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

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Date:	Fri, 14 Mar 2008 09:31:38 -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 12:08:16 -0400
Yan <rottled@...il.com> wrote:

> Can you point me to where in the source that self-patching happens? I
> tried grep'ing
> through the source for anything relevant, and came out empty handed.


in the apply_alternatives() function

> 
> -yan
> 
> On Fri, Mar 14, 2008 at 12:00 PM, Arjan van de Ven
> <arjan@...radead.org> wrote:
> > 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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