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

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Message-ID: <2a49231c-496d-132a-93e1-8a2071a35b08@6wind.com>
Date:   Fri, 23 Sep 2022 17:42:33 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Guillaume Nault <gnault@...hat.com>,
        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




Le 23/09/2022 à 15:48, Jakub Kicinski a écrit :
> Let me clarify one more time in case Hangbin is waiting for 
> the discussion to resolve...
> 
> On Fri, 23 Sep 2022 10:43:53 +0200 Nicolas Dichtel wrote:
>> Le 22/09/2022 à 16:51, Guillaume Nault a écrit :
>>> 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).
> 
> In NEWLINK, right? In NEWLINK there is no way to pass it back 
> at the moment. A newly added command can just respond with the handle
> always. The problem with NEWLINK is that it _used to_ not respond so 
> we can't make it start responding because it will confuse existing user
> space.
> 
> At the protocol level NEW is no different than GET, whether it sends 
> a response back is decided by whoever implements the command.
> 
> So yes, for NEWLINK we need a way to inform the kernel that user space
> wants a reply. It can be via ECHO, it could be via a new attr.
> 
> What I'm trying to argue about is *not* whether NEWLINK should support
> ECHO but whether requiring ECHO to get a response for newly added
> CREATE / NEW commands is a good idea. I think it is not, and new
> commands should just always respond with the handle.
Sure, but I don't see how we can enforce this.

> 
> My main concern with using ECHO is that it breaks the one-to-one
> relationship between a request and a response. There may be multiple
> notifications generated due to a command, and if we want to retain 
> the "ECHO will loop back to you all resulting notifications" semantics,
> which I think we should, then there can be multiple "responses".
Thanks for the detailed explanation.

> 
> This also has implications for the command IDs used in notifications.
> A lot of modern genl families use different IDs for notifications to
> make it easily distinguishable from responses.
I didn't know that. Indeed, it's a good idea.