[debian-knoppix] [HOWTO] How to boot a chroot directory ...

[Fabian Franz <FabianFranz@gmx.de>]

Hi,

in my last mail I said, that there are problems with chroot, that you are not 
able to unmount or remount ...

I also had an idea, and this idea did work ...

What you need:

- Linux partition
- Lilo (Grub could also be possible)

Create an ext2-image with 2560000 bytes. (Should be enough). 

Mount it to /mnt.

-rwxr-xr-x    1 root     root       150580 2002-07-26 14:09 /mnt/static/ash
lrwxrwxrwx    1 root     root            3 2003-04-01 23:46 /mnt/static/init 
-> ash
lrwxrwxrwx    1 root     root            3 2003-04-01 23:46 /mnt/static/sh -> 
ash

Copy ash to /mnt/static and create symlinks ...

Copy needed modules (for ext3 that are jbd.o and ext3.o) to /mnt/modules

-rw-r--r--    1 root     root        87626 2003-04-01 23:46 
/mnt/modules/ext3.o
-rw-r--r--    1 root     root        74090 2003-04-01 23:46 /mnt/modules/jbd.o

Copy device that needs to be mounted to /mnt/dev (or copy whole /dev)

cp libraries for non-static binaries ("real" mount/pivot_root are needed)

-rwxr-xr-x    1 root     root        82456 2003-05-07 09:27 
/mnt/lib/ld-linux.so.2
-rwxr-xr-x    1 root     root      1104072 2003-05-07 09:27 /mnt/lib/libc.so.6

mkdir -p /mnt/mnt /mnt/mnt2 /mnt/proc

copy mount and pivot_root to /bin:


-rwsr-xr-x    1 root     root        81080 2003-05-07 09:27 /mnt/bin/mount
-rwsr-xr-x    1 root     root          4832 2003-05-07 09:27 
/mnt/bin/pivot_root

Last but not least create the /mnt/linuxrc the heart of our operation :-))

#!/static/sh

# load needed modules
insmod /modules/jbd.o
insmod /modules/ext3.o

# mount proc, say that root is pivot_rooted and unmount it
mount -t proc proc /proc
echo 256 > /proc/sys/kernel/real-root-dev
umount /proc

# mount the root-filesystem
mount -t ext3 /dev/hda5 /mnt

# tricky: We now bind our chroot environment to /mnt2
/bin/mount --bind /mnt/home/knoppix/newroot /mnt2

# now we unmount the root-filesystem; believe me, its possible and /mnt2 is 
still there
umount /mnt

# Now we change the root permanently to the chroot-dir and we can do every 
operation that is needed (remount, unmount, ...)

/bin/pivot_root /mnt2 /mnt2/initrd

Of course you can sue the comments and just have the script so compact :

#!/static/sh
insmod /modules/jbd.o
insmod /modules/ext3.o
mount -t proc proc /proc
echo 256 > /proc/sys/kernel/real-root-dev
umount /proc
mount -t ext3 /dev/hda5 /mnt
/bin/mount --bind /mnt/home/knoppix/newroot /mnt2
umount /mnt
/bin/pivot_root /mnt2 /mnt2/initrd

I tried this and it did work ... I was really suprised, but I think its an 
kind of easy way to test several copied knoppix systems or bootstrapped and 
then knoppised debian's :-)).

If anyone knows an easier way, I'm interested to learn it :-) (Or if anyone 
knows this way from another source on the web ...)

btw. lilo.conf:

image=/boot/vmlinuz-2.4.20-xfs
        label=Chroot
        append="hda=scsi hdb=scsi hdc=scsi hdd=scsi hde=scsi hdf=scsi hdg=scsi 
hdh=scsi apm=power-off nomce ramdisk_size=100000 lang=de init=/etc/init"
        initrd=/boot/chroot_initrd
        read-only

I think it would be possible to also have all this pointing to 
/home/knoppix/newroot/vmlinuz|boot/chroot_initrd ...

But as I had the same kernel, I'm lazy ... :-))

cu

Fabian

PS: I hope this is useful to someone ...
PPS: So you can finally start your changed system and such USE sortlists and 
so optimise the system more and more without the need of an extra-partition 
...
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