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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B70F5@saturn3.aculab.com>
Date:	Tue, 18 Dec 2012 16:50:52 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	<nicolas.dichtel@...nd.com>, "Thomas Graf" <tgraf@...g.ch>
Cc:	<bhutchings@...arflare.com>, <netdev@...r.kernel.org>,
	<davem@...emloft.net>
Subject: RE: [PATCH v2] netlink: align attributes on 64-bits

> 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).

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)).

	David



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