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

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Date:	Wed, 19 Dec 2012 17:20:12 +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 12/19/12 at 09:17am, David Laight wrote:
> 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.

I am not sure I understand this correctly. Are you saying
that the following does not work on i386?

struct foo {
  uint32_t a;
  uint64_t b;
};

struct foo buf;

memcpy(&buf, nla_data(attr), nla_len(attr));
printf([...], buf.b);


> I think:
> 1) Alignment is only needed on systems that have 'strict alignment'
>    requirements (maybe disable for testing?)

Right, what about mixed 32bit/64bit environments?

> 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).

Good point. I'll fix this in the next iteration of the patch.

> 3) You need to add NA_HDR_LEN to the write pointer before
>    determining the size of the pad.

Right, I'm doing this in the patch I proposed. Or are you referring
to something else?
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