Re: GRUB and the risk of block list corruption in extX

[Martin Wilck]

Andrey,

> Here is example how using filesystem blocklists may lead to unbootable
> system without any extX corruption involved.
> 
> - user sets up multiboot system with Windows as primary bootloader
> - standard technique to add Linux loaders has always been - copy
>   partition boot sector and "launch" it from Windows loader
> - user copies Linux partition boot sector which points to core.imng
>   absolute disk position
> - user updates grub in Linux. core.img is rewritten and its position
>   changes
> - next time user tries to boot Linux (s)he gets blinking cursor
> 
> So *any* third party bootloader that relies on being able to
> "chainload" *copy* of boot sector will give you the same issue.

I understand. It's generally understood that updating core.img without
updating the boot sector is a bad idea. In this particular case updating
the boot sector is not enough because the copy needs to be updated, too.

The background for my question was a different scenario, with a
chainload-capable boot loader in the MBR and secondary boot loaders in
partition boot sectors. It is that scenario that the new anaconda
installer doesn't support any more, and the major argument from the
Fedora devs for this (apart from sparing dev and QA resources) was the
warning emitted by GRUB when users try to install using block lists.

I am still convinced that the risk of boot loader corruption in that
scenario is extremely low.

Martin

-- 
Dr. Martin Wilck
PRIMERGY System Software Engineer
x86 Server Engineering

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