Linux System Overview: From Boot To Panic

Boot process

Under BIOS-based systems:

1. Power-on self-test (POST) and peripheral initializations

2. Jump to the boot code in the first 440 bytes of Master Boot Record (MBR)

3. MBR boot code locates and launches boot loader – ex) GRUB, Syslinux

4. Boot loader reads its configuration and possibly presents menu

5. Boot loader loads the kernel and launches it

6. Kernel unpacks the initramfs (initial RAM filesystem)

   • initial temporary root filesystem
   • contains device drivers needed to mount real root filesystem

7. Kernel switches to real root filesystem

8. Kernel launches the first user-level process /sbin/init (pid == 1)

   • /sbin/init is the mother of all processes
   • traditional SysV-style init reads /etc/inittab
   • in Arch, /sbin/init is a symlink to systemd

User session

1. init calls getty

   • getty is called for each virtual terminal (normally 6 of them)
   • getty checks user name and password against /etc/passwd

2. getty calls login, which then runs the user’s shell – ex) bash

   • login sets the user’s environment variables
   • the user’s shell is specified in /etc/passwd

Process control

Displaying process hierarchy

ps axf     # display process tree

ps axfj    # with more info

ps axfjww  # even if lines wrap around terminal

Creating processes

Simple shell program from APUE3, 1.6:

#include "apue.h"
#include <sys/wait.h>

int
main(void)
{
        char	buf[MAXLINE];	/* from apue.h */
        pid_t	pid;
        int		status;

        printf("%% ");	/* print prompt (printf requires %% to print %) */
        while (fgets(buf, MAXLINE, stdin) != NULL) {
                if (buf[strlen(buf) - 1] == '\n')
                        buf[strlen(buf) - 1] = 0; /* replace newline with null */

                if ((pid = fork()) < 0) {
                        err_sys("fork error");
                } else if (pid == 0) {		/* child */
                        execlp(buf, buf, (char *)0);
                        err_ret("couldn't execute: %s", buf);
                        exit(127);
                }

                /* parent */
                if ((pid = waitpid(pid, &status, 0)) < 0)
                        err_sys("waitpid error");
                printf("%% ");
        }
        exit(0);
}

Questions:

• What happens if the parent process terminates before its children?

  • create this condition and see it in ps output

• What happens if a child process has terminated, but the parent never calls waitpid()?

  • create this condition and see it in ps output

Signals

How do you terminate the following program?

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

static void sig_int(int signo)
{
    printf("stop pressing ctrl-c!\n");
}

int main()
{
    if (signal(SIGINT, &sig_int) == SIG_ERR) {
        perror("signal() failed");
        exit(1);
    }

    int i = 0;
    for (;;) {
        printf("%d\n", i++);
        sleep(1);
    }
}

How about this one?

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

static void sig_int(int signo)
{
    printf("stop pressing ctrl-c!\n");
}

static void sig_term(int signo)
{
    printf("stop trying to kill me!\n");
}

int main()
{
    if (signal(SIGINT, &sig_int) == SIG_ERR) {
        perror("signal() failed");
        exit(1);
    }

    if (signal(SIGTERM, &sig_term) == SIG_ERR) {
        perror("signal() failed");
        exit(1);
    }

    int i = 0;
    for (;;) {
        printf("%d\n", i++);
        sleep(1);
    }
}

X session

Let’s take a look at the processes in your X session:

 145 ?        Ss     0:00 login -- jae     
 406 tty1     Ss     0:00  \_ -bash
 421 tty1     S+     0:00      \_ xinit /etc/xdg/xfce4/xinitrc -- /etc/X11/xinit/xserverrc
 422 ?        Ss     0:03          \_ /usr/bin/X -nolisten tcp :0 vt1
 425 tty1     S      0:00          \_ sh /etc/xdg/xfce4/xinitrc
 430 tty1     Sl     0:00              \_ xfce4-session
 448 tty1     S      0:00                  \_ xfwm4 --display :0.0 --sm-client-id 27c950fe4-6
 450 tty1     Sl     0:00                  \_ Thunar --sm-client-id 2ed5a75a5-2f67-42c8-9765-
 452 tty1     Sl     0:01                  \_ xfce4-panel --display :0.0 --sm-client-id 24a77
 494 tty1     S      0:00                  |   \_ /usr/lib/xfce4/panel/wrapper /usr/lib/xfce4
 498 tty1     S      0:00                  |   \_ /usr/lib/xfce4/panel/wrapper /usr/lib/xfce4
 454 tty1     Sl     0:00                  \_ xfdesktop --display :0.0 --sm-client-id 29a760d
 456 tty1     Sl     0:01                  \_ xfce4-terminal --geometry=100x36 --display :0.0
 505 tty1     S      0:00                      \_ gnome-pty-helper
 507 pts/0    Ss+    0:00                      \_ bash
 518 pts/1    Ss     0:00                      \_ bash
2330 pts/1    R+     0:00                      |   \_ ps afx
1218 pts/2    Ss+    0:00                      \_ bash

Explorations:

• Identify the function of each process

• Understand the network client-server architecture of X window system

• Try running a simpler X session – twm or even just xterm

Kernel module

Here is a simple kernel module:

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

/* This function is called when the module is loaded. */
int simple_init(void)
{
       printk(KERN_INFO "Loading Module\n");

       return 0;
}

/* This function is called when the module is removed. */
void simple_exit(void) {
        printk(KERN_INFO "Removing Module\n");
}

/* Macros for registering module entry and exit points. */
module_init( simple_init );
module_exit( simple_exit );

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Simple Module");
MODULE_AUTHOR("SGG");

Here is the Makefile:

obj-m += simple.o
all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Can you modify the code to cause the kernel panic?

References

• Source code for this lecture

Last updated: 2019–01–24