Re: netlink, RTM_NEWTCLASS, nested attributes

From: Stephen Hemminger
Date: Wed Feb 20 2013 - 18:21:47 EST

On Tue, 19 Feb 2013 23:45:25 +0200
Denys Fedoryshchenko <denys@xxxxxxxxxxx> 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,
> 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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