linux-kernel - Re: [PATCH] Document randomize_va_space and CONFIG_COMPAT_BRK (was Re: [PATCH 2/2] ASLR: add possibility for more fine-grained tweaking)

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Date:	Thu, 7 Feb 2008 16:01:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Hugh Dickins <hugh@...itas.com>, Pavel Machek <pavel@....cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Document randomize_va_space and CONFIG_COMPAT_BRK (was
	Re: [PATCH 2/2] ASLR: add possibility for more fine-grained tweaking)


* Jiri Kosina <jkosina@...e.cz> wrote:

> On Thu, 7 Feb 2008, Ingo Molnar wrote:
> 
> > i'm wondering about the following detail: i guess on 64-bit x86 
> > kernels we could default to !CONFIG_COMPAT_BRK? In 1997 there was no 
> > 64-bit x86. Maybe for compat 32-bit binaries we could keep it off, 
> > but always do it for 64-bit binaries.
> 
> So what do you think is proper behavior in situation when 
> CONFIG_COMPAT_BRK=N on 64bit kernel, and 32bit-binary is loaded in 
> 32bit emulation?
> 
> We can either leave the brk as-is, but that is in contradiction to 
> user explictly specifying CONFIG_COMPAT_BRK=N. Is this what you 
> propose?
> 
> Or we can randomize brk start in such situation, but that is the 
> behavior we currently automatically have due to CONFIG_COMPAT_BRK=N, 
> so no change is needed.

thinking about it ... i think we should just keep this simple, and when 
COMPAT_BRK=y then we disable brk randomization globally. If !COMPAT_BRK 
then we do brk randomization globally as well. (and that is probably 
what users want the sysctl to do anyway - users wont necessarily know 
whether the app breakage they want to solve is due to 32-bit or 64-bit.)

	Ingo
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