We ship boxes configured with overlayroot, using the following overlayroot.conf:

overlayroot=device:dev=/dev/sda6,timeout=20,recurse=0

Which produces the following mount configuration:

$ 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)

As you can see, three key physical partitions: sda1 is /boot, sda5 is a read-only "factory" root, and sda6 is a "user" root which can be wiped at any point to restore the machine to its original factory state.

Now, the problem arises when update-grub is run for any reason:

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

Understandable, since / is an overlayfs.

The contents of /usr/sbin/update-grub are:

#!/bin/sh
set -e
exec grub-mkconfig -o /boot/grub/grub.cfg "$@"

With /usr/sbin/grub-mkconfig being the business part of things. But the actual problem is in /usr/sbin/grub-probe, called by grub-mkconfig, and grub-probe is a binary.

So my question is, is there a parameter or whatever which can make grub-probe do the right thing in the face of / being an overlayfs? And secondly, is there a way to hack/patch that in so that the update-grub script just does the right thing?

Thanks.