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

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Message-ID: <20080205161246.GE24331@elf.ucw.cz>
Date:	Tue, 5 Feb 2008 17:12:46 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Andrew Morton <akpm@...l.org>, "Rafael J. Wysocki" <rjw@...k.pl>,
	kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	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 13:50:51, Jiri Kosina 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 -- are you able to verify that?

I'm in same position as you here. I only have few old binaries :-(.

> > You say it is wrong. Manpages imply otherwise:
> > 
> >        int brk(void *end_data_segment);
> > ...
> > DESCRIPTION
> >        brk()  sets  the  end  of  the  data  segment  to  the  value specified  by
> >        end_data_segment, when that value is reasonable, the system does have enough
> >        memory and the process does not exceed its max data size (see setrlimit(2)).
> > Note it talks about data segment, not about heap, and that seems to
> > imply that BSS and heap are actually one area. 2.6.25 broke that.
> 
> Single Unix Specification talks only about manipulating the break section, 
> see http://opengroup.org/onlinepubs/007908775/xsh/brk.html

SuS:
# The brk() and sbrk() functions are used to change the amount of space
# allocated for the calling process.

It talks about "space for calling process". It does not talk about
heap, and I think it implicitely assumes "bss and heap" are
continuous... 

(It also says return 0 or success, and we return address).

> > > Still, it will probably not fix your particular program crashes, just 
> > > because it will always assume that brk starts immediately after the end of 
> > > the bss, which is plain wrong and has never been assured. Could you please 
> > Can you quote docs that tells me it is plain wrong? 
> 
> See the Single Unix Specification. It doesn't seem to allow you to assume 
> *anything* about start_brk location, seems to me.

I believe it implicitely assumes start_brk is well known,
actually. Otherwise it should have told us how to get start_brk.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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