HOWTO set up a 32-bit chroot in AMD64 the easy way

[hkoster1]

Many users of 64-bit Debian GNU/Linux also (still) have a 32-bit version of the OS installed. One reason may be that some programmes are not yet ported to 64-bit, or don't yet have the full functionality in 64-bit. Multimedia programmes requiring special codecs and plugins (Flash, Shockwave) come to mind; their use would force one to "adulterate" an otherwise pure 64-bit system with 32-bit libraries and wrapper programmes. So, this HOWTO is for those who (1) want to run a pure 64-bit system; or (2) want to run favourite programmes installed in their old 32-bit OS but not (for any reason) in their new 64-bit OS.

This HOWTO describes an easy way to run any programme installed in the 32-bit OS in a chroot jail running under the 64-bit OS. It is so simple to implement that people might even consider installing a new 32-bit OS for exactly that purpose in a spare partition (don't even think of messing with debootstrap).

I'm running Debian Sid in both 32- and 64-bit versions, but clearly Etch or Lenny will do just as well. I do recommend, though, that you run the same distro in both cases, dist-upgrading one of them as needed. I'll use Etch in the examples below, change that to Lenny or Sid as appropriate. My 32-bit OS root partition is /dev/sda8, change that also to match your installation.

The following setup must be run from the 64-bit OS. All commands prefixed with # are to be run as root; user commands use $.

Mount points
First make a mount point for the 32-bit root partition,

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# mkdir /mnt/etch32

and edit /etc/fstab by adding the lines

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/dev/sda8         /mnt/etch32                       ext3   defaults    0  0
/home             /mnt/etch32/home                  none   bind        0  0
/tmp              /mnt/etch32/tmp                   none   bind        0  0
/media/cdrom0     /mnt/etch32/media/cdrom0          none   bind        0  0
/dev              /mnt/etch32/dev                   none   bind        0  0
proc-chroot       /mnt/etch32/proc                  proc   defaults    0  0

A separate boot partition for 32-bit Etch, if you have one, need not be mounted. The new additions are mounted with

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# mount -a

which will also happen automatically at boot from now on. Note that several directories of the 64-bit host OS have been mounted in the chroot, taking the place of similarly named directories in the 32-bit OS.

This completes the setting up of the chroot, what remains is making it simple to use.

Schroot users
Install the schroot package with

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# apt-get install schroot

and configure it by editing /etc/schroot/schroot.conf to contain the following minimal contents:

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[etch32]     # some name for the chroot environment
description=Etch 32-bit chroot
location=/mnt/etch32
aliases=default
personality=linux32
priority=1
users=henk

Of course, you should substitute your own user name.

How to use
Schroot allows the user to run any programme in the 32-bit OS from the 64-bit OS without ever having to enter the chroot. What's more, this can optionally be done with the current 64-bit environment (as in /etc/profile) preserved. For example,

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$ schroot -p iceweasel

starts the 32-bit version of IceWeasel with the user's own settings (bookmarks, etc) for the 64-bit version (provided that IceWeasel is installed in the 32-bit OS). Next, hit on a site playing Flash content or content encoded with w32codecs, and all should be well if it already was set up properly in the 32-bit OS to start with. Note that any changes made inside IceWeasel, like added bookmarks, will also carry over to the 64-bit version of this programme.

Note that maintenance of the 32-bit OS can also be done this way,

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# schroot apt-get update

or even with

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$ sudo schroot apt-get dist-upgrade

so there's no need to shutdown the 64-bit OS and reboot into the 32-bit OS to do so.

We'll leave it to the reader as an exercise to provide icons on the 64-bit desktop that will start 32-bit IceWeasel, Kaffeine, Mplayer, etc, this way.

Removing the chrooted environment
This must be done with care, since some regular directories in the host OS (like /home) are bound to it -- these should not be removed inadvertently! The proper way to remove the chroot environment is by first removing the added lines in /etc/fstab (see above), and then to reboot. Let's hope you do this because Debian GNU/Linux has become fully multi-arch bootable and/or all the multimedia programmes and codecs have become fully functional in 64-bit (that'll be the day...).

Have fun!

Acknowledgements: I started with the Howto by michael7, see http://forums.debian.net/viewtopic.php?t=12061, and consulted recursively the sources listed there. Those sources say that the system libraries of the 32-bit OS should be linked with those of the host 64-bit OS, but this is clearly not needed if all you want to do is run 32-bit programmes in a chroot jail. I have also upgraded from dchroot to schroot. Consult michael7's Howto if you can't solve the icon exercise...

Edited 13 August 2007: rewrote the introductory paragraphs, no changes to the code.

[dmn_clown]

It is also possible to skip editing /etc/fstab and use schroot to mount and umount depending on how you configure schroot. See: man schroot and man schroot.conf for more details.

[hkoster1]

Good point! I'll leave the details as an exercise...