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

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Date:	Fri, 29 Feb 2008 20:49:16 +0100
From:	"Michael Kerrisk" <michael.kerrisk@...glemail.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>, aaw <aaw@...gle.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	carlos@...esourcery.com, "Alan Cox" <alan@...rguk.ukuu.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>, drepper@...hat.com,
	mtk.manpages@...il.com, "Geoff Clare" <gwc@...ngroup.org>
Subject: Re: [RFC/PATCH] RLIMIT_ARG_MAX

On Fri, Feb 29, 2008 at 7:39 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
>  On Fri, 29 Feb 2008, Michael Kerrisk wrote:
>  >
>  > My reading of POSIX.1 (and POSIX doesn't seem very explicit on this point), is
>  > that the limits on argv+environ and on stack are decoupled, since POSIX
>  > specifies RLIMIT_STACK and sysconf(_SC_ARG_MAX) and doesn't specify any
>  > relationship between the two.
>
>  I agree. And clearly there _are_ relationships and always have been, but
>  equally clearly they simply haven't been a big issue in practice, and
>  nobody really cares.

Do we know that for sure?

>  Usually, _SC_ARG_MAX is just so much smaller than RLIMIT_STACK that it
>  makes no possible difference.  Which I would actually argue we should just
>  continue with: just keep _SC_ARG_MAX a smallish, irrelevant constant.
>
>  We still have to have the compile-time ARG_MAX constant (as in *real*
>  constant - a #define) anyway, for traditional programs, and you might as
>  well make sysconf(_SC_ARG_MAX) always just match ARG_MAX.
>
>  It's not like there is likely a single user of _SC_ARG_MAX that cares.

In my initial reply, I pointed out one example where users *may* care:
NPTL uses RLIMIT_STACK to determine the size of per-thread stacks.  It
is conceivable that users might want to set RLIMIT_STACK < 512k, and
that would have the effect of lowering the amount of space for
argv+eviron below what the kernel has historically guaranteed.  That's
an ABI change, though it's unclear whether it would impact anyone in
practice.
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