mjhjmtu

I am trying to set up a read-only filesystem at /root directory on my embedded device.
I have following in code in my boot init command file /sbin/init-overlay:

rootmnt=/root

ro_mount_point="$rootmnt%/.ro"
rw_mount_point="$rootmnt%/.rw"

# For local system rearranged from init
/bin/mkdir -p "$ro_mount_point" "$rw_mount_point"

# Move the already-mounted root filesystem to the ro mount point:
/bin/mount --move $rootmnt $ro_mount_point

# Mount the read/write filesystem:
/bin/mount -t tmpfs root.rw "$rw_mount_point"

# Mount the union:
/bin/mount -t aufs -o dirs=$rw_mount_point=rw:$ro_mount_point=ro aufs $rootmnt

# Correct the permissions of /:
/bin/chmod 755 "$rootmnt"

# Make sure the individual ro and rw mounts are accessible from within the root
# once the union is assumed as /. This makes it possible to access the
# component filesystems individually.
/bin/mkdir "$rootmnt/ro" "$rootmnt/rw"
/bin/mount --bind "$ro_mount_point" "$rootmnt/ro"
/bin/mount --bind "$rw_mount_point" "$rootmnt/rw"

This is my boot command:

console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rootfstype=ext4 rootwait fsck.repair=$fsck.repair panic=10 $extra fbcon=$fbcon rw init=/sbin/init-overlay

But while booting, I am getting the following error:

mount: mounting /root on /root.ro failed: Invalid argument

Can anyone point-out what's wrong here?

The most likely explanation for a mount --move failing with "Invalid argument" is that the source isn't a mount point itself. If $rootmnt isn't a mount point, you have two choices: either properly make it one, or fake it with mount --bind "$rootmnt" "$rootmnt".