netdev - Re: [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink

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Date:	Tue, 11 Dec 2012 16:03:57 +0100
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Thomas Graf <tgraf@...g.ch>
CC:	David Miller <davem@...emloft.net>, David.Laight@...LAB.COM,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next 0/7] Allow to monitor multicast cache event via
 rtnetlink

Le 06/12/2012 18:49, Thomas Graf a écrit :
> On 12/06/12 at 09:43am, Nicolas Dichtel wrote:
>> Le 05/12/2012 18:54, David Miller a écrit :
>>> From: "David Laight" <David.Laight@...LAB.COM>
>>> Date: Wed, 5 Dec 2012 11:41:33 -0000
>>>
>>>> Probably worth commenting that the 64bit items might only be 32bit aligned.
>>>> Just to stop anyone trying to read/write them with pointer casts.
>>>
>>> Rather, let's not create this situation at all.
>>>
>>> It's totally inappropriate to have special code to handle every single
>>> time we want to put 64-bit values into netlink messages.
>>>
>>> We need a real solution to this issue.
>>>
>> The easiest way is to update *_ALIGNTO values (maybe we can keep
>> NLMSG_ALIGNTO to 4). But I think that many userland apps have these
>> values hardcoded and, the most important thing, this may increase
>> size of many netlink messages. Hence we need probably to find
>> something better.
>
> We can't do this, as you say, ALIGNTO is compiled into all the
> binaries.
>
> A simple backwards compatible workaround would be to include an
> unknown, empty padding attribute if needed. That would be 4 bytes
> in size and could be used to include padding as needed.
>
> We could use nla_type = 0 as it is a reserved value that should
> be available in all protocols. All readers (kernel and user space)
> must ignore such an attribute just like any other unknown
> attribute they encounter.
>
> We could easily extend nla_put_u64() and variants to automatically
> include such a padding attribute as needed.
In fact, it seems not so easy because most users of nlmsg_new() calculate
the exact needed length, thus if we add an unpredicted attribute, the message 
will be too small.
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