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

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Date: Sun, 2 Jun 2024 14:59:16 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: David Ahern <dsahern@...nel.org>
Cc: 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, 1 Jun 2024 20:23:17 -0600 David Ahern wrote:
> > 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.

That seems like a good solution, with the helper marking the "legacy"
handlers - I hope it should be trivial to add such option and change
the helper's behavior based on the socket state.

> Could even throw a `pr_warn_once("modernize app %s/%d\n")`
> to help old apps understand they need to move forward.

Hm, do you think people would actually modernize all the legacy apps?

Coincidentally, looking at Jaroslav's traces it appears that the app
sets ifindex for the link dump, so it must not be opting into strict
checking, either.