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Date: Wed, 01 Oct 2014 08:31:38 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andrey Ryabinin <a.ryabinin@samsung.com>, linux-kernel@vger.kernel.org
CC: Dmitry Vyukov <dvyukov@google.com>, Konstantin Serebryany <kcc@google.com>,
        Dmitry Chernenkov <dmitryc@google.com>,
        Andrey Konovalov <adech.fo@gmail.com>,
        Yuri Gribov <tetra2005@gmail.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Sasha Levin <sasha.levin@oracle.com>, Christoph Lameter <cl@linux.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>, Andi Kleen <andi@firstfloor.org>,
        Vegard Nossum <vegard.nossum@gmail.com>, x86@kernel.org,
        linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [RFC/PATCH v2 02/10] x86_64: add KASan support
References: <1404905415-9046-1-git-send-email-a.ryabinin@samsung.com> <1410359487-31938-1-git-send-email-a.ryabinin@samsung.com> <1410359487-31938-3-git-send-email-a.ryabinin@samsung.com> <54111E99.7080309@zytor.com> <5411339E.8080007@samsung.com>
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On 09/10/2014 10:31 PM, Andrey Ryabinin wrote:
> On 09/11/2014 08:01 AM, H. Peter Anvin wrote:
>> On 09/10/2014 07:31 AM, Andrey Ryabinin wrote:
>>> This patch add arch specific code for kernel address sanitizer.
>>>
>>> 16TB of virtual addressed used for shadow memory.
>>> It's located in range [0xffff800000000000 - 0xffff900000000000]
>>> Therefore PAGE_OFFSET has to be changed from 0xffff880000000000
>>> to 0xffff900000000000.
>>
>> NAK on this.
>>
>> 0xffff880000000000 is the lowest usable address because we have agreed
>> to leave 0xffff800000000000-0xffff880000000000 for the hypervisor or
>> other non-OS uses.
>>
>> Bumping PAGE_OFFSET seems needlessly messy, why not just designate a
>> zone higher up in memory?
>>
> 
> I already answered to Dave why I choose to place shadow bellow PAGE_OFFSET (answer copied bellow).
> In short - yes, shadow could be higher. But for some sort of kernel bugs we could have confusing oopses in kasan kernel.
> 

Confusing how?  I presume you are talking about something trying to
touch a non-canonical address, which is usually a very blatant type of bug.

	-hpa