netdev - Re: [PATCH v2] netlink: align attributes on 64-bits

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Date:	Tue, 18 Dec 2012 17:11:03 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	nicolas.dichtel@...nd.com, bhutchings@...arflare.com,
	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v2] netlink: align attributes on 64-bits

On 12/18/12 at 04:50pm, David Laight wrote:
> > 2/ Suppose that the attribute is:
> > 
> >    struct foo {
> >    	__u64 bar1;
> >    	__u32 bar2;
> >    }
> >    => sizeof(struct foo) = 12 (= payload)
> 
> That is only true if the host architecture aligns 64bit items
> on 32 it boundaries (as i386 does).
> Otherwise there are 4 bytes of padding at the end and the
> size is 16.
> 
> Actually it is worse than that.
> Consider the structure:
> 	struct bar {
> 		__u32 foo1;
> 		__u64 foo2;
> 	}
> On i386 it will have size 12 and foo2 will be at offset 4.
> On sparc32 (and most 64bit) it will have size 16 with foo2
> at offset 8 (and 4 bytes of pad after foo1).

This is a known problem and I can't think of anything
that can be done about it except for memcpy()ing the
data before accessing it.

If you have ideas, I'm more that willing to listen :)

> Do these messages move between systems?
> If they do then any 64bit items need an explicit alignment
> eg tag with __attribute__((aligned(8))) (or aligned(4)).

They don't. Netlink has and will be host bound. It also
uses host byte order for that reason.
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