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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 17 Dec 2012 18:35:24 +0100
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	bhutchings@...arflare.com, tgraf@...g.ch, netdev@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [PATCH v2] netlink: align attributes on 64-bits

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

>
> Eg what happens if you add a 4-byte item after an 8 byte one.
>
> Are there are attributes that consist of a pair of 4 byte values?
>
> ...
>> +	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.
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...
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html