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diff --git a/Documentation/admin-guide/initrd.rst b/Documentation/admin-guide/initrd.rst new file mode 100644 index 000000000..67bbad880 --- /dev/null +++ b/Documentation/admin-guide/initrd.rst @@ -0,0 +1,383 @@ +Using the initial RAM disk (initrd) +=================================== + +Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and +Hans Lermen <lermen@fgan.de> + + +initrd provides the capability to load a RAM disk by the boot loader. +This RAM disk can then be mounted as the root file system and programs +can be run from it. Afterwards, a new root file system can be mounted +from a different device. The previous root (from initrd) is then moved +to a directory and can be subsequently unmounted. + +initrd is mainly designed to allow system startup to occur in two phases, +where the kernel comes up with a minimum set of compiled-in drivers, and +where additional modules are loaded from initrd. + +This document gives a brief overview of the use of initrd. A more detailed +discussion of the boot process can be found in [#f1]_. + + +Operation +--------- + +When using initrd, the system typically boots as follows: + + 1) the boot loader loads the kernel and the initial RAM disk + 2) the kernel converts initrd into a "normal" RAM disk and + frees the memory used by initrd + 3) if the root device is not ``/dev/ram0``, the old (deprecated) + change_root procedure is followed. see the "Obsolete root change + mechanism" section below. + 4) root device is mounted. if it is ``/dev/ram0``, the initrd image is + then mounted as root + 5) /sbin/init is executed (this can be any valid executable, including + shell scripts; it is run with uid 0 and can do basically everything + init can do). + 6) init mounts the "real" root file system + 7) init places the root file system at the root directory using the + pivot_root system call + 8) init execs the ``/sbin/init`` on the new root filesystem, performing + the usual boot sequence + 9) the initrd file system is removed + +Note that changing the root directory does not involve unmounting it. +It is therefore possible to leave processes running on initrd during that +procedure. Also note that file systems mounted under initrd continue to +be accessible. + + +Boot command-line options +------------------------- + +initrd adds the following new options:: + + initrd=<path> (e.g. LOADLIN) + + Loads the specified file as the initial RAM disk. When using LILO, you + have to specify the RAM disk image file in /etc/lilo.conf, using the + INITRD configuration variable. + + noinitrd + + initrd data is preserved but it is not converted to a RAM disk and + the "normal" root file system is mounted. initrd data can be read + from /dev/initrd. Note that the data in initrd can have any structure + in this case and doesn't necessarily have to be a file system image. + This option is used mainly for debugging. + + Note: /dev/initrd is read-only and it can only be used once. As soon + as the last process has closed it, all data is freed and /dev/initrd + can't be opened anymore. + + root=/dev/ram0 + + initrd is mounted as root, and the normal boot procedure is followed, + with the RAM disk mounted as root. + +Compressed cpio images +---------------------- + +Recent kernels have support for populating a ramdisk from a compressed cpio +archive. On such systems, the creation of a ramdisk image doesn't need to +involve special block devices or loopbacks; you merely create a directory on +disk with the desired initrd content, cd to that directory, and run (as an +example):: + + find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img + +Examining the contents of an existing image file is just as simple:: + + mkdir /tmp/imagefile + cd /tmp/imagefile + gzip -cd /boot/imagefile.img | cpio -imd --quiet + +Installation +------------ + +First, a directory for the initrd file system has to be created on the +"normal" root file system, e.g.:: + + # mkdir /initrd + +The name is not relevant. More details can be found on the +:manpage:`pivot_root(2)` man page. + +If the root file system is created during the boot procedure (i.e. if +you're building an install floppy), the root file system creation +procedure should create the ``/initrd`` directory. + +If initrd will not be mounted in some cases, its content is still +accessible if the following device has been created:: + + # mknod /dev/initrd b 1 250 + # chmod 400 /dev/initrd + +Second, the kernel has to be compiled with RAM disk support and with +support for the initial RAM disk enabled. Also, at least all components +needed to execute programs from initrd (e.g. executable format and file +system) must be compiled into the kernel. + +Third, you have to create the RAM disk image. This is done by creating a +file system on a block device, copying files to it as needed, and then +copying the content of the block device to the initrd file. With recent +kernels, at least three types of devices are suitable for that: + + - a floppy disk (works everywhere but it's painfully slow) + - a RAM disk (fast, but allocates physical memory) + - a loopback device (the most elegant solution) + +We'll describe the loopback device method: + + 1) make sure loopback block devices are configured into the kernel + 2) create an empty file system of the appropriate size, e.g.:: + + # dd if=/dev/zero of=initrd bs=300k count=1 + # mke2fs -F -m0 initrd + + (if space is critical, you may want to use the Minix FS instead of Ext2) + 3) mount the file system, e.g.:: + + # mount -t ext2 -o loop initrd /mnt + + 4) create the console device:: + + # mkdir /mnt/dev + # mknod /mnt/dev/console c 5 1 + + 5) copy all the files that are needed to properly use the initrd + environment. Don't forget the most important file, ``/sbin/init`` + + .. note:: ``/sbin/init`` permissions must include "x" (execute). + + 6) correct operation the initrd environment can frequently be tested + even without rebooting with the command:: + + # chroot /mnt /sbin/init + + This is of course limited to initrds that do not interfere with the + general system state (e.g. by reconfiguring network interfaces, + overwriting mounted devices, trying to start already running demons, + etc. Note however that it is usually possible to use pivot_root in + such a chroot'ed initrd environment.) + 7) unmount the file system:: + + # umount /mnt + + 8) the initrd is now in the file "initrd". Optionally, it can now be + compressed:: + + # gzip -9 initrd + +For experimenting with initrd, you may want to take a rescue floppy and +only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you +can try the experimental newlib environment [#f2]_ to create a small +initrd. + +Finally, you have to boot the kernel and load initrd. Almost all Linux +boot loaders support initrd. Since the boot process is still compatible +with an older mechanism, the following boot command line parameters +have to be given:: + + root=/dev/ram0 rw + +(rw is only necessary if writing to the initrd file system.) + +With LOADLIN, you simply execute:: + + LOADLIN <kernel> initrd=<disk_image> + +e.g.:: + + LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw + +With LILO, you add the option ``INITRD=<path>`` to either the global section +or to the section of the respective kernel in ``/etc/lilo.conf``, and pass +the options using APPEND, e.g.:: + + image = /bzImage + initrd = /boot/initrd.gz + append = "root=/dev/ram0 rw" + +and run ``/sbin/lilo`` + +For other boot loaders, please refer to the respective documentation. + +Now you can boot and enjoy using initrd. + + +Changing the root device +------------------------ + +When finished with its duties, init typically changes the root device +and proceeds with starting the Linux system on the "real" root device. + +The procedure involves the following steps: + - mounting the new root file system + - turning it into the root file system + - removing all accesses to the old (initrd) root file system + - unmounting the initrd file system and de-allocating the RAM disk + +Mounting the new root file system is easy: it just needs to be mounted on +a directory under the current root. Example:: + + # mkdir /new-root + # mount -o ro /dev/hda1 /new-root + +The root change is accomplished with the pivot_root system call, which +is also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)` +man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher +[#f3]_). ``pivot_root`` moves the current root to a directory under the new +root, and puts the new root at its place. The directory for the old root +must exist before calling ``pivot_root``. Example:: + + # cd /new-root + # mkdir initrd + # pivot_root . initrd + +Now, the init process may still access the old root via its +executable, shared libraries, standard input/output/error, and its +current root directory. All these references are dropped by the +following command:: + + # exec chroot . what-follows <dev/console >dev/console 2>&1 + +Where what-follows is a program under the new root, e.g. ``/sbin/init`` +If the new root file system will be used with udev and has no valid +``/dev`` directory, udev must be initialized before invoking chroot in order +to provide ``/dev/console``. + +Note: implementation details of pivot_root may change with time. In order +to ensure compatibility, the following points should be observed: + + - before calling pivot_root, the current directory of the invoking + process should point to the new root directory + - use . as the first argument, and the _relative_ path of the directory + for the old root as the second argument + - a chroot program must be available under the old and the new root + - chroot to the new root afterwards + - use relative paths for dev/console in the exec command + +Now, the initrd can be unmounted and the memory allocated by the RAM +disk can be freed:: + + # umount /initrd + # blockdev --flushbufs /dev/ram0 + +It is also possible to use initrd with an NFS-mounted root, see the +:manpage:`pivot_root(8)` man page for details. + + +Usage scenarios +--------------- + +The main motivation for implementing initrd was to allow for modular +kernel configuration at system installation. The procedure would work +as follows: + + 1) system boots from floppy or other media with a minimal kernel + (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and + loads initrd + 2) ``/sbin/init`` determines what is needed to (1) mount the "real" root FS + (i.e. device type, device drivers, file system) and (2) the + distribution media (e.g. CD-ROM, network, tape, ...). This can be + done by asking the user, by auto-probing, or by using a hybrid + approach. + 3) ``/sbin/init`` loads the necessary kernel modules + 4) ``/sbin/init`` creates and populates the root file system (this doesn't + have to be a very usable system yet) + 5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and + execs - via chroot - a program that continues the installation + 6) the boot loader is installed + 7) the boot loader is configured to load an initrd with the set of + modules that was used to bring up the system (e.g. ``/initrd`` can be + modified, then unmounted, and finally, the image is written from + ``/dev/ram0`` or ``/dev/rd/0`` to a file) + 8) now the system is bootable and additional installation tasks can be + performed + +The key role of initrd here is to re-use the configuration data during +normal system operation without requiring the use of a bloated "generic" +kernel or re-compiling or re-linking the kernel. + +A second scenario is for installations where Linux runs on systems with +different hardware configurations in a single administrative domain. In +such cases, it is desirable to generate only a small set of kernels +(ideally only one) and to keep the system-specific part of configuration +information as small as possible. In this case, a common initrd could be +generated with all the necessary modules. Then, only ``/sbin/init`` or a file +read by it would have to be different. + +A third scenario is more convenient recovery disks, because information +like the location of the root FS partition doesn't have to be provided at +boot time, but the system loaded from initrd can invoke a user-friendly +dialog and it can also perform some sanity checks (or even some form of +auto-detection). + +Last not least, CD-ROM distributors may use it for better installation +from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk +via initrd from CD; or by booting via a loader like ``LOADLIN`` or directly +from the CD-ROM, and loading the RAM disk from CD without need of +floppies. + + +Obsolete root change mechanism +------------------------------ + +The following mechanism was used before the introduction of pivot_root. +Current kernels still support it, but you should _not_ rely on its +continued availability. + +It works by mounting the "real" root device (i.e. the one set with rdev +in the kernel image or with root=... at the boot command line) as the +root file system when linuxrc exits. The initrd file system is then +unmounted, or, if it is still busy, moved to a directory ``/initrd``, if +such a directory exists on the new root file system. + +In order to use this mechanism, you do not have to specify the boot +command options root, init, or rw. (If specified, they will affect +the real root file system, not the initrd environment.) + +If /proc is mounted, the "real" root device can be changed from within +linuxrc by writing the number of the new root FS device to the special +file /proc/sys/kernel/real-root-dev, e.g.:: + + # echo 0x301 >/proc/sys/kernel/real-root-dev + +Note that the mechanism is incompatible with NFS and similar file +systems. + +This old, deprecated mechanism is commonly called ``change_root``, while +the new, supported mechanism is called ``pivot_root``. + + +Mixed change_root and pivot_root mechanism +------------------------------------------ + +In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_root +mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd +image. + +``/linuxrc`` would contain only the following:: + + #! /bin/sh + mount -n -t proc proc /proc + echo 0x0100 >/proc/sys/kernel/real-root-dev + umount -n /proc + +Once linuxrc exited, the kernel would mount again your initrd as root, +this time executing ``/sbin/init``. Again, it would be the duty of this init +to build the right environment (maybe using the ``root= device`` passed on +the cmdline) before the final execution of the real ``/sbin/init``. + + +Resources +--------- + +.. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future" + https://www.almesberger.net/cv/papers/ols2k-9.ps.gz +.. [#f2] newlib package (experimental), with initrd example + https://www.sourceware.org/newlib/ +.. [#f3] util-linux: Miscellaneous utilities for Linux + https://www.kernel.org/pub/linux/utils/util-linux/ |