1. introto linux kernel

8/6/2019 1. IntroTo Linux Kernel

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What is Linux?

Is it different from Windows?

Before moving along, let's make sure thatwe understand the difference between aGUI and an OS.

Windows is an operating system that includesa GUI.

Mac OS X is an operating system thatincludes a GUI.

Linux is an operating system that may or may

not include a GUI.


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what is an OS?

Computers are really dumb and don't think. They blindly followwhatever instructions they are given.

That said, the operating system is a program that exists to

control access to hardware and software resources like yourmemory (RAM), hard-drives, network, display, keyboard, mouseand so on.

Thus, a program like OpenOffice needs to ask the OS which

keys were pressed on the keyboard. OpenOffice also needs to ask the operation system to draw

letters on the screen.

OpenOffice also needs to ask the operating system to allocatememory, load and save information to the hard disk, and send

printing instructions to your printer.


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Putting all this into a picture..
http://farstarnet.com/jfbilodeau/uploaded_images/os-layers-705763.png


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Back to the original question:

What is Linux?The answer is: Linux is a kernel. Nomore. No less.

It is the fundamental piece of code thatcontrols the resources on your machine.

Windows has a kernel.

Mac OS X has a kernel.

Pretty much every operating system has akernel.


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So, whats inside a kernel?


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But, I have never seen akernel!!

End users do not interact directly with the kernel. To allow users to interact with their computer, a set of

tools are provided with the operating system. In the world of Windows, you are familiar with the Start

menu, the Control Panel and the Windows Explorer. These are some of the operating system's tools that one

uses everyday to control the machine. These programs are part of the operating system, but run

on top of the kernel. Such tools are sometimes called shell

programs. A kernel provides API that computer programmers use to write

these shell programs.


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So, is Linux not an OS?

Linux is only a kernel.

It does not include a shell or programs.

However, Linux is most commonly packaged

with a combination of free programs fromanother--as of yet incomplete--operatingsystem called GNU.

Because it borrows from the GNU project,you may hear people refer to the operatingsystem as GNU/Linux.
http://en.wikipedia.org/wiki/GNUhttp://en.wikipedia.org/wiki/GNU


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Another view..

With the combinationof the GNU tools andthe Linux kernel, we

now have a completeoperating system.Lets visualize it


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A little bit more on GNU

GNU: Open Source OS before Linux

The concept of free software

Through 70s: Richard Stallman advocates free

software free as in free speech not free beer (not zero

cost) - free to use, distribute (for a profit), and modify.

1984: Richard Stallman founded GNU

Goal: to produce free software

GPL: ensure software freedom by copyrightterms

GNU software: Unix-like programs (no kernel)


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Linux History

1991: Finnish student Linus Torvalds started working onan update of Minix (released in 1987 by Andrew STanenbaum).

From the start, Linus asked volunteers on the Internet tohelp him develop Linux People started using and publicizing Linux A number of programmers joined the project Linux Community: created by a loose worldwide community of

programmers collaborating mainly through the internet

From the start, the source code has been freely available

on the Internet Linux provides a kernel for GNU software

GNU has lots of user-space programs (like libraries, compilers,text editors and a Windowing System) but no kernel

Linux kernel + GNU software = Linux distribution


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Linux Kernel Evolution Linux kernel has undergone massive development in the last one and

a half decade : 0 (Apr 1991): First e-mail from Linus about his new OS. 0.01 (Sept 1991) 1.0 (March 94), user: 100,000 1.2 (March 95), user: 500,000 2.0 (June 96), user: 1,500,000

2.2 (January 99) 2.4 (January 2001) 2.6 (December 2003)

Odd/Even versioning system 2.4.x: stable kernels 2.5.x: development kernels

Starting with the Linux 2.6.x series, there is no significance to even or odd numbers, with new feature

development going on in the same kernel series. Linus Torvalds has statedthat this will be the model for the foreseeable future.

The version number of the Linux kernel has changed from a three-numberversioning scheme to a four-number one. A.B.C[.D] (e.g. 2.2.1, 2.4.13 or2.6.12.3).


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Linux Kernel versions


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The Kernel of Our Study

Using Linux on Windows By using partitioning and a dual boot system

By running Linux in a virtual machine (usingVMWare, sun virtiual box or Virtual PC) on a

Windows system.


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Recall..


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Kernel basic facilities The kernel's primary purpose is to manage the

computer's resources and allow other programs to runand use these resources

The Central Processing Unit (CPU, the processor). Kernels role - deciding at any time which of the many

running programs should be allocated to the processor orprocessors .

The computer's memory. Kernels role - deciding which memory each process can

use, and determining what to do when not enough isavailable.

Input/Output (I/O) devices. Kernels role - allocates requests from applications to

perform I/O to an appropriate device and providesconvenient methods for using the device (typicallyabstracted to the point where the application does not needto know implementation details of the device).


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The Linux Kernel

Linux kernel is written in C programminglanguage

It implements POSIX standards in terms ofinterface, commands etc, but the sourcecode implementation is not UNIX.


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A. Frank - P. Weisberg

UNIX System Structure


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Kernel design decisions

1. Monolithic

Examples: Linux, Unix

2. Microkernel

Examples: Minix

3. Hybrid

Examples: Windows NT


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CPU modes At a minimum, any CPU architecture offers two

distinct operating modes; Kernel mode or supervisor mode or privilegedmode In kernel mode, the CPU may perform any operation

allowed by its architecture; any instruction may be

executed, any I/O operation initiated, any area ofmemory accessed, and so on

User Mode or Slave Mode certain restrictions on CPU operations are enforced

by the hardware. Typically, certain instructions are not

permitted (especially thoseincluding I/Ooperationsthat could alter the global state of themachine), some memory areas cannot be accessed,etc.


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Monolithic Kernel

As the name suggests kernel is oneunstructured program.

It runs every basic system service likeprocess and memory management,interrupt handling and I/O communication,file system, etc. as a single process in the

same address space (in kernel space).


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Monolithic Operating System


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Microkernel System Structure

Move as much functionality as possible fromthe kernel into user space.

Only a few essential functions in the kernel primitive memory management (address space) I/O and interrupt management Inter-Process Communication (IPC) basic scheduling

Other OS services are provided by processesrunning in user mode (referred to as servers) device drivers, file system, virtual memory,

networking


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A. Frank - P. Weisberg

Layered vs. MicrokernelArchitecture


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Pros and Cons of Monolithickernels

Pros increase the

performance of thesystem.

Cons : the kernel size,

lack of extensibility

bad maintainability. a bug in a device

driver might crash theentire system.


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Pros and Cons of Microkernel

Pros easier to maintain

Cons

the large number ofsystem calls andcontext switchesslow down thesystem becausethey generate moreoverhead than plainfunction calls.


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The Linux Kernel

Linux kernel is monolithic Implemented in a single executable module

Data structure for any aspect of the kernel isavailable to any other part of the kernel.

Design Philosophy: Main part of kernel should never change

To account for advanced devices (esp. bitmapdisplays and networks) Linux uses modules as containers to implement

extensions to the main part of the kernel.

Module an independent software unit that can bedesigned and implemented after the kernel and

dynamically loaded into kernel space.


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Introduction to System Calls


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The System Call Path


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System Call Stub Functions The system call stub functions provide a high-level

language interface to a function whose main job isto generate the software interrupt (trap) needed toget the kernel's attention.

These functions are often called wrappers.

The stub functions do the following:

set up the parameters,

trap to the kernel,

check the return value when the kernel returns, and

if no error: return immediately, else

if there is an error: set a global error number variable (called"errno") and return a value of -1.


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x86 Linux read (glibc 2.1.3)

read: push %ebx

mov 0x10(%esp,1),%edx ; put the 3 parms in registers

mov 0xc(%esp,1),%ecx

mov 0x8(%esp,1),%ebx

mov $0x3,%eax ; 3 is the syscall # for readint $0x80 ; trap to kernel

pop %ebx

cmp $0xfffff001,%eax ; check return value

jae read_err

read_ret: ret ; return if OK.read_err:


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The dilemma?

How can a user mode program switch theCPU to supervisor mode with theassurance that once the switch is done the

CPU will be executing trusted kernel codeand not untrusted user code?


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The trap instruction

A trap instruction is not a privilegedinstruction, so any program can execute atrap.

However, the destination of the branchinstruction is predetermined by a set ofaddresses that are kept in supervisory

space and that are configured to point tokernel code


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The Trap Instruction Operation

S

trap

Mode

Branch Table

TrustedCode

1

2

3

User Space Kernel Space


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Sequence of steps.

The trap instruction does three things:

1. Switch the CPU to supervisor mode.

2. Look up a branch address in a kernel space table.

3. Branch to a trusted OS function.

Because the trap instruction is compiled into the stubprocedure, a user-space program cannot easilydetermine the traps destination address.

Also, it cannot branch directly to the kernel functiononly through the system-provided stub.


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Daemons There is no special kernel process that executes the

kernel code. Kernel code is actually induced by normal user processes.

However, there are several user-transparentprocesses called daemons, that are started whena Linux machine is started, and which must exist

for correct operation of the operating system. They do not belong to a user.

Used for functioning in areas such as for Processing network packets

Logging of system and error messages etc. Normally, filename ends with d, such as inetd, syslogd, crond, lpd etc

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