netdev - Re: [Patch 4/5] Network Drop Monitor: Adding drop monitor implementation & Netlink protocol

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Message-ID: <20090402095957.GA30799@gondor.apana.org.au>
Date:	Thu, 2 Apr 2009 17:59:57 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	kaber@...sh.net, nhorman@...driver.com, zbr@...emap.net,
	netdev@...r.kernel.org, kuznet@....inr.ac.ru, pekkas@...core.fi,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org
Subject: Re: [Patch 4/5] Network Drop Monitor: Adding drop monitor
	implementation & Netlink protocol

On Thu, Apr 02, 2009 at 02:52:23AM -0700, David Miller wrote:
>
> It has to do with how the netlink attributes get packed together
> into the messages.  It's this crap:
> 
> #define NLMSG_ALIGNTO	4
> #define NLMSG_ALIGN(len) ( ((len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )
>  ...
> #define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
> 				  (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
> 
> Thus, nothing in an nlmsg can ever be more than 4 byte aligned.
> And this is hard coded into the protocol.

Ah yes, this is the killer.

I suppose we can modify in-kernel helpers like nla_parse to deal
with that by copying if it's unaligned.  Though we'll have to
figure out how to free the memory.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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