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

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Date:	Fri, 10 Sep 2010 18:43:57 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	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, pageexec@...email.hu,
	Eugene Teo <eugene@...hat.com>
Subject: Re: [PATCH 1/3] setup_arg_pages: diagnose excessive argument size

> > 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.

Hmm...
Probably my poor english leaded to misunderstood. I didn't intent glibc
is very crazy. I only intended to "even if userland is crazy, I disagree
to break userland".

And yes, we obviously need to expose ARG_MAX limit to libc. a duplicated
heuristic code easily makes confusion and mistake. nobody want such 
fragile state. however, it's a bit offtopic. anyway.


Thanks.

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