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

Le 17/12/2012 10:59, David Laight a écrit :

quoted

-	if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen)))
+	int align = IS_ALIGNED((unsigned long)skb_tail_pointer(skb), sizeof(void *)) ? 0 : 4;
+
+	if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen) + align))
  		return -EMSGSIZE;

+	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);
+	}
+

Shouldn't the size of the dummy parameter be based on the value
of 'align' - and that be based on the amount of padding needed?

Align is 4 or 0. Instead of the comment and 0, I can put 'NLA_HDRLEN - align', 
which will always be 0, because we made this patch because we don't want to 
change values like NLA_HDRLEN, because many user apps have these values 
/structures hardcoded.

That aligns the write pointer, what guarantees the alignment of
the start of the buffer - so that the reader will find aligned data?

As Thomas said, skb->head will be aligned, am I wrong?

What guarantees that the reader will read the data into an
8-byte aligned buffer.

There is also the lurking issue of items that require more
than 8-byte alignment.
(x86/amd64 requires 16-byte alignment for 16-byte SSE2 regs and
32-byte alignment for the AVX regs.)

Will anyone ever want to put such items into a netlink message?

	David