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

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Message-ID: <20110103101533.3f98b311@nehalam>
Date:	Mon, 3 Jan 2011 10:15:33 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
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()

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.

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