linux-kernel - Re: [PATCH 1/3] setup_arg_pages: diagnose excessive argument size

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Message-ID: <4C8B8692.7649.3E0895B@pageexec.freemail.hu>
Date:	Sat, 11 Sep 2010 15:39:30 +0200
From:	pageexec@...email.hu
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Roland McGrath <roland@...hat.com>
CC:	Brad Spengler <spender@...ecurity.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, oss-security@...ts.openwall.com,
	Solar Designer <solar@...nwall.com>,
	Kees Cook <kees.cook@...onical.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Oleg Nesterov <oleg@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	linux-fsdevel@...r.kernel.org, Eugene Teo <eugene@...hat.com>
Subject: Re: [PATCH 1/3] setup_arg_pages: diagnose excessive argument size

On 10 Sep 2010 at 2:25, Roland McGrath wrote:

> > Brad, sorry, I have bad news. glibc sysconf(_SC_ARG_MAX) is implemented
> > by hard coded RLIMIT_STACK/4 heuristics. That said, at least _now_, we
> > can't change this even though you disliked. That said, we can't break
> > userland even though userland library is very crazy.
> 
> I'm sorry you think it's "very crazy" to implement the required
> functionality in the only way available.  POSIX requires that execve
> fail with E2BIG when the ARG_MAX limit is exceeded.  sysconf has to
> return the correct actual limit that execve will enforce so that a
> conforming application knows how much it can safely attempt to use.
> Since the kernel uses the hard-coded RLIMIT_STACK/4 heuristic and does
> not expose the true manifest limit any other way, sysconf has to
> parallel the kernel's calculation.

no it doesn't have to, similarly to how it doesn't have to hardcode
_SC_PAGESIZE either, AT_PAGESZ tells userland what it needs to know
and i think AT_ARGMAX could exist just as well.

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