s6-linux-init
Software
skarnet.org

The s6-linux-init program

s6-linux-init is a program that is meant to run as pid 1, as a stage 1 init: it performs the necessary early system preparation and execs into s6-svscan.

Interface

     s6-linux-init [ -c basedir ] [ -p initial_path ] [ -s env_store ] [ -m umask ] [ -d dev_style ] [ -D initdefault ] [ args... ]

• If s6-linux-init isn't pid 1, it execs into s6-linux-init-telinit with the same arguments.
• Else, it performs some early preparation, spawns a process that will run the rc.init script, then execs into s6-svscan.

Options

These options should exactly mirror the options with the same name that have been given to s6-linux-init-maker. If there is a discrepancy, the system might not boot.

• -c basedir : read all its initialization data from basedir. If the data has not indeed been copied to basedir, the system will not boot.
• -p initial_path : the value to set the initial PATH environment variable to.
• -s env_store : the place where to dump kernel environment variables.
• -m initial_umask : the value of the initial file umask.
• -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.
• -D initdefault : the initial runlevel to boot to, if it isn't overridden by the kernel command line. This is only given as a first argument to rc.init. Default is default.

Early preparation

When booting a system, s6-linux-init performs the following operations:

• It prints a banner to /dev/console.
• It chdirs to /.
• It sets the umask to initial_umask.
• It becomes a session leader.
• It mounts a devtmpfs on /dev, if necessary.
• It uses /dev/null as its stdin (instead of /dev/console). /dev/console is still used, for now, as stdout and stderr.
• It unmounts /run (or the directory you have given to the --tmpfsdir configure option at package build time), just in case.
• It creates a tmpfs on /run (or your chosen tmpfsdir).
• It copies the whole basedir/run-image hierarchy to /run (or your chosen tmpfsdir).
• It reads the initial environment from basedir/env.
• If required, it stores the kernel environment into env_store.
• It performs "the fifo trick", i.e. it redirects its stdout to the catch-all logger's fifo, without blocking, before the catch-all logger is even up (because it's a service that will be spawned a bit later, when s6-svscan is executed).
• It forks a child.
  • The child scans the kernel command line to find a suitable runlevel (default, 2, 3, 4, or 5). If it doesn't find any kernel command line argument that defines a runlevel, it uses initdefault.
  • The child becomes a session leader.
  • The child blocks until the catch-all logger runs.
• It also makes the catch-all logger's fifo its stderr.
• It execs into s6-svscan with /run/service as its scandir (or tmpfsdir/service).
• s6-svscan spawns the early services that are defined in basedir/run-image/service, and have been copied into /run/service (or tmpfsdir/service).
• One of those early services is s6-svscan-log, which is the catch-all logger. When this service is up, s6-linux-init's child unblocks.
• The child execs into basedir/scripts/rc.init. The first argument to rc.init is the chosen runlevel. The kernel command line, as given by the kernel to s6-linux-init (i.e. without the key=value arguments, which were passed into s6-linux-init's environment and were stored into env_store), makes for the rest of the arguments given to rc.init.

By the time rc.init runs, s6-svscan is running as pid 1 and has spawned its early services - at least the catch-all logger, and the other services, including the early getty if it has been defined, are started in parallel and will be ready instantly. rc.init can then perform stage 2 of the initialization process, i.e. the initialization sequence per se.

Notes

• The s6-linux-init binary is not meant to be called directly, or be linked to /sbin/init directly, because it takes command-line options. Instead, after a s6-linux-init-maker invocation, the bin/ subdirectory of the target will contain a script called init, which execs into s6-linux-init with the appropriate command-line options, and is suitable as a /sbin/init program. The bin/ subdirectory should be copied by the administrator into /sbin for full interface compatibility with sysvinit.