Re: [B.A.T.M.A.N.] [PATCH 1/9] batman-adv: add UNICAST_4ADDR packet type

From: Sven Eckelmann <sven@narfation.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.