[Bug 1786992] [NEW] Existing bios mode Windows install left unbootable after installing in EFI mode

[Phillip Susi]

Public bug reported:

Bugs #1668148 and #1766945 changed the installation process to install
grub-efi instead of grub-pc when the installer is booted in EFI mode but
the existing OS on the disk is in bios mode.  Currently, this renders
the existing Windows or FreeDOS install unbootable.  Windows installs at
least may be able to be salvaged by patching os-prober to generate a
grub.cfg that still has the windows bios mode chainloader command when
interpreted by grub-pc ( if the user tells their firmware to boot in
legacy mode ), or to chainload the windows EFI boot loader when booting
grub-efi.  Normally os-prober looks for the windows bootmgr in the ESP,
but in the case of a bios mode install, a copy can also be found in the
windows\boot directory and at least in theory should be loadable to boot
Windows even though it was originally installed in bios mode.

As things currently stand, this will be a major breakage for users in
18.10 and 18.04.1, so unless the changes to os-prober can be made in
time, the other two changes should be reverted and simply stick with
grub-pc in the case where the existing OS is in bios mode.

** Affects: os-prober (Ubuntu)
     Importance: Critical
         Status: Triaged

** Affects: partman-auto (Ubuntu)
     Importance: Critical
         Status: Triaged

** Affects: partman-efi (Ubuntu)
     Importance: Critical
         Status: Triaged

** Affects: ubiquity (Ubuntu)
     Importance: Critical
         Status: Triaged

** Also affects: partman-efi (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: partman-auto (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: ubiquity (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: os-prober (Ubuntu)
    Milestone: None => ubuntu-18.04.1

** Changed in: partman-auto (Ubuntu)
    Milestone: None => ubuntu-18.04.1

** Changed in: partman-efi (Ubuntu)
    Milestone: None => ubuntu-18.04.1

** Changed in: ubiquity (Ubuntu)
    Milestone: None => ubuntu-18.04.1

** Changed in: os-prober (Ubuntu)
       Status: New => Triaged

** Changed in: partman-auto (Ubuntu)
       Status: New => Triaged

** Changed in: partman-efi (Ubuntu)
       Status: New => Triaged

** Changed in: ubiquity (Ubuntu)
       Status: New => Triaged

** Changed in: os-prober (Ubuntu)
   Importance: Undecided => Critical

** Changed in: partman-auto (Ubuntu)
   Importance: Undecided => Critical

** Changed in: partman-efi (Ubuntu)
   Importance: Undecided => Critical

** Changed in: ubiquity (Ubuntu)
   Importance: Undecided => Critical

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https://bugs.launchpad.net/bugs/1786992

Title:
  Existing bios mode Windows install left unbootable after installing in
  EFI mode

Status in os-prober package in Ubuntu:
  Triaged
Status in partman-auto package in Ubuntu:
  Triaged
Status in partman-efi package in Ubuntu:
  Triaged
Status in ubiquity package in Ubuntu:
  Triaged

Bug description:
  Bugs #1668148 and #1766945 changed the installation process to install
  grub-efi instead of grub-pc when the installer is booted in EFI mode
  but the existing OS on the disk is in bios mode.  Currently, this
  renders the existing Windows or FreeDOS install unbootable.  Windows
  installs at least may be able to be salvaged by patching os-prober to
  generate a grub.cfg that still has the windows bios mode chainloader
  command when interpreted by grub-pc ( if the user tells their firmware
  to boot in legacy mode ), or to chainload the windows EFI boot loader
  when booting grub-efi.  Normally os-prober looks for the windows
  bootmgr in the ESP, but in the case of a bios mode install, a copy can
  also be found in the windows\boot directory and at least in theory
  should be loadable to boot Windows even though it was originally
  installed in bios mode.

  As things currently stand, this will be a major breakage for users in
  18.10 and 18.04.1, so unless the changes to os-prober can be made in
  time, the other two changes should be reverted and simply stick with
  grub-pc in the case where the existing OS is in bios mode.

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