COMS W4118 Operating Systems I

Linux System Overview: From Boot To Panic

Boot process

Under BIOS-based systems:

Power-on self-test (POST) and peripheral initializations
Jump to the boot code in the first 440 bytes of Master Boot Record (MBR)
MBR boot code locates and launches boot loader – ex) GRUB, Syslinux
Boot loader reads its configuration and possibly presents menu
Boot loader loads the kernel and launches it
Kernel unpacks the initramfs (initial RAM filesystem)
- initial temporary root filesystem
- contains device drivers needed to mount real root filesystem
Kernel switches to real root filesystem
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

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

?        Ss     0:00 login -- jae     
tty1     Ss     0:00  \_ -bash
tty1     S+     0:00      \_ xinit /etc/xdg/xfce4/xinitrc -- /etc/X11/xinit/xserverrc
?        Ss     0:03          \_ /usr/bin/X -nolisten tcp :0 vt1
tty1     S      0:00          \_ sh /etc/xdg/xfce4/xinitrc
tty1     Sl     0:00              \_ xfce4-session
tty1     S      0:00                  \_ xfwm4 --display :0.0 --sm-client-id 27c950fe4-6
tty1     Sl     0:00                  \_ Thunar --sm-client-id 2ed5a75a5-2f67-42c8-9765-
tty1     Sl     0:01                  \_ xfce4-panel --display :0.0 --sm-client-id 24a77
tty1     S      0:00                  |   \_ /usr/lib/xfce4/panel/wrapper /usr/lib/xfce4
tty1     S      0:00                  |   \_ /usr/lib/xfce4/panel/wrapper /usr/lib/xfce4
tty1     Sl     0:00                  \_ xfdesktop --display :0.0 --sm-client-id 29a760d
tty1     Sl     0:01                  \_ xfce4-terminal --geometry=100x36 --display :0.0
tty1     S      0:00                      \_ gnome-pty-helper
pts/0    Ss+    0:00                      \_ bash
pts/1    Ss     0:00                      \_ bash
pts/1    R+     0:00                      |   \_ ps afx
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