[Bug 1247905] Re: update-grub fails in the face of overlayroot

[Matthias Kruzenski]

WORKING SOLUTION
================

After encountering this problem, I developed a very simple solution:


nano /usr/sbin/grub-probe-overlay

-----[BEGIN]--------------------------------------------------------------
#!/bin/sh

ARGS=

for ARG in $@; do
	if [ "${ARG%%[!/]*}" = "/" ]; then

                OPTIONS=

		if FSTYPE=$( findmnt -n -o FSTYPE "$ARG" ); then
			if [ "$FSTYPE" = "overlay" ]; then
				OPTIONS=$(findmnt -n -o OPTIONS "$ARG")
			fi
		elif FSTYPE=$( findmnt -n -o FSTYPE / ); then
			if [ "$FSTYPE" = "overlay" ]; then
				OPTIONS=$(findmnt -n -o OPTIONS /)
			fi
		fi

		if LOWERDIR=$(echo "$OPTIONS" | grep -m 1 -oP 'lowerdir=\K[^,]+'); then
#			echo "[DEBUG] Overlay filesystem detected \${ARG} --> \${LOWERDIR}\${ARG%*/}" 1>&2
			ARG="${LOWERDIR}${ARG%*/}"
		fi
	fi
	ARGS="$ARGS $ARG"
done

grub-probe-default $ARGS

exit $?
-----[END]----------------------------------------------------------------


mv /usr/sbin/grub-probe /usr/sbin/grub-probe-default

ln -s /usr/sbin/grub-probe-overlay /usr/sbin/grub-probe


OPTIONAL STEP:
==============

apt-mark hold grub-pc

NOTE: package name can also be "grub-efi-amd64" or "grub-efi-ia32" or
something else, depending on what you use

PROS
====
- works perfectly with overlay filesystem
- shows a general solution (maybe someone can use it to fix the grub-probe source code)
- no patching of grub scripts required (some people tried patches as solution, but grub-probe is used in many grub scripts --> patching nightmare)

CONS
====
- grub-probe must be renamed (for this reason, I recommend that you set the status of your grub package to "hold")

-- 
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Foundations Bugs, which is subscribed to grub2 in Ubuntu.
https://bugs.launchpad.net/bugs/1247905

Title:
  update-grub fails in the face of overlayroot

Status in grub2 package in Ubuntu:
  Triaged
Status in grub2 source package in Trusty:
  Triaged

Bug description:
  Info on overlayroot: http://blog.dustinkirkland.com/2012/08
  /introducing-overlayroot-overlayfs.html

  Mount configuration for an overlayroot-configured box:

  $ mount
  overlayroot on / type overlayfs (rw,errors=remount-ro)
  /dev/sda5 on /media/root-ro type ext3 (ro,relatime,errors=continue,user_xattr,acl,barrier=1,data=ordered)
  /dev/sda6 on /media/root-rw type ext3 (rw,relatime,errors=continue,user_xattr,acl,barrier=1,data=ordered)
  /dev/sda1 on /boot type ext3 (rw)

  When attempting to update-grub:

  $ sudo update-grub
  [sudo] password for administrator: 
  /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).

  The issue comes about in this line of the /usr/sbin/grub-mkconfig
  script:

  GRUB_DEVICE="`${grub_probe} --target=device /`"

  In the short term, I'm hacking that script to pass in a value for
  GRUB_DEVICE, but the ideal would be to have the grub-probe binary
  detect when an overlayfs is / and return the device which is the
  bottom of the overlay.

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