netdev - Re: [PATCH net-next] rtnetlink: Honour NLM_F_ECHO flag in rtnl

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Date:   Thu, 22 Sep 2022 16:51:42 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Ido Schimmel <idosch@...dia.com>,
        Petr Machata <petrm@...dia.com>,
        Florent Fourcot <florent.fourcot@...irst.fr>,
        Nikolay Aleksandrov <razor@...ckwall.org>
Subject: Re: [PATCH net-next] rtnetlink: Honour NLM_F_ECHO flag in rtnl_{new,
 set}link

On Thu, Sep 22, 2022 at 06:03:46AM -0700, Jakub Kicinski wrote:
> On Thu, 22 Sep 2022 13:09:51 +0200 Guillaume Nault wrote:
> > That's why I complained when RTM_NEWNSID tried to implement its own
> > notification mechanism:
> > https://lore.kernel.org/netdev/20191003161940.GA31862@linux.home/
> > 
> > I mean, let's just use the built-in mechanism, rather than reinventing
> > a new one every time the need comes up.
> 
> See, when you say "let's just use the built-in mechanism" you worry 
> me again. Let's be clear that no new API should require the use of
> ECHO for normal operation, like finding out what the handle of an
> allocated object is.

I've always thought the lack of NLM_F_ECHO support in many subsystems
was just an oversight, as it shouldn't take a lot of plumbing to make
it work. But if you prefer to deprecate the feature then okay.

I just don't see any way to pass a handle back to user space at the
moment. The echo mechanism did that and was generic to all netlink
families (as long as nlmsg_notify() was called with the right
parameters).