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

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Message-Id: <20110103.121336.212692696.davem@davemloft.net>
Date:	Mon, 03 Jan 2011 12:13:36 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...ux-foundation.org
Cc:	xiaosuo@...il.com, 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: Stephen Hemminger <shemminger@...ux-foundation.org>
Date: Mon, 3 Jan 2011 10:15:33 -0800

> On Mon, 03 Jan 2011 09:22:14 -0800 (PST)
> David Miller <davem@...emloft.net> wrote:
> 
>> 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.
> 
> Because PPPOE happens afterwards and is not part the calculation.
> The check should be moved until after skb has reached final form.

That works for me.
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