[BusyBox] OT: Using CRAMFS and DEVFS on BB system?

[Erik Andersen]

On Mon Apr 30, 2001 at 07:00:07PM -0400, Tom Cameron wrote:
> Hello all,
> 	I have a problem.  Essentially, I want a read-write filesystem
> on my BB system, but I use cramfs for my bootimage.  Now, my question
> is: How would I go about mounting a, lets say ramdrive, formatting it,
> and getting my BB system copied to it...and after all that, mounting the
> new ramdrive as / ?  Keep in mind that I'm using DEVFS, which is making
> things a bit difficult for me (as I can't even mount the root now
> because the devices haven't been created).  Anyone with any ideas would
> be a GREAT help...or anyone with a better way even.  Thanks in
> advance!!!


I personally would leave the root fs read-only, but mount ramdisks
over the top of places you need to write to -- like /tmp.  Then make 
other places that need to be written to symlinks to dirs under /tmp.
But anyways, here is a rough implementation of what you asked for:

    1) The kernel mounts root fs (cramfs)
    2) The kernel starts /sbin/init (busybox)
    3) busybox init reads a line like "::sysinit:/etc/init.d/rcS" from /etc/inittab
    4) /etc/init.d/rcS runs ash and looks something like:
	    #!/bin/sh

	    # disable modprobe calls
	    if [ -f "/proc/sys/kernel/modprobe" ] ; then
		    echo "/bin/true" >/proc/sys/kernel/modprobe
	    fi

	    # Start all init scripts in /etc/init.d
	    # executing them in numerical order.
	    #
	    for i in /etc/init.d/S??* ;do

		 # Ignore dangling symlinks (if any).
		 [ ! -f "$i" ] && continue

		 case "$i" in
		    *.sh)
			# Source shell script for speed.
			(
			    trap - INT QUIT TSTP
			    set start
			    . $i
			)
			;;
		    *)
			# No sh extension, so fork subprocess.
			$i start
			;;
		esac
	    done
    5) Then you would have a S01mount script to do things like

	    #!/bin/sh
	    mount -t devfs devfs /dev
	    
	    # I have no idea if devfs subsumes devpts or not.
	    # Assuming not...
	    mkdir -p /dev/pts
	    mount devpts /dev/pts -t devpts -o rw,gid=5,mode=620 
	     
    6) Then you need to create a ramdisk to work in and pivot_root to it.

	    #!/bin/sh

	    SIZE=8192
	    INODES=850

	    # Build ramdisks to overlay the underlying filesystem
	    echo -n "Building device ramdisk: "

	    dd if=/dev/zero of=/dev/ram1 bs=1k count=$SIZE > /dev/null 2>&1
	    mkfs.minix -n30 -i $INODES /dev/ram1 $SIZE > /dev/null 2>&1

	    mount /dev/ram1 /mnt -t minix -o rw > /dev/null 2>&1
	    if [ ! $? -eq 0 ] ; then
		    echo "failed."
	    fi;

	    cp -a /usr /bin /sbin /lib <whatever> /mnt > /dev/null 2>&1
	    if [ ! $? -eq 0 ] ; then
		    echo "failed."
	    fi;

	    pivot_root /mnt /opt

 -Erik

--
Erik B. Andersen   email:  andersen at lineo.com
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