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

Le 18/12/2012 10:19, David Laight a écrit :

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Le 17/12/2012 18:06, David Laight a écrit :

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   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.

Even if you cast the structure in a buffer and read bar3 (without any memcpy 
before)?

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...

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+	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.

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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.

Yes, it is the goal ;-)