umount /dev: Invalid argument

Question

I would like to chroot to another rootfs and unmount the former one so, after mounting a SD card containing a rootfs (on /mnt/disk/) and killing almost every process, I tried to run:

umount /tmp
mount --move /proc /mnt/disk/proc
mount --move /sys /mnt/disk/sys

and then, mount --move /dev /mnt/disk/dev which fails with the error message:

mount: mounting /dev/ on /mnt/disk/dev failed: No such file or directory

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Some more information:

• I also tried umount /dev giving

  umount: can't umount /dev/: Invalid argument
  # (same error with /dev/root)

• The former rootfs is on a NAND partition (JFFS2)

• /proc/cmdline contains:

  root=/dev/mtdblock3

• mount -t jffs2 /dev/mtdblock3 /mnt/disk/dev works (mounts the former root) Could it be related to a filesystem conflict between the SD card and the root partition ?

• cat /mnt/disk/proc/mounts

  rootfs / rootfs rw 0 0
  /dev/root / jffs2 rw 0 0
  /proc /mnt/disk/proc proc rw 0 0
  /sys /mnt/disk/sys sysfs rw 0 0
  /dev/mmcblk0p0 /mnt/disk ext2 rw,errors=continue 0 0

• Kernel: 2.6.29.4

• busybox: 1.16.1

Answer 1

# cat /mnt/disk/proc/mounts
rootfs / rootfs rw 0 0
/dev/root / jffs2 rw 0 0
/proc /mnt/disk/proc proc rw 0 0
/sys /mnt/disk/sys sysfs rw 0 0
/dev/mmcblk0p0 /mnt/disk ext2 rw,errors=continue 0 0

umount /dev: Invalid argument

mount: mounting /dev/ on /mnt/disk/dev failed: No such file or directory

There is no filesystem mounted at /dev, therefore you cannot unmount it or move it. That is what these error messages mean. (Possibly busybox mount uses more generic error messages than its heavyweight counterpart).

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If the target /dev/ isn't going to have the device nodes you need, hopefully you have a MAKEDEV in there you can use.

If you try copying them from the original /dev/ using cp -a, there's a risk that the UIDs will not match the target rootfs, and hence permissions won't work as desired for non-root users. (Less secure or "so secure it doesn't work").