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

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Date:	Fri, 14 Dec 2012 17:04:01 +0100
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	tgraf@...g.ch, netdev@...r.kernel.org, davem@...emloft.net,
	David.Laight@...LAB.COM
Subject: Re: [PATCH] netlink: align attributes on 64-bits

Le 14/12/2012 16:49, Ben Hutchings a écrit :
> On Fri, 2012-12-14 at 14:16 +0100, Nicolas Dichtel wrote:
>> On 64 bits arch, we must ensure that attributes are always aligned on 64-bits
>> boundary. We do that by adding attributes of type 0, size 4 (alignment on
>> 32-bits is already done) when needed. Attribute type 0 should be available and
>> unused in all netlink families.
> [...]
>> --- a/lib/nlattr.c
>> +++ b/lib/nlattr.c
>> @@ -450,9 +450,18 @@ EXPORT_SYMBOL(__nla_put_nohdr);
>>    */
>>   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), sizeof(void *)) ? 0 : 4;
>
> The assumption here is that nothing needs to be aligned to a greater
> width than that of a pointer.  However, for most 32-bit architectures
> (i386 being an exception) the C ABI requires 64-bit alignment for 64-bit
> types.  There may be cases where a mostly 32-bit processor really
> requires 64-bit alignment, e.g. to load or save a pair of registers.
Ok, I will wait other comments to send a v2 (which will align these attributes 
for all arch).

Nicolas
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