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 as well as a script suitable as an init program.

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_user ] \
       [ -g early_getty ] \
       [ -2 stage2 ] \
       [ -r ] \
       [ -3 stage3 ] \
       [ -p initial_path ] \
       [ -m initial_umask ] \
       [ -t timestamp_style ] \
       [ -d dev_style ] \
       [ -e initial_envvar ] ... \
       dir > stage1

• 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 then prints a script to its standard output.
• It exits 0 if everything went well, 100 if a user error occurred, and 111 if a problem occurred during the directory or script creation.

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 stage1 script printed by s6-linux-init-maker on its stdout is then suitable as an init program. The administrator should copy it to /sbin/init and make it executable.

Boot sequence

When the kernel boots, it runs the stage1 script, and this is what happens:

• stage1 is an execline script, so the first process run by the kernel is the execlineb program launcher.
• stage1 mounts a tmpfs filesystem on tmpfsdir.
• stage1 copies basedir/run-image verbatim to tmpfsdir.
• stage1 reads a global set of environment variables from the basedir/env environment directory.
• stage1 forks a child that will block until s6-svscan is running.
• stage1 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 stage1.
• 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 stage1 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

When s6-svscan is told to exit via an appropriate s6-svscanctl command, it executes into the stage3 script, which, like stage2, is the responsibility of the administrator. stage3 is run in the following state:

• It 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 s6-svscanctl command that terminated s6-svscan, and what has happened before that command was sent, there may or may not be long-running services that remain alive. The catch-all logger and its supervisor will always be alive, which is not a problem because they do not hold any file descriptor to a filesystem that would need to be unmounted.

s6-linux-init-maker options

• -c basedir : at boot time, stage1 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_user : the catch-all logger will not run as root, but as log_user. Default is nobody.
• -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.
• -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 stage1. 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 the value that has been compiled in skalibs via the --with-default-path configure option, i.e. by default /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.
• -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.