finit/doc/plugins.md

Hooks & Plugins

• Plugins
• Hooks
  • Bootstrap Hooks
  • Runtime Hooks
  • Shutdown Hooks

Finit can be extended to add general functionality in the form of I/O monitors, or hook plugins.

The following sections detail existing plugins and hook points. For more information, see the plugins listed below.

Plugins

For your convenience a set of optional plugins are available:

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

• bootmisc.so: Setup necessary files and system directories for, e.g., UTMP (tracks logins at boot). This plugin is central to get a working system and runs at HOOK_BASEFS_UP. The /var, /run, and /dev file systems must be writable for this plugin to work.

  Note: On an embedded system both /var and /run can be tmpfs RAM disks and /dev is usually a devtmpfs. This must be defined in the /etc/fstab file and in the Linux kernel config.

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

• hook-scripts.so: Trigger the execution of scripts from plugin hook points (see Hooks). Optional plugin.

  This plugin is particularly useful for early boot debugging that needs to take place before regular services are available.

  For example, say that you want to enable some kernel tracing before modules are loaded. With hook scripts, you can just drop in a shell script in /libexec/finit/hook/mount/all/ that will poke the right control files in tracefs.

  Scripts are located in /libexec/finit/hook by default, this can be customized at build-time using the --with-hook-scripts-path=PATH argument to configure.

• hotplug.so: Setup and start either udev or mdev hotplug daemon, if available. Enabled by default.

• mdevd.so: Set up and start mdevd (mdev replacement). Enabling this also changes how hotplug.so plugin operates. Optional plugin.

• rtc.so: Restore and save system clock from/to RTC on boot/halt. Enabled by default.

• modules-load.so: Scans /etc/modules-load.d/*.conf for modules to load using modprobe. Each file can contain multiple lines with the name of the module to load. Any line starting with the standard UNIX comment character, #, is skipped.

  Modules are loaded when entering runlevel [2345] using the task stanza. Each module gets a unique name:modprobe.foo, and optional:ID. The :ID is a globally incremented index, which can be disabled per file (anywhere) using the following config line:

    set noindex
  

  Note: unlike the traditional .conf module directive, which load any listed module immediately, this plugin creates standard taks directives which load the module(s) in the background. As long as the modprobe program is found in the path, these tasks will always return [ OK ] at boot. Check the actual status using initctl.

• netlink.so: Listens to Linux kernel Netlink events for gateway and interfaces. These events are then sent to the Finit service monitor for services that may want to be SIGHUP'ed on new default route or interfaces going up/down. Enabled by default.

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

• 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. Enabled by default.

• urandom.so: Setup random seed at startup. Enabled by default.

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

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:

Hooks

In the below listings, the first label is the hook point for a C plugin, the second is the condition name and hook script path. A hook script is a plain shell script, or program, that does a very small dedicated job at the below hook points.

Example:

$ mkdir -p /libexec/finit/hook/sys/down
$ cat <<EOF >/libexec/finit/hook/sys/foo.sh
#!/bin/sh
echo 'I run just before the reboot() syscall at shutdown/reboot'
echo 'I have access to /dev since devtmpfs is exempt from umount'
exit 0
EOF
$ chmod +x /libexec/finit/hook/sys/foo.sh

Note: to use hook scripts, even for pre-bootstrap and pre-shutdown tasks, you must build with configure --enable-hook-scripts-plugin.

Bootstrap Hooks

• HOOK_BANNER, hook/sys/banner: The very first point at which a plugin can run. Intended to be used as a banner replacement. Essentially this runs just before entering runlevel S. Assume nothing is available, so be prepared to use absolute paths, etc.

• HOOK_ROOTFS_UP, hook/mount/root: When finit.conf has been read and / has is mounted — very early

• HOOK_MOUNT_ERROR, hook/mount/error: executed if mount -a fails

• HOOK_MOUNT_POST, hook/mount/post: always executed after mount -a

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

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

• HOOK_SVC_UP, hook/svc/up: All services in the active runlevel have been launched

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

Runtime Hooks

• HOOK_SVC_RECONF, N/A: Called when the user has changed something in the /etc/finit.d directory and issued SIGHUP. The hook is called when all modified/removed services have been stopped. When the hook has completed, Finit continues to start all modified and new services.

• HOOK_RUNLEVEL_CHANGE, N/A: Called when the user has issued a runlevel change. The hook is called when services not matching the new runlevel have been been stopped. When the hook has completed, Finit continues to start all services in the new runlevel.

Shutdown Hooks

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

• HOOK_SVC_DN, hook/svc/down: In shutdown/reboot, all services and non-reserved processes have been killed. Note: only hook scripts can run here!

• HOOK_SYSTEM_DN, hook/sys/down: In shutdown/reboot, called after all non-reserved file systems have been unmounted, just before Finit tells the kernel to reboot or shut down. Note: only hook scripts can run here!

Plugins like 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.