Chapter 10 : Case Study - UNIX

• History • Overview • Processes • Memory management • Input/output in • The unix file system • Security Note: This case study covers only

UNIX.

Please read chapter 10 of the text book for

LINUX

. 1

History of UNIX

 Originated from the MULTICS (Multiplexed Information and Computing Service) operating system by M.I.T, Bell Labs and GE  There are two main versions:  AT&T System V Release 4 (SVR4)  Originally developed by AT&T, now SCO  BSD (Berkeley Software Distribution) 2

Overview of UNIX

         Supports various architectures Structure varies Supports preemptive multitasking Multiuser environment - generally secure Supports multithreaded applications Protection/Security is high on modern versions Supports symmetric multiprocessing Highly scalable/portable to various systems Many types/flavours of UNIX exist 3

UNIX Layers

The layers of a UNIX system.

Interface

4

UNIX Utility Programs

A few of the more common UNIX utility programs required by POSIX 5

UNIX Kernel (1)

Approximate structure of generic UNIX kernel 6

UNIX Kernel (2)

 Bottom layer  

Device drivers

for character and block devices

Process dispatcher

which stops the current process, saves its state and starts the appropriate driver when an

interrupt occurs

7

Process Creation in UNIX - fork

Process creation in UNIX.

8

POSIX Shell

A highly simplified shell 9

The

ls

Command

Steps in executing the command ls type to the shell 10

Some UNIX Process Concepts

  

Daemons

 (background processes)

Cron daemon

which wakes up once a minute to check scheduled events (eg., disk backup)

Pipes

- syncronized channels between processes to pass byte streams

Signals

– software interrupts used for interprocess communication. Choices: catch, ignore, or kill process 11

Signals Required By POSIX

The signals required by POSIX.

12

System Calls for Process Management

s is an error code pid is a process ID residual is the remaining time from the previous alarm 13

Thread Calls in POSIX

The principal POSIX thread calls.

14

Thread Calls in POSIX

     Threads were not in the first versions of UNIX There are many thread packages in use which are standardized in POSIX Thread calls are the same for user-space or kernel space In kernel-space implementation calls are system calls In user-space implementation calls are to a run-time library 15

Thread Communication mutexes

   Threads use locks called

mutexes

locking a resource (say a shared buffer) for short-time A mutex must be first

created

(and finally

destroyed)

Mutual exclusion is implemented by

locking a mutex

before accessing a resource and

unlock it

when they are done (like binary semaphores which is “0” or “1”, a mutex is either locked or unlocked) 16

Thread Communication – condition variables

   For long-term synchronization (such as waiting for a tape to become free)

condition variables

are used Condition variables have to be

destroyed

like mutexes

created

first and later A condition variable is used by having one thread

wait

on it, and another thread

signal

it. If no thread is waiting when a signal is sent, the signal is lost 17

UNIX Scheduler (1)

  The UNIX scheduler is based on a multilevel queue structure (highest priority queue first, round-robin in each queue) In this scheme, a process which was blocked and waiting for an event joins the appropriate queue when blocking is over (a process whose disk I/O is finished joins,say, queue –4) 18

UNIX Scheduler (2)

     Once a second the priority of all processes are recalculated to avoid starvation using

priority = CPU_usage + nice + base

CPU_usage, represents the average number of clock ticks per second that the process has had during the past few seconds Nice is a value between –20 to 20 (default 0). Nice system call can be used to set this value 0-20 Base is a system parameter in UNIX source code The scheduler forces CPU bound (on positive queues) get any service that is left over when all I/O bound and interactive processes are blocked 19

Booting UNIX (1)

     The first sector of the boot disk (

master boot record

) is read in and executed This sector loads the

boot

program Boot reads root directory, loads execution

kernel

and starts its Kernel reads the rest of the operating system (

C-code section

)

main

C code does some initialization, allocates system data structures, loads device drivers and handcrafts the first process,

process 0

20

Booting UNIX (2)

cp The sequences of processes used to boot some systems 21

Handling Memory

Process B Process A

 

Each process has three segments ( shown as one segment in the figure, but if hardware supports they can be separate ):



Text : executable code ( which is shared in the figure )



Data : variables, strings, arrays etc.

 

initialized data – variables which must be initialized to some value when program starts Uninitialized data (BSS) – not initialized but has value 0 as default



Stack Text is fixed in length, data and stack can grow and shrink

22

Paging in UNIX (1)

  Prior to 3BSD, UNIX systems used swapping (if memory is full, swap processes to disk) To run a process, all that is needed is the

structure

demand and

page table.

user

The pages of the text, data and stack segments are brought in on 23

Paging in UNIX (2) – Core Map

Page on Disk Process table entry   Main memory: kernel, core map, pages

Core map

has an entry for each page and contains information about the contents of the page frames 24

Page Replacement Algorithm (1)

   The page replacement algorithm is executed by the

page daemon

(process 2)

Page daemon

wakes up every 250 msec and transfers pages to disk if the amount of free memory is less than the system parameter

lotsfree

(typically set to ¼ of memory) Page daemon uses the

algorithm two-handed clock

25

Basic Clock Algorithm (1)

  The pointer (hand) points to the oldest page When a page fault occurs,   if the

R

bit of the pointed page is 0 (page not referred), this page is evicted (the new page replaces this page - written to disk first if it is dirty) if the

R

bit is 1 (page accessed), to the next page

R

bit is cleared and the hand is advanced 26

Two-handed Clock Algorithm (2)

    Page daemon has to do two passes with pointer. Pass 1 clears all pages (

R R one

bits are set between pass 1 and 2) core map bits, second pass removes Page daemon maintains two pointers into the core map to speed up the process (one pass instead of two) for large memories When page daemon runs, it first clears the

front hand

, and then checks the

R hand

, after which it advances both hands

R

bit at the bit at the

back

Each time the page daemon runs, the hands rotate less than a full revolution, the amount depending on the number of pages needed to reach

lotsfree

27

I/O in UNIX

  All I/O devices are integrated into the file system as

special files

These special files are accessed like ordinary files (ie., file operations such as read, write, open are the same for special files 28

Networking in UNIX (1)



Sockets

are used to establish a connection between network nodes 29

    

Networking in UNIX (2)

Sockets are created and destroyed dynamically Creating a socket returns a file descriptor, which is needed for establishing a connection, reading data, writing data, and releasing the connection One party makes a

listen

call on a local socket, which creates a buffer and blocks until data arrive The other party makes a

connect

parameters the file descriptor of the local socket and the address of a remote socket (a sockets has an address in the network like the internet) call giving as Once a connection is established, a socket functions like a

pipe

30

UNIX I/O (1)

   When a user accesses a special file, the file system determines the major and minor device numbers and whether it is a block or character special file Major device number is used to index into either array for block special or

cdevsw bdevsw

for character special files These structures contain pointers to the procedures to open the device, read, write etc., Some of the fields of a typical

cdevsw

table are shown below 31

UNIX I/O (2)

C-list The UNIX I/O system in BSD 32

UNIX I/O (3)

    For block special files (eg., disks) the blocks are cached in a

buffer cache

The buffer cache works for both reads and writes Usually dirty (modified) blocks are written to the disk in every 30 seconds For character special devices, data is buffered in a chain of

C-lists

. A C-list block is 64 characters long, plus a count and a pointer to the next block (BSD method of character buffering) 33

The UNIX File System (1)

Some important directories found in most UNIX systems 34

The UNIX File System (2)

 Before linking.

 After linking.

(a) Before linking. (b) After linking 35

The UNIX File System (3)

  Separate file systems After mounting (a) Before mounting (b) After Mounting 36

Locking Files in UNIX (1)

    Accessing a file by several processes need some critical section management This is done by

locks

A lock is defined by a file name, the starting byte and the number of bytes When placing a lock, the process specifies to 

Block

: when the existing block is removed, the process is unblocked and the lock is placed 

Not to block

: the system call returns with a status code telling whether the lock succeeded or not 37

Locking Files (2)

( a) File with one lock (b) Addition of a second lock (c) A third lock 38

System Calls for File Management

   s is an error code (-1 if an error has occured) fd is a file descriptor (a positive number: 0 standard input) position is a file offset 39

The

stat

System Call

Fields returned by the stat system call 40

System Calls for Directory Management

   s is an error code dir identifies a directory stream dirent is a directory entry 41

UNIX File System (1)

Disk layout in classical UNIX systems   Block 0 is the boot block Block 1 is the

superblock

about the layout of the file system, including the number of i-nodes, number of disk blocks, and start of the list of free disk blocks which contains information 42

UNIX File System (2)

Directory entry fields.

Structure of the i-node in System V 43

UNIX File System (3)

File descriptor table is indexed by the

fd

parameter and has one entry for each file The relation between the file descriptor table, the open file description 44

UNIX File System (4)

A BSD directory with three files.The same directory after the file voluminous has been removed   File name can be 255 characters long The first 4 fields are fixed length 45

Security in UNIX (1)

   Each UNIX user has a

UID

UID is an integer between 0 and 65536. Files, processes and other resources are marked with the UID of their owner (User ID). A The user with UID 0 is the

superuser

Users can be organized in groups, which are also numbered with 16-bit

GID

’s (Group ID) 46

Security in UNIX (2)

Some examples of file protection modes 47

System Calls for File Protection

  s is an error code uid and gid are the UID and GID, respectively 48