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Chapter 21: The Linux

System

Chapter 21: The Linux System

 Linux History

 Design Principles

 Kernel Modules

 Process Management

 Scheduling

 Memory Management

 File Systems

 Input and Output

 Interprocess Communication

 Network Structure

 Security

History

 Linux is a modern, free operating system based on UNIX standards

 First developed as a small but self-contained kernel in 1991 by Linus Torvalds, with the major design goal of UNIX compatibility

 Its history has been one of collaboration by many users from all around the world, corresponding almost exclusively over the Internet

 It has been designed to run efficiently and reliably on common PC hardware, but also runs on a variety of other platforms

 The core Linux operating system kernel is entirely original, but it can run much existing free UNIX software, resulting in an entire UNIX- compatible operating system free from proprietary code

 Many, varying Linux Distributions including the kernel, applications, and management tools

The Linux Kernel

 Version 0.01 (May 1991) had no networking, ran only on 80386- compatible Intel processors and on PC hardware, had extremely limited device-drive support, and supported only the Minix file system

 Linux 1.0 (March 1994) included these new features:

 Support for UNIX’s standard TCP/IP networking protocols  BSD-compatible socket interface for networking programming  Device-driver support for running IP over an Ethernet  Enhanced file system  Support for a range of SCSI controllers for high-performance disk access  Extra hardware support

 Version 1.2 (March 1995) was the final PC-only Linux kernel

The Linux System

 Linux uses many tools developed as part of Berkeley’s BSD operating system, MIT’s X Window System, and the Free Software Foundation's GNU project

 The min system libraries were started by the GNU project, with improvements provided by the Linux community

 Linux networking-administration tools were derived from 4.3BSD code; recent BSD derivatives such as Free BSD have borrowed code from Linux in return

 The Linux system is maintained by a loose network of developers collaborating over the Internet, with a small number of public ftp sites acting as de facto standard repositories

Linux Distributions

 Standard, precompiled sets of packages, or distributions, include the basic Linux system, system installation and management utilities, and ready-to-install packages of common UNIX tools

 The first distributions managed these packages by simply providing a means of unpacking all the files into the appropriate places; modern distributions include advanced package management

 Early distributions included SLS and Slackware

 Red Hat and Debian are popular distributions from commercial and noncommercial sources, respectively

 The RPM Package file format permits compatibility among the various Linux distributions

Design Principles

 Linux is a multiuser, multitasking system with a full set of UNIX- compatible tools

 Its file system adheres to traditional UNIX semantics, and it fully implements the standard UNIX networking model

 Main design goals are speed, efficiency, and standardization

 Linux is designed to be compliant with the relevant POSIX documents; at least two Linux distributions have achieved official POSIX certification

 The Linux programming interface adheres to the SVR4 UNIX semantics, rather than to BSD behavior

Components of a Linux System

Components of a Linux System (Cont)

 The system libraries define a standard set of functions through which applications interact with the kernel, and which implement much of the operating-system functionality that does not need the full privileges of kernel code

 The system utilities perform individual specialized management tasks

Kernel Modules

 Sections of kernel code that can be compiled, loaded, and unloaded independent of the rest of the kernel

 A kernel module may typically implement a device driver, a file system, or a networking protocol

 The module interface allows third parties to write and distribute, on their own terms, device drivers or file systems that could not be distributed under the GPL

 Kernel modules allow a Linux system to be set up with a standard, minimal kernel, without any extra device drivers built in

 Three components to Linux module support:

 module management  driver registration  conflict resolution

Driver Registration

 Allows modules to tell the rest of the kernel that a new driver has become available

 The kernel maintains dynamic tables of all known drivers, and provides a set of routines to allow drivers to be added to or removed from these tables at any time

 Registration tables include the following items:

 Device drivers  File systems  Network protocols  Binary format

Conflict Resolution

 A mechanism that allows different device drivers to reserve hardware resources and to protect those resources from accidental use by another driver

 The conflict resolution module aims to:

 Prevent modules from clashing over access to hardware resources  Prevent autoprobes from interfering with existing device drivers  Resolve conflicts with multiple drivers trying to access the same hardware

Process Identity

 Process ID (PID). The unique identifier for the process; used to specify processes to the operating system when an application makes a system call to signal, modify, or wait for another process

 Credentials. Each process must have an associated user ID and one or more group IDs that determine the process’s rights to access system resources and files

 Personality. Not traditionally found on UNIX systems, but under Linux each process has an associated personality identifier that can slightly modify the semantics of certain system calls  Used primarily by emulation libraries to request that system calls be compatible with certain specific flavors of UNIX

Process Environment

 The process’s environment is inherited from its parent, and is composed of two null-terminated vectors:  The argument vector lists the command-line arguments used to invoke the running program; conventionally starts with the name of the program itself  The environment vector is a list of “NAME=VALUE” pairs that associates named environment variables with arbitrary textual values

 Passing environment variables among processes and inheriting variables by a process’s children are flexible means of passing information to components of the user-mode system software

 The environment-variable mechanism provides a customization of the operating system that can be set on a per-process basis, rather than being configured for the system as a whole