linux-kernel - Re: [regression] Re: brk randomization breaks columns

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Date:	Tue, 5 Feb 2008 17:18:10 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...l.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	kernel list <linux-kernel@...r.kernel.org>,
	Abel Bernabeu <abel.bernabeu@...il.com>,
	Hugh Dickins <hugh@...itas.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [regression] Re: brk randomization breaks columns

On Tue 2008-02-05 08:05:46, Jakub Jelinek wrote:
> On Tue, Feb 05, 2008 at 01:54:26PM +0100, Ingo Molnar wrote:
> > * Jiri Kosina <jkosina@...e.cz> wrote:
> > 
> > > On Tue, 5 Feb 2008, Pavel Machek wrote:
> > > 
> > > > > Actually, this clearly shows that either prehistoric libc.so.5 or the 
> > > > > program itself are broken.
> > > > I believe it shows clear regression in latest 2.6.25 kernel.
> > > 
> > > I am still not completely sure. It might be a regression, but it also 
> > > might just trigger the bug in ancient version in libc.so.5 which might 
> > > be fixed in some later version [...]
> > 
> > which too is a regression ...
> > 
> > really, lets add a sysctl for this, and a .config option that either 
> > disables or enables it. Then we will default to disabled. (but users can 
> > enable it - and distros can build their kernels with this .config option 
> > enabled)
> 
> I don't think kernel should care about programs which are buggy and make invalid
> assumptions, and that's the case here.  I remember we have been

Those "invalid assumptions" crept into documentation. Everybody knew
heap starts at the end of bss in 1995.

> 5 years ago when brk randomization has been added to Red Hat kernels.  There was
> one or two broken programs which made assumptions on what brk(0) is supposed
> to return at program startup, everything else was ok.

That's not the problem. Problem is that programs assume
brk(0x12345678) allocates space between end of bss and 0x12345678;
which is no longer the case.

And actually even
http://opengroup.org/onlinepubs/007908775/xsh/brk.html only talks
about "ammount of space"... implying begging of that space is well
known.

								Pavel
PS: It would be nice to fix linux man pages to say that it brk() moves
end of the heap, only, and that any usage of brk() is invalid w/o
doing brk(0) before.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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