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[[!meta copyright="Copyright © 2020 Free Software Foundation, Inc."]]
[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
id="license" text="Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
is included in the section entitled [[GNU Free Documentation
License|/fdl]]."]]"""]]
[[!meta title="System bootstrap"]]
[[!tag open_issue_documentation]] <!-- Someone still needs to make a pass over
this text. -->
[[!toc]]
# State at the beginning of the bootstrap
After initializing itself, GNU Mach sets up tasks for the various bootstrap
translators (which were loader by the GRUB bootloader). It notably makes
variables replacement on their command lines and boot script function calls (see
the details in `gnumach/kern/boot_script.c`). For instance, if the GRUB
bootloader has the following configuration:
multiboot /boot/gnumach-1.8-486-dbg.gz root=device:hd1 console=com0
module /hurd/ext2fs.static ext2fs --readonly \
--multiboot-command-line='${kernel-command-line}' \
--host-priv-port='${host-port}' \
--device-master-port='${device-port}' \
--exec-server-task='${exec-task}' -T typed '${root}' \
'$(task-create)' '$(task-resume)'
module /lib/ld.so.1 exec /hurd/exec '$(exec-task=task-create)'
GNU Mach will first make the `$(task-create)` function calls, and thus create a
task for the ext2fs module and a task for the exec module (and store a port on
that task in the `exec-task` variable).
It will then replace the variables (`${foo}`), i.e.
* `${kernel-command-line}` with its own command line (`root=device:hd1 console=com0`),
* `${host-port}` with a reference to the GNU Mach host port,
* `${device-port}` with a reference to the GNU Mach device port,
* `${exec-task}` with a reference to the exec task port.
* `${root}` with `device:hd1`
This typically results in:
task loaded: ext2fs --readonly --multiboot-command-line=root="device:hd1 console=com0" --host-priv-port=1 --device-master-port=2 --exec-server-task=3 -T typed device:hd1
task loaded: exec /hurd/exec
(You will have noticed that `/hurd/exec` is not run directly, but through
`ld.so.1`: Mach only knows to run statically-linked ELF binaries, so we could
either load `/hurd/exec.static`, or load the dynamic loader `ld.so.1` and tell
it to load `/hurd/exec`)
GNU Mach will eventually make the `$(task-resume)` function calls, and thus
resume the ext2fs task only.
# ext2fs initialization
ext2fs's `main` function starts by calling `diskfs_init_main`.
`diskfs_init_main` parses the ext2fs command line with `argp_parse`, to record
the parameters set up by the kernel. It makes sure to have a working stdout by
opening the Mach console.
Since the multiboot command line is available, `diskfs_init_main` sets the
ext2fs bootstrap port to `MACH_PORT_NULL`: it is the bootstrap filesystem which
will be in charge of dancing with the exec translator.
`diskfs_init_main` then initializes the libdiskfs library and spawns a thread to
manage libdiskfs RPCs.
ext2fs continues its initialization: creating a pager, opening the
hypermetadata, opening the root inode to be set as root by libdiskfs.
ext2fs then calls `diskfs_startup_diskfs` to really run the startup.
# diskfs bootstrap
Since the bootstrap port is `MACH_PORT_NULL`, `diskfs_startup_diskfs` calls
`diskfs_start_bootstrap`.
TODO: we want `diskfs_startup_diskfs` to also call `task_get_bootstrap_port` to
call `fsys_startup` on its real bootstrap port once `diskfs_start_bootstrap` is
finished, for bootstrap translators before the root filesystem to know when the
root filesystem is ready, and register themselves as translators in the root
filesystem, register for shutdown notification, etc.
`diskfs_start_bootstrap` starts by creating a open port on itself for the
current and root directory, all other processes will inherit it.
`diskfs_start_bootstrap` does have a port on the exec task, so it can dance with
it. It calls `start_execserver` that sets the bootstrap port of the exec task
to a port of the `diskfs_execboot_class`, and resumes the exec task.
`diskfs_start_bootstrap` then waits for `execstarted`.
# exec bootstrap
exec's `main` function starts and calls `task_get_bootstrap_port` to get
its bootstrap port and `getproc` to get a port on the proc translator (thus
`MACH_PORT_NULL` at this point since the proc translator is not started yet).
exec initializes the trivfs library, and eventually calls `trivfs_startup` on
its bootstrap port.
`trivfs_startup` creates a control port for the exec translator, and calls
`fsys_startup` on the bootstrap port to notify ext2fs that it is ready, give it
the control port, and get back a port on the underlying node for the exec
translator (we want to make it show up on `/servers/exec`).
# diskfs taking back control
`diskfs_execboot_fsys_startup` is thus called. It calls `dir_lookup` on
`/servers/exec` to return the underlying node for the exec translator, and
stores the control port in `diskfs_exec_ctl`. It can then signal `execstarted`.
`diskfs_start_bootstrap` thus takes back control, It calls `fsys_getroot` on the
control port of exec, and uses `dir_lookup` and `file_set_translator` to attach
it to `/servers/exec`.
`diskfs_start_bootstrap` then looks for which startup process to run. It may
be specified on the multiboot command line, but otherwise it will default to
`/hurd/startup`.
Now that exec is up and running, the startup process can be created with
`exec_exec`. `diskfs_start_bootstrap` takes a lot of care in this: this is
the first unix-looking process, it notably inherits the root directory and
current working directory initialized above, it gets stdin/out/err on the mach
console. It is passed as bootstrap port a port from the `diskfs_control_class`.
# startup
startup's `main` function starts and calls `task_get_bootstrap_port` to get its
bootstrap port, and `fsys_getpriv` to get a port on the ext2fs translator. It
clears the bootstrap port so children do not inherit it. It sets itself up with
output on the Mach console, and wires itself against swapping. It requests
notification for ext2fs translator dying to detect it and print a warning in
case that happens during boot. It creates a `startup` port which it will get
RPCs on.
startup can then complete the unixish initialization, and run `/hurd/proc` and
`/hurd/auth`, giving them as bootstrap port the `startup` port.
# proc
proc's `main` function starts. It initializes itself, and calls
`task_get_bootstrap_port` to get a port on startup. It can then call
`startup_procinit` to pass it the proc port that will represent the startup
task, and get ports on the auth server, the host privileged port, and device
master port.
Eventually, proc calls `startup_essential_task` to tell startup that it is
ready.
# auth
auth's `main` function starts. It creates the initial root auth handle (all
permissions allowed). It calls `task_get_bootstrap_port` to get a port on
startup. It can then call `startup_authinit` to pass the initial root auth
handle, and get a port on the proc server. It can then register itself to proc.
Eventually, auth calls `startup_essential_task` to tell startup that it is ready.
# startup getting back control
startup notices initialization of auth and proc from `S_startup_procinit` and
`S_startup_authinit`. Once both have been called, it calls `launch_core_servers`.
`launch_core_servers` starts by calling `startup_procinit_reply` to actually
reply to the `startup_procinit` RPC with a port on auth.
`launch_core_servers` then tells proc that the bootstrap processes are
important, and how they relate to each other.
`launch_core_servers` then calls `install_as_translator` to show up in the
filesystem on `/servers/startup`.
`launch_core_servers` calls `startup_authinit_reply` to actually reply to the
`startup_authinit` RPC with a port on proc.
`launch_core_servers` eventually calls `fsys_init` on its bootstrap port
# diskfs taking back control
diskfs' `diskfs_S_fsys_init` gets called, it thus knows that proc and auth are
ready, and can call `exec_init`. It initializes the default proc and auth ports
to be given to processes.
diskfs calls `startup_essential_task` to tell startup that it is
ready.
Eventually, it calls `_diskfs_init_completed` to finish its initialization, and
notably call `startup_request_notification` to get notified by startup when the
system is shutting down.
# exec taking back control
exec's `S_exec_init` gets called, it can register itself with proc, and
eventually call `startup_essential_task` to tell startup that it is ready.
# startup monitoring bootstrap progress
As mentioned above, the different essential tasks (ext2fs, proc, auth, exec)
call `startup_essential_task` when they are ready. startup's
`S_startup_essential_task` function thus gets called each time, and startup
records each of them as essential, monitoring their death to crash the whole
system.
Once all of proc, auth, exec have called `startup_essential_task`, startup
replies to their respective RPCs, so they actually start working altogether. It
also calls `launch_system`, which calls `launch_something`, which "launches
something", which by default is `/libexec/runsystem`, but if that can not be
found, launches a shell instead, so the user can fix it.
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