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The Loopback Root Filesystem HOWTO by Andrew M. Bishop, amb@gedanken.demon.co.uk v1.0.0, 12 June 1998 This HOWTO explains how to use the Linux loopback device to create a Linux native filesystem format installation that can be run from a DOS partition without re-partitioning. Other uses of this same technique are also discussed. ______________________________________________________________________ Table of Contents 1. Principles of Loopback Devices and Ramdisks 1.1 Loopback Devices 1.2 Ramdisk Devices 1.3 The Initial Ramdisk Device 1.4 The Root Filesystem 1.5 The Linux Boot Sequence 2. How To Create a Loopback Root Device 2.1 Requirements 2.2 Creating the Linux Kernel 2.3 Creating the Initial Ramdisk Device 2.4 Creating The Root Device 2.5 Creating the Swap Device 2.6 Creating the MSDOS Directory 2.7 Creating the Boot Floppy 3. Booting the System 3.1 Possible Problems With Solutions 3.2 Reference Documents 4. Other Loopback Root Device Possibilities 4.1 DOS Hard-disk Only Installation 4.2 LILO Booted Installation 4.3 VFAT / NTFS Installation 4.4 Installing Linux without Re-partitioning 4.5 Booting From a Non-bootable device ______________________________________________________________________ 1. Principles of Loopback Devices and Ramdisks First I will describe some of the general principles that are used in the setting up of a loopback filesystem as the root device. 1.1. Loopback Devices A loopback device in Linux is a virtual device that can be used like any other media device. Examples of normal media devices are hard disk partitions like /dev/hda1, /dev/hda2, /dev/sda1, or entire disks like the floppy disk /dev/fd0 etc. They are all devices that can be used to hold a files and directory structures. They can be formatted with the filesystem that is required (ext2fs, msdos, ntfs etc.) and then mounted. The loopback filesystem associates a file on another filesystem as a complete device. This can then be formatted and mounted just like any of the other devices listed above. To do this the device called /dev/loop0 or /dev/loop1 etc is associated with the file and then this new virtual device is mounted. 1.2. Ramdisk Devices In Linux it is also possible to have another type of virtual device mounted as a filesystem, this is the ramdisk device. In this case the device does not refer to any physical hardware, but to a portion of memory that is set aside for the purpose. The memory that is allocated is never swapped out to disk, but remains in the disk cache. A ramdisk can be created at any time by writing to the ramdisk device /dev/ram0 or /dev/ram1 etc. This can then be formatted and mounted in the same way that the loopback device is. When a ramdisk is used to boot from (as is often done on Linux installation disks or rescue disks) then the disk image (the entire contents of the disk as a single file) can be stored on the boot floppy in a compressed form. This is automatically recognised by the kernel when it boots and is uncompressed into the ramdisk before it is mounted. 1.3. The Initial Ramdisk Device The initial ramdisk device in Linux is another important mechanism that we need to be able to use a loopback device as a the root filesystem. When the initial ramdisk is used the filesystem image is copied into memory and mounted so that the files on it can be accessed. A program on this ramdisk (called /linuxrc) is run and when it is finished a different device is mounted as the root filesystem. The old ramdisk is still present though and is mounted on the directory /initrd if present or available through the device /dev/initrd. This is unusual behaviour since the normal boot sequence boots from the designated root partition and keeps on running. With the initial ramdisk option the root partition is allowed to change before the main boot sequence is started. 1.4. The Root Filesystem The root filesystem is the device that is mounted first so that it appears as the directory called / after booting. There are a number of complications about the root filesystem that are due to the fact that it contains all files. When booting the rc scripts are run, these are either the files in /etc/rc.d or /etc/rc?.d depending on the version of the /etc/init program. When the system has booted it is not possible to unmount the root partition or change it since all programs will be using it to some extent. This is why the initial ramdisk is so useful because it can be used so that the final root partition is not the same as the one that is loaded at boot time. 1.5. The Linux Boot Sequence To show how the initial ramdisk operates in the boot sequence, the order of events is listed below. 1. The kernel is loaded into memory, this is performed by LILO or LOADLIN. You can see the Loading... message as this happens. 2. The ramdisk image is loaded into memory, again this is performed by LILO or LOADLIN. You can see the Loading... message again as this happens. 3. The kernel is initialised, including parsing the command line options and setting of the ramdisk as the root device. 4. The program /linuxrc is run on the initial ramdisk. 5. The root device is changed to that specified in the kernel parameter. 6. The init program /etc/init is run which will perform the user configurable boot sequence. This is just a simplified version of what happens, but is sufficient to explain how the kernel starts up and where the initial ramdisk is used. 2. How To Create a Loopback Root Device Now that the general principles are explained the method of creating the loopback device can be explained. 2.1. Requirements To create the loopback root device will require a number of things. ╖ A working Linux system. ╖ A way to copy large files onto the target DOS partition. Most important is access to an installed Linux system. This is because the loop device can only be created under Linux. This will mean that it is not possible to bootstrap a working system from nothing. The requirements of the Linux system that you use is that you can compile a kernel on it. Once the loopback device is created it will be a large file. I have used an 80 MB files, but while this was sufficient for an X terminal it may not be enough if you want to use it for much else. This file must be copied onto the DOS partition, so either a network or a lot of floppy disks must be used. The software that you will require includes ╖ LOADLIN version 1.6 or above ╖ A version of mount that supports loopback devices ╖ A version of the kernel that supports the required options. All of these should be standard for recent Linux installations. 2.2. Creating the Linux Kernel I created the loopback device using Linux kernel version 2.0.31, other versions should also work, but they must have at least the options listed below. The kernel options that you will need to enable are the following: ╖ RAM disk support (CONFIG_BLK_DEV_RAM). ╖ Initial RAM disk (initrd) support (CONFIG_BLK_DEV_INITRD). ╖ Loop device support (CONFIG_BLK_DEV_LOOP). ╖ fat fs support (CONFIG_FAT_FS). ╖ msdos fs support (CONFIG_MSDOS_FS). The first two are for the RAM disk device itself and the initial ram disk device. The next one is the loop back filesystem option. The last two are the msdos filesystem support which is required to mount the DOS partitition. Compiling a kernel without modules is the easiest option, although if you do want modules then it should be possible although I have not tried it. If modules are used then you should have the options above compiled in and not as modules themselves. The kernel source code itself will need to be changed in one very simple way. The version 2.0.34 kernel as supplied does not allow the loopback device to be used as the root filesystem. One very small kernel patch can make this possible. The file /init/main.c needs to have a single line added to it as shown by the modified version below. The line that says "loop", 0x0700 is the one that was added. static void parse_root_dev(char * line) { int base = 0; static struct dev_name_struct { const char *name; const int num; } devices[] = { { "nfs", 0x00ff }, { "loop", 0x0700 }, { "hda", 0x0300 }, ... { "sonycd", 0x1800 }, { NULL, 0 } }; ... } Once the kernel is configured it should be compiled to produce a zImage file (make zImage). This file will be arch/i386/boot/zImage when compiled. 2.3. Creating the Initial Ramdisk Device The initial ramdisk is most easily created as a loopback device from the start. You will need to do this as root, the commands that you need to execute are listed below, they are assumed to be run from root's home directory (/root). mkdir /root/initrd dd if=/dev/zero of=initrd.img bs=1k count=1024 mke2fs -i 1024 -b 1024 -m 5 -F -v initrd.img mount initrd.img /root/initrd -t ext2 -o loop cd initrd [create the files] cd .. umount /root/initrd gzip -c -9 initrd.img > initrdgz.img There are a number of steps to this, but they can be described as follows. 1. Create a mount point for the initial ramdisk (an empty directory). 2. Create an empty file of the size required. Here I have used 1024kB, you may need less or more depending on the contents, (the size is the last parameter). 3. Make an ext2 filesystem on the empty file. 4. Mount the file onto the mount point, this uses the loopback device. 5. Change to the mounted loopback device. 6. Create the files that are required (see below for details). 7. Move out of the mounted loopback device. 8. Unmount the device. 9. Create a compressed version for use later. Contents Of The Initial Ramdisk The files that you will need on the ramdisk are the minimum requirements to be able to execute any commands. ╖ /linuxrc The script that is run to mount the msdos file system (see below). ╖ /lib/* The dynamic linker and the libraries that the programs need. ╖ /etc/* The cache used by the dynamic linker (not strictly needed, but does stop it complaining). ╖ /bin/* A shell interpreter (ash because it is smaller than bash. The mount and losetup programs for handling the DOS disk and setting up the loopback devices. ╖ /dev/* The devices that will be used. You need /dev/zero for ld- linux.so, /dev/hda* to mount the msdos disk and /dev/loop* for the lopback device. ╖ /mnt An empty directory to mount the msdos disk on. The initial ramdisk that I used is listed below, the contents come to about 800kB when the overhead of the filesystem are taken into account. total 18 drwxr-xr-x 2 root root 1024 Jun 2 13:57 bin drwxr-xr-x 2 root root 1024 Jun 2 13:47 dev drwxr-xr-x 2 root root 1024 May 20 07:43 etc drwxr-xr-x 2 root root 1024 May 27 07:57 lib -rwxr-xr-x 1 root root 964 Jun 3 08:47 linuxrc drwxr-xr-x 2 root root 12288 May 27 08:08 lost+found drwxr-xr-x 2 root root 1024 Jun 2 14:16 mnt ./bin: total 168 -rwxr-xr-x 1 root root 60880 May 27 07:56 ash -rwxr-xr-x 1 root root 5484 May 27 07:56 losetup -rwsr-xr-x 1 root root 28216 May 27 07:56 mount lrwxrwxrwx 1 root root 3 May 27 08:08 sh -> ash ./dev: total 0 brw-r--r-- 1 root root 3, 0 May 20 07:43 hda brw-r--r-- 1 root root 3, 1 May 20 07:43 hda1 brw-r--r-- 1 root root 3, 2 Jun 2 13:46 hda2 brw-r--r-- 1 root root 3, 3 Jun 2 13:46 hda3 brw-r--r-- 1 root root 7, 0 May 20 07:43 loop0 brw-r--r-- 1 root root 7, 1 Jun 2 13:47 loop1 crw-r--r-- 1 root root 1, 3 May 20 07:42 null crw-r--r-- 1 root root 5, 0 May 20 07:43 tty crw-r--r-- 1 root root 4, 1 May 20 07:43 tty1 crw-r--r-- 1 root root 1, 5 May 20 07:42 zero ./etc: total 3 -rw-r--r-- 1 root root 2539 May 20 07:43 ld.so.cache ./lib: total 649 lrwxrwxrwx 1 root root 18 May 27 08:08 ld-linux.so.1 -> ld-linux.so.1.7.14 -rwxr-xr-x 1 root root 21367 May 20 07:44 ld-linux.so.1.7.14 lrwxrwxrwx 1 root root 14 May 27 08:08 libc.so.5 -> libc.so.5.3.12 -rwxr-xr-x 1 root root 583795 May 20 07:44 libc.so.5.3.12 ./lost+found: total 0 ./mnt: total 0 The only complex steps about this are the devices in dev. Use the mknod program to create them, use the existing devices in /dev as a template to get the required parameters. The /linuxrc file The /linuxrc file on the initial ramdisk is required to do all of the preparations so that the loopback device can be used for the root partition when it exits. The example below tries to mount /dev/hda1 as an msdos partition and if it succeeds then sets up the files /linux/linuxdsk.img as /dev/loop0 and /linux/linuxswp.img as /dev/loop1. #!/bin/sh echo INITRD: Trying to mount /dev/hda1 as msdos if /bin/mount -n -t msdos /dev/hda1 /mnt; then echo INITRD: Mounted OK /bin/losetup /dev/loop0 /mnt/linux/linuxdsk.img /bin/losetup /dev/loop1 /mnt/linux/linuxswp.img exit 0 else echo INITRD: Mount failed exit 1 fi The first device /dev/loop0 will become the root device and the second one /dev/loop1 will become the swap space. If you want to be able to write to the DOS partition as a non-root user when you have finished then you should use mount -n -t msdos /dev/hda1 /mnt -o uid=0,gid=0,umask=000 instead. This will map all accesses to the DOS partition to root and set the permissions appropriately. 2.4. Creating The Root Device The root device that you will be using is the file linuxdsk.img. You will need to create this in the same way that the initial ramdisk was created, but bigger. You can install any Linux installation that you like onto this disk. The easiest way might be to copy an existing Linux installation into it. An alternative is to install a new Linux installation onto it. Assuming that you have done this, there are some minor changes that you must make. The /etc/fstab file must reference the root partition and the swap using the two loopback devices that are setup on the initial ramdisk. /dev/loop0 / ext2 defaults 1 1 /dev/loop1 swap swap defaults 1 1 This will ensure that when the real root device is used the kernel will not be confused about where the root device is. It will also allow the swap space to be added in the same way a swap partition is normally used. You should remove any other reference to a root disk device or swap partition. If you want to be able to read the DOS partition after Linux has started then you will need to make a number of extra small changes. Create a directory called /initrd, this is where the initial ramdisk will be mounted once the loopback root filesystem is mounted. Create a symbolic link called /DOS that points to /initrd/mnt where the real DOS parition will be mounted. Add a line into the rc file that mounts the disks. This should run the command mount -f -t msdos /dev/hda1 /initrd/mnt, this will create a 'fake' mount of the DOS partition so that all programs (like df) will know that the DOS partition is mounted and where to find it. If you used different options in the /linuxrc file that obviously you should use them here also. There is no need to have a Linux kernel on this root device since that is already loaded earlier. If you are using modules however then you should include them on this device as normal. 2.5. Creating the Swap Device The root device that you will be using is the file linuxswap.img. The swap device is very simple to create. Create an empty file as was done for the initial ramdisk and then run mkswap linuxswap.img to intialise it. The size of the swap space will depend on what you plan to do with the installed system, but I would recommend between 8MB and the amount of RAM that you have. 2.6. Creating the MSDOS Directory The files that are going to be used need to be moved onto the DOS partition. The files that are required in the DOS directory called C:\LINUX are the following: ╖ LINUXDSK.IMG The disk image that will become the root device. ╖ LINUXSWP.IMG The swap space. 2.7. Creating the Boot Floppy The boot floppy that is used is just a normal DOS format bootable floppy. This is created using format a: /s from DOS. Onto this disk you will need to create an AUTOEXEC.BAT file (as below) and copy the kernel, compressed initial ramdisk and LOADLIN executable. ╖ AUTOEXEC.BAT The DOS automatically executed batch file. ╖ LOADLIN.EXE The LOADLIN program executable. ╖ ZIMAGE The Linux kernel. ╖ INITRDGZ.IMG The compressed initial ramdisk image. The AUTOEXEC.BAT file should contain just one line as below. \loadlin \zImage initrd=\initrdgz.img root=/dev/loop0 ro This specifies the kernel image to use, the initial ramdisk image, the root device after the initial ramdisk has finished and that the root partition is to be mounted read-only. 3. Booting the System To boot from this new root device all that is required is that the floppy disk prepared as described above is inserted for the PC to boot from. You will see the following sequence of events. 1. DOS boots 2. AUTOEXEC.BAT starts 3. LOADLIN is run 4. The Linux kernel is copied into memory 5. The initial ramdisk is copied into memory 6. The Linux kernel is started running 7. The /linuxrc file on the initial ramdisk is run 8. The DOS partition is mounted and the root and swap devices set up 9. The boot sequence continues from the loopback device When this is complete you can remove the boot floppy and use the Linux system. 3.1. Possible Problems With Solutions There are a number of stages where this process could fail, I will try to explain what they are and what to check. DOS booting is easy to recognise by the message that it prints MS-DOS Starting ... on the screen. If this is not seen then the floopy disk is either not-bootable or the PC is not bootable from the floppy disk drive. When the AUTOEXEC.BAT file is run the commands in it should be echoed to the screen by default. In this case there is just the single line in the file that starts LOADLIN. When LOADLIN executes it will do two very visible things, firstly it will load the kernel into memory, secondly it will copy the ramdisk into memory. Both of these are indicated by a Loading... message. The kernel starts by uncompressing itself, this can give crc errors if the kernel image is corrupted. Then it will start running the initialisation sequence which is very verbose with diagnostic messages. Loading of the initial ramdisk device is also visible during this phase. When the /linuxrc file is run there is no diagnostic messages, but you can add these yourself as an aid to debugging. If this stage fails to set up the loopback device as the root device then you may see a message that there is no root device and the kernel aborts. The normal boot sequence of the new root device will now continue and this is quite verbose. There may be problems about the root device being mounted read-write, but the LOADLIN command line option 'ro' should stop that. Other problems that can occur are that the boot sequence is confused about where the root device is, this is probably due to a problem with /etc/fstab. When the boot sequence has completed, the remaining problem is that programs are confused about whether the DOS partition is mounted or not. This is why it is a good idea to use the fake mount command described earlier. This makes life a lot easier if you want to access the files on the DOS device. 3.2. Reference Documents The documents that I used to create my first loopback root filesystem were: ╖ The Linux kernel source, in particular init/main.c ╖ The Linux kernel documentation, in particular Documentation/initrd.txt and Documentation/ramdisk.txt. ╖ The LILO documentation. ╖ The LOADLIN documentation 4. Other Loopback Root Device Possibilities Once the principle of booting a filesystem in a file on a DOS partition has been established there are many other things that you can now do. 4.1. DOS Hard-disk Only Installation If it is possible to boot Linux from a file on a DOS harddisk by using a boot floppy then it is obviously also possible to do it using the harddisk itself. A configuration boot menu can be used to give the option of running LOADLIN from within the AUTOEXEC.BAT. This will give a much faster boot sequence, but is otherwise identical. 4.2. LILO Booted Installation Using LOADLIN is only one option for booting a Linux kernel. There is also LILO that does much the same but without needing DOS. In this case the DOS format floppy disk can be replaced by an ext2fs format one. Otherwise the details are very similar, with the kernel and the initial ramdisk being files on that disk. The reason that I chose the LOADLIN method is that the arguments that need to be given to LILO are slightly more complex. Also it is more obvious to a casual observer what the floppy disk is since it can be read under DOS. 4.3. VFAT / NTFS Installation I have tried the NTFS method, and have had no problems with it. The NTFS filesystem driver is not a standard kernel option, but you must use the patch from Martin von L÷wis which is available from his web page. http://www.informatik.hu-berlin.de/~loewis/ntfs/. This is alpha software and requires a patch to the kernel which is not quite trivial but not too difficult. The only changes for the VFAT or NTFS options are in the initial ramdisk, the file /linuxrc needs to mount a file system of type vfat or ntfs rather that msdos. I know of no reason why this should not also work on a VFAT partition. 4.4. Installing Linux without Re-partitioning The process of installing Linux on a PC from a standard distribution requires booting from a floppy disk and re-partitioning the disk. This stage could instead be accomplished by a boot floppy that creates an empty loopback device and swap file. This would allow the installation to proceed as normal, but it would install into the loopback device rather than a partition. This could be used as an alternative to a UMSDOS installation, it would be more efficient in disk usage since the minimum allocation unit in the ext2 filesystem is 1kB instead of up to 32kB on a DOS partition. It can also be used on VFAT and NTFS formatted disks which are otherwise a problem. 4.5. Booting From a Non-bootable device This method can also be used to boot a Linux system from a device that is not normally bootable. ╖ CD-Rom ╖ Zip Disks ╖ Parallel port disk drives Obviously there are many other devices that could be used, NFS root filesystems are already included in the kernel as an option, but the method described here might also be used instead.