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

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Date:	Wed, 19 Dec 2012 19:07:11 +0100
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Thomas Graf <tgraf@...g.ch>
CC:	bhutchings@...arflare.com, netdev@...r.kernel.org,
	davem@...emloft.net, David.Laight@...LAB.COM
Subject: Re: [PATCH v2] netlink: align attributes on 64-bits

Le 19/12/2012 18:09, Thomas Graf a écrit :
> On 12/19/12 at 12:22pm, Nicolas Dichtel wrote:
>> Here padlen will return 4, which is wrong: padlen + NLA_HDRLEN = 8,
>> alignment is the same than before. Here is a proposal fix:
>>
>> diff --git a/lib/nlattr.c b/lib/nlattr.c
>> index e4f0329..1556313 100644
>> --- a/lib/nlattr.c
>> +++ b/lib/nlattr.c
>> @@ -338,7 +338,10 @@ struct nlattr *__nla_reserve(struct sk_buff
>> *skb, int attrtype, int attrlen)
>>   		struct nlattr *pad;
>>   		size_t padlen;
>>
>> -		padlen = nla_total_size(offset) - offset -  NLA_HDRLEN;
>> +		/* We need to remove NLA_HDRLEN two times: one time for the
>> +		 * attribute hdr and one time for the pad attribute hdr.
>> +		 */
>> +		padlen = nla_total_size(offset) - offset -  2 * NLA_HDRLEN;
>>   		pad = (struct nlattr *) skb_put(skb, nla_attr_size(padlen));
>>   		pad->nla_type = 0;
>>   		pad->nla_len = nla_attr_size(padlen);
>>
>> With this patch, it seems goods. attribute are always aligned on 8 bytes. Also
>> I did not notice any problem with size calculation (I try some ip
>> link, ip xfrm, ip [m]route).
>>
>> Do you want to make more tests? Or will your repost the full patch?
>> I can do it if you don't have time.
>
> Thanks.
>
> I would like to do some testing as well. I do expect some fallout from
> this. There is likely some interface abuse that will now be exposed
> due to this.
>
> We'll have to wait for the next merge window to open anyway. I'd
> consider this a new feature and not a bugfix based on the possible
> regression impact it could have.
>
> I'll post a new version of the patch integrating your fix above so
> others (especially subsystem maintainers depending on netlink) can run
> the patch as well.
>
Ok, sounds good.
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