Re: [RFC/PATCH] RLIMIT_ARG_MAX

From: "Michael Kerrisk" <michael.kerrisk@googlemail.com>
To: "Linus Torvalds" <torvalds@linux-foundation.org>
Cc: "Peter Zijlstra" <a.p.zijlstra@chello.nl>, aaw <aaw@google.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	carlos@codesourcery.com, "Alan Cox" <alan@lxorguk.ukuu.org.uk>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	drepper@redhat.com, mtk.manpages@gmail.com,
	"Geoff Clare" <gwc@opengroup.org>
Subject: Re: [RFC/PATCH] RLIMIT_ARG_MAX
Date: Fri, 29 Feb 2008 21:43:45 +0100	[thread overview]
Message-ID: <c49095e30802291243x6087399w4b98ae251f57319b@mail.gmail.com> (raw)
In-Reply-To: <alpine.LFD.1.00.0802291200580.17889@woody.linux-foundation.org>

On Fri, Feb 29, 2008 at 9:07 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>
>  On Fri, 29 Feb 2008, Michael Kerrisk wrote:
>
>  > On Fri, Feb 29, 2008 at 7:39 PM, Linus Torvalds
>  > <torvalds@linux-foundation.org> wrote:
>  >
>
> > >  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?
>
>  We *do* know for sure that the relationship has always been there. At
>  least in Linux, and I bet in 99% of all other Unixes too. The arguments
>  simply have traditionally been counted as part of the stack size.
>
>  Or did you mean the latter part?

I meant: do we know for sure that no one really cares?

>  The fact is, we *also* know for sure that anybody that depends on
>  _SC_ARG_MAX being exact has always - and will continue to be - broken.
>  Again, because of not only older kernels but also because even with the
>  patch in question, we don't count argument sizes exactly.
>
>
>  > 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.
>
>  I do agree that we should at least make the "MAX(stacksize/4, 128k)"
>  change for backwards compatibility.

Good -- because that's probably the most important point, IMO.

> That is actually a potential
>  regression, but it has nothing to do with a new _SC_ARG_SIZE, because
>  quite frankly, it's a regression *regardless* of whether we'd expose a new
>  rlimit or not!

Agreed.

The new rlimit is primarily for the (supposed) applications that care
about knowing (at least approximately) what _SC_ARG_MAX is.  I raised
the initial bug report against glibc because applications can no
longer (post 2.6.23) do this, but I haven't done the investigation
about how many applications actually care.

Cheers,

Michael