Busybox initrd and initialization process

Initrd File system loading process ：
    1. Boot loader , for example GRUB, Defines the kernel to load , And the kernel image and related initrd Copy to memory . We can do it in Linux In the kernel source directory ./init Many of these features are found in subdirectories .
    2. In the kernel and initrd After the image is decompressed and copied into memory , The kernel will be called . It performs different initialization operations , Eventually you will find yourself in init/main.c:init()（subdir/file:function） Function .
    3. This function performs a lot of subsystem initialization operations . A pair of... Will be executed here init/do_mounts.c:prepare_namespace() Call to , This function is used to prepare the namespace （ mount dev file system 、RAID or md、 Equipment and final initrd）.
    4. load initrd By calling init/do_mounts_initrd.c:initrd_load() Realized .
    5. initrd_load() The function is called init/do_mounts_rd.c:rd_load_image(), It uses the init/do_mounts_rd.c:identify_ramdisk_image() To determine which... To load RAM disk .
    6. This function will check the of the image file magic Number to make sure it's minux、etc2、romfs、cramfs or gzip Format .
    7. Before returning to initrd_load_image Before , It also calls init/do_mounts_rd:crd_load(). This function is responsible for RAM Disk allocation space , And calculate the cyclic redundancy check code （CRC）, Then on RAM Decompress the disk image , And load it into memory .
    8. Now? , We have this in a block device suitable for mounting initrd image .
    9. Now use a init/do_mounts.c:mount_root() Call to mount the block device to the root file system . It will create the root device , And call init/do_mounts.c:mount_block_root().
    10. Call it here init/do_mounts.c:do_mount_root(), The latter will call fs/namespace.c:sys_mount() To actually mount the root file system , then chdir Into this file system .
    11. Last , Back to init Function , And call init/main.c:run_init_process. This results in a call to execve To start up init process （ In this case, it's /linuxrc）.linuxrc It can be an executable program , It can also be a script （ If it has a script interpreter available ）.

Busybox init The sequence of program execution is as follows ：

   1. establish init Signal processing of
　2. Initialize console
　3. analysis inittab file ,/etc/inittab
　4. Execute the system initialization script ,/etc/init.d/rcS
　5. To perform all inittab The type in is wait The program
　6. To perform all inittab The type in is once The program
　7. After completing the above steps ,init Loop executes all inittab The type in is respawn and askfirst The program .

Busybox inittab File example ：

::sysinit:/etc/init.d/rcS

# /sbin/getty respawn shell invocations for selected ttys.
tty1::respawn:/sbin/getty -nl /sbin/autologin 38400 tty1
tty2::askfirst:/sbin/getty 38400 tty2
tty3::askfirst:/sbin/getty 38400 tty3
#tty4::askfirst:/sbin/getty 38400 tty4
#tty5::askfirst:/sbin/getty 38400 tty5
#tty6::askfirst:/sbin/getty 38400 tty6

# Stuff to do when restarting the init
# process, or before rebooting.
::restart:/etc/init.d/rc.shutdown
::restart:/sbin/init
::ctrlaltdel:/sbin/reboot
::shutdown:/etc/init.d/rc.shutdown

Busybox inittab Document interpretation ：

/etc/inittab Each entry in the file is used to define a child process , And determine its startup method , The format is as follows ：
<id>:<runlevels>:<action>:<process>
for example ：
ttySAC0:askfirst:-/bin/sh
（1）<id>: Indicates the console to be used by this process （ Standard input 、 standard output 、 Standard error device ）. If omitted , Then use and init Process like console .
（2）<runlevels>: about Busybox init Program , This field means , It can be omitted .
（3）<action>: Express init How does the program control this subprocess ,
（4）<process>: The program to be executed , It can be an executable program , It can also be a script

If <process>: Field has "-" character , This means that this program is called “ The interaction of ”. stay /etc/inittab/ Under the control of documents ,init The behavior of the process is summarized as follows ：
（1） In the early stage of system startup ,init The process starts first <action> by sysinit wait once Of 3 Class subprocess .
（2） During normal system operation ,init The program starts first ,<action> by respawn askfirst Two kinds of subprocesses , And watch them , Restart a child process when it is found to exit .
（3） When the system exits , perform <action> by shutdown restart ctrlaltdel Of 3 One or all of the class subprocesses .

Action            conditions for execution                               explain
Sysinit          After the system is started, execute first              Only once ,init The process waits for it to finish before continuing with other actions
Wait              The system has finished executing sysinit After the process       Only once ,init The process waits for it to finish before continuing with other actions
Once            The system has finished executing wait After the process          Only once ,init The process does not wait for it to end
Respawn       It's over once After the process                Init Process monitoring found that the subprocess exited , Restart it
Askfirst          It's over respawn After the process          And respawn similar , however init Process first output “ Please press Enter to actvie this console ”, Wait for the user to enter the driving key before starting the sub process
Shutdown      When the system is shut down                         That is, when restarting and shutting down the system command
Restart          Busybox Configuration of the CONFIG_FEATURE_USE_INITTAB, also init Process receive SIGHUP Signal time      Read again first , analysis /etc/inittab file , Re execution  restart  Program
Ctrlatldel        Press down  Ctr+Alt+del  When you combine keys