@davemloft.net>
Date:	Mon, 03 Jan 2011 09:22:14 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	xiaosuo@...il.com
Cc:	shemminger@...ux-foundation.org, bridge@...ts.linux-foundation.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] net: bridge: check the length of skb after
 nf_bridge_maybe_copy_header()

From: Changli Gao <xiaosuo@...il.com>
Date: Mon, 3 Jan 2011 18:44:59 +0800

> On Sat, Jan 1, 2011 at 3:10 AM, David Miller <davem@...emloft.net> wrote:
>> From: Changli Gao <xiaosuo@...il.com>
>> Date: Sat, 25 Dec 2010 21:41:30 +0800
>>
>>> Since nf_bridge_maybe_copy_header() may change the length of skb,
>>> we should check the length of skb after it to handle the ppoe skbs.
>>>
>>> Signed-off-by: Changli Gao <xiaosuo@...il.com>
>>
>> This is really strange.
>>
>> packet_length() subtracts VLAN_HLEN from the value it returns, so the
>> correct fix seems to be to make this function handle the PPPOE case
>> too.
>>
> 
> It is correct. The actual MTU of 802.1q frame is 4 bytes larger. For
> example, the MTU of ethernet is normally 1500, however the actual MTU
> of the 802.1Q is 1504.

Yes, I understand this, but I don't see why packet_length() cannot
simply account for PPPOE's encapsulation overhead just as it does for
VLAN's special MTU considerations.
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