Re: [PATCH net-next] rtnetlink: Honour NLM_F_ECHO flag in rtnl_{new, set}link

On Wed, Sep 21, 2022 at 03:56:40PM -0700, Jakub Kicinski wrote:

On Wed, 21 Sep 2022 18:14:09 +0200 Guillaume Nault wrote:

quoted

quoted

I'd love to hear what others think. IMO we should declare a moratorium
on any use of netlink flags and fixed fields, push netlink towards
being a simple conduit for TLVs.  

At my previous employer, we had a small program inserting and removing
routes depending on several external events (not a full-fledged routing
daemon). NLM_F_ECHO was used at least to log the real kernel actions (as
opposed to what the program intended to do) and link that to the events
that triggered these actions. That was really helpful for network
administrators. Yes, we were lucky that the RTM_NEWROUTE and
RTM_DELROUTE message handlers supported NLM_F_ECHO. I was surprised when
I later realised that RTM_NEWLINK and many others didn't.

Then, a few years ago, I had questions from another team (maybe Network
Manager but I'm not sure) who asked how to reliably retrieve
informations like the ifindex of newly created devices. That's the use
case NLM_F_ECHO is for, but lacking this feature this team had to
rely on a more convoluted and probably racy way. That was the moment
I decided to expose the problem to our team. Fast-forwarding a couple
of years and Hangbin picked up the task.

Looking closer at the code it seems like what NLM_F_ECHO does in most
places is to loop notifications resulting from the command back onto
the requesting socket. See nlmsg_notify(), report is usually passed 
as nlmsg_report(req).

Yes, this is how it's supposed to work. NLM_F_ECHO is already handled
by the core netlink code. Netlink message handlers only have to pass
the right parameters to nlmsg_notify() (or the rtnl_notify() wrapper
for rtnetlink) to make it work.

That's why I complained when RTM_NEWNSID tried to implement its own
notification mechanism:
https://lore.kernel.org/netdev/20191003161940.GA31862@linux.home/ (local)

I mean, let's just use the built-in mechanism, rather than reinventing
a new one every time the need comes up.

I guess that answers Hangbin's question - yes, I'd vote that we just
pass the nlh to rtnl_notify() and let the netlink core do its thing.

Definitely.

In general I still don't think NLM_F_ECHO makes for a reasonable API.
It may seem okay to those who are willing to write manual netlink
parsers but for a normal programmer the ability to receive directly
notifications resulting from a API call they made is going to mean..
nothing they can have prior experience with. NEWLINK should have
reported the allocated handle / ifindex from the start :(

I also don't know of any other API that works this way. But given the
current situation, I also can't really see that change.

The "give me back the notifications" semantics match well your use
case to log what the command has done, in that case there is no need
to "return" all the notifications from the API call.

It's not really _my_ use case anymore :), but I'm pretty sure this
piece of software is still in use. Anyway I used this example just to
illustrate why a programmer would use this feature. The RTM_NEWNSID
is another practical use case.