platform_system_core/init/readme.txt

Android Init Language
---------------------

The Android Init Language consists of four broad classes of statements,
which are Actions, Commands, Services, and Options.

All of these are line-oriented, consisting of tokens separated by
whitespace.  The c-style backslash escapes may be used to insert
whitespace into a token.  Double quotes may also be used to prevent
whitespace from breaking text into multiple tokens.  The backslash,
when it is the last character on a line, may be used for line-folding.

Lines which start with a # (leading whitespace allowed) are comments.

Actions and Services implicitly declare a new section.  All commands
or options belong to the section most recently declared.  Commands
or options before the first section are ignored.

Actions and Services have unique names.  If a second Action or Service
is declared with the same name as an existing one, it is ignored as
an error.  (??? should we override instead)


Actions
-------
Actions are named sequences of commands.  Actions have a trigger which
is used to determine when the action should occur.  When an event
occurs which matches an action's trigger, that action is added to
the tail of a to-be-executed queue (unless it is already on the
queue).

Each action in the queue is dequeued in sequence and each command in
that action is executed in sequence.  Init handles other activities
(device creation/destruction, property setting, process restarting)
"between" the execution of the commands in activities.

Actions take the form of:

on <trigger>
   <command>
   <command>
   <command>


Services
--------
Services are programs which init launches and (optionally) restarts
when they exit.  Services take the form of:

service <name> <pathname> [ <argument> ]*
   <option>
   <option>
   ...


Options
-------
Options are modifiers to services.  They affect how and when init
runs the service.

critical
  This is a device-critical service. If it exits more than four times in
  four minutes, the device will reboot into recovery mode.

disabled
  This service will not automatically start with its class.
  It must be explicitly started by name.

setenv <name> <value>
  Set the environment variable <name> to <value> in the launched process.

socket <name> <type> <perm> [ <user> [ <group> [ <seclabel> ] ] ]
  Create a unix domain socket named /dev/socket/<name> and pass
  its fd to the launched process.  <type> must be "dgram", "stream" or "seqpacket".
  User and group default to 0.
  'seclabel' is the SELinux security context for the socket.
  It defaults to the service security context, as specified by seclabel or
  computed based on the service executable file security context.

user <username>
  Change to username before exec'ing this service.
  Currently defaults to root.  (??? probably should default to nobody)
  Currently, if your process requires linux capabilities then you cannot use
  this command. You must instead request the capabilities in-process while
  still root, and then drop to your desired uid.

group <groupname> [ <groupname> ]*
  Change to groupname before exec'ing this service.  Additional
  groupnames beyond the (required) first one are used to set the
  supplemental groups of the process (via setgroups()).
  Currently defaults to root.  (??? probably should default to nobody)

seclabel <seclabel>
  Change to 'seclabel' before exec'ing this service.
  Primarily for use by services run from the rootfs, e.g. ueventd, adbd.
  Services on the system partition can instead use policy-defined transitions
  based on their file security context.
  If not specified and no transition is defined in policy, defaults to the init context.

oneshot
  Do not restart the service when it exits.

class <name>
  Specify a class name for the service.  All services in a
  named class may be started or stopped together.  A service
  is in the class "default" if one is not specified via the
  class option.

onrestart
  Execute a Command (see below) when service restarts.

writepid <file...>
  Write the child's pid to the given files when it forks. Meant for
  cgroup/cpuset usage.


Triggers
--------
Triggers are strings which can be used to match certain kinds
of events and used to cause an action to occur.

boot
   This is the first trigger that will occur when init starts
   (after /init.conf is loaded)

<name>=<value>
   Triggers of this form occur when the property <name> is set
   to the specific value <value>.

   One can also test multiple properties to execute a group
   of commands. For example:

   on property:test.a=1 && property:test.b=1
       setprop test.c 1

   The above stub sets test.c to 1 only when
   both test.a=1 and test.b=1


Commands
--------

bootchart_init
   Start bootcharting if configured (see below).
   This is included in the default init.rc.

chmod <octal-mode> <path>
   Change file access permissions.

chown <owner> <group> <path>
   Change file owner and group.

class_start <serviceclass>
   Start all services of the specified class if they are
   not already running.

class_stop <serviceclass>
   Stop and disable all services of the specified class if they are
   currently running.

class_reset <serviceclass>
   Stop all services of the specified class if they are
   currently running, without disabling them. They can be restarted
   later using class_start.

copy <src> <dst>
   Copies a file. Similar to write, but useful for binary/large
   amounts of data.

domainname <name>
   Set the domain name.

enable <servicename>
   Turns a disabled service into an enabled one as if the service did not
   specify disabled.
   If the service is supposed to be running, it will be started now.
   Typically used when the bootloader sets a variable that indicates a specific
   service should be started when needed. E.g.
     on property:ro.boot.myfancyhardware=1
        enable my_fancy_service_for_my_fancy_hardware

exec [ <seclabel> [ <user> [ <group> ]* ] ] -- <command> [ <argument> ]*
   Fork and execute command with the given arguments. The command starts
   after "--" so that an optional security context, user, and supplementary
   groups can be provided. No other commands will be run until this one
   finishes. <seclabel> can be a - to denote default.

export <name> <value>
   Set the environment variable <name> equal to <value> in the
   global environment (which will be inherited by all processes
   started after this command is executed)

hostname <name>
   Set the host name.

ifup <interface>
   Bring the network interface <interface> online.

import <filename>
   Parse an init config file, extending the current configuration.

insmod <path>
   Install the module at <path>

load_all_props
   Loads properties from /system, /vendor, et cetera.
   This is included in the default init.rc.

load_persist_props
   Loads persistent properties when /data has been decrypted.
   This is included in the default init.rc.

loglevel <level>
   Sets the kernel log level to level. Properties are expanded within <level>.

mkdir <path> [mode] [owner] [group]
   Create a directory at <path>, optionally with the given mode, owner, and
   group. If not provided, the directory is created with permissions 755 and
   owned by the root user and root group. If provided, the mode, owner and group
   will be updated if the directory exists already.

mount_all <fstab>
   Calls fs_mgr_mount_all on the given fs_mgr-format fstab.

mount <type> <device> <dir> [ <flag> ]* [<options>]
   Attempt to mount the named device at the directory <dir>
   <device> may be of the form mtd@name to specify a mtd block
   device by name.
   <flag>s include "ro", "rw", "remount", "noatime", ...
   <options> include "barrier=1", "noauto_da_alloc", "discard", ... as
   a comma separated string, eg: barrier=1,noauto_da_alloc

powerctl
   Internal implementation detail used to respond to changes to the
   "sys.powerctl" system property, used to implement rebooting.

restart <service>
   Like stop, but doesn't disable the service.

restorecon <path> [ <path> ]*
   Restore the file named by <path> to the security context specified
   in the file_contexts configuration.
   Not required for directories created by the init.rc as these are
   automatically labeled correctly by init.

restorecon_recursive <path> [ <path> ]*
   Recursively restore the directory tree named by <path> to the
   security contexts specified in the file_contexts configuration.

rm <path>
   Calls unlink(2) on the given path. You might want to
   use "exec -- rm ..." instead (provided the system partition is
   already mounted).

rmdir <path>
   Calls rmdir(2) on the given path.

setprop <name> <value>
   Set system property <name> to <value>. Properties are expanded
   within <value>.

setrlimit <resource> <cur> <max>
   Set the rlimit for a resource.

start <service>
   Start a service running if it is not already running.

stop <service>
   Stop a service from running if it is currently running.

swapon_all <fstab>
   Calls fs_mgr_swapon_all on the given fstab file.

symlink <target> <path>
   Create a symbolic link at <path> with the value <target>

sysclktz <mins_west_of_gmt>
   Set the system clock base (0 if system clock ticks in GMT)

trigger <event>
   Trigger an event.  Used to queue an action from another
   action.

verity_load_state
   Internal implementation detail used to load dm-verity state.

verity_update_state <mount_point>
   Internal implementation detail used to update dm-verity state and
   set the partition.<mount_point>.verified properties used by adb remount
   because fs_mgr can't set them directly itself.

wait <path> [ <timeout> ]
   Poll for the existence of the given file and return when found,
   or the timeout has been reached. If timeout is not specified it
   currently defaults to five seconds.

write <path> <content>
   Open the file at <path> and write a string to it with write(2).
   If the file does not exist, it will be created. If it does exist,
   it will be truncated. Properties are expanded within <content>.


Properties
----------
Init updates some system properties to provide some insight into
what it's doing:

init.action
   Equal to the name of the action currently being executed or "" if none

init.command
   Equal to the command being executed or "" if none.

init.svc.<name>
   State of a named service ("stopped", "running", "restarting")


Bootcharting
------------
This version of init contains code to perform "bootcharting": generating log
files that can be later processed by the tools provided by www.bootchart.org.

On the emulator, use the -bootchart <timeout> option to boot with bootcharting
activated for <timeout> seconds.

On a device, create /data/bootchart/start with a command like the following:

  adb shell 'echo $TIMEOUT > /data/bootchart/start'

Where the value of $TIMEOUT corresponds to the desired bootcharted period in
seconds. Bootcharting will stop after that many seconds have elapsed.
You can also stop the bootcharting at any moment by doing the following:

  adb shell 'echo 1 > /data/bootchart/stop'

Note that /data/bootchart/stop is deleted automatically by init at the end of
the bootcharting. This is not the case with /data/bootchart/start, so don't
forget to delete it when you're done collecting data.

The log files are written to /data/bootchart/. A script is provided to
retrieve them and create a bootchart.tgz file that can be used with the
bootchart command-line utility:

  sudo apt-get install pybootchartgui
  # grab-bootchart.sh uses $ANDROID_SERIAL.
  $ANDROID_BUILD_TOP/system/core/init/grab-bootchart.sh

One thing to watch for is that the bootchart will show init as if it started
running at 0s. You'll have to look at dmesg to work out when the kernel
actually started init.


Comparing two bootcharts
------------------------
A handy script named compare-bootcharts.py can be used to compare the
start/end time of selected processes. The aforementioned grab-bootchart.sh
will leave a bootchart tarball named bootchart.tgz at /tmp/android-bootchart.
If two such barballs are preserved on the host machine under different
directories, the script can list the timestamps differences. For example:

Usage: system/core/init/compare-bootcharts.py base_bootchart_dir
       exp_bootchart_dir

process: baseline experiment (delta)
 - Unit is ms (a jiffy is 10 ms on the system)
------------------------------------
/init: 50 40 (-10)
/system/bin/surfaceflinger: 4320 4470 (+150)
/system/bin/bootanimation: 6980 6990 (+10)
zygote64: 10410 10640 (+230)
zygote: 10410 10640 (+230)
system_server: 15350 15150 (-200)
bootanimation ends at: 33790 31230 (-2560)


Debugging init
--------------
By default, programs executed by init will drop stdout and stderr into
/dev/null. To help with debugging, you can execute your program via the
Android program logwrapper. This will redirect stdout/stderr into the
Android logging system (accessed via logcat).

For example
service akmd /system/bin/logwrapper /sbin/akmd

For quicker turnaround when working on init itself, use:

  mm -j
  m ramdisk-nodeps
  m bootimage-nodeps
  adb reboot bootloader
  fastboot boot $ANDROID_PRODUCT_OUT/boot.img

Alternatively, use the emulator:

  emulator -partition-size 1024 -verbose -show-kernel -no-window

You might want to call klog_set_level(6) after the klog_init() call
so you see the kernel logging in dmesg (or the emulator output).