[Unionfs] UnionFS as root filesystem

[Francesco Carsana]

Tomas M wrote:
> > mount -o dirs=/mnt/hda1=ro:/mnt/hda2=rw unionfs /mnt/hda2
>
> That mount command is wrong, at first, leftmost branch must be 
> writable, not ro.
> I don't know if you can overmount the directory used in unionfs as a 
> branch,
> I believe that the result is undefined and you should not do that.
>
> Tomas


I'm working with Knoppix 3.7 (Kernel 2.4.27) and UnionFS 1.0.14.
I must use kernel 2.4.27 because it's necessary for other purposes 
(RTLinux...)
My linuxrc is like this:

   #!/bin/sh
   set -x
   export PATH=.:/:/usr/sbin:/usr/bin:/sbin:/bin:/usr/X11R6/bin
   mount -t proc /proc /proc
   mkdir -p /mnt
   mkdir -p /mnt/hda2
   mount -nt ext3 -o rw /dev/hda2 /mnt/hda2
   mkdir -p /mnt/hda1
   mount -nt ext3 -o ro /dev/hda1 /mnt/hda1
   mkdir -p /mnt/union
   modprobe unionfs
   mount -t unionfs -o dirs="/mnt/hda2=rw:/mnt/hda1=ro" unionfs /mnt/union
   cd /mnt/union
   mkdir -p initrd
   pivot_root . initrd
   exec chroot . /sbin/init  <dev/console >dev/console 2>&1

And grub's menu.lst is:

   title        Debian GNU/Linux, kernel 2.4.27 (Safe Mode)
   kernel       (hd0,0)/boot/vmlinuz-2.4.27 rw root=/dev/hda2
                  init=/etc/init ramdisk_size=100000 lang=us
                  screen=1280x1024 vga=0x31B nomce apm=power-off
   initrd       (hd0,0)/boot/roinitrd.img.gz
   savedefault
   boot

If I write "/bin/bash" insted of "/sbin/init" in the last life of linuxrc,
I obtain the root filesystem that I want (union between hda1 and hda2) but
the init process doesn't start, so I think that pivot root works well...
If I leave "/sbin/init" the init process remount /dev/hda2 as root 
filesystem
(hda2 is a clean ext3 partition) and obviously can't find /sbin/init.
Infact the kernel prints these messages:

   Usage: init 0123456SsQqAaBbCcUu
   VFS: Mounted root (ext3 filesystem).
   Trying to move old root to /initrd ... okay
   Freeing unused kernel memory: 144k freed
   Warning: unable to open an initial console.
   Kernel panic: No init found. Try passing init= option to kernel