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Date:   Mon, 2 Aug 2021 12:58:25 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: How to find out name or id of newly created interface

On Monday 02 August 2021 12:02:38 Guillaume Nault wrote:
> On Sat, Jul 31, 2021 at 10:30:54PM +0200, Pali Rohár wrote:
> > 
> > And now I would like to know, how to race-free find out interface name
> > (or id) of this newly created interface?
> > 
> > Response to RTM_NEWLINK/NLM_F_CREATE packet from kernel contains only
> > buffer with struct nlmsgerr where is just error number (zero for
> > success) without any additional information.
> 
> You'd normally pass the NLM_F_ECHO flag on the netlink request, so the
> kernel would echo back a netlink message with all information about the
> device it created.
> 
> Unfortunately, many netlink handlers don't implement this feature. And
> it seems that RTM_NEWLINK is part of them (rtmsg_ifinfo_send() doesn't
> provide the 'nlh' argument when it calls rtnl_notify()).

I see...

> So the proper solution is to implement NLM_F_ECHO support for
> RTM_NEWLINK messages (RTM_NEWROUTE is an example of netlink handler
> that supports NLM_F_ECHO, see rtmsg_fib()).

Do you know if there is some workaround / other solution which can be
used by userspace applications now? And also with stable kernels (which
obviously do not receive this new NLM_F_ECHO support for RTM_NEWLINK)?

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