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:

quoted

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.

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).