[syslinux] Stumped by extlinux After OS Upgrade

[Gene Cumm]

On Wed, Nov 3, 2010 at 17:53, H. Peter Anvin <hpa at zytor.com> wrote:
> On 11/03/2010 09:22 AM, John.Florian at dart.biz wrote:
>>
>> I've never had to reinstall grub unless something happened to the MBR.  It
>> just seems to be able to find its various stages.  I'm still not clear
>> what extlinux was doing in my case.  I suspect the first stage was loading
>> fine, but it couldn't find the next stage due to the shuffling my update
>> script performs.  I also suspect the extlinux --update repairs that link.
>> Maybe that's all wrong.  Maybe the stage transfer went fine, but the odd
>> behavior was simply version incompatibility between the stages.
>>
>
> That's because Grub by default stashed information in unreserved parts of
> your disk (specifically, the gap between the MBR and the first partition,
> which most partition programs create.)  I consider this unsafe, since a lot
> of tools are known to muck with this space in a completely uncoordinated
> fashion.
>
>        -hpa

As an example, I've seen some rather common desktop applications stuff
license authorization data there, breaking a boot loader that I was
using at the time.

If I recall correctly, the default was for Grub's stage 1 to load a
stage 1.5, stuffed in the unreserved sectors, which can understand
ext2/3 and load stage 2, avoiding the issue when the stage 2 file is
touched.  Optionally, stage 1 can load stage 2 but touching the stage
2 file would then have the same effect as touching ldlinux.sys of
potentially breaking the boot loader.

-- 
-Gene