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

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Date:	Tue, 18 Dec 2012 09:19:04 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	<nicolas.dichtel@...nd.com>
Cc:	<bhutchings@...arflare.com>, <tgraf@...g.ch>,
	<netdev@...r.kernel.org>, <davem@...emloft.net>
Subject: RE: [PATCH v2] netlink: align attributes on 64-bits

> Le 17/12/2012 18:06, David Laight a écrit :
> >>   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.
> As said in the commit log, I want to align all attributes. An attribute can be
> like this:
> 
> struct foo {
> 	__u32 bar1;
> 	__u32 bar2;
> 	__u64 bar3;
> }
> 
> nla_put() don't know what is contained in the attribute.

Put there is no need to 8-byte align something whose size isn't a
multiple of 8 bytes.

> > ...
> >> +	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.

> I thought if you were to research why we use 0, you would know that the first 0
> is the type and the second is the payload size...

I can tell that one is the type and the other the size, you've
implied that the 'type+size' actually total 4 bytes.
I don't need to find out which is which!
Now you've told me I'd have written:
	_nla_reserve(skb, 0, align - NLA_HDRLEN);

The compiler could well have tracked the value - so know it is 4.
OTOH you might want to generate the size of 'align' without
using a conditional.

	David



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