linux-kernel - Re: netlink, RTM_NEWTCLASS, nested attributes

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Message-ID: <20130220152130.72df1258@samsung-9>
Date:	Wed, 20 Feb 2013 15:21:30 -0800
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Denys Fedoryshchenko <denys@...p.net.lb>
Cc:	<netdev@...r.kernel.org>, <pablo@...filter.org>,
	<jhs@...atatu.com>, <davem@...emloft.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: netlink, RTM_NEWTCLASS, nested attributes

On Tue, 19 Feb 2013 23:45:25 +0200
Denys Fedoryshchenko <denys@...p.net.lb> wrote:

> Hi
> 
> I tried recently to write my own tool based on amazing libmnl (which 
> makes understanding of netlink - easy), written
> by Pablo Neira Ayuso, to manage QoS in Linux and faced problem, which i 
> think probably
> a bug in handling netlink messages in kernel.
> 
> For example if i send message, RTM_NEWTCLASS, after attribute 
> TCA_OPTIONS i have nested attributes,
> for example in HTB: TCA_HTB_PARMS, TCA_HTB_RTAB, TCA_HTB_CTAB.
> libmnl, if i use nested attribute, adding a bit to it, by OR -  
> NLA_F_NESTED(1 << 15).
> If i remove this flag - everything works fine. And here is the case, 
> iproute2 tools
> just update length of TCA_OPTIONS, without setting flag, and it works 
> because of that fine too.
> 
> So there is basically 3 solutions:
> 1)New function in libmnl to do nested attributes without setting by OR 
> flag
> 2)AND-ing attribute type in kernel to ignore nested flag
> 3)Keeping as is, who cares?
> 

Several legacy netlink interfaces don't use NESTED flag. These are by
now enshrined in ABI and can't change. In code, that uses libmnl, I just
manually clear the flag as needed and document why. This could
be added to libmnl.

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