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9chroot

I've recently set up and automated a process for building nightly ISO files for 9front, mostly for use in using them or installing virtual machines that I would like to be up to date but don't expect to live long. One thing I wanted with this machine was to use a dedicated disk for building and keep a clean system to build the ISO files from. However it would also be nice to have the build machine using my existing file server such that it could do internal builds as well. To solve this I decided to see if I it was possible to have specific programs on a cpu server run under a different root filesystem.

First up was getting the machine alive and on 9front. Putting a disk in the machine and running through the typical install procedure left me with a terminal using a local hjfs disk as it's root. To add it in to my existing network I configured plan9.ini properly and initialized nvram to make this new machine a standard cpu node for my network.

However using this for building the ISO files left me with three problems:

To solve this I figured I could make use of the existing clean hjfs filesystem on the disk for building. The first step of this is starting hjfs on system boot.

#start hjfs on boot and post to /srv/hjfs
echo 'hjfs -f /dev/sdE2/fs -n hjfs -m 2011' > /cfg/$sysname/cpustart

#bind it in by default when I rcpu in
echo 'bind -c #s/hjfs /n/hjfs' > /cfg/$sysname/namespace

Next we construct a namespace file for the build script to use.

# Replace the use of '#s/boot' to instead use the hjfs instance
sed 's/boot/hjfs/ /lib/namespace > /lib/namespace.build

# Test that everything works by changing in to the new namespace
auth/newns -n /lib/namespace.build

This leaves us with what looks like a typical 'chroot' enviornment that can be invoked for specific programs. With this I can set something up in the cron file of my auth server like this:

40 5 * * * una auth/newns -n /lib/namespace.build /usr/glenda/bin/rc/nightly > /sys/log/build

This will run the nightly script every morning under the new namespace while saving the output to my normal cwfs filesystem. It's worth noting that if you plan to have this namespace be usable for the none user then /srv/hjfs must be read-writable from the none user, adding a chmod o+rw /srv/hjfs after hjfs is started to /cfg/$sysname/cpustart will fix this issue.