netdev - Re: [PATCHv3 net-next] rtnetlink: Honour NLM_F_ECHO flag in rtnl

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Message-ID: <20220928071609.5af4bb4f@kernel.org>
Date:   Wed, 28 Sep 2022 07:16:09 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     "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>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Guillaume Nault <gnault@...hat.com>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCHv3 net-next] rtnetlink: Honour NLM_F_ECHO flag in
 rtnl_{new, set, del}link

On Wed, 28 Sep 2022 11:22:11 +0800 Hangbin Liu wrote:
> Hi Jakub,
> On Tue, Sep 27, 2022 at 12:13:03PM +0800, Hangbin Liu wrote:
> > @@ -3009,6 +3012,11 @@ static int do_setlink(const struct sk_buff *skb,
> >  		}
> >  	}
> >  
> > +	nskb = rtmsg_ifinfo_build_skb(RTM_NEWLINK, dev, 0, 0, GFP_KERNEL, NULL,
> > +				      0, pid, nlh->nlmsg_seq);
> > +	if (nskb)
> > +		rtnl_notify(nskb, dev_net(dev), pid, RTNLGRP_LINK, nlh, GFP_KERNEL);  
> 
> BTW, in do_setlink() I planed to use RTM_SETLINK. But I found iproute2 use
> RTM_NEWLINK to set links. And I saw an old doc[1] said
> 
> """
> - RTM_SETLINK does not follow the usual rtnetlink conventions and ignores
>   all netlink flags
> 
> The RTM_NEWLINK message type is a superset of RTM_SETLINK, it allows
> to change both driver specific and generic attributes of the device.
> """

Interesting, so we actually do use this "NEW as SET" thing.

> So I just use RTM_NEWLINK for the notification. Do you think if we should
> use RTM_SETLINK?

FWIW I think it's typical for rtnl / classic netlink to generate 
"new object" notification whenever object is changed, rather than
a notification about just the change.