From cd946f0b9c51d4dcc1ab174daf853bdf5c828855 Mon Sep 17 00:00:00 2001 From: David Oberhollenzer Date: Sat, 14 Apr 2018 19:14:08 +0200 Subject: Add basic documentation stub Signed-off-by: David Oberhollenzer --- docs/bootup.md | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 71 insertions(+) create mode 100644 docs/bootup.md (limited to 'docs/bootup.md') diff --git a/docs/bootup.md b/docs/bootup.md new file mode 100644 index 0000000..1f107bd --- /dev/null +++ b/docs/bootup.md @@ -0,0 +1,71 @@ +# System Bootup Process + +## Initial Ram Disk to Rootfs transition + +After mounting the root filesystem, either the kernel or the initial ram disk +startup process is expected to exec the init program from the root filesystem. + +At the current time, there is no support for re-scanning the service files +*yet*, so when init is started, the final configuration in `/etc/init.d` has to +be present. As a result, we currently cannot perform mounting of `/etc/` or +packing init into the initial ram disk and doing the rootfs transition. + +Also, as a result of this, changing the service configuration requires a system +reboot to be effective. + +This _will_ change in the future. + + +## Processing Service Descriptions + +The init process reads service description files from `/etc/init.d` which are +usually symlinks to actual files in `/usr/share/init`. + +The exact locations may be changed through configure flags when compiling init. + +Service files specify a *target* which is basically like a SystemV runlevel and +can be one of the following: + +* boot +* reboot +* shutdown +* ctrlaltdel + +After parsing the configuration files, the init process starts running the +services for the `boot` target in a topological order as determined by their +*before* and *after* dependencies. + +Services can be of one of the following *types*: + +* wait +* once +* respawn + +Services of type `wait` are started exactly once and the init process waits +until they terminate before continuing with other services. + +The type `once` also only runs services once, but immediately continues +starting other services in the mean time without waiting. + +Services of type `respawn` also don't stall the init process and are re-started +whenever they terminate. + +## Service Process Setup + +If a service description contains only a single `exec` line, the init process +forks and then execs the command directly in the child process. + +If the service description contains a `tty` field, the specified device file +is opened in the child process and standard I/O is redirected to it before +calling exec. Also, a new session is created. + + +If a service description contains multiple `exec` lines, the init process forks +off to a single child process that does the same setup as above, and then runs +the command lines sequentially by forking a second time for each one, followed +by an exec in the grand child and a wait in the original child. + +If a single command line returns something other than `EXIT_SUCCESS`, +processing of multiple command lines is immediately stopped and the offending +exit status is returned to init. + -- cgit v1.2.3