===================================
Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
- Hans Lermen <lermen@fgan.de>
+Hans Lermen <lermen@fgan.de>
initrd provides the capability to load a RAM disk by the boot loader.
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 [1].
+discussion of the boot process can be found in [#f1]_.
Operation
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)
+ 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
+ 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
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
+ 8) init execs the ``/sbin/init`` on the new root filesystem, performing
the usual boot sequence
9) the initrd file system is removed
Boot command-line options
-------------------------
-initrd adds the following new options:
+initrd adds the following new options::
initrd=<path> (e.g. LOADLIN)
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):
+example)::
-find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
+ find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
-Examining the contents of an existing image file is just as simple:
+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
+ 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.
+"normal" root file system, e.g.::
-# mkdir /initrd
+ # mkdir /initrd
-The name is not relevant. More details can be found on the pivot_root(2)
-man page.
+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.
+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:
+accessible if the following device has been created::
-# mknod /dev/initrd b 1 250
-# chmod 400 /dev/initrd
+ # 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
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
+ 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:
+ 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 that /sbin/init's permissions must include "x" (execute).
+ 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
+ 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
+ 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
+ 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 [2] to create a small
+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:
+have to be given::
root=/dev/ram0 rw
(rw is only necessary if writing to the initrd file system.)
-With LOADLIN, you simply execute
+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.
+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
+and run ``/sbin/lilo``
For other boot loaders, please refer to the respective documentation.
- 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:
+a directory under the current root. Example::
-# mkdir /new-root
-# mount -o ro /dev/hda1 /new-root
+ # 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 pivot_root(8) man
-page; pivot_root is distributed with util-linux version 2.10h or higher
-[3]). pivot_root moves the current root to a directory under the new
+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:
+must exist before calling ``pivot_root``. Example::
-# cd /new-root
-# mkdir initrd
-# pivot_root . initrd
+ # 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:
+following command::
-# exec chroot . what-follows <dev/console >dev/console 2>&1
+ # exec chroot . what-follows <dev/console >dev/console 2>&1
-Where what-follows is a program under the new root, e.g. /sbin/init
+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.
+``/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:
- 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:
+disk can be freed::
-# umount /initrd
-# blockdev --flushbufs /dev/ram0
+ # umount /initrd
+ # blockdev --flushbufs /dev/ram0
It is also possible to use initrd with an NFS-mounted root, see the
-pivot_root(8) man page for details.
+:manpage:`pivot_root(8)` man page for details.
Usage scenarios
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
+ 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
+ 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
+ 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
+ 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)
+ ``/dev/ram0`` or ``/dev/rd/0`` to a file)
8) now the system is bootable and additional installation tasks can be
performed
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
+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
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
+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.
+floppies.
Obsolete root change mechanism
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
+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.
+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".
+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.
+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:
+``/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
+ #! /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.
+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
---------
-[1] Almesberger, Werner; "Booting Linux: The History and the Future"
+.. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"
http://www.almesberger.net/cv/papers/ols2k-9.ps.gz
-[2] newlib package (experimental), with initrd example
- http://sources.redhat.com/newlib/
-[3] util-linux: Miscellaneous utilities for Linux
- http://www.kernel.org/pub/linux/utils/util-linux/
+.. [#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/
projects / GitHub / moto-9609 / android_kernel_motorola_exynos9610.git / commitdiff