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

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Date: Sat, 1 Jun 2024 20:23:17 -0600
From: David Ahern <dsahern@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>,
 Stephen Hemminger <stephen@...workplumber.org>
Cc: 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 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.