cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
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initrd.rst (14627B)


      1Using the initial RAM disk (initrd)
      2===================================
      3
      4Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
      5Hans Lermen <lermen@fgan.de>
      6
      7
      8initrd provides the capability to load a RAM disk by the boot loader.
      9This RAM disk can then be mounted as the root file system and programs
     10can be run from it. Afterwards, a new root file system can be mounted
     11from a different device. The previous root (from initrd) is then moved
     12to a directory and can be subsequently unmounted.
     13
     14initrd is mainly designed to allow system startup to occur in two phases,
     15where the kernel comes up with a minimum set of compiled-in drivers, and
     16where additional modules are loaded from initrd.
     17
     18This document gives a brief overview of the use of initrd. A more detailed
     19discussion of the boot process can be found in [#f1]_.
     20
     21
     22Operation
     23---------
     24
     25When using initrd, the system typically boots as follows:
     26
     27  1) the boot loader loads the kernel and the initial RAM disk
     28  2) the kernel converts initrd into a "normal" RAM disk and
     29     frees the memory used by initrd
     30  3) if the root device is not ``/dev/ram0``, the old (deprecated)
     31     change_root procedure is followed. see the "Obsolete root change
     32     mechanism" section below.
     33  4) root device is mounted. if it is ``/dev/ram0``, the initrd image is
     34     then mounted as root
     35  5) /sbin/init is executed (this can be any valid executable, including
     36     shell scripts; it is run with uid 0 and can do basically everything
     37     init can do).
     38  6) init mounts the "real" root file system
     39  7) init places the root file system at the root directory using the
     40     pivot_root system call
     41  8) init execs the ``/sbin/init`` on the new root filesystem, performing
     42     the usual boot sequence
     43  9) the initrd file system is removed
     44
     45Note that changing the root directory does not involve unmounting it.
     46It is therefore possible to leave processes running on initrd during that
     47procedure. Also note that file systems mounted under initrd continue to
     48be accessible.
     49
     50
     51Boot command-line options
     52-------------------------
     53
     54initrd adds the following new options::
     55
     56  initrd=<path>    (e.g. LOADLIN)
     57
     58    Loads the specified file as the initial RAM disk. When using LILO, you
     59    have to specify the RAM disk image file in /etc/lilo.conf, using the
     60    INITRD configuration variable.
     61
     62  noinitrd
     63
     64    initrd data is preserved but it is not converted to a RAM disk and
     65    the "normal" root file system is mounted. initrd data can be read
     66    from /dev/initrd. Note that the data in initrd can have any structure
     67    in this case and doesn't necessarily have to be a file system image.
     68    This option is used mainly for debugging.
     69
     70    Note: /dev/initrd is read-only and it can only be used once. As soon
     71    as the last process has closed it, all data is freed and /dev/initrd
     72    can't be opened anymore.
     73
     74  root=/dev/ram0
     75
     76    initrd is mounted as root, and the normal boot procedure is followed,
     77    with the RAM disk mounted as root.
     78
     79Compressed cpio images
     80----------------------
     81
     82Recent kernels have support for populating a ramdisk from a compressed cpio
     83archive. On such systems, the creation of a ramdisk image doesn't need to
     84involve special block devices or loopbacks; you merely create a directory on
     85disk with the desired initrd content, cd to that directory, and run (as an
     86example)::
     87
     88	find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
     89
     90Examining the contents of an existing image file is just as simple::
     91
     92	mkdir /tmp/imagefile
     93	cd /tmp/imagefile
     94	gzip -cd /boot/imagefile.img | cpio -imd --quiet
     95
     96Installation
     97------------
     98
     99First, a directory for the initrd file system has to be created on the
    100"normal" root file system, e.g.::
    101
    102	# mkdir /initrd
    103
    104The name is not relevant. More details can be found on the
    105:manpage:`pivot_root(2)` man page.
    106
    107If the root file system is created during the boot procedure (i.e. if
    108you're building an install floppy), the root file system creation
    109procedure should create the ``/initrd`` directory.
    110
    111If initrd will not be mounted in some cases, its content is still
    112accessible if the following device has been created::
    113
    114	# mknod /dev/initrd b 1 250
    115	# chmod 400 /dev/initrd
    116
    117Second, the kernel has to be compiled with RAM disk support and with
    118support for the initial RAM disk enabled. Also, at least all components
    119needed to execute programs from initrd (e.g. executable format and file
    120system) must be compiled into the kernel.
    121
    122Third, you have to create the RAM disk image. This is done by creating a
    123file system on a block device, copying files to it as needed, and then
    124copying the content of the block device to the initrd file. With recent
    125kernels, at least three types of devices are suitable for that:
    126
    127 - a floppy disk (works everywhere but it's painfully slow)
    128 - a RAM disk (fast, but allocates physical memory)
    129 - a loopback device (the most elegant solution)
    130
    131We'll describe the loopback device method:
    132
    133 1) make sure loopback block devices are configured into the kernel
    134 2) create an empty file system of the appropriate size, e.g.::
    135
    136	# dd if=/dev/zero of=initrd bs=300k count=1
    137	# mke2fs -F -m0 initrd
    138
    139    (if space is critical, you may want to use the Minix FS instead of Ext2)
    140 3) mount the file system, e.g.::
    141
    142	# mount -t ext2 -o loop initrd /mnt
    143
    144 4) create the console device::
    145
    146    # mkdir /mnt/dev
    147    # mknod /mnt/dev/console c 5 1
    148
    149 5) copy all the files that are needed to properly use the initrd
    150    environment. Don't forget the most important file, ``/sbin/init``
    151
    152    .. note:: ``/sbin/init`` permissions must include "x" (execute).
    153
    154 6) correct operation the initrd environment can frequently be tested
    155    even without rebooting with the command::
    156
    157	# chroot /mnt /sbin/init
    158
    159    This is of course limited to initrds that do not interfere with the
    160    general system state (e.g. by reconfiguring network interfaces,
    161    overwriting mounted devices, trying to start already running demons,
    162    etc. Note however that it is usually possible to use pivot_root in
    163    such a chroot'ed initrd environment.)
    164 7) unmount the file system::
    165
    166	# umount /mnt
    167
    168 8) the initrd is now in the file "initrd". Optionally, it can now be
    169    compressed::
    170
    171	# gzip -9 initrd
    172
    173For experimenting with initrd, you may want to take a rescue floppy and
    174only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you
    175can try the experimental newlib environment [#f2]_ to create a small
    176initrd.
    177
    178Finally, you have to boot the kernel and load initrd. Almost all Linux
    179boot loaders support initrd. Since the boot process is still compatible
    180with an older mechanism, the following boot command line parameters
    181have to be given::
    182
    183  root=/dev/ram0 rw
    184
    185(rw is only necessary if writing to the initrd file system.)
    186
    187With LOADLIN, you simply execute::
    188
    189     LOADLIN <kernel> initrd=<disk_image>
    190
    191e.g.::
    192
    193	LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw
    194
    195With LILO, you add the option ``INITRD=<path>`` to either the global section
    196or to the section of the respective kernel in ``/etc/lilo.conf``, and pass
    197the options using APPEND, e.g.::
    198
    199  image = /bzImage
    200    initrd = /boot/initrd.gz
    201    append = "root=/dev/ram0 rw"
    202
    203and run ``/sbin/lilo``
    204
    205For other boot loaders, please refer to the respective documentation.
    206
    207Now you can boot and enjoy using initrd.
    208
    209
    210Changing the root device
    211------------------------
    212
    213When finished with its duties, init typically changes the root device
    214and proceeds with starting the Linux system on the "real" root device.
    215
    216The procedure involves the following steps:
    217 - mounting the new root file system
    218 - turning it into the root file system
    219 - removing all accesses to the old (initrd) root file system
    220 - unmounting the initrd file system and de-allocating the RAM disk
    221
    222Mounting the new root file system is easy: it just needs to be mounted on
    223a directory under the current root. Example::
    224
    225	# mkdir /new-root
    226	# mount -o ro /dev/hda1 /new-root
    227
    228The root change is accomplished with the pivot_root system call, which
    229is also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)`
    230man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher
    231[#f3]_). ``pivot_root`` moves the current root to a directory under the new
    232root, and puts the new root at its place. The directory for the old root
    233must exist before calling ``pivot_root``. Example::
    234
    235	# cd /new-root
    236	# mkdir initrd
    237	# pivot_root . initrd
    238
    239Now, the init process may still access the old root via its
    240executable, shared libraries, standard input/output/error, and its
    241current root directory. All these references are dropped by the
    242following command::
    243
    244	# exec chroot . what-follows <dev/console >dev/console 2>&1
    245
    246Where what-follows is a program under the new root, e.g. ``/sbin/init``
    247If the new root file system will be used with udev and has no valid
    248``/dev`` directory, udev must be initialized before invoking chroot in order
    249to provide ``/dev/console``.
    250
    251Note: implementation details of pivot_root may change with time. In order
    252to ensure compatibility, the following points should be observed:
    253
    254 - before calling pivot_root, the current directory of the invoking
    255   process should point to the new root directory
    256 - use . as the first argument, and the _relative_ path of the directory
    257   for the old root as the second argument
    258 - a chroot program must be available under the old and the new root
    259 - chroot to the new root afterwards
    260 - use relative paths for dev/console in the exec command
    261
    262Now, the initrd can be unmounted and the memory allocated by the RAM
    263disk can be freed::
    264
    265	# umount /initrd
    266	# blockdev --flushbufs /dev/ram0
    267
    268It is also possible to use initrd with an NFS-mounted root, see the
    269:manpage:`pivot_root(8)` man page for details.
    270
    271
    272Usage scenarios
    273---------------
    274
    275The main motivation for implementing initrd was to allow for modular
    276kernel configuration at system installation. The procedure would work
    277as follows:
    278
    279  1) system boots from floppy or other media with a minimal kernel
    280     (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
    281     loads initrd
    282  2) ``/sbin/init`` determines what is needed to (1) mount the "real" root FS
    283     (i.e. device type, device drivers, file system) and (2) the
    284     distribution media (e.g. CD-ROM, network, tape, ...). This can be
    285     done by asking the user, by auto-probing, or by using a hybrid
    286     approach.
    287  3) ``/sbin/init`` loads the necessary kernel modules
    288  4) ``/sbin/init`` creates and populates the root file system (this doesn't
    289     have to be a very usable system yet)
    290  5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and
    291     execs - via chroot - a program that continues the installation
    292  6) the boot loader is installed
    293  7) the boot loader is configured to load an initrd with the set of
    294     modules that was used to bring up the system (e.g. ``/initrd`` can be
    295     modified, then unmounted, and finally, the image is written from
    296     ``/dev/ram0`` or ``/dev/rd/0`` to a file)
    297  8) now the system is bootable and additional installation tasks can be
    298     performed
    299
    300The key role of initrd here is to re-use the configuration data during
    301normal system operation without requiring the use of a bloated "generic"
    302kernel or re-compiling or re-linking the kernel.
    303
    304A second scenario is for installations where Linux runs on systems with
    305different hardware configurations in a single administrative domain. In
    306such cases, it is desirable to generate only a small set of kernels
    307(ideally only one) and to keep the system-specific part of configuration
    308information as small as possible. In this case, a common initrd could be
    309generated with all the necessary modules. Then, only ``/sbin/init`` or a file
    310read by it would have to be different.
    311
    312A third scenario is more convenient recovery disks, because information
    313like the location of the root FS partition doesn't have to be provided at
    314boot time, but the system loaded from initrd can invoke a user-friendly
    315dialog and it can also perform some sanity checks (or even some form of
    316auto-detection).
    317
    318Last not least, CD-ROM distributors may use it for better installation
    319from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
    320via initrd from CD; or by booting via a loader like ``LOADLIN`` or directly
    321from the CD-ROM, and loading the RAM disk from CD without need of
    322floppies.
    323
    324
    325Obsolete root change mechanism
    326------------------------------
    327
    328The following mechanism was used before the introduction of pivot_root.
    329Current kernels still support it, but you should _not_ rely on its
    330continued availability.
    331
    332It works by mounting the "real" root device (i.e. the one set with rdev
    333in the kernel image or with root=... at the boot command line) as the
    334root file system when linuxrc exits. The initrd file system is then
    335unmounted, or, if it is still busy, moved to a directory ``/initrd``, if
    336such a directory exists on the new root file system.
    337
    338In order to use this mechanism, you do not have to specify the boot
    339command options root, init, or rw. (If specified, they will affect
    340the real root file system, not the initrd environment.)
    341
    342If /proc is mounted, the "real" root device can be changed from within
    343linuxrc by writing the number of the new root FS device to the special
    344file /proc/sys/kernel/real-root-dev, e.g.::
    345
    346  # echo 0x301 >/proc/sys/kernel/real-root-dev
    347
    348Note that the mechanism is incompatible with NFS and similar file
    349systems.
    350
    351This old, deprecated mechanism is commonly called ``change_root``, while
    352the new, supported mechanism is called ``pivot_root``.
    353
    354
    355Mixed change_root and pivot_root mechanism
    356------------------------------------------
    357
    358In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_root
    359mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd
    360image.
    361
    362``/linuxrc`` would contain only the following::
    363
    364	#! /bin/sh
    365	mount -n -t proc proc /proc
    366	echo 0x0100 >/proc/sys/kernel/real-root-dev
    367	umount -n /proc
    368
    369Once linuxrc exited, the kernel would mount again your initrd as root,
    370this time executing ``/sbin/init``. Again, it would be the duty of this init
    371to build the right environment (maybe using the ``root= device`` passed on
    372the cmdline) before the final execution of the real ``/sbin/init``.
    373
    374
    375Resources
    376---------
    377
    378.. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"
    379    https://www.almesberger.net/cv/papers/ols2k-9.ps.gz
    380.. [#f2] newlib package (experimental), with initrd example
    381    https://www.sourceware.org/newlib/
    382.. [#f3] util-linux: Miscellaneous utilities for Linux
    383    https://www.kernel.org/pub/linux/utils/util-linux/