Customization Method of Linux Peripheral File System

Preface

Generally speaking, when we talk about Linux systems, we are referring to various operating system distributions based on the Linux Kernel and GNU Project. In order to master the use of the Linux operating system, understand the operation process of the Linux operating system, understand the relationship between the kernel and the peripheral support system, and deepen my understanding of open source operating systems, I decided to reinvent the wheel - customize a Linux file system myself.

There are two ways to do this:

Implement init directly by yourself**\*（M1）***

Load the hardware information of bios -> read MBR –> execute Grub -> load kernel –> load driver –> init –> execute bash

Utilize system/sbin/init**\*（M2）***

Load the hardware information of bios -> Read MBR –> Execute Grub -> Load kernel –> Load driver –> init –> /sbin/init -> Get run-level information -> /etc/rc.d/rc.sysinit -> services –> /etc/rc.d/rc.local –> mingetty –> login

Let’s select *M1* first.

Ideas

• Use the original system to copy the necessary components to the new storage
• Testing in RAM Disk using initrd.img mechanism
• Start with the original file kernel and module

Step 1: Get the shell version of initrd.img

First, we can write a script init so that the kernel can directly obtain a Bash after starting with the file system.

Create script init

Among them: the /bin directory contains commonly used commands, init is a script written by yourself, and the /lib64 directory contains the dynamic libraries that the application depends on.

init content

Now we need to use the command line, create bin and sbin directories, and add basic commands such as bash, ls, rm, cp, mv, echo, cat, less, etc. Since these commands need to rely on some dynamically linked shared libraries in directories such as /lib64, you need to copy the dependent libraries to the directory corresponding to the mini-system and use the ldd command to query the application and its dependent dynamic libraries. Once completed, execute:

find . | cpio -H newc -o | gzip > /boot/initrd.img

Package the root file system into initrd.img and put it in the /boot directory. The system will automatically execute init in initrd.img when it starts.

After spending so much effort to generate initrd.img, how do you test the newly created initrd.img? You need to add an entry in the grub startup configuration file for testing.

title CentOS 6 Mini
root (hd0,0)
kernel/vmlinuz-2.6.32-642.el6.x86_64
initrd /initrd.img

After restarting, the startup options will appear.

Step 2: Complete the ability to mount the original system

In order to mount the original system, the driver modules required for the original system to run must be loaded in initrd.img, such as the driver of the ext4 file system, the related drivers of the scsi device, etc. /sbin/modinfo cooperates with /sbin/insmod, and the driver is placed in /module

Step 3: Complete the ability to manage devices (udev)

Using udevd, a service program that manages and monitors host devices, to automatically load the required driver modules is more reliable than implementing it ourselves. The udevd rule file is in the /lib/udev/ directory, and the configuration file is in the /etc/udev/ directory. At the same time, the name service switch configured in /etc/nsswitch.conf is also required. The dependent library is the file starting with libnss in the /lib directory. Copy the above files to our directory, and then use the /sbin/start\_udev command to start the udevd service. (udevd needs to call some other system commands, such as /sbin/modprobe, which can be tracked and obtained using strace).

Mini-system directory file

The /dev directory is where the system stores available devices, and /log is the log file generated by the strace command.

Step 4: Complete the login capability

Since the login mechanism is relatively complex, involving many aspects such as process management mechanism, process group, console, etc., we use *M2* to copy the /sbin/init command to the small system directory and change the init script to

#!/bin/bash
exec /sbin/init

After handing control to /sbin/init, when the system starts up, the user must wait until it completes a series of calls and enters the login interface before the user can regain control.

The process of /sbin/init is roughly divided into three parts: the first part is udevd loading the driver module, file system check and root switching, and the relevant configuration is in /etc/rc.sysinit; the second part is starting various services, and the relevant configuration is in the /etc/rc.d/ directory; the third part is the login part, which requires calling commands such as /sbin/mingetty and /bin/login. Copy the above commands and files to the directory corresponding to the small system, and modify the configuration.

Since initrd.img runs directly in memory as a temporary root file system after the mini-system is started, and our mini-system does not need to perform root switching, the remount_needed() function body in /etc/rc.sysinit is commented out, so that there will be no root switching.

Since the system uses the new Upstart startup method (the /sbin/init program has been changed to be provided by the upstart software package), copy the configuration files related to Upstart startup to the mini-system directory:

/etc/inittab configures the default run level
/etc/init/rcS.conf loads the rc.sysinit script to complete the system initialization task
/etc/init/rc.conf compatible script, responsible for the call processing of each run level
/etc/init/rcS-sulogin.conf starts the /sbin/sushell environment for single-user mode
/etc/init/control-alt-delete.conf Control terminal Ctrl+Alt+Del hotkey operation
/etc/init/start-ttys.conf configures the number of tty terminals opened and device files
/etc/sysconfig/init controls the number of open tty terminals and the terminal color scheme
/etc/init/tty.conf controls the opening of tty terminals

Change the running priority of bootmini/etc/inittab to 2. Then when the system starts, /sbin/init will execute the files starting with S in the bootmini/etc/rc.d/rc2.d/ directory. Change the names of some service files that do not need to be enabled to start with K.

In the bootmini/etc/rc.d/rc.local file, you can add operations that users need to be automatically executed after the system starts.

The login program is based on the authentication system PAM. The configuration files are in the /etc/pam.d/ directory. The related library files are /lib64/security/ and its dependent library files. Login also involves user group management /bin/chgrp, /bin/chown, /bin/chmod, etc. The files that save user names are /etc/passwd and /etc/group, and the user password file is /etc/shadow. Some other files involved can be analyzed with the help of strace.

A complete mini-system that can run on a real machine

Some directory files:

/etc

/bin

/sbin

/usr/bin

/usr/sbin

At this point, the file system can be considered to be running. In the next article, we will reinvent the wheel and tailor the Linux kernel. The real machine effect will also be seen in the next article.

Summarize

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