[HOWTO] How to boot a chroot directory ...

[Fabianx]

This is actuaqlly a copy of a mail I sent to debian-knoppix earlier this day ...

I hope this is interesting for some of you:

---

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
...

[garyng]

I am confused. What do you want to achieve ? I use pivot_root in the normal initrd to boot into a real root fs(say a clooped device, or a loopback device on FAT/NTFS) but don't quite understand what you want to do in this post. pivot_root is sort of like a 'chain boot' to me and there is no way one can get back to the previous root system. I believe UML is a better way to test out a new distribution within a running system.

BTW, in general people seldom do a pivot_root to absolute path "/..." but rather :

cd /your_new_root
pivot_root . etc/init(this is a under the new root system and note the absence of / before etc)

[Fabianx]

I am confused. What do you want to achieve ? I use pivot_root in the normal initrd to boot into a real root fs(say a clooped device, or a loopback device on FAT/NTFS) but don't quite understand what you want to do in this post. pivot_root is sort of like a 'chain boot' to me and there is no way one can get back to the previous root system. I believe UML is a better way to test out a new distribution within a running system.

BTW, in general people seldom do a pivot_root to absolute path "/..." but rather :

cd /your_new_root
pivot_root . etc/init(this is a under the new root system and note the absence of / before etc)

If you want to test Knoppix-features like hw-detection, I think UML is no choice as you need real test-circumstances ...

I want to boot a directory /home/knoppix/newroot as if everything in newroot was in its own partition!

So /mnt/home/knoppix/newroot -> /

But with pivot_root this is not possible ... (Neither with relative nor absolute path)

You need the trick, I've described above ...

cu

Fabian

PS: Btw and yes, you CAN change the System, while running with pivot_root and you can also go back (if you don
't unmount the initrd) ...