efibootmgr -w overwrites existing unique MBR signature

Bug #1065261 reported by Colin Watson on 2012-10-10
This bug affects 1 person
Affects Status Importance Assigned to Milestone
efibootmgr (Ubuntu)
Colin Watson
Colin Watson

Bug Description

Kano reported on IRC that efibootmgr -w overwrites an existing unique MBR signature, and that this can break existing Windows installations. I had assumed that -w was "write unique MBR signature if that field is zero", but code inspection confirms that it's "write unique MBR signature no matter what".

However, I don't want to just drop -w; I'd much rather have the best of both worlds and leave existing signatures in place but also write a new signature if it's zero. My reading of the UEFI spec strongly encourages writing a signature; it notes for example that this field is always set by the OS, never by the firmware.

Related branches

Colin Watson (cjwatson) on 2012-10-10
Changed in grub2 (Ubuntu Quantal):
status: New → Triaged
importance: Undecided → Critical
assignee: nobody → Colin Watson (cjwatson)
milestone: none → ubuntu-12.10
Colin Watson (cjwatson) wrote :

-w is consistently documented as "write unique sig to MBR *if needed*" (emphasis mine), and the original ChangeLog entry that introduced it said:

- Added test for non-zero signature on MBR-style disks,
  and new -w flag to write a unique signature to the disk if so.

So I think this is an efibootmgr bug for failing to meet its own documentation, and since that documentation happens to describe a much more convenient behaviour, I'm going to fix efibootmgr rather than add some quite complex code to grub-install to parse the MBR.

affects: grub2 (Ubuntu Quantal) → efibootmgr (Ubuntu Quantal)
Changed in efibootmgr (Ubuntu Quantal):
status: Triaged → Fix Committed
Launchpad Janitor (janitor) wrote :

This bug was fixed in the package efibootmgr - 0.5.4-2ubuntu2

efibootmgr (0.5.4-2ubuntu2) quantal; urgency=low

  * Fix implementation of -w option to match documentation by leaving an
    existing unique MBR signature intact (LP: #1065261).
 -- Colin Watson <email address hidden> Wed, 10 Oct 2012 23:25:29 +0100

Changed in efibootmgr (Ubuntu Quantal):
status: Fix Committed → Fix Released
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