Openwrt学习笔记（四）——系统开机启动 (续)

迈不过友情╰ 2022-07-08 17:59 607阅读 0赞

下面是英文原版，如果访问慢，也可以直接查看这里

[https://wiki.openwrt.org/doc/techref/init.detail.cc][https_wiki.openwrt.org_doc_techref_init.detail.cc]

[  
][https_wiki.openwrt.org_doc_techref_init.detail.cc]

# Init (User space boot) reference for Chaos Calmer: procd #

Analysis of how the user space part of the boot sequence is implemented in OpenWrt, Chaos Calmer release.

## Procd replaces init ##

On a fully booted Chaos Calmer system, pid 1 is `/sbin/procd`:

root@openwrt:~# ps
      PID USER       VSZ STAT COMMAND
        1 root      1440 S    /sbin/procd
        ...

At boot, Linux kernel starts `/sbin/init` as the first user process. In Chaos Calmer, `/sbin/init` does the preinit/failsafe steps, those that depend only on the read-only partition in flashed image, then execs (that is: is replaced by) `/sbin/procd` to continue boot as specified by the configuration in writable flash partition. Procd started as pid 1 assumes several roles: service manager, hotplug events handler; this as of February 2016, when this research was done. [Procd techref wiki page][] at this point in time is a design document and work in progress, if you are reading here and know/understand procd's semantics and API, please update that page.

Procd sources:  
[http://git.openwrt.org/?p=project/procd.git;a=tree;hb=0da5bf2ff222d1a499172a6e09507388676b5a08][http_git.openwrt.org_p_project_procd.git_a_tree_hb_0da5bf2ff222d1a499172a6e09507388676b5a08]  
at the commit used to build the procd package in Chaos Calmer release:  
`PKG_SOURCE_VERSION:=0da5bf2ff222d1a499172a6e09507388676b5a08`

`/sbin/init` source:  
[http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/init.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08\#l71][http_git.openwrt.org_p_project_procd.git_a_blob_f_initd_init.c_hb_0da5bf2ff222d1a499172a6e09507388676b5a08_l71]

## Life and death of a Chaos Calmer system ##

This is the source code path followed in logical order of execution by the processor in user space while booting Chaos Calmer.

![:!:][icon_exclaim.gif] All links to source repositories should show the code at the commit used in Chaos Calmer release.  
![:!:][icon_exclaim.gif] Pathnames evaluated at preinit time when / is read only have "(/rom)" prepended, to signify the path where the file is found on a fully booted system.

1.  `main(int argc, char **argv)` in /sbin/init, line 71  
    User space life begins here. OpenWrt calls this phase "preinit".
    
    1.  `early()` [(definition)][definition]  
        Mount filesystems: `/proc`, `/sys`, `/sys/fs/cgroup`, `/dev` (a tmpfs), `/dev/pts`  
        Populate `/dev` with entries from `/sys/dev/{char;block}`  
        Open `/dev/console` as STDIN/STDOUT/STDERR  
        Make directories `/tmp` (optionally on zram), `/tmp/run`, `tmp/lock`, `/tmp/state`  
          
        This accounts for most of the filesystem layout, observed that `/etc/fstab` is a [broken symlink, line 161][broken symlink_ line 161], with the following additions:  
        \- `procd_coldplug()` [invoked at hotplug setup time][] will recreate `/dev` from scratch.  
        \- `/etc/rc.d/S10boot` will invoke `mount_root` to setup a writable filesystem based on extroot or jffs2 overlay or a tmpfs backed [snapshot capable][] overlay, add some directories and files, and mount debugfs.
    2.  `cmdline()` [(definition)][definition 1]  
        Check kernel cmdline for boot parameter " `init_debug={1,2,3,4}`".
    3.  Fork `/sbin/kmodloader (/rom)/etc/modules-boot.d/` [kmodloader source][]  
        Wait up to 120 seconds for `/sbin/kmodloader` to probe the kernel modules declared in `(/rom)/etc/modules-boot.d/`  
        At this point in the boot sequence, '/etc/modules-boot.d' is the one from the rom image ( `/rom/etc/…` when boot is done). The overlay filesystem is mounted later.  
          
        kmodloader is a multicall binary, invoked as  
        `kmodloader`  
        does  
        `main_loader()`  
        which reads files in `(/rom)/etc/modules-boot.d/`, looking for lines starting with the name of a module to load, optionally followed by a space and module parameters. There appear to be [special treatment for files with names beginning with a number][]: the modules they list are immediately loaded, then modules from files with name beginning with an ascii char greater than "9" are loaded all together in a final load\_modprobe call.
    4.  `uloop_init()` line 116 [(definition)][definition 2]  
        [Documentation of libubox/uloop.h][Documentation of libubox_uloop.h] says:  
        *Uloop is a loop runner for i/o. Gets in charge of polling the different file descriptors you have added to it, gets in charge of running timers, and helps you manage child processes. Supports epoll and kqueue as event running backends.*  
        [uloop.c source in libubox][] says uloop's process management duty is assigned by a call to  
        `int uloop_process_add(struct uloop_process *p)`  
        `p->pid` is the process id of a child process to monitor and `p->cb` a pointer to a callback function.  
        When the managed child process will exit, uloop\_run, running in parent context to receive SIGCHLD signal, will trigger execution of the callback.
    5.  `preinit()` line 117 [(definition)][definition 3]
        
        1.  [Forks a "plugd instance"][Forks a _plugd instance], line 94  
            `/sbin/procd -h (/rom)/etc/hotplug-preinit.json`  
            to listen to kernel uevents for any required firmware or for notification of button pressed, handled by `(/rom)/etc/rc.button/failsafe`  
            as the request to enter failsafe mode. A flag file `/tmp/failsafe_button` containing the value of `${BUTTON}` is created if failsafe has been requested.
        2.  Forks, [at lines 106-111][],  
            `PREINIT=1 /bin/sh (/rom)/etc/preinit`  
            a shell to execute `(/rom)/etc/preinit` with `PREINIT=1` in its environment. Submits the child process to uloop management with the callback  
            `spawn_procd()`  
            that will exec procd to replace init as pid 1 at completion of `(/rom)/etc/preinit`.
            
            1.  `/etc/preinit`  
                A shell script, fully documented here [preinit\_operation][preinit_operation]. In short, parse files in `(/rom)/lib/preinit` to build 5 lists of hooks and an environment, then run the hooks from some of the lists depending on the state of the environment.  
                One of the steps in a successful boot sequence is to mount the overlay file system with a hook setup by  
                `(/rom)/lib/preinit/80_mount_root`  
                to call  
                `mount_root`  
                which if extroot is not configured, mounts the writable data partition "rootfs\_data" as overlay over the / partition "rootfs". If the data partition is being prepared, overlays a tmpfs in ram.  
                Filesystem snapshots are supported; this is a feature listed in Barrier Breaker announce, shell wrapper is `/sbin/snapshot` script. The " `SNAPSHOT=magic`" environment variable is set in `mount_snapshot()` line 330.
    6.  `uloop_run()`, line 118  
        At exit of the `(/rom)/etc/preinit` shell script, invokes the callback spawn\_procd()
    7.  `spawn_procd()`  
        As a callback by uloop\_run in pid 1, this is pid 1; execs `/sbin/procd`  
          
        
2.  `/sbin/procd`  
    Execed by pid 1 `/sbin/init`, `/sbin/procd` replaces it as pid 1.
    
    1.  `setsid()`, line 67  
        *The process group ID and session ID of the calling process are set to the PID of the calling process: [man 2 setsid][]* See also [man 7 credentials][].
    2.  `uloop_init()`, line 68  
        The uloop instance set up before by `/sbin/init` is gone. Creates a new one.
    3.  `procd_signal()`, line 69 [(definition)][definition 4], line 82.  
        Setup signal handlers. Reboot on SIGTERM or SIGINT, poweroff on SIGUSR2 or SIGUSR2.
    4.  `trigger_init()`, line 70 [(definition)][definition 5]  
        Procd triggers on config file/network interface changes, see [procd\_triggers\_on\_config\_filenetwork\_interface\_changes][procd_triggers_on_config_filenetwork_interface_changes]  
        Initialise a run queue. An [example][] is the sole documentation. A queued task has an uloop callback invoked when done, here sets the empty queue callback to do nothing.
    5.  `procd_state_next()`, line 74 [(definition)][definition 6]  
        Transitions from NONE to EARLY the state of a state machine implemented in `state_enter(void)` used to sequence the remaining boot steps.
    6.  `STATE_EARLY` in `state_enter()`
        
        1.  Emits "- early -" to syslog,
        2.  Initialise the watchdog,
        3.  `hotplug("/etc/hotplug.json")` [(definition)][definition 7]  
            User space device hotplugging handler setup.  
            Static variables in file scope are important. The filename of the script to execute is kept in hotplug.c global scope: `static char * rule_file;`.  
            Opens a netlink socket ( [man 7 netlink][]) and handles the file descriptor to uloop, to listen to uevents: kernel messages informing *u*serspace of kernel *events*. See [https://www.kernel.org/doc/pending/hotplug.txt][https_www.kernel.org_doc_pending_hotplug.txt]  
            The uloop instance in pid 1 [uses epoll\_wait][uses epoll_wait] to monitor file descriptors, the kernel netlink socket FD is one of them, and is instructed to invoke the callback `hotplug_handler()` on uevent arrival.  
            This `hotplug_handler` callback stays active after coldplug, and will handle all uevents the kernel will emit.
        4.  `procd_coldplug()` [(definition)][definition 8]  
            Umounts `/dev/pts` and `/dev`, mounts a tmpfs on `/dev`, creates directories `/dev/shm` and `/dev/pts`, forks `udevtrigger` to reconstruct kernel uevents went unheard before netlink socket opening ("coldplug").
            
            1.  `udevtrigger`  
                Scans `/sys/bus/*/devices`, `/sys/class`; and `/sys/block` if it isn't a subdir of `/sys/class`, writing "add" to the uevent file of all devices. Then the kernel synthesizes an "add" uevent message on netlink. See Injecting events into hotplug via "uevent" in [https://www.kernel.org/doc/pending/hotplug.txt][https_www.kernel.org_doc_pending_hotplug.txt]  
                  
                A callback chain, `udevtrigger_complete()` followed by `coldplug_complete()` is attached to completion of the child udevtrigger process, such that the still to be reached `uloop_run()` in procd `main()` function, after all uevents will have been processed, will advance procd state to STATE\_UBUS, [line 31][].
    7.  `uloop_run`, line 75  
        Solicited by udevtrigger in another process, the kernel emits uevents and uloop invokes the user space hotplug handler: the callback
        
        1.  `hotplug_handler`  
            to run `/etc/hotplug.json`.
            
            1.  The `/etc/hotplug.json` script  
                \- creates and removes devices files, assigns them permissions,  
                \- loads firmware,  
                \- handles buttons by calling scripts in `/etc/rc.button/%BUTTON%` if the uevent has the " `BUTTON`" value,  
                \- and invokes `/sbin/hotplug-call "%SUBSYSTEM%"` to handle all other subsystem related actions.  
                Subystems are: "platform" "net", "input", "usb", "usbmisc", "ieee1394", "block", "atm", "zaptel", "tty", "button" (without BUTTON value, possible?), "usb-serial". "usb-serial" is aliased to "tty" in hotplug.json.  
                Documentation of json script syntax? Offline. [Use the source][]. It is the json representation of the abstract syntax tree of a script in a fairly intuitive scripting language.  
                There are 2 levels at which decisions are taken: hotplug.json acts as fast path executor or lightweight dispatcher, the subsystem scripts in /etc/hotplug.d/%SUBSYSTEM%/ do the heavy lifting.  
                Uevent messages from the kernel contain key-value pairs passed as environment variables to the scripts. The kernel function  
                `int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...)`  
                creates them. This link [http://lxr.free-electrons.com/ident?v=3.18;i=add\_uevent\_var][http_lxr.free-electrons.com_ident_v_3.18_i_add_uevent_var] provides a list of all places in the Linux kernel where it is used. It is an authoritative reference of the upstream defined uevent variables. Button events are generated by the out of tree kernel modules `button-hotplug` `gpio-button-hotplug` specific to OpenWrt.
                
                1.  `/sbin/hotplug-call "%SUBSYSTEM%"`  
                    is a shell script that scans `/etc/hotlug.d/%SUBSYSTEM%/*` and sources all scripts assigned to a subsystem. "button" subsystem is handled here if the uevent lacks the "BUTTON" value, unlikely or impossible?.
        2.  `STATE_UBUS`  
            At end of coldplug uevents processing, the callback [coldplug\_complete calls][coldplug_complete calls] `procd_state_next` which results in advancing procd to STATE\_UBUS.  
            "- ubus -" is logged to console, the services infrastructure is initialised, then procd schedules connect to after 1" ( [line 67][]) and starts `/sbin/ubus` as the system [ubus][] service.  
            Transition to next state is triggered by the callback `ubus_connect_cb` that at the end, line 118, calls `procd_state_ubus_connect()`, line 186, that calls `procd_state_next` to transition to
        3.  `STATE_INIT`  
            "- init -" is logged, `/etc/inittab` is parsed and entries  
            `::askconsole:/bin/ash –login`  
            `::sysinit:/etc/init.d/rcS S boot`  
            executed. inittab format is the same as the one from busybox ( [Busybox example inittab][]).  
            The " `sysinit` action" handler
            
            1.  `runrc`  
                instantiates a queue, whose empty handler `rcdone` will advance procd state.  
                `runrc` [ignores][] the process specification " `/etc/init.d/rcS`" (there is no such a script!), and runs
                
                1.  `rcS(pattern="S" , param="boot", rcdone)` (line 159)  
                    that invokes the equivalent of  
                    `_rc(&q, *path="/etc/rc.d", *file="S", *pattern="*", *param="boot")`  
                    to enqueue in glob sort order the scripts  
                    `/etc/rc.d/S* boot`  
                    with " `boot`" as the action. `/etc/rc.d/S*` are [symlinks made by rc.common enable][] to files in `/etc/init.d`, that are shell scripts with the shebang `#!/bin/sh /etc/rc.common`.  
                    Invoking a `/etc/rc.d/S*` script runs `rc.common` that sources the /etc/rc.d/S\* script to set up a context, then invokes the function named as the action parameter (" `boot()`"), in that context.
        4.  `STATE_RUNNING`  
            Execution arrives here after rcS scripts are done.  
            "- init complete -" is logged.  
            This is a stable state, keeping uloop\_run in procd.c main() running, mostly waiting on epoll\_wait. [Upon receipt of a signal][] in SIGTERM, SIGINT (reboot), or SIGUSR2, SIGUSR2 (poweroff), procd transitions to
        5.  `STATE_SHUTDOWN`  
            "- shutdown -" is logged, /etc/inittab shutdown entry is executed, and procd sleeps [at line 169][] while the kernel does poweroff or reboot.
    8.  `uloop_done`  
        `return 0`  
        lines 75 & 76 are never reached by pid 1, kernel would panic if init exited.

[https_wiki.openwrt.org_doc_techref_init.detail.cc]: https://wiki.openwrt.org/doc/techref/init.detail.cc
[Procd techref wiki page]: https://wiki.openwrt.org/doc/techref/procd
[http_git.openwrt.org_p_project_procd.git_a_tree_hb_0da5bf2ff222d1a499172a6e09507388676b5a08]: http://git.openwrt.org/?p=project/procd.git;a=tree;hb=0da5bf2ff222d1a499172a6e09507388676b5a08
[http_git.openwrt.org_p_project_procd.git_a_blob_f_initd_init.c_hb_0da5bf2ff222d1a499172a6e09507388676b5a08_l71]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/init.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l71
[icon_exclaim.gif]: /images/20220708/a6e5786a400346a49189a39d472aaf64.png
[definition]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/early.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l92
[broken symlink_ line 161]: http://git.openwrt.org/?p=15.05/openwrt.git;a=blob;f=package/base-files/Makefile;hb=483dac821788b457d349233e770329186a0aa860#l161
[invoked at hotplug setup time]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=state.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l105
[snapshot capable]: https://wiki.openwrt.org/inbox/snapshot
[definition 1]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/init.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l56
[kmodloader source]: http://git.openwrt.org/?p=project/ubox.git;a=blob;f=kmodloader.c;hb=907d046c8929fb74e5a3502a9498198695e62ad8#l830
[special treatment for files with names beginning with a number]: http://git.openwrt.org/?p=project/ubox.git;a=blob;f=kmodloader.c;hb=907d046c8929fb74e5a3502a9498198695e62ad8#l788
[definition 2]: http://git.openwrt.org/?p=project/libubox.git;a=blob;f=uloop.c;hb=d1c66ef1131d14f0ed197b368d03f71b964e45f8#l211
[Documentation of libubox_uloop.h]: https://wiki.openwrt.org/doc/techref/libubox#libuboxulooph
[uloop.c source in libubox]: http://git.openwrt.org/?p=project/libubox.git;a=blob;f=uloop.c;hb=d1c66ef1131d14f0ed197b368d03f71b964e45f8#l666
[definition 3]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/preinit.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l86
[Forks a _plugd instance]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/preinit.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l94
[at lines 106-111]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=initd/preinit.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l106
[preinit_operation]: https://wiki.openwrt.org/doc/techref/preinit_mount#preinit_operation
[man 2 setsid]: http://man7.org/linux/man-pages/man2/setsid.2.html
[man 7 credentials]: http://man7.org/linux/man-pages/man7/credentials.7.html
[definition 4]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=signal.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l82
[definition 5]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=service/trigger.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l319
[procd_triggers_on_config_filenetwork_interface_changes]: https://wiki.openwrt.org/inbox/procd-init-scripts#procd_triggers_on_config_filenetwork_interface_changes
[example]: http://git.openwrt.org/?p=project/libubox.git;a=blob;f=examples/runqueue-example.c;hb=d1c66ef1131d14f0ed197b368d03f71b964e45f8
[definition 6]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=state.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l179
[definition 7]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=plug/hotplug.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l568
[man 7 netlink]: http://man7.org/linux/man-pages/man7/netlink.7.html
[https_www.kernel.org_doc_pending_hotplug.txt]: https://www.kernel.org/doc/pending/hotplug.txt
[uses epoll_wait]: http://git.openwrt.org/?p=project/libubox.git;a=blob;f=uloop.c;hb=d1c66ef1131d14f0ed197b368d03f71b964e45f8#l259
[definition 8]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=plug/coldplug.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l40
[line 31]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=plug/coldplug.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l31
[Use the source]: http://git.openwrt.org/?p=project/libubox.git;a=blob;f=json_script.c;hb=d1c66ef1131d14f0ed197b368d03f71b964e45f8
[http_lxr.free-electrons.com_ident_v_3.18_i_add_uevent_var]: http://lxr.free-electrons.com/ident?v=3.18;i=add_uevent_var
[coldplug_complete calls]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=plug/coldplug.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l34
[line 67]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=ubus.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l64
[ubus]: https://wiki.openwrt.org/doc/techref/ubus
[Busybox example inittab]: https://git.busybox.net/busybox/tree/examples/inittab?h=1_23_1&id=1ecfe811fe2f70380170ef7d820e8150054e88ca
[ignores]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=inittab.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l151
[symlinks made by rc.common enable]: http://git.openwrt.org/?p=15.05/openwrt.git;a=blob;f=package/Makefile;hb=483dac821788b457d349233e770329186a0aa860#l117
[Upon receipt of a signal]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=signal.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l33
[at line 169]: http://git.openwrt.org/?p=project/procd.git;a=blob;f=state.c;hb=0da5bf2ff222d1a499172a6e09507388676b5a08#l169