From: admin Date: Tue, 29 Mar 2011 22:06:54 +0000 (+0000) Subject: (no commit message) X-Git-Url: http://www.opopop.net/cgi-bin/gitweb.cgi?p=reminder;a=commitdiff_plain;h=664b5eefa4828079c1f0c6ad3dc9bdfcb43d196b --- diff --git a/booting_linux_from_a_loop_file_system.mdwn b/booting_linux_from_a_loop_file_system.mdwn new file mode 100644 index 0000000..9148938 --- /dev/null +++ b/booting_linux_from_a_loop_file_system.mdwn @@ -0,0 +1,213 @@ +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: + 
+    kernel ... root=/dev/sdb5 loop=/somedir/myownrootfs ...
+
+ +Now the modifications to /etc/initramfs-tools: + +* File /etc/initramfs-tools/modules: + 
+    loop
+    ntfs
+
+ +This ensures loop.ko and ntfs.ko are included in initrd. + +* File /etc/initramfs-tools/scripts/init-top/loopboot: + 
+     #!/bin/sh     
+     PREREQ=""
+     prereqs()
+     {
+       	 echo "$PREREQ"
+     }
+     case $1 in
+     # get pre-requisites
+     prereqs)
+       	 prereqs
+       	 exit 0
+       	 ;;
+     esac       
+     [ -d ${rootmnt}2 ] || mkdir --mode=0700 ${rootmnt}2
+
+ +This ensures some intermediate mounting point /root2 is created +(rootmnt having value /root from /sbin/init). + +* File /etc/initramfs-tools/scripts/local-premount/loopboot: + 
+     #!/bin/sh      
+     PREREQ=""
+     prereqs()
+     {
+         echo "$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}
+
+ +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: + 
+    menuentry "GNU/Linux" {
+	   set root=(cd)
+	   linux /vmlinuz root=/dev/sdb5 loop=/somedir/myownrootfs
+	   initrd /initrd.img
+    }
+
+1. create a bootable iso with a command like: + 
+    genisoimage -o /tmp/new.iso /tmp/newiso
+
+1. burn the iso: + 
+    wodim /tmp/new.iso
+
+ +# 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)