s6-linux-init
Software
skarnet.org

The s6-linux-init-maker program

s6-linux-init-maker reads configuration options on the command line, and outputs a directory to place in the root filesystem. That directory contains a script suitable as an init program, as well as support file hierarchies to get a complete s6 infrastructure running when the system is booted on that script.

s6-linux-init-maker only writes scripts. At boot time, these scripts will call commands provided by other skarnet.org packages such as execline or s6. It is the responsibility of the administrator to make sure that all the dependencies are properly installed at boot time, and that the correct options have been given to s6-linux-init-maker so that the programs are found on the root filesystem of the machine - else the scripts will crash.

Interface and usage

     s6-linux-init-maker \
       [ -c basedir ] \
       [ -l tmpfsdir ] \
       [ -b execline_bindir ] \
       [ -u log_uid -g log_gid | -U ] \
       [ -G early_getty ] \
       [ -2 stage2 ] \
       [ -r ] \
       [ -Z ] stage2_finish \
       [ -3 stage3 ] \
       [ -p initial_path ] \
       [ -m initial_umask ] \
       [ -t timestamp_style ] \
       [ -d dev_style ] \
       [ -s env_store ] \
       [ -e initial_envvar ] ... \
       dir

• s6-linux-init-maker should be run as root.
• s6-linux-init-maker parses options on its command line.
• It writes data into a directory dir, which must not exist beforehand.
• It exits 0 if everything went well, 100 if a user error occurred, and 111 if a problem occurred during the creation of the directory or its contents.

dir should then be copied by the administrator to the place declared as basedir. Be careful: it contains fifos, files with precise uid/gid permissions, and files with non-standard access rights, so be sure to copy it verbatim. The s6-hiercopy tool can do it, as well as the GNU or busybox cp -a or mv commands.

The basedir/init script is then suitable as a "stage 1" init program, i.e. the first program run by the kernel. The administrator should make a symbolic link from /sbin/init to basedir/init; the machine will then be ready to boot

Boot sequence

When the kernel boots, it runs the basedir/init script, also known as stage 1. and this is what happens:

• stage 1 is an execline script, so the first process run by the kernel is the execlineb program launcher.
• stage 1 mounts a tmpfs filesystem on tmpfsdir.
• stage 1 copies basedir/run-image verbatim to tmpfsdir.
• stage 1 empties its environment, then reads a global set of environment variables from the basedir/env environment directory.
• stage 1 forks a child that will block until s6-svscan is running.
• stage 1 executes, as process 1, into s6-svscan, with tmpfsdir/service as a scan directory.
• This scan directory already contains at least one service, which is the catch-all logger: error messages from the supervision tree, and from services that do not have a dedicated logger, are handled by a special s6-log instance and made available in tmpfsdir/uncaught-logs instead of clogging the system console.
• If the -G option has been given to s6-linux-init-maker, the scan directory will also contain a service for an early getty.
• s6-svscan starts all the services defined in the scan directory, and unblocks the child forked by stage 1.
• This child executes into stage2.

stage2 is the responsibility of the administrator - it will not be written automatically! It should contain all the necessary initialization sequence to bring up a proper system. When stage2 is executed, the machine state is as follows:

• stage2's working directory is / and its stdin is /dev/null. Its stdout and stderr both point either to /dev/console or to the pipe to the catch-all logger, depending on the -r option.
• The system has a valid device directory mounted on /dev.
• Depending on the kernel boot command line, the root filesystem may be in read-only mode.
• There is a tmpfs available for root only in tmpfsdir.
• s6-svscan is running as process 1. At any time, it is possible to make it supervise a long-lived process by linking the appropriate service directory into tmpfsdir/service, then running the command s6-svscanctl -a tmpfsdir/service. Services without a dedicated logger will send their output to the catch-all logger.
• A getty service may already be available. The point of this early getty is essentially to make it easier to debug if stage2 fails.

There is nothing else. In particular, no filesystem has been mounted yet, including /proc and /sys; and no one-time initialization has been performed. The point of stage 1 is only to make it possible to run stage2 with a logging infrastructure and a supervision infrastructure already available, and all the real machine and service initialization should happen in stage2.

Shutdown sequence

• A shutdown is performed when the administrator runs one of the s6-halt, s6-poweroff or s6-reboot commands.
• Those commands send a signal to the s6-svscan process running as pid 1; this signal is caught and s6-svscan runs the corresponding "signal handler" script that has been placed by s6-linux-init-maker into the basedir/run-image/service/.s6-svscan directory (and that has been copied at boot time to tmpfsdir/service/.s6-svscan).
• That script first spawns the stage2_finish script, who must have been written by the administrator. The purpose of stage2_finish is to perform the high-level shutdown sequence while the supervision tree is still alive. Typically, when using a service manager, stage2_finish would tell the service manager to bring all services down. When using s6-rc, a typical stage2_finish script just contains s6-rc -da change. More generally speaking, stage2_finish should undo what stage2 has done at boot time.
• The "signal handler" script then tells s6-svscan to exit via an appropriate s6-svscanctl command: s6-svscan then executes into the stage3 script, which, like stage2 and stage2_finish, is the responsibility of the administrator. When stage3 runs, the machine is in the following state:
  • The supervision tree has been torn down: it is not operational anymore. (So, commands such as s6-rc, which require a live supervision tree, will not work.)
  • stage3 runs as process 1. Doing so makes it easier to recover after killing all processes by kill -9 -1 or s6-nuke.
  • Its working directory is / and its stdin is /dev/null
  • Its stdout and stderr are both /dev/console
  • Depending on the exact configuration and what the administrator has written in stage2_finish, there may or may not be long-running services that remain alive. The catch-all logger and its supervisor will always be alive; this is not a problem because they do not hold any file descriptor to a filesystem that would need to be unmounted.
• The last command that stage3 executes should be s6-$1 -f, $1 being the first argument that has been given to it. This command will instantly execute the hard system halt, poweroff or reboot that has initially been asked by the admin.

The examples/ subdirectory of the s6-linux-init package contains an example of /etc/rc.init, /etc/rc.tini and /etc/rc.shutdown scripts, suitable for stage2, stage2_finish and stage3 respectively. Those scripts can practically be used as is if the machine is managed by the s6-rc service manager.

s6-linux-init-maker options

• -c basedir : at boot time, stage 1, which should be accessible as basedir/init, will read its read-only data from basedir. After running s6-linux-init-maker, the administrator should make sure to copy the created directory dir to basedir. basedir must be absolute. Default is /etc/s6-linux-init.
• -l tmpfsdir : at boot time, a tmpfs will be mounted on tmpfsdir. The directory should already exist in the root filesystem, and be empty. tmpfsdir must be absolute. Default is /run.
• -b execline_bindir : init is run by the kernel without a PATH, and since it is a script, it is necessary to tell it where to find the execlineb launcher and the first few early commands before PATH can be set. execline_bindir is the location where the execline binaries can be found. It must be absolute. Default is /bin.
• -u log_uid : the catch-all logger will run with the uid log_uid. Default is 0.
• -g log_gid : the catch-all logger will run with the gid log_gid. Default is 0.
• -U : the correct log_uid and log_gid values for the catch-all logger will be read from the UID and GID environment variables that have been passed to s6-linux-init-maker. This allows for invocations such as s6-envuidgid nobody s6-linux-init-maker -U ... so that the catch-all logger runs as the nobody user. Be aware that this option is only safe when the user database on the boot-time machine is the same as on the run-time machine, else the catch-all logger may run with an unexpected uid and gid.
• -G early_getty : if this option is set, s6-linux-init-maker will define a service that will run very early, before stage2 is executed. This early service should be a getty, to allow logins even if stage2 fails. early_getty should be a simple command line: for instance, "/sbin/getty 38400 tty1". By default, no early service is defined.
• -2 stage2 : stage2 is the location of the stage 2 script that will be run when the system has an operational supervision tree. It must be absolute. Default is /etc/rc.init.
• -r : redirect. By default, stage2 is run with stdout and stderr pointing to /dev/console, so that users can see what init scripts print. However, it may conflict with an early getty, or be undesirable for other reasons. The -r option redirects stage2's stdout and stderr to the catch-all logger, so the output will be made available in the tmpfsdir/uncaught-logs directory.
• -Z stage2_finish : stage2_finish is the location of the script that will be run when s6-svscan receives a signal that tells it to stop the machine, before it executes into stage3. It must be absolute. Default is /etc/rc.tini. Note that this script is run with its stdout and stderr redirected to the tmpfsdir/uncaught-logs logging directory, so its output will not appear on the system's console.
• -3 stage3 : stage3 is the location of the stage 3 script that will be run at the end of the machine lifetime, when s6-svscan is told to terminate. It must be absolute. Default is /etc/rc.shutdown.
• -p initial_path : the value to set the PATH environment variable to, for all the starting processes. This will be done as early as possible in stage 1. It is absolutely necessary for execline, s6, s6-portable-utils and s6-linux-utils binaries to be accessible via initial_path, else the machine will not boot. Default is /usr/bin:/bin.
• -m initial_umask : the value of the initial file umask for all the starting processes, in octal. Default is 022.
• -t timestamp_style : how logs are timestamped by the catch-all logger. 0 means no timestamp, 1 means external TAI64N format, 2 means ISO 8601 format, and 3 means both. Default is 1.
• -d dev_style : how /dev is handled on this system. 0 means a static /dev, 1 means devtmpfs but not automounted by the kernel at boot time, and 2 means devtmpfs automounted by the kernel at boot time. Default is 2.
• -s env_store : stage 1 init sometimes inherits a few environment variables from the kernel. It empties its environment before spawning stage2 and executing into s6-svscan, in order to prevent those "kernel" environment variables from leaking into the whole process tree. However, sometimes those variables are needed at a later time; in that case, giving the -s option to s6-linux-init-maker makes stage 1 init dump the "kernel" environment variables into the env_store directory, via the s6-dumpenv program, before erasing them. env_store should obviously be a writable directory, so it should be located under tmpfsdir! If this option is not given (which is the default), the environment inherited from the kernel isn't saved anywhere.
• -e initial_envvar : this option can be repeated. For every initial_envvar, s6-linux-init-maker will adjust the global environment directory in dir/env. initial_envvar must either be of the form VAR, to make sure that VAR does not appear in the global environment, or of the form VAR=VALUE, to add an environment variable VAR with the value VALUE. The TZ variable, for instance, is a good candidate to be set in the global environment.

Notes

The difficult parts of running s6-svscan as process 1 are:

• The fact that the supervision tree requires writable directories, so in order to accommodate read-only root filesystems, there needs to be a tmpfs mounted before s6-svscan is run.
• The catch-22 coming for the need to redirect the supervision tree's output away from /dev/console (which is fine for a first process invocation but impractical for log management of a whole process tree) and into a logger that is itself managed by the supervision tree it's reading data from.

The main benefit of s6-linux-init-maker is that it automates those parts. This means that it has been designed for real hardware where the above issues apply. If you are building an init system for a virtual machine, a container, or anything similar that does not have the /dev/console issue or the read-only rootfs issue, you will probably not reap much benefit from using s6-linux-init-maker: you could probably invoke s6-svscan directly as your process 1, or build a script by hand, which would result in a simpler init with less dependencies.