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

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Date:	Wed, 15 Sep 2010 00:27:58 +0200
From:	pageexec@...email.hu
To:	Roland McGrath <roland@...hat.com>
CC:	KOSAKI Motohiro <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, Eugene Teo <eugene@...hat.com>
Subject: Re: [PATCH 1/3] setup_arg_pages: diagnose excessive argument size

On 14 Sep 2010 at 14:16, Roland McGrath wrote:

> > obviously an AT_ARGMAX computed at execve time would be based on the rlimits
> > as well and if later userland changed the rlimits, it'd be userland's problem,
> > not that of the kernel (or the kernel could refuse a change that would violate
> > its earlier promise).
> 
> This would thoroughly defeat the purpose of adding the thing.  The
> only reason to have a new thing is so that userland does not have to
> mirror the kernel's policy (as it attempts to do now, with the 1/4
> calculation).  If the new thing is not something that userland can
> use consistently so as not to have to know what the kernel's actual
> policy is, then I don't see the point of it at all.

userland could never rely on the kernel's policy at all since get_arg_page
could have failed for more reasons than overstepping the currently hardcoded
ARG_MAX check in there. so what AT_ARGMAX would buy us is to allow the kernel
policy to change over time, but it's never been about guarantees, whether
POSIX wants such a thing or not.

> > >  auxv is only appropriate for things that
> > > are known at the time of the exec and won't change thereafter.
> > 
> > you mean stuff like AT_EUID et al.? ;)
> 
> The information that these give is about the conditions at startup.
> That's what they mean to userland, and userland only uses them to know
> the situation before it has made any calls.  The definition of AT_EUID
> is "effective user ID at program startup", and that fact does not
> change.

just for my own curiosity, where does this definition come from?

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