Knoppix anatomy in 60 seconds
Wed, 22 Oct 2003 15:39:37 +0000
Knoppix anatomy in 60 seconds. You are not expected to remember this.
- It's a CDROM. It contains an ISO filesystem
- The ISO filesystem contains
- A "compressed loop image" in KNOPPIX/KNOPPIX
- A boot.img file, which is an bootable floppy disk image. This is what
actually boots when someone tries to boot off the CD. It contains
- SYSLINUX and stuff like that
- A kernel
- An initrd image. This is a gzipped ext2 filesystem, containing
- Some modules (mostly for scsi controllers)
- a shell script /linuxrc
- various binaries that /linuxrc needs to call
- So, when you boot, SYSLINUX runs, starts the kernel with appropriate initrd options.
- The kernel unzips and loads the initrd, then runs /linuxrc.
- /linuxrc finds the appropriate scsi modules for the cdrom (which might be the ide-scsi driver)
- /linuxrc mounts /cdrom, then loop mounts /cdrom/KNOPPIX/KNOPPIX as /KNOPPIX
- Given sufficient memory, /linuxrc adds a tmpfs ramdisk and puts some stuff in it. In either case, it populates the root fs with stuff and symlinks
- This is the bit I don't understand too well: /linuxrc appears to use the obsolete /proc/sys/kernel/real-root-dev ("change_root") mechanism to change to the ramdisk that it's already got as root (no, not to the tmpfs ramdisk).
- /sbin/init runs. By symlink wizardry, this is actually on the compressed loop fs
- The system comes up
Recipe for a happy life
This is the rough order I did things in. The end result is Makefile-driven and completely repeatable (except for some questions from debconf that I haven't provided answers for yet, so are still being asked interactively)
- set up apt-proxy. You're probably going to be doing a lot of apt-get
- debootstrap. doen't seem to work on sid, so debootstrap woody then chroot apt-get dist-upgrade. I warned you you wanted apt-proxy
- chroot apt-get install lots more stuff
- exciting messing around with debconf for xserver-xfree86 and etherconf to remove fake answers on the build machine
- mkisofs, createcompressedfs
- loopback mount a knoppix iso, copy its contents into a scratch area, drop the fs from the previous step in as KNOPPIX/KNOPPIX, mkisofs it all back together
- boot it
- fiddle around some more with debconf and attempting to run /var/lib/dpkg/info/foo.config scripts directly. Observe that a lot of stuff is read-only. So, it's time to properly sort out what should be on the cloop image and what should be in RAM
- which is easy enough. create a file listing all the directories
that we don't mind being read-only (/usr, /etc/X11, /bin, /lib, ...),
use tar magic to produce a rootfs.tgz that excludes this, and mkisofs magic
to produce an iso9660 image that includes only this
- add a symlink from /etc/X11/XF86Config -> /var so this can be written after the cd boots
- Hack up /linuxrc significantly. Now instead of creating a zillion symlinks on the fly into the current root then changerooting to it, it creates a tmpfs on /ramdisk, untars the rootfs.tgz (which we can store on the outer iso9660 image) into it, then pivotroots to it
- Swear a bit because linuxrc doesn't run as pid 1, so when we start the real init it thinks it's being run as telinit. Pass the --init option to make it reconsider. Frankly, this sucks, but it'll do for now.
- Cope with random fallout occasioned by "when do we mount /proc" and similar questions.
Right now I am staring at a freshly booted (in vmware) cd image. It seems to have found my ethernet card, sound (not that Common Music is on the shortlist for cirCLe just yet, but eventually this'll be fun to play with), network, and usb card. If I run my cobbled together script for X configuration, I have a working X server. If I run my cobbled together script for ethernet configuration, it runs a dhcp client. I'm not overly impressed by etherconf, though. I think something ought to be able to dhcp first and set the default hostname/domain name based on what comes back. It's been years since I last ran a debian install from scratch, but I'm pretty sure it did this.
Next, write proper init.d scripts to do this detection, start X, run a session/window manager, and stuff. Next also, replace some more of the remaining bits of Knoppix: at the moment, we're copying /lib/modules straight out of the knoppix cloop fs, which is kind of cheesy - would much prefer to have (or at least know) the sources that correspond to our installed kernel.
Was this all a good idea? I think so, probably. We've lost some of the fabled Knoppix detect-all-hardware-on-the-planet juice (no support for isdn or pppoe any longer, nor for graphics cards that XFree86 4.2 doesn't support), but the whole thing is a lot more understandable, and (after we get our own kernel) runs from a Makefile with no need to start by untarring someone else's ISO image and changing it incrementally.