Re: mysterious changes in post-MBR gap

[webslim]

Andrei Borzenkov <address@hidden> wrote [on Mon, Aug 17, 2015 at  
06:48:51AM +0300]:
> 16.08.2015 15:53, address@hidden пишет:
> > Hi folks,
> >
> > Once upon a time I wrote a small init script that computes a checksum
> > of the MBR and the gap behind it where GRUB2 lives, and also of the
> > boot partition, and compares them to already saved values.
> > Everytime I do a Kernel or GRUB update, I also make an image and save
> > the checksum. Up until grub-2.02 these values did not change if I would
> > not touch anything and always boot the default entry.
> > But recently, the post-MBR gap check started to fail every time.
> > A diff of the hexdump of the first 2MB of the disk showed some small
> > discrepancies (at around 0x00100420 and 0x00100570). Before rebooting,
> > they did not exist. After each boot, there are some small changes.
> > So my question is:
> > - Does grub keep some kind of counter or status information when
> >    booting? This would calm me somehow.
> > - Or does this mean there is something strange going on on my machine?
> >
>
> Do you mean that you do grub-install, compute checksum, reboot,  
> compute checksume and both checksums are different?

Yes.

> Bugs withstanding, the only time when grub writes anything to disk  
> is when saving environment variables. So checking grub.cfg for  
> save_env commands makes sens.

And according to the documentation this is stored at /boot/grub/, not
outside the file system.

> Also if you compute checksum after reboot it could be operating  
> system (anything is startup sequence) that does it. Did you try  
> booting in single user (or even with init=/bin/sh to rule out  
> everything)?

You were right.
This is not a grub issue. I checked from the initramfs: no changes there.
And this happens also only on this particular machine.
Time for a fresh install... Thanks for the advice!
<p>Quoting Andrei Borzenkov &lt;<a  
href="mailto:address@hidden";>address@hidden</a>&gt;:</p><blockquote  
type="cite" style="border-left:2px solid  
blue;margin-left:2px;padding-left:12px;">16.08.2015 15:53, <a  
href="mailto:address@hidden";>address@hidden</a>  
пишет:<blockquote type="cite" style="border-left:2px solid  
blue;margin-left:2px;padding-left:12px;">Hi folks,<br />
<br />
Once upon a time I wrote a small init script that computes a checksum<br />
of the MBR and the gap behind it where GRUB2 lives, and also of the<br />
boot partition, and compares them to already saved values.<br />
Everytime I do a Kernel or GRUB update, I also make an image and save<br />
the checksum. Up until grub-2.02 these values did not change if I would<br />
not touch anything and always boot the default entry.<br />
But recently, the post-MBR gap check started to fail every time.<br />
A diff of the hexdump of the first 2MB of the disk showed some small<br />
discrepancies (at around 0x00100420 and 0x00100570). Before rebooting,<br />
they did not exist. After each boot, there are some small changes.<br />
So my question is:<br />
- Does grub keep some kind of counter or status information when<br />
&nbsp; booting? This would calm me somehow.<br />
- Or does this mean there is something strange going on on my machine?<br />
</blockquote>Do you mean that you do grub-install, compute checksum,  
reboot, compute checksume and both checksums are different?<br />
<br />
Bugs withstanding, the only time when grub writes anything to disk is  
when saving environment variables. So checking grub.cfg for save_env  
commands makes sens.<br />
Also if you compute checksum after reboot it could be operating system  
(anything is startup sequence) that does it. Did you try booting in  
single user (or even with init=/bin/sh to rule out  
everything)?</blockquote><br /><br />



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