Boot from FAT32 error and return to grub menu

I've duplicated this problem - installing on a FAT32 partition with size > 4GB. This creates a separate usr.disk and this seems to be resulting in a problem in processing grub.cfg when loop0 is reset from usr.disk to root.disk.

This also happens when changing the root.disk (e.g. through a custom boot entry). In my case, I added a custom menu entry to boot a copy of my root.disk (new.disk). First time I select it I get the "You have to load the kernel", hit Enter to return to the grub menu. If I then select the custom entry it boots fine. If I didn't select the custom entry (i.e. the normal root.disk) I get the same error, and then after returning it boots.

So it seems to be the act of replacing the loop device that causes the issue.

I'll try experimenting. e.g. does the initial loop device have to be the same as the one in the menu entries? I'll play around a bit and see if I can get a workaround.

Modifying to use loop1 in the menu entry works:
menuentry 'Ubuntu - backup new.disk' --class ubuntu --class gnu-linux
--class gnu --class os {
        set gfxpayload=$linux_gfx_mode
        insmod part_msdos
        insmod ntfs
        set root='(hd0,msdos3)'
        search --no-floppy --fs-uuid --set=root 18B4B7BBB4B799A8
        loopback loop1 /ubuntu/disks/new.disk
        set root=(loop1)
        linux /vmlinuz root=UUID=18B4B7BBB4B799A8
loop=/ubuntu/disks/new.disk ro quiet splash vt.handoff=7
        initrd /initrd.img
}

Not sure if this has memory implications, but it seems like it would solve
this problem for FAT32 if the usr.disk was setup on loop1.

The patch is for file: /usr/lib/grub/grub-mkconfig_lib
This is from package: grub-common

It produces no diff for my own wubi install's grub.cfg (single virtual disk), I'll test it on the FAT32 install and confirm that it resolves it when I have time.

Added a task for grub2 in Ubuntu as the patch above is for it.

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I've just tested the patch on a FAT32 wubi install and it worked as intended.

In order to test it, I manually edited the /usr/share/grub/grub-mkconfig_lib file during the installation (patch wasn't installed), used 'diff' to confirm it matched the patch, and then copied it to /target/usr/share/grub/grub_mkconfig_lib (/target/usr is the mount point for /dev/loop4 which is for the usr.disk virtual disk). And root.disk was mounted as /dev/loop2 on /target.

The grub.cfg that was generated during the install is attached, and it shows that usr.disk will be mounted on loop4 by grub and root.disk is mounted as loop2:
insmod part_msdos
insmod fat
set root='(hd1,msdos5)'
search --no-floppy --fs-uuid --set=root 2dab-fe25
loopback loop4 /ubuntu/disks/usr.disk
set root=(loop4)
if loadfont /share/grub/unicode.pf2 ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  insmod part_msdos
  insmod fat
  set root='(hd1,msdos5)'
  search --no-floppy --fs-uuid --set=root 2dab-fe25
  loopback loop2 /ubuntu/disks/root.disk
  set root=(loop2)
  set locale_dir=($root)/boot/grub/locale
  set lang=en_US
  insmod gettext
fi
...

After rebooting and selecting the Wubi install, there was no error or return to the menu. Then running "sudo update-grub" resulted in grub.cfg changing the loop device for usr.disk to loop1 and root.disk to loop0 (as expected):
insmod part_msdos
insmod fat
set root='(hd1,msdos5)'
search --no-floppy --fs-uuid --set=root 2dab-fe25
loopback loop1 /ubuntu/disks/usr.disk
set root=(loop1)
if loadfont /share/grub/unicode.pf2 ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  insmod part_msdos
  insmod fat
  set root='(hd1,msdos5)'
  search --no-floppy --fs-uuid --set=root 2dab-fe25
  loopback loop0 /ubuntu/disks/root.disk
  set root=(loop0)
  set locale_dir=($root)/boot/grub/locale
  set lang=en_US
  insmod gettext
fi

Please use 'diff -u' when generating diffs. Context diffs are difficult to read, and are especially painful when there's no actual context included. Also, put the previous version on the diff command line before the new version so that the patch doesn't come out reversed.

As for the substance of the patch, I had to think about it a bit to convince myself that it was safe to use Linux's loop device ordering as a unique key for GRUB's loop device naming, but after consideration I agree that it's fine. I'll apply this patch for my next upload.

Okay, I see the difference in the diff -u output, and my ordering mistake. I'll make a note of that.
Thanks

This bug was fixed in the package grub2 - 1.99-18ubuntu1

---------------
grub2 (1.99-18ubuntu1) precise; urgency=low

  * Resynchronise with Debian. Remaining changes:
    - Adjust for default Ubuntu boot options ("quiet splash").
    - Default to hiding the menu; holding down Shift at boot will show it.
    - Set a monochromatic theme and an appropriate background for Ubuntu.
    - Apply Ubuntu GRUB Legacy changes to legacy update-grub script.
    - Fix backslash-escaping in merge_debconf_into_conf.
    - Remove "GNU/Linux" from default distributor string.
    - Add crashkernel option.
    - Bypass menu unless other OSes are installed or Shift is pressed.
    - Allow Shift to interrupt 'sleep --interruptible'.
    - Reduce visual clutter in normal mode.
    - Remove verbose messages printed before reading configuration.
    - Suppress kernel/initrd progress messages, except in recovery mode.
    - Show the boot menu if the previous boot failed.
    - Don't generate device.map during grub-install or grub-mkconfig.
    - Adjust upgrade version checks for Ubuntu.
    - Suppress "GRUB loading" message unless Shift is held down.
    - Adjust versions of grub-doc and grub-legacy-doc conflicts.
    - Fix LVM/RAID probing in the absence of /boot/grub/device.map.
    - Look for .mo files in /usr/share/locale-langpack first.
    - Build-depend on qemu-kvm rather than qemu-system for grub-pc tests.
    - Check hardware support before using gfxpayload=keep.
    - Put second and subsequent Linux menu entries in a submenu.
    - Preferred resolution detection for VBE.
    - Set vt.handoff=7 for smooth handoff to kernel graphical mode.
    - Update default/grub.md5sum to include maverick's default md5sum.
    - In recovery mode, add nomodeset to the Linux kernel arguments, and
      remove the 'set gfxpayload=keep' command.
    - Skip Windows os-prober entries on Wubi systems, and suppress the menu
      by default if those are the only other-OS entries.
    - Handle probing striped DM-RAID devices.
    - Replace 'single' by 'recovery' when friendly-recovery is installed.
    - Use qemu -no-kvm in tests for now to work around LP #947597.
    - Disable cursor as early as possible in grub_main.

grub2 (1.99-18) unstable; urgency=low

  [ Colin Watson ]
  * Ensure that /sbin and /usr/sbin are in $PATH when running tests (closes:
    #662916).
  * mkconfig_loopback.patch: Use different GRUB loop devices for different
    OS loop devices (thanks, bcbc; LP: #888281).
  * Backport from upstream:
    - Add support for LZO compression in btrfs (LP: #727535).
    - Fix efiemu configure check.

  [ Ilya Yanok ]
  * Backport from upstream:
    - Make FAT UUID uppercase to match Linux (LP: #948716).

  [ Debconf translations ]
  * Norwegian Bokmål (Hans Fredrik Nordhaug).
  * Gujarati (Kartik Mistry). Closes: #663542
 -- Colin Watson <email address hidden> Mon, 19 Mar 2012 20:00:22 +0000