dracut/man/dracut.modules.7.asc

DRACUT.MODULES(7)
=================
:doctype: manpage
:man source:   dracut
:man manual:   dracut
:man version:  {version}

NAME
----
dracut.modules - dracut modules

DESCRIPTION
-----------

dracut uses a modular system to build and extend the initramfs image. All
modules are located in _/usr/lib/dracut/modules.d_ or in _<git-src>/modules.d_.
The most basic dracut module is _99base_. In _99base_ the initial shell script
init is defined, which gets run by the kernel after initramfs loading. Although
you can replace init with your own version of _99base_, this is not encouraged.
Instead you should use, if possible, the hooks of dracut. All hooks, and the
point of time in which they are executed, are described in <<stages>>.

The main script, which creates the initramfs is dracut itself. It parses all
arguments and sets up the directory, in which everything is installed. It then
executes all check, install, installkernel scripts found in the modules, which
are to be processed. After everything is installed, the install directory is
archived and compressed to the final initramfs image. All helper functions used
by check, install and installkernel are found in in the file _dracut-functions_.
These shell functions are available to all module installer (install,
installkernel) scripts, without the need to source _dracut-functions_.

A module can check the preconditions for install and installkernel with the
check script. Also dependencies can be expressed with check. If a module passed
check, install and installkernel will be called to install all of the necessary
files for the module. To split between kernel and non-kernel parts of the
installation, all kernel module related parts have to be in installkernel. All
other files found in a module directory are module specific and mostly are hook
scripts and udev rules.


[[stages]]
== Boot Process Stages

dracut modules can insert custom script at various points, to control the boot
process.
These hooks are plain directories containing shell scripts ending with ".sh",
which are sourced by init.
Common used functions are in _dracut-lib.sh_, which can be sourced by any script.

=== Hook: cmdline

The _cmdline_ hook is a place to insert scripts to parse the kernel command line
and prepare the later actions, like setting up udev rules and configuration
files.

In this hook the most important environment variable is defined: root. The
second one is rootok, which indicates, that a module claimed to be able to parse
the root defined. So for example, **root=**__iscsi:....__ will be claimed by the
iscsi dracut module, which then sets rootok.

=== Hook: pre-udev

This hook is executed right after the cmdline hook and a check if root and
rootok were set. Here modules can take action with the final root, and before
udev has been run.

=== Start Udev

Now udev is started and the logging for udev is setup.

=== Hook: pre-trigger

In this hook, you can set udev environment variables with **udevadm control
--property=KEY=_value_** or control the further execution of udev with
udevadm.

=== Trigger Udev

udev is triggered by calling udevadm trigger, which sends add events for all
devices and subsystems.

=== Main Loop

In the main loop of dracut loops until udev has settled and
all scripts in _initqueue/finished_ returned true.
In this loop there are three hooks, where scripts can be inserted
by calling /sbin/initqueue.

==== Initqueue

This hook gets executed every time a script is inserted here, regardless of the
udev state.

==== Initqueue settled

This hook (initqueue/settled) gets executed every time udev has settled.

==== Initqueue timeout

This hook (initqueue/timeout) gets executed, when the main loop counter becomes
half of the rd.retry counter.

==== Initqueue online

This hook (initqueue/online) gets executed whenever a network interface comes online
(that is, once it is up and configured by the configured network module).

==== Initqueue finished

This hook (initqueue/finished) is called after udev has settled and
if all scripts herein return 0 the main loop will be ended.
Arbitrary scripts can be added here, to loop in the
initqueue until something happens, which a dracut module wants to wait for.

=== Hook: pre-mount

Before the root device is mounted all scripts in the hook pre-mount are
executed. In some cases (e.g. NFS) the real root device is already mounted,
though.

=== Hook: mount

This hook is mainly to mount the real root device.

=== Hook: pre-pivot

This hook is called before cleanup hook, This is a good place for
actions other than cleanups which need to be called before pivot.

=== Hook: cleanup

This hook is the last hook and is called before init finally switches root to
the real root device. This is a good place to clean up and kill processes not
needed anymore.


=== Cleanup and switch_root

Init (or systemd) kills all udev processes, cleans up the environment,
sets up the arguments for the real init process and finally calls switch_root.
switch_root removes the whole filesystem hierarchy of the initramfs,
chroot()s to the real root device and calls /sbin/init with the specified
arguments.

To ensure all files in the initramfs hierarchy can be removed, all processes
still running from the initramfs should not have any open file descriptors left.

== Network Infrastructure

FIXME

== Writing a Module

A simple example module is _90kernel-modules_, which modprobes a kernel module
after udev has settled and the basic device drivers have been loaded.

All module installation information is in the file module-setup.sh.

First we create a check() function, which just exits with 0 indicating that this
module should be included by default.

check():
----
return 0
----

Then we create the install() function, which installs a cmdline hook with
priority number 20 called _parse-insmodpost.sh_. It also installs the
_insmodpost.sh_ script in _/sbin_.

install():
----
inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh
----

The _parse-instmodpost.sh_ parses the kernel command line for a argument
rd.driver.post, blacklists the module from being autoloaded and installs the
hook _insmodpost.sh_ in the _initqueue/settled_.

_parse-insmodpost.sh_:
----
for p in $(getargs rd.driver.post=); do
    echo "blacklist $p" >> /etc/modprobe.d/initramfsblacklist.conf
    _do_insmodpost=1
done

[ -n "$_do_insmodpost" ] && /sbin/initqueue --settled --unique --onetime /sbin/insmodpost.sh
unset _do_insmodpost

----

_insmodpost.sh_, which is called in the _initqueue/settled_ hook will just
modprobe the kernel modules specified in all rd.driver.post kernel command line
parameters. It runs after udev has settled and is only called once (--onetime).

_insmodpost.sh_:
----
. /lib/dracut-lib.sh

for p in $(getargs rd.driver.post=); do
    modprobe $p
done

----


=== module-setup.sh: check()

_check()_ is called by dracut to evaluate the inclusion of a dracut module in
the initramfs.

$hostonly:: If the $hostonly variable is set, then the module check() function
should be in "hostonly" mode, which means, that the check() should only return
0, if the module is really needed to boot this specific host.

check() should return with:

0:: Include the dracut module in the initramfs.

1:: Do not include the dracut module. The requirements are not fulfilled
(missing tools, etc.)

255:: Only include the dracut module, if another module requires it or if
explicitly specified in the config file or on the argument list.


=== module-setup.sh: depends()

The function depends() should echo all other dracut module names the module
depends on.

=== module-setup.sh: cmdline()

This function should print the kernel command line options needed to boot the
current machine setup. It should start with a space and should not print a
newline.

=== module-setup.sh: install()

The install() function is called to install everything non-kernel related.
To install binaries, scripts, and other files, you can use the functions
mentioned in <<creation>>.

To address a file in the current module directory, use the variable "$moddir".

=== module-setup.sh: installkernel()

In installkernel() all kernel related files should be installed. You can use all
of the functions mentioned in <<creation>> to install files.

=== [[creation]]Creation Functions

==== inst_multiple [-o] <file> [ <file> ...]

installs multiple binaries and files. If executables are specified without a
path, dracut will search the path PATH=/usr/sbin:/sbin:/usr/bin:/bin for the
binary. If the option "-o" is given as the first parameter, a missing file does
not lead to an error.

==== inst <src> [<dst>]

installs _one_ file <src> either to the same place in the initramfs or to an
optional <dst>. inst with more than two arguments is treated the same as
inst_multiple, all arguments are treated as files to install and none as
install destinations.

==== inst_hook <hookdir> <prio> <src>

installs an executable/script <src> in the dracut hook <hookdir> with priority
<prio>.

==== inst_rules <udevrule> [ <udevrule> ...]

installs one or more udev rules. Non-existant udev rules are reported, but do
not let dracut fail.

==== instmods <kernelmodule> [ <kernelmodule> ... ]

instmods should be used only in the installkernel() function.

instmods installs one or more kernel modules in the initramfs. <kernelmodule>
can also be a whole subsystem, if prefixed with a "=", like "=drivers/net/team".

instmods will not install the kernel module, if $hostonly is set and the kernel
module is not currently needed by any /sys/*...*/uevent MODALIAS.
To install a kernel module regardless of the hostonly mode use the form:
----
hostonly='' instmods <kernelmodule>
----

=== Initramfs Functions

FIXME


=== Network Modules

FIXME

AUTHOR
------
Harald Hoyer

SEE ALSO
--------
*dracut*(8)