linux-kernel - Re: 2.6.22.6 - Discrepancy between running and on-disk kernels

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Message-ID: <20080314090049.346d7428@laptopd505.fenrus.org>
Date:	Fri, 14 Mar 2008 09:00:49 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Yan <rottled@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: 2.6.22.6 - Discrepancy between running and on-disk kernels

On Fri, 14 Mar 2008 10:51:43 -0400
Yan <rottled@...il.com> wrote:

> Hello,
> 
> I have been trying to compare the code from the on-disk compressed
> kernel that was booted and the running kernel extracted
> from /dev/kmem. I extracted the kernel's code from the disk image by
> stripping the head.S and similar and gunzipping it, and extracted the
> kernel from /dev/kmem by reading data between _text and _etext symbol
> offsets.
> 
> I then ran both through a disassembler and diff'ed the outputs.
> Predictably, the disassembly was similar, but not identical. Some
> instructions (e.g. bts) had a 'lock' prefix, where as others had a
> 'nop' in its place.
> 
> There were other differences with some instructions like mfence.
> Everything else matched just fine, the differences were mostly in
> memory-referencing instructions.
> 
> My question is, what can be changing the kernel between being on
> static storage and being loaded?

the kernel code is self-patching, it gets modified to match your system
during boot time. So you cannot assume that the kernel on disk and the kernel
in memory are identical.
(Same goes for modules)
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