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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B70F6@saturn3.aculab.com>
Date:	Wed, 19 Dec 2012 09:17:41 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Thomas Graf" <tgraf@...g.ch>
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

> > 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.

You can't use memcpy() to copy a pointer to a misaligned
structure into an aligned buffer. The compiler assumes
the pointer is aligned and will use instructions that
depend on the alignment.

> If you have ideas, I'm more that willing to listen :)
> ... Netlink has and will be host bound. It also
> uses host byte order for that reason.

I think:
1) Alignment is only needed on systems that have 'strict alignment'
   requirements (maybe disable for testing?)
2) Alignment is only needed for parameters whose size is a
   multiple of the alignment (a structure containing a
   field that needs 8 byte alignment will always be a multiple
   of 8 bytes long).
3) You need to add NA_HDR_LEN to the write pointer before
   determining the size of the pad.

So a structure of three uint32_t will never need aligning.

	David

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