32bits_sandbox_howto.txt

USING THE WORKER IN A 32 BITS CHROOT
====================================

by Giovanni Mascellani <mascellani@poisson.phc.unipi.it>


  If you're using a 64 bits environment you may experience some
problems in running correctly the sanbox. Although the sandbox is
perfectly capable of running in 64 bits mode, the way the Worker calls
it at the moment could fail in 64 bits environment, because it
requests to allow some syscalls that are not available in 64 bits
mode. Moreover, the Worker is mostly tested in 32 bits environment, so
you should use in 32 bits to achieve more consistent results (it is
strongly advised to install 32 bits operating systems when doing a
real contest).

  To mitigate these problems you can make the Worker running in a 32
bits chroot. This tutorial briefly explains how to do it. It is
assumed that you're using a Debian or Ubuntu operating system.


1. CREATION OF THE CHROOT

  Start by installing the package deboostrap:

# apt-get install debootstrap

  Then make some directory where install chroots and fire debootstrap:

# mkdir /srv/chroot
# debootstrap --arch=i386 sid /srv/chroot/sid32

  You can also add another argument that will be interpreted as the
mirror to use. Otherwise some default one will be used. Of course you
also have to substitute "sid" with the distribution you want to
install: the most sensible ones are sid and squeeze for Debian, and
lucid, maverick and natty for Ubuntu. You can also install an Ubuntu
chroot on Debian and viceversa, but you have to specify the relevan
mirror to use.

  Debootstrap will have to download some hundreds of megabytes of data
from the Internet, so be sure to have a sufficient Internet
connection. Hopefully everything will work and you'll find a brand new
minimal system in the directory you specified.


2. USING THE CHROOT

  In principle you could just use the chroot by executing:

# chroot /srv/chroot/sid32

  This has some disadvantages, though: you can do it only if you're
root and you may have to manually do a lot of things (for example,
mounting /dev, /proc/, ... filesystems, copy things from your home and
other things). It's better to use the schroot program, that automates
lots of these things.

# apt-get install schroot

  Then edit the file /etc/schroot/schroot.conf, adding a stanza like
this:

---
[sid32]
type=directory
description=Debian sid 32 bits
directory=/srv/chroot/sid32
users=<your username>
script-config=default/config
---

  Of course, as before, change description, directory and users as
relevant on your system. Then the specified unprivileged user will
have the power to enter the chroot with this command:

$ schroot -c sid32

  The /home directory (like many others) will be bind-mounted, so
you'll see exactly the same files inside the chroot, without having to
copy anything. To do administrative things (mostly installing
packages: the standard chroot is really essential!) inside the chroot
you can execute the same command as root, and you'll become root
inside the chroot.


3. CAVEATS WITH MO-BOX

  Inside the 32 bits chroot you're using a 64 bits kernel anyway, so
you cannot use a mo-box version compiled for 32 bits kernel (it would
be unsafe, and the mo-box itself wouldn't start unless you manually
change the code). When you compile the mo-box with the compile64.sh
script, two executables get created: mo-box and mo-box32. Just use the
second (overwriting the first with it).