linux-ext4 - Re: GRUB and the risk of block list corruption in extX

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Message-ID: <5113CD2F.9010908@redhat.com>
Date:	Thu, 07 Feb 2013 09:50:07 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Martin Wilck <martin.wilck@...fujitsu.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: GRUB and the risk of block list corruption in extX

On 2/7/13 4:18 AM, Martin Wilck wrote:
> Hello,
> 
> you may have seen the following warning that is displayed when
> someone tries to install GRUB2 on in a extX partition:
> 
>   "/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be
>   installed in this setup by using blocklists. However, blocklists are
>   UNRELIABLE and their use is discouraged."

To be clear, this is only the case when installing the bootloader itself to
a partition containing a filesystem, not when installing to the MBR, correct?

Which is different than saying "/boot is on ext4" - it's putting the bootloader
itself on a partition containing a filesystem, something which is a bit more
unusual, I think.

-Eric

> Recently I have been involved in discussions about this on
> https://bugzilla.redhat.com/show_bug.cgi?id=872826.
> 
> The Grub manual says "installing to a filesystem means that GRUB is
> vulnerable to its blocks being moved around by filesystem features such
> as tail packing, or even by aggressive fsck implementations".
> 
> My question to the extX experts: Under what circumstances (except
> modifying, overwriting, deleting the bootloader image "core.img" itself)
> can a block list referencing "core.img" be corrupted? In particular:
> 
>  1) could it happen during ordinary operation, filesystem code silently
>    moving blocks around?
>  2) could it happen in an e2fsck run?
>  3) could it be caused by e4defrag?
>  4) could it happen with resize2fs even if the blocks occupied by the
> file fit in the size that the FS is resized to (otherwise obviously "yes")?
>  5) Anything else?
>  6) if the file was protected with the IMMUTABLE flag, would any of 1-5
> still be able to corrupt the file?
> 
> Regards
> Martin
> 

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