cloudxtreme/finit

Small, fast and extensible /sbin/init for Linux systems. Cookies included.

Finit | Fast & Extensible init for Linux

Table of Contents

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

Introduction

Init is the first userland process started by the UNIX kernel, therefore it always has PID 1 and is responsible for starting up the rest of the system.

Finit is a plugin-based init with 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.

Finit basically does everything you need from its main() function!

Hence, there is no /etc/init.d/rcS script, or similar, instead Finit reads its configuration from /etc/finit.conf. This file details what kernel modules to load, programs to run, daemons to supervise, and services to launch on demand.

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

Features

Process Supervision

Start, monitor and restart processes (daemons) if they fail.

Inetd

Finit comes with an inetd server built-in. No need to maintain a separate config file for services that you want to start on demand.

Runlevels

Runlevels are 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, service callbacks, or pure extensions. A plugin is a C file that compiles into a dynamic library that is loaded automatically by finit at boot. 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.

The command line arguments given in /etc/finit.conf to each service provide a default. A plugin can be used to register a callback to a service and then 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.

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.

• inetd SVC[@iface[:port]]/PROTO <wait|nowait> [LVLS] /path/to/daemon args

  Launch daemon on demand when a client initiates a connection to SVC. Services (SVC) are specified in the standard UNIX /etc/services file. With optional filtering for iface and possible custom port. The following example opens port 2323 and only allows inbound telnet connections from eth0:

  inetd telnet@eth0:2323/tcp nowait [2345] /sbin/telnetd -i -F

  The inetd directive can also have -- Optional Description, only Finit does not output this text on the console when launching inetd services. Instead this text is sent to syslog.

• 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 | /path/to/cmd [args]>

  Start a getty on the given TTY device DEV, in the given runlevels. If 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.

  It is also possible to supply the full command line, with arguments, to your getty. In this case finit will simply use that command.

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

• console <DEV | /path/to/cmd [args]>

  Some embedded systems have a dedicated serial console/service port. This command tells finit to not start getty directly, since there may not be anyone there. To save RAM and CPU finit instead displays a friendly message and waits for the user to activate the console with a key press before starting getty. Finit also does some other magic and changes the process name to "console".

  Like the tty command, the console command can also be used to simply start a UNIX shell, e.g. /bin/sh.

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!

Every run, task, service, or inetd can also list the privileges the /path/to/cmd should be executed with. Simply prefix the path with [@USR[:GRP]] like this:

run [2345] @joe:users /usr/bin/logger "Hello world"

/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 inetd 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 statements.

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.

Inetd

A built-in Internet Super Server support was added in Finit v1.12, along with an internal time inetd service, RFC 868 (rdate). The latter is supplied as a plugin to illustrate how simple it is to extend finit with more internal inetd services.

Please note, not all UNIX daemons are prepared to run as inetd services. In the example below sshd also need the command line argument -i.

The inetd support in finit is quite advanced. Not only does it launch services on demand, it can do so on custom ports and also filter inbound traffic using a poor man's tcpwrappers. The syntax is very similar to traditional /etc/inetd.conf, yet keeping with the style of Finit:

# Launch SSH on demand, in runlevels 2-5 as root
inetd ssh/tcp nowait [2345] @root:root /usr/sbin/sshd -i

A more advanced example:

# Start sshd if inbound connection on eth0, port 222, or
# inbound on eth1, port 22.  Ignore on other interfaces.
inetd ssh@eth0:222/tcp nowait [2345] /usr/sbin/sshd -i
inetd ssh@eth1:22/tcp  nowait [2345] /usr/sbin/sshd -i

If eth0 is your Internet interface you may want to avoid using the default port. To run ssh on port 222, and all others on port 22:

inetd ssh@eth0:222/tcp nowait [2345] /usr/sbin/sshd -i
inetd ssh/tcp          nowait [2345] /usr/sbin/sshd -i

This actually adds a deny rule for eth0 on ssh/tcp, implicitly. You can even list the services in the reverse order with the same result:

inetd ssh/tcp          nowait [2345] /usr/sbin/sshd -i
inetd ssh@eth0:222/tcp nowait [2345] /usr/sbin/sshd -i

There is no specific deny syntax available yet, see the TODO file for more details on how this can be implemented.

The time.so inetd plugin setup as follows. Notice the keyword internal which applies to all built-in inetd services:

inetd time/tcp         nowait [2345] internal

Then call rdate from a remote machine (or use localhost):

rdate -p <IP>

If you use time/udp you must use the standard rdate implementation and then call it with rdate -up to connect using UDP. Without the -p argument rdate will try to set the system clock. Please note that rdate has been deprecated by the NTP protocol and this plugin should only be used for testing or environments where NTP for some reason is blocked. Also, remember the UNIX year 2038 bug, or in the case of RFC 868 (and some NTP implementations), year 2036!

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. Useful when plugging in a usb2serial converter to login to your embedded device.

• urandom.so: Setup random seed at startup.

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

• time.so: RFC 868 (rdate) plugin. Start as inetd service. Useful for testing inetd filtering -- BusyBox has an rdate (TCP) client.

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 & 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

Finit comes with a lightweight configure script to control which features to enable an plugins to build. Below are a few of the main switches to configure:

• --prefix=: Base prefix path for all files, except --sbindir 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.

• --plugindir=: Absolute path to where finit should search for dynamically loadable plugins at runtime. Can also be controlled with the --libdir switch.

  Defaults to /lib/finit/plugins.

• --with-plugins=: List of stock finit plugins to build and install. The build system adds .o or .so extension to the listed plugins depending on the --enable-static switch.

• --enable-embedded: Target finit for BusyBox getty and mdev instead of a standard Linux distribution with GNU tools and udev.

• --enable-debug: Add GDB symbols and disable code optimization.

• --enable-static: Build Finit statically. The plugins will be built-in (.o files) instead. Note: very untested and not all plugins can be built static. It is recommended to use --with-plugins and select only the plugins really needed.

• --disable-inetd: Disables the built-in inetd server.

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.

• 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-1.13.tar.xz; cd finit-1.13/

Then configure, build and install:

  $ ./configure --with-plugins="initctl.so hwclock.so"
  $ make
  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
  $ DESTDIR=/tmp/finit make install
  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-1.13.tar.xz archive is unpacked to the user's home directory, configured, built and installed to a temporary staging directory. The environment variable DESTDIR controls the destination directory when installing, very useful for building binary standalone packages.

To target an embedded Linux system, usally a system that use BusyBox tools instead of udev & C:o, add --enable-embedded to the configure command above.

For more configure flags, see ./configure --help

Note: If you run into problems starting Finit, take a look at finit.c. One of the most common problems is a custom Linux kernel build that lack CONFIG_DEVTMPFS. Another is too much cruft in the system /etc/fstab.

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.