From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751786Ab1AFJIH (ORCPT ); Thu, 6 Jan 2011 04:08:07 -0500 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:59430 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751214Ab1AFJID (ORCPT ); Thu, 6 Jan 2011 04:08:03 -0500 Date: Thu, 6 Jan 2011 09:07:41 +0000 From: Russell King - ARM Linux To: Stephen Boyd Cc: linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] arm: mm: Poison freed init memory Message-ID: <20110106090741.GQ8638@n2100.arm.linux.org.uk> References: <1294256845-29517-1-git-send-email-sboyd@codeaurora.org> <20110105202645.GL8638@n2100.arm.linux.org.uk> <4D255263.7040106@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4D255263.7040106@codeaurora.org> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 05, 2011 at 09:25:55PM -0800, Stephen Boyd wrote: > On 01/05/2011 12:26 PM, Russell King - ARM Linux wrote: > > On Wed, Jan 05, 2011 at 11:47:25AM -0800, Stephen Boyd wrote: > >> Poisoning __init marked memory can be useful when tracking down > >> obscure memory corruption bugs. When a pointer is 0xCCCCCCCC in an > > > > That's a bad idea for a value. With a 3GB page offset and 256MB or > > more memory, accesses to such an address will always succeed. > > > > There's two things to be considered when selecting a possible poison > > value: > > > > 1. what value is guaranteed to provoke an undefined instruction exception? > > 2. what value when used as an address and dereferenced is mostly always > > going to abort? > > > > 1 for ARM mode implies an 0xe7fXXXfX value. For Thumb mode 0xdeXX. We > > use this space for breakpoints. > > > > 2 unfortunately depends on the platform. > > A coworker proposed we use a SWI instruction. We could do that if the > poison is 0xEF and then do something in the SWI handler where that > number causes us to blow up? Doesn't work with EABI - the comment field in the SWI instruction is ignored on EABI. > If I'm following correctly, point 1 is about __init functions and point > 2 is about __initdata. I'm more concerned about __initdata because > __init functions called from non __init marked functions are usually > caught with section mismatch checks. Also, if we're jumping to > 0xCCCCCCCC we're probably not in the text section of the kernel with a But, as I pointed out, you don't know that 0xCCCCCCCC isn't a valid address _and_ on modern platforms it won't fault. So it's pointless to use it as a poison value.