netdev - RE: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get

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Date:	Mon, 13 Oct 2014 08:32:10 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'David Miller' <davem@...emloft.net>,
	"alexander.h.duyck@...hat.com" <alexander.h.duyck@...hat.com>
CC:	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"alexander.duyck@...il.com" <alexander.duyck@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH] flow-dissector: Fix alignment issue in
 __skb_flow_get_ports

From: David Miller
> From: Alexander Duyck <alexander.h.duyck@...hat.com>
> Date: Fri, 10 Oct 2014 09:50:17 -0700
> 
> > If I just use get_unaligned that is pretty easy in terms of cleanup
> > for the ports and IPv4 addresses, the IPv6 will still be a significant
> > hurdle to overcome though.
> 
> Actually, it's not that simple.
> 
> When the compiler sees things like "th->doff" it will load the 32-bit
> word that 4-bit field contains and extract the value using shifts and
> masking.
> 
> So we might need to sprinkle a "attribute((packed))" here and there
> to make it work.

Marking a structure 'packed' forces the compiler to generate byte accesses.
It is enough to mark the 32bit members with __attribute__((aligned(2)))
(or use a typedef for u32 that includes that attribute).
Then the compiler will use 16bit accesses for that field.

	David



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