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GRUB and the risk of block list corruption in extX

From: Martin Wilck
Subject: GRUB and the risk of block list corruption in extX
Date: Thu, 07 Feb 2013 11:47:33 +0100
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this is a question about the long-running topic of installing GRUB in
partitions or partitionless disks.

Recently I have been involved in discussions about this on The Fedora boot
loader can't be installed in partition headers any more. The major
reason given by the Fedora developers is the famous GRUB warning
"blocklists are UNRELIABLE and their use is discouraged."

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".

I'd like to understand how this blocklist corruption might come to pass
(except for cases where "core.img" itself is moved, deleted, or
overwritten by user space tools). Also, it has been recommended to
prevent accidental corruption by setting the IMMUTABLE flag on core.img,
and I'd like to ask for the GRUB experts' opinion about that.
Finally I'd like to know if it's true that the GRUB team plans to drop
block list support altogether in a future version.

Martin Wilck

PS: It has been stated that of recent filesystems, this matters most for
extX, because btrfs has a 64k header where embedding GRUB is usually
possible. Therefore I asked a similar question on ext4-devel.

Dr. Martin Wilck
PRIMERGY System Software Engineer
x86 Server Engineering

Fujitsu Technology Solutions GmbH
Heinz-Nixdorf-Ring 1
33106 Paderborn, Germany
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