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

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Message-ID: <20240412192200.662d92ae@elisabeth>
Date: Fri, 12 Apr 2024 19:22:00 +0200
From: Stefano Brivio <sbrivio@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: David Ahern <dsahern@...nel.org>, davem@...emloft.net,
 netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com, Ilya
 Maximets <i.maximets@....org>, donald.hunter@...il.com
Subject: Re: [PATCH net] inet: bring NLM_DONE out to a separate recv() again

On Thu, 11 Apr 2024 14:01:54 -0700
Jakub Kicinski <kuba@...nel.org> wrote:

> On Thu, 11 Apr 2024 13:45:42 -0600 David Ahern wrote:
> > > +	/* Don't let NLM_DONE coalesce into a message, even if it could.
> > > +	 * Some user space expects NLM_DONE in a separate recv().    
> > 
> > that's unfortunate  
> 
> Do you have an opinion on the sysfs/opt-in question?
> Feels to me like there shouldn't be that much user space doing raw
> netlink, without a library. Old crufty code usually does ioctls, right?

I think so too -- if there were more (maintained) applications with
this issue, we would have noticed by now.

> So maybe we can periodically reintroduce this bug to shake out all 
> the bad apps? :D

Actually, I had half a mind of proposing something on these lines: add
a TODO comment here and revisit in, say, two years.

I guess it's definitely more painful for libreswan, but for passt, I
think it's quite unlikely that distribution users could get the
"breaking" kernel change without a fixed version of the application: we
made a new release relatively close to the NLM_DONE change.

There might be substantial value in keeping this type of short netlink
exchanges fast, for example for container engines that need to be able
to spawn a bazillion containers per second.

-- 
Stefano