linux-kernel - Re: [PATCH 1/2] Use structs instead of hardcoded offsets in x86 boot decompressor.

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Message-ID: <20080525042527.GA12989@bitplanet.net>
Date:	Sun, 25 May 2008 00:25:27 -0400
From:	Kristian Høgsberg <krh@...planet.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Kristian Høgsberg <krh@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Use structs instead of hardcoded offsets in x86
	boot decompressor.

On Sat, May 24, 2008 at 09:06:44PM -0700, H. Peter Anvin wrote:
> Rik van Riel wrote:
>> On Fri, 23 May 2008 23:17:01 -0700
>> "H. Peter Anvin" <hpa@...or.com> wrote:
>>
>>> Uhm... except it doesn't live at address zero, at all.
>>>
>>> It's called "zeropage" because we used to recycle it into 
>>> empty_zero_page, a long long time ago.
>>>
>>> The bootparms structure is pointed to by %esi being passed from the setup 
>>> code to the decompressor to the kernel.
>>
>> Then why does dereferencing the uninitialized
>> pointer work, both in the code before and after
>> the patch?
>>
>> What am I misssing?
>
> Heck if I know.  It definitely *shouldn't* work...

Damn, I lost a chunk of the patch.  There's supposed to be a

        rm_screen_info = &real_mode->screen_info;

after the real_mode = rmode assignment.  In the code before,
RM_SCREEN_INFO was a macro that just referenced the real_mode pointer:

	#define RM_SCREEN_INFO (*(struct screen_info *)(real_mode+0))

Thanks, wil resend.
Kristian


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