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

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Date:	Wed, 12 Dec 2012 18:30:51 +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 11/12/2012 19:40, Thomas Graf a écrit :
> On 12/11/12 at 04:03pm, Nicolas Dichtel wrote:
>> 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.
>
> True, we would either need to fix the calculations by accounting
> for an additional 4 bytes for each 64bit arg or just reserve an
> additional fixed amount for padding per message in nlmsg_new().
>
I would say that reserving additional space is better, because we also
need to align attributes that contain u64 fields:

struct attribute_foo {
    __u32 bar;
    __u32 bar2;
    __u64 foo;
};

I wonder if it is better to align all attribute on 64-bits or only u64 and
add a new function nla_put_align64() for attribute with u64 fields.
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