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

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Date:	Mon, 17 Dec 2012 17:06:59 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Nicolas Dichtel" <nicolas.dichtel@...nd.com>,
	<bhutchings@...arflare.com>
Cc:	<tgraf@...g.ch>, <netdev@...r.kernel.org>, <davem@...emloft.net>
Subject: RE: [PATCH v2] netlink: align attributes on 64-bits

>  int nla_put(struct sk_buff *skb, int attrtype, int attrlen, const void *data)
>  {
> -	if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen)))
> +	int align = IS_ALIGNED((unsigned long)skb_tail_pointer(skb), 8) ? 0 : 4;

I've just realised where you are adding this!
You only want to add pad if the attribute is a single 64bit item,
not whenever the destination is misaligned.

Eg what happens if you add a 4-byte item after an 8 byte one.

Are there are attributes that consist of a pair of 4 byte values?

...
> +	if (align) {
> +		/* Goal is to add an attribute with size 4. We know that
> +		 * NLA_HDRLEN is 4, hence payload is 0.
> +		 */
> +		__nla_reserve(skb, 0, 0);

One of those zeros should be 'align - 4', then the comment
can be more descriptive.

	David



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