netdev - Re: [PATCH net] inet: bring NLM_DONE out to a separate recv() in inet_dump

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Date: Mon, 3 Jun 2024 10:05:40 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: David Ahern <dsahern@...nel.org>
Cc: Jakub Kicinski <kuba@...nel.org>, Stephen Hemminger <stephen@...workplumber.org>, davem@...emloft.net, 
	netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com, 
	Jaroslav Pulchart <jaroslav.pulchart@...ddata.com>
Subject: Re: [PATCH net] inet: bring NLM_DONE out to a separate recv() in inet_dump_ifaddr()

On Sat, Jun 1, 2024 at 10:23 PM David Ahern <dsahern@...nel.org> wrote:
>
> On 6/1/24 5:48 PM, Jakub Kicinski wrote:
> > On Sat, 1 Jun 2024 16:10:13 -0700 Stephen Hemminger wrote:
> >> Sorry, I disagree.
> >>
> >> You can't just fix the problem areas. The split was an ABI change, and there could
> >> be a problem in any dump. This the ABI version of the old argument
> >>   If a tree falls in a forest and no one is around to hear it, does it make a sound?
> >>
> >> All dumps must behave the same. You are stuck with the legacy behavior.
>
> I don't agree with such a hard line stance. Mistakes made 20 years ago
> cannot hold Linux back from moving forward. We have to continue
> searching for ways to allow better or more performant behavior.
>
> >
> > The dump partitioning is up to the family. Multiple families
> > coalesce NLM_DONE from day 1. "All dumps must behave the same"
> > is saying we should convert all families to be poorly behaved.
> >
> > Admittedly changing the most heavily used parts of rtnetlink is very
> > risky. And there's couple more corner cases which I'm afraid someone
> > will hit. I'm adding this helper to clearly annotate "legacy"
> > callbacks, so we don't regress again. At the same time nobody should
> > use this in new code or "just to be safe" (read: because they don't
> > understand netlink).
>
> What about a socket option that says "I am a modern app and can handle
> the new way" - similar to the strict mode option that was added? Then
> the decision of requiring a separate message for NLM_DONE can be based
> on the app. Could even throw a `pr_warn_once("modernize app %s/%d\n")`
> to help old apps understand they need to move forward.
>

Sorry, being a little lazy so asking instead:
NLMSG_DONE is traditionally the "EOT" (end of transaction) signal, if
you get rid of it  - how does the user know there are more msgs coming
or the dump transaction is over? In addition to the user->kernel "I am
modern", perhaps set the nlmsg_flag in the reverse path to either say
"there's more coming" which you dont set on the last message or "we
are doing this the new way". Backward compat is very important - there
are dinosaur apps out there that will break otherwise.

cheers,
jamal