Re: netlink, RTM_NEWTCLASS, nested attributes

From: Denys Fedoryshchenko
Date: Wed Feb 20 2013 - 18:24:13 EST

On 2013-02-21 01:21, Stephen Hemminger wrote:
On Tue, 19 Feb 2013 23:45:25 +0200
Denys Fedoryshchenko <denys@xxxxxxxxxxx> wrote:


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
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.

Thank you for clarification!

Denys Fedoryshchenko, Network Engineer, Virtual ISP S.A.L.
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