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Subject2.6.22.6 - Discrepancy between running and on-disk kernels
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? Or am I just being fooled into thinking something
changed it? I'm thinking that it had something to do with mutex locking
and SMP, but I looked through boot code and tried googling and came up with
nothing. Another possibility is me interpreting non-instructions as
instructions.

Example:

The following pattern repeats a lot:

1325c1326,1327
< 1089: f0 0f b3 04 24 lock btr %eax,(%esp)
---
> 1089: 90 nop
> 108a: 0f b3 04 24 btr %eax,(%esp)

As well as other changes:

94383,94384c94832,94835
< 47bb3: f0 83 04 24 00 lock addl $0x0,(%esp)
< 47bb8: f0 ff 05 7c 30 80 c0 lock incl 0xc080307c
---
> 47bb3: 0f ae f0 mfence
> 47bb6: 89 f6 mov %esi,%esi
> 47bb8: 90 nop
> 47bb9: ff 05 7c 30 80 c0 incl 0xc080307c


(I'm not subscribed; please cc any responses to me)

Thanks in advance,
Yan


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