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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B70F0@saturn3.aculab.com>
Date:	Mon, 17 Dec 2012 09:59:01 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Nicolas Dichtel" <nicolas.dichtel@...nd.com>, <tgraf@...g.ch>
Cc:	<netdev@...r.kernel.org>, <davem@...emloft.net>
Subject: RE: [PATCH] netlink: align attributes on 64-bits

> -	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?

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

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

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