netdev - Re: [PATCH] rtnl_wilddump_request: fix alignment issue for embedded platforms

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Message-ID: <20120830160948.GA26387@lutz.bln.innominate.local>
Date:	Thu, 30 Aug 2012 18:09:48 +0200
From:	Lutz Jaenicke <ljaenicke@...ominate.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] rtnl_wilddump_request: fix alignment issue for
 embedded platforms

On Thu, Aug 30, 2012 at 04:51:49PM +0100, David Laight wrote:
> > +		/* attribute has to be NLMSG aligned */
> > +		struct rtattr ext_req
> __attribute__((aligned(NLMSG_ALIGNTO)));
> 
> Would it be better to apply the attribute to the definition
> of 'struct rtattr' itself ?

'struct rtattr' is defined in a kernel header and I am not really sure
what kind of side effects it would have.

The problem I intend to solve with my page is rather caused by the use case:
The request is sent as a single structure, the processing is however
performed step by step: first the nlmsg is parsed, then the pointer
is moved on to the parsing of the content. This pointer is hoever not
oriented at the structure element but by using size information which
is then adjust by NLMSG_ALIGN() etc. Consequently one should actually
use the same method in sending (generate the header first, then addattr_()
which enforces protocol conform alignment). This is the way all other
instances in iproute2 generate their messages btw.

I hence consider my proposed patch to be the least intrusive solution.

Best regards,
	Lutz
-- 
Dr.-Ing. Lutz Jänicke
CTO
Innominate Security Technologies AG  /protecting industrial networks/
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