Finit | Fast & Extensible init for Linux

Table of Contents

• Introduction
• Features
• /etc/finit.conf
• /etc/finit.d
• Bootstrap
• Runlevels
• Hooks, Callbacks & Plugins
• Rebooting and Halting
• Building
• Running
• Debugging
• Origin & References

Introduction

Init is the first process to run once a UNIX kernel has booted, it always has PID 1 and is responsible for starting up the rest of the system. Finit is plugin-based with built in process supervision similar to that of D.J. Bernstein's daemontools and Gerrit Pape's runit. The main focus of Finit is on small and embedded GNU/Linux systems, yet fully functional on standard server and desktop installations as well.

Traditional SysV init style systems are scripted. For low-resource embedded systems this can be quite resource intensive and cause longer boot times. Finit is optimized to reduce context switches and forking of processes to provide a very basic bootstrap written entirely in C. Hence, there is no /etc/init.d/rcS script, or similar, but instead a human-readable /etc/finit.conf. This file details what kernel modules to load, programs to run and daemons to supervise.

The command line arguments given in /etc/finit.conf to each service provide a default. Each service can register callbacks, using plugins, to override and modify the behavior to suit the current runlevel and system configuration. For instance, before starting a heavily resource intensive service like IPsec or OpenVPN, a callback can check if the outbound interface is up and has an IP address, or just check if the service is disabled -- much like what a SysV init start script usually does.

See TroglOS for an example of how to boot a small embedded system with Finit.

Features

Process Supervision

Start, monitor and restart services if they fail.

Runlevels

Runlevels is optional in Finit, but support for SysV runlevels is available if needed. All services in runlevel S(1) are started first, followed by the desired run-time runlevel. Runlevel S can be started in sequence by using run [S] cmd. Changing runlevels at runtime is done like any other init, e.g. init 4.

Plugins

Finit plugins can be either boot hooks into different stages of the boot process or service callbacks, or both. A basic set of plugins that extend and modify the basic behavior are bundled. See examples in the plugins/ directory.

Plugin capabilities:

• Service callbacks -- modify service arguments, run/restart/stop
• Task/Run callbacks -- a one-shot commands, executed in sequence
• Hooks -- hook into the boot at predefined points to extend finit
• I/O -- listen to external events and control finit behavior/services

Extensions and functionality not purely related to what an /sbin/init needs to start a system are available as a set of plugins that either hook into the boot process or respond to various I/O.

/etc/finit.conf

Contrary to most other script based init alternatives (SysV init, upstart, systemd, OpenRC, etc.) Finit reads its entire configuration from /etc/finit.conf.

Syntax:

• check <DEV>

  Run fsck on a file system before mounting it

• module <MODULE>

  Load a kernel module, with optional arguments

• network <PATH>

  Script or program to bring up networking, with optional arguments

• runlevel <N>

  N is the runlevel number 1-9, where 6 is reserved for reboot.

  Default is 2.

• run [LVLS] /path/to/cmd ARGS -- Optional description

  One-shot command to run in sequence when entering a runlevel, with optional arguments and description. This command is guaranteed to be completed before running the next command.

• task [LVLS] /path/to/cmd ARGS -- Optional description

  One-shot like 'run', but starts in parallel with the next command

• service [LVLS] /path/to/daemon ARGS -- Optional description

  Service, or daemon, to be monitored and automatically restarted if it exits prematurely. Please note that you often need to provide a --foreground or --no-background argument to most daemons to prevent them from forking off to the background.

• runparts <PATH>

  Call run-parts(8) on a directory other than default /etc/finit.d

• include <CONF>

  Include another configuration file, /etc/finit.d, or runparts path is prepended to file if the file is not found or an absolute path is not given

• tty [LVLS] <DEV | /bin/sh>

  Start a getty on the given TTY device, in the given runlevels. When no tty setting is given in finit.conf, or if /bin/sh is given as argument instead of a device path, a single shell is started on the default console. Useful for really bare-bones systems

  See finit.h for the #define GETTY that is called, along with the default baud rate.

• console <DEV>

  Some embedded systems have a dedicated console port. This command tells finit to not start getty, but instead print a friendly message and wait for the user to activate the console with a key press before starting getty.

When running make install no default /etc/finit.conf will be installed since system requirements differ too much. Try out the Debian 6.0 example /usr/share/doc/finit/finit.conf configuration that is capable of service monitoring SSH, sysklogd, gdm and a console getty!

/etc/finit.d

At the end of the boot, when networking and all services are up, finit calls its built-in run-parts(8) on the /etc/finit.d/ directory, if it exists. Similar to how the /ec/rc.local file works in most other init daemons, only finit runs a directory of scripts. This replaces the earlier support for a /usr/sbin/services.sh script in the original finit.

Bootstrap

1. Setup /dev
2. Parse /etc/finit.conf
3. Load all .so plugins
4. Remount/Pivot / to get R+W
5. Call 1st level hooks, HOOK_ROOTFS_UP
6. Mount /etc/fstab and swap, if available
7. Cleanup stale files from /tmp/* et al
8. Enable SysV init signals
9. Call 2nd level hooks, HOOK_BASEFS_UP
10. Start all 'S' runlevel tasks and services
11. Setup /etc/sysctl.conf
12. Setup hostname and loopback
13. Call network script, if set in /etc/finit.conf
14. Call 3rd level hooks, HOOK_NETWORK_UP
15. Switch to active runlevel, as set in /etc/finit.conf, default 2. Here is where the rest of all tasks and services are started.
16. Call 4th level hooks, HOOK_SVC_UP
17. Run-parts in /etc/finit.d, if any
18. Call 5th level (last) hooks, HOOK_SYSTEM_UP
19. Start TTYs defined in /etc/finit.conf, or rescue on /dev/console
20. Enter main monitor loop

In (10) and (15) tasks and services defined in /etc/finit.conf are started. Remember, all service and task stanzas are started in parallel and run in sequence, and in the order listed. Hence, to emulate a SysV /etc/init.d/rcS one could write a long file with only run statments.

Notice the five hook points that are called at various point in the bootstrap process. This is where plugins can extend the boot in any way they please.

For instance, at HOOK_BASEFS_UP a plugin could read an XML file from a USB stick, convert/copy its contents to the system's /etc/ directory, well before all 'S' runlevel tasks are started. This could be used with system images that are created read-only and all configuration is stored on external media.

Runlevels

Basic support for runlevels is included in Finit from v1.8. By default all services, tasks, run commands and TTYs listed without a set of runlevels get a default set [234] assigned. The default runlevel after boot is 2.

To specify an allowed set of runlevels for a service, run command, task, or tty, add [NNN] to it in your /etc/finit.conf, like this:

service [S12345] /sbin/syslogd -n -x     -- System log daemon
run     [S]      /etc/init.d/acpid start -- Starting ACPI Daemon
task    [S]      /etc/init.d/kbd start   -- Preparing console
service [S12345] /sbin/klogd -n -x       -- Kernel log daemon
tty     [12345]  /dev/tty1
tty     [2]      /dev/tty2
tty     [2]      /dev/tty3
tty     [2]      /dev/tty4
tty     [2]      /dev/tty5
tty     [2]      /dev/tty6

In this example syslogd is first started, in parallel, and then acpid is called using a conventional SysV init script. It is called with the run command, meaning the following task command to start the kbd script is not called until the acpid init script has fully completed. Then the keyboard setup script is called in parallel with klogd as a monitored service.

Again, tasks and services are started in parallel, while run commands are called in the order listed and subsequent commands are not started until a run command has completed.

Switching between runlevels can be done by calling init with a single argument, e.g. init 5 switches to runlevel 5.

Hooks, Callbacks & Plugins

Finit provides only the bare necessities for starting and supervising processes, with an emphasis on bare -- for your convenience it does however come with support for hooks, service callbacks and plugins that can used to extend finit with. For your convenience a set of optional plugins are available:

• alsa-utils.so: Restore and save ALSA sound settings on startup/shutdown.

• bootmisc.so: Setup necessary files for UTMP, tracks logins at boot.

• dbus.so: Setup and start system message bus, D-Bus, at boot.

• hwclock.so: Restore and save system clock from/to RTC on startup/shutdown.

• initctl.so: Extends finit with a traditional initctl functionality.

• resolvconf.so: Setup necessary files for resolvconf at startup.

• tty.so: Watches /dev, using inotify, for new device nodes (TTY's) to start/stop getty consoles on them on demand.

• urandom.so: Setup random seed at startup.

• x11-common.so: Setup necessary files for X-Window.

Usually you want to hook into the boot process once, simple hook plugins like bootmisc.so are great for that purpose. They are called at each hook point in the boot process, useful to insert some pre-bootstrap mechanisms, like generating configuration files, restoring HW device state, etc. Available hook points are:

• HOOK_ROOTFS_UP: When finit.conf has been read and / has is mounted -- very early

• HOOK_BASEFS_UP: All of /etc/fstab is mounted, swap is available and default init signals are setup

• HOOK_NETWORK_UP: System bootstrap, runlevel S, has completed and networking is up (lo is up and the network script has run)

• HOOK_SVC_UP: All services in the active runlevel has been launched

• HOOK_SYSTEM_UP: All services and everything in /etc/finit.d has been launched

• HOOK_SHUTDOWN: Called at shutdown/reboot, right before all services are sent SIGTERM

Plugins like initctl.so and tty.so extend finit by acting on events, they are called I/O plugins and are called from the finit main loop when poll() detects an event. See the source code for plugins/*.c for more help and ideas.

Callback plugins are called by finit them right before a process is started, or restarted if it exits. The callback receive a pointer to the svc_t (defined in svc.h) of the service, with all command line parameters free to modify if needed. All the callback needs to do is respond with one of: SVC_STOP (0) tells finit to not start the service, SVC_START (1) to start the service, or SVC_RELOAD (2) to have finit signal the process with SIGHUP.

Rebooting and Halting

Finit handles SIGUSR1 and SIGUSR2 for reboot and halt, and listens to /dev/initctl so system reboot and halt commands should also work. This latter functionality is implemented in the optional initctl.so plugin.

Building

The finit build system does not employ the GNU Configure and Build System, instead standard makefiles are used. The user is encouraged to make source code changes, using defines and conditionally building plugins instead to alter the behavior of finit.

The following environment variables are checked by the makefiles and control what is built and where resulting binaries are installed.

• ROOTDIR=: Top directory for building complete system, used in pretty printing.

• VERSION=: Defaults to the currently released version of finit, e.g., 1.3 but can be overridden by packagers using this variable to add a suffix or completely alter the version.

• CFLAGS=: Default CFLAGS are inherited from the environment.

• CPPFLAGS=: Default CPPFLAGS are inherited from the environment.

• LDFLAGS=: Default LDFLAGS are inherited from the environment.

• LDLIBS=: Default LIBLIBS are inherited from the environment.

• prefix=: Base prefix path for all files, except sbinbdir and sysconfdir. Used in concert with the DESTDIR variable.

  Defaults to /usr.

• sbindir=: Path to where resulting binaries should install to. Used in concert with the DESTDIR variable.

  Defaults to /sbin.

• sysconfdir=: Path to where finit configuration files should install to. Used in concert with the DESTDIR variable.

  Defaults to /etc, but is currently unused.

• PLUGINS=: List of stock finit plugins to build and install.

• plugindir=: Absolute path to where finit should look for dynamically loadable plugins at runtime. At installation prepended by DESTDIR and prefix.

  Defaults to /lib/finit/plugins.

• DESTDIR=: Used by packagers and distributions when building a relocatable bundle of files. Always prepended to the prefix destination directory.

Example

First, unpack the archive:

$ tar xfJ finit-1.3.tar.xz; cd finit-1.3/

Then build and install:

$ PLUGINS="initctl.so hwclock.so" DESTDIR=/tmp/finit make install

  CC      finit.o
  CC      conf.o
  CC      helpers.o
  CC      sig.o
  CC      svc.o
  CC      plugin.o
  CC      strlcpy.o
  LINK    finit
  CC      plugins/initctl.o
  PLUGIN  plugins/initctl.so
  CC      plugins/hwclock.o
  PLUGIN  plugins/hwclock.so
  INSTALL /tmp/finit/sbin/finit
  INSTALL /tmp/finit/lib/finit/plugins/initctl.so
  INSTALL /tmp/finit/lib/finit/plugins/hwclock.so

In this example the finit-1.3.tar.xz archive is unpacked to the user's home directory, built and installed to a temporary staging directory. The environment variables DESTDIR and PLUGINS are changed to suit this particular build.

Running

The default install does not setup finit as the system default /sbin/init, neither does it setup an initial /etc/finit.conf.

It is assumed that users of finit are competent enough to either setup finit as their default /sbin/init or alter their respective Grub, LOADLIN, LILO, U-Boot/Barebox or RedBoot boot loader configuration to give the kernel the following extra command line:

init=/sbin/finit

Debugging

Add finit_debug, or --debug, to the kernel command line to enable trace messages. A console getty is always started, see the file finit.h for more useful compile-time tweaks:

init=/sbin/finit --debug

Origin & References

This is the continuation of the original finit by Claudio Matsuoka, which in turn was reverse engineered from syscalls of the EeePC fastinit -- "gaps filled with frog DNA ...".

Finit is currently being developed and maintained by Joachim Nilsson at GitHub. Please file bug reports, clone it, or send pull requests for bug fixes and proposed extensions.