--- /dev/null
+No general provision seems to exist to boot a Linux system and loop
+mount as root a file containing a file system, albeit even more
+complex scenarii, involving the device mapper and its implementations,
+are supported. It can be useful to have that facility, if
+repartitioning or using a live CD is not practical. Of course, this
+makes sense mostly if you don't already run Linux on the target host.
+
+So in the example below, I will suppose you want to mount a root file
+system contained in a NTFS file. This NTFS file is part of an
+underlying Windows machine. Except for the creation of that NTFS
+file, the rule of the game is to not have to use Windows for the
+installation step and to not mess with the Windows setup, especially
+not it's boot process. Booting will happen through an external device
+(network, CD or USB stick).
+
+# Similar approaches
+
+The above constraint obviously rules out approaches such as
+[CoLinux](http://www.colinux.org/) and
+[friends](http://www.topologilinux.com). Other methods which run
+the installer on Windows such as [Wubi](http://wubi.sourceforge.net) or the [Debian Windows installer](http://en.wikipedia.org/wiki/Win32-loader_(Debian\)) are also out, even if they probably use the
+same kind of tricks which I will describe below. So this was actually
+a way for me to learn these!
+
+# Booting principles
+
+The current standard way of booting Debian is recalled briefly:
+
+1. the boot loader (GRUB) loads the kernel image and an initramfs file (line initrd in the GRUB configuration)
+1. the boot loader uncompresses the kernel and pass control to it
+1. the initramfs file (a cpio gzipped archive) is uncompressed and mounted as the root file system
+1. /sbin/init is run and does it's job: essentially loading needed
+kernel modules which have been prepared inside the initramfs and which
+are needed for mounting the real root file system, which name is given
+by kernel options root=
+1. the real root file system is mounted, presence of initramfs is cleaned up
+and control is passed to the /sbin/init there
+
+More information about initramfs and it's predecessor initrd can be
+found on [Wikipedia](http://en.wikipedia.org/wiki/Initrd).
+
+Our strategy will be to modify the initramfs file system content to be able to
+loop mount an NTFS file. But first, you will have to create and populate that file.
+
+# Installing a Linux root file system in a NTFS file
+
+The Debian installer has no direct option for that. You could proceed
+as follows, in case you have only one file system and no swap:
+
+1. create some big NTFS file, say /somedir/myownrootfs; this is the only step requiring running Windows
+1. boot a Debian installer CD, and start a shell when asked to define the root partition
+1. mount the NTFS partition, and create a file system on /somedir/myownrootfs
+1. exit the shell and continue the installation as normal, naming /somedir/myownrootfs as the root partition and giving the loop option
+1. at the end of the installation, don't let GRUB write on your NTFS partition
+
+If you want swap or separate file systems for /usr, /home..., you will have to adapt the above procedure.
+
+# Booting in a Linux system whose root file system is a NTFS file
+
+At that point, your only option to boot in the previously installed
+system is to run the CD installer in rescue mode, loop mount the NTFS
+file and pivot_root in it. You will have to do that at least once, as
+the following procedure requires modifications to some files inside
+/somedir/myownrootfs, and running a command to generate a new
+initramfs while the system is live.
+
+So from now on, I will suppose you have booted in your system as
+explained just above.
+
+# initramfs-tools
+
+The Debian specific package initramfs-tools contains all that is
+necessary to manipulate and build the initramfs file. The man page of
+initramfs-tools explains the organization of the Debian initramfs. In
+particular, /etc/initramfs-tools is where to put host specific scripts
+to run inside initramfs, while /usr/share/initramfs-tools contains
+generic stuff.
+
+You are encouraged to explore the initramfs file created by the
+installation process (called like /boot/initrd.img-2.6...) to
+understand fully how the following works.
+
+# Modifying initramsfs content
+
+The root file system is to be given as the root= option for the
+kernel, normally a partition name. In our case, we need 2 pieces of
+information: the name of the file containing the root file system and
+it's NTFS partition name. So we need to add an additional kernel
+option providing that file name; I will use loop=/somedir/myownrootfs,
+while the NTFS partition will be called /dev/sdb5.
+
+This implies editing the GRUB configuration file, for instance the kernel line would be like:
+ <pre>
+ kernel ... root=/dev/sdb5 loop=/somedir/myownrootfs ...
+ </pre>
+
+Now the modifications to /etc/initramfs-tools:
+
+* File /etc/initramfs-tools/modules:
+ <pre>
+ loop
+ ntfs
+ </pre>
+
+This ensures loop.ko and ntfs.ko are included in initrd.
+
+* File /etc/initramfs-tools/scripts/init-top/loopboot:
+ <pre>
+ #!/bin/sh
+ PREREQ=""
+ prereqs()
+ {
+ echo "$PREREQ"
+ }
+ case $1 in
+ # get pre-requisites
+ prereqs)
+ prereqs
+ exit 0
+ ;;
+ esac
+ [ -d ${rootmnt}2 ] || mkdir --mode=0700 ${rootmnt}2
+ </pre>
+
+This ensures some intermediate mounting point /root2 is created
+(rootmnt having value /root from /sbin/init).
+
+* File /etc/initramfs-tools/scripts/local-premount/loopboot:
+ <pre>
+ #!/bin/sh
+ PREREQ=""
+ prereqs()
+ {
+ echo "$PREREQ"
+ }
+ case $1 in
+ # get pre-requisites
+ prereqs)
+ prereqs
+ exit 0
+ ;;
+ esac
+ modprobe -k ntfs
+ mount -n -t ntfs -o nodiratime,noatime ${ROOT} ${rootmnt}2
+ modprobe -k loop
+ mount -n -t ext2 -o loop ${rootmnt}2${loop} ${rootmnt}
+ </pre>
+
+This loop mounts effectively the file from loop= option at the
+expected point. Notice that I have here hardcoded the file system
+types. It's certainly possible to autodetect them, but then a larger
+set of file systems modules are to be included in
+/etc/initramfs-tools/modules
+
+# Creating a bootable device
+
+Now you have to run update-initramfs to get the new
+initrd.img-2.6... file in place. At that point, if you want to reboot
+without the installer CD, you have these choices:
+
+* create a bootable device: USB stick, CD-ROM... including the kernel
+ image and initramfs file; GRUB on that device has to be configured
+ to load it from there
+* in case of network booting, upload kernel
+ image and initramfs file to another host
+ configured as a bootp or dhcp server.
+
+Notice how the /boot directory content on /somedir/myownrootfs will
+never be used when booting; it is only there to feed the booting
+method just chosen.
+
+For the sake of completeness, here is how to create an iso for a bootable CD-ROM:
+
+1. install package grub-rescue; it contains an iso file
+/usr/lib/grub-rescue/grub-rescue-cdrom.iso
+1. mount that iso as e.g. /media/cdrom
+1. copy all the content of /media/cdrom to a temporary place, e.g. /tmp/newiso,
+while keeping the permissions of the files and directories
+1. you can now unmount /media/cdrom
+1. in /tmp/newiso, you can remove boot.catalog, but add to it kernel
+ image and initramfs file
+1. in /tmp/newiso/boot/grub, edit grub.cfg to boot from
+the 2 files just added above, which would look like:
+ <pre>
+ menuentry "GNU/Linux" {
+ set root=(cd)
+ linux /vmlinuz root=/dev/sdb5 loop=/somedir/myownrootfs
+ initrd /initrd.img
+ }
+ </pre>
+1. create a bootable iso with a command like:
+ <pre>
+ genisoimage -o /tmp/new.iso /tmp/newiso
+ </pre>
+1. burn the iso:
+ <pre>
+ wodim /tmp/new.iso
+ </pre>
+
+# The moment of truth
+
+Then, you can reboot with the just burned CD. In case of problems,
+[use the break= or debug
+kernel options](http://wiki.debian.org/InitramfsDebug).
+
+Related links:
+
+* comparing initramfs with an [alternative](http://wiki.debian.org/InitrdReplacementOptions)
+* what is said about initramfs by the [Debian competition](http://en.gentoo-wiki.com/wiki/Initramfs)
+* as noted at the beginning, there is support in initramfs-tools for the
+[device mapper](http://linuxgazette.net/114/kapil.html), so
+a more generic solution to the problem solved here
+may involve some work on top of that, such as [dm-loop](http://sources.redhat.com/lvm2/wiki/DMLoop)
projects / reminder / blobdiff