linux-kernel - Re: [RFC/PATCH] RLIMIT_ARG

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Message-Id: <1204306934.6243.119.camel@lappy>
Date:	Fri, 29 Feb 2008 18:42:14 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Michael Kerrisk <michael.kerrisk@...glemail.com>,
	aaw <aaw@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
	michael.kerrisk@...il.com, carlos@...esourcery.com,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>, drepper@...hat.com,
	mtk.manpages@...il.com
Subject: Re: [RFC/PATCH] RLIMIT_ARG_MAX


On Fri, 2008-02-29 at 09:29 -0800, Linus Torvalds wrote:
> 
> On Fri, 29 Feb 2008, Peter Zijlstra wrote:
> >
> > > ... and what's the point? We've never had it before, nobody has ever cared, 
> > > and the whole notion is just stupid. Why would we want to limit it? The 
> > > only thing that the kernel *cares* about is the stack size - any other 
> > > size limits are always going to be arbitrary.
> > 
> > Well, don't think of limiting it, but querying the limit.
> > 
> > Programs like xargs would need to know how much to stuff into argv
> > before starting a new invocation.
> 
> But they already can't really do that. 

I think they used to use sysconf(_SC_ARG_MAX) to do that.

> More importantly, isn't it better to just use the whole stack size then 

Well, we ran into trouble of freshly spawned tasks faulting on the first
stack grow. The /4 thing was to avoid that situation.

> (or just return "stack size / 4" or whatever)?

I'm all for that, trouble is that the POSIX folks specified that the
sysconf() value must be consistent during the lifetime of a process.
Which isn't true, because we can change rlimit_stack after asking. And
the linux implementation doesn't even seem to bother asking the kernel -
so there just isn't much we _can_ do here.

My suggestion was a kernel version check along with sysconf or
rlimit_stack. But I guess that made the userspace people puke :-)

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