update-grub fails in the face of overlayroot

[Mike Purvis]

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.

[Mike Friedrich (mfriedrich74)]

This affects all releases, not just trusty. e.g. xenial and bionic