netdev - Re: [B.A.T.M.A.N.] [PATCH 1/9] batman-adv: add UNICAST

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Message-Id: <20121104.122611.1457246059414191903.davem@davemloft.net>
Date:	Sun, 04 Nov 2012 12:26:11 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	sven@...fation.org
Cc:	b.a.t.m.a.n@...ts.open-mesh.org, ordex@...istici.org,
	netdev@...r.kernel.org
Subject: Re: [B.A.T.M.A.N.] [PATCH 1/9] batman-adv: add UNICAST_4ADDR
 packet type

From: Sven Eckelmann <sven@...fation.org>
Date: Sun, 04 Nov 2012 11:29:29 +0100

> Now also assume following really unusual situation: We get our data from a 
> ethernet driver and the skb stores the ethernet header. The start of the 
> ethernet header is perfectly aligned (4 or even 16 byte boundary aligned). The 
> the header is 14/18 byte long (6 byte src, 6 byte dst, 2 byte ethertype and 
> maybe 4 byte vlan). Now the payload starts only on a 2 byte boundary -> it is 
> never 4 byte boundary aligned. A 32 bit read now causes different variations 
> of problems (reminder: bus error).

Every ethernet driver must provide the networking stack with a modulo 2
aligned ethernet header, which makes sure that everything after the
ethernet header is at least 4 byte aligned.
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