From 1bfba3b0be32306b078f5ee527b864e758b2c77b Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Fri, 26 Apr 2019 15:52:54 +0000 Subject: Make a single hpr. Full doc, first draft. --- doc/s6-linux-init.html | 154 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 154 insertions(+) create mode 100644 doc/s6-linux-init.html (limited to 'doc/s6-linux-init.html') diff --git a/doc/s6-linux-init.html b/doc/s6-linux-init.html new file mode 100644 index 0000000..a4bb636 --- /dev/null +++ b/doc/s6-linux-init.html @@ -0,0 +1,154 @@ + + + + + + s6-linux-init: the s6-linux-init program + + + + + + +

+ +

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. + + + + -- cgit v1.3.1

The s6-linux-init program

Interface

Options

Early preparation

Notes

The `s6-linux-init` program