netdev - Re: alignment faults in 3.6

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Message-ID: <yw1xpq4ppcju.fsf@unicorn.mansr.com>
Date:	Thu, 11 Oct 2012 10:45:25 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	"David Laight" <David.Laight@...LAB.COM>
Cc:	Måns Rullgård <mans@...sr.com>,
	"Jon Masters" <jonathan@...masters.org>,
	<linux-arm-kernel@...ts.infradead.org>, <netdev@...r.kernel.org>
Subject: Re: alignment faults in 3.6

"David Laight" <David.Laight@...LAB.COM> writes:

>> -----Original Message-----
>> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On Behalf Of Måns Rullgård
>> Sent: 11 October 2012 03:27
>> To: Jon Masters
>> Cc: linux-arm-kernel@...ts.infradead.org; netdev@...r.kernel.org
>> Subject: Re: alignment faults in 3.6
>> 
>> Jon Masters <jonathan@...masters.org> writes:
>> 
>> > Hi everyone,
>> >
>> > On 10/05/2012 10:33 AM, Rob Herring wrote:
>> >> On 10/05/2012 09:05 AM, Russell King - ARM Linux wrote:
>> >>> On Fri, Oct 05, 2012 at 07:24:44AM -0500, Rob Herring wrote:
>> >>>> On 10/05/2012 03:24 AM, Russell King - ARM Linux wrote:
>> >>>>> Does it matter?  I'm just relaying the argument against adding __packed
>> >>>>> which was used before we were forced (by the networking folk) to implement
>> >>>>> the alignment fault handler.
>> >>>>
>> >>>> It doesn't really matter what will be accepted or not as adding __packed
>> >>>> to struct iphdr doesn't fix the problem anyway. 
> ...
>> There are exactly two possible solutions:
>> 
>> 1. Change the networking code so those structs are always aligned.  This
>>    might not be (easily) possible.
>> 2. Mark the structs __packed and fix any typecasts like the ones seen in
>>    this thread.  This will have an adverse effect in cases where the
>>    structs are in fact aligned.
>> 
>> Both solutions lie squarely in the networking code.  It's time to
>> involve that list, or we'll never get anywhere.
>
> It might be enough to use __attribute__((aligned(2))) on some structure
> members (actually does 'ldm' need 8 byte alignment?? - in which case
> aligned(4) is enough).

The aligned attribute can only increase alignment.

-- 
Måns Rullgård
mans@...sr.com
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