linux-kernel - Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic

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Date:	Wed, 09 Oct 2013 07:13:26 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
CC:	Dave Anderson <anderson@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	kernel-hardening@...ts.openwall.com,
	Aaron Durbin <adurbin@...gle.com>,
	Eric Northup <digitaleric@...gle.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...gle.com>,
	Mathias Krause <minipli@...glemail.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"Discussion list for crash utility usage, maintenance and development" 
	<crash-utility@...hat.com>
Subject: Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic

On 10/09/2013 03:04 AM, HATAYAMA Daisuke wrote:
>>>
>>> Sorry, I don't understand why relocation size cannot be calculated when
>>> CONFIG_PHYSICALSTART > CONFIG_PHYSICAL_ALIGN. Could you explain that?
>>
>> I just meant that when CONFIG_PHYSICAL_START > CONFIG_PHYSICAL_ALIGN,
>> the 32-bit x86 kernel
>> gets relocated (like the secondary kdump kernel), but that information
>> is not readily available
>> from the vmlinux/vmcore pair.
>>
> 
> My understanding on CONFIG_PHYSICAL_ALIGN was that starting address of
> kernel text area
> is always rounded up to CONFIG_PHYSICAL_ALIGN, only. Your explanation
> would be part I don't
> understand well. I'll reconsider it locally...
> 

If CONFIG_PHYSICAL_START == CONFIG_PHYSICAL_ALIGN, then it is very
likely that the kernel (in the absence of kASLR) will be run at the
CONFIG_PHYSICAL_START address, as the initial loading address, usually 1
MB, will be rounded up to CONFIG_PHYSICAL_ALIGN.

Since CONFIG_PHYSICAL_START is the unrelocated linking address, they end
up matching.

	-hpa


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