The document discusses file protection in operating systems. It covers the need for reliability and security of files, types of access control like read, write and execute permissions, access lists that specify permissions for individual users and groups, and approaches to access control including password protection and user classifications of owner, group, and others. Problems with long access lists and password protection are also summarized.

1. Recap
In the last class, you have learnt:
• Issues regarding deletion of files in a cyclic –
graph structure
• Advantages & disadvantage of a cyclic – graph
structure
• Understand the structure of general graph
directories & problems
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2. • HOME PREVIOUS TOPIC NEXT
• PREVIOUS QUESTION PAPERS FOR
OS
• CPP TUTORIALS
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3. Objectives
On completion of this period, you would be able to
• Understand the concept of file protection
• Know types of access
• Understand Access Lists and Groups
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4. PROTECTION
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5. A computer file needs protection which is of
two types
1. Reliability
Protection from physical damage
File systems can be damaged by
Hardware problems(errors in r/w)
Power surges or failures
Head crashes
Dirt and temperature
Bugs in file system software
2. Security
Protection from improper access
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6. A computer file needs protection which is of
two types
1. Reliability
• Provided by duplicate copies of files
• Take backup's at regular intervals
(daily/weekly/monthly)
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7. A computer file needs protection which
is of two types
2. Security
• Protecting files from unauthorized access
• More important in a multi user system
• Provided by controlling access to files
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8. Need for protecting files
• Need for protection is due to the ability to
access files
• Two ways to tackle the problem
1. Prohibit access providing complete protection
2. Provide free access without protection
• Both approaches are too extreme for general
use
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9. Controlled Access
• Desirable
• Provided by limiting the types of file access that
can be made
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10. Types of Access
• Read : Read from the file
• Write : Write or rewrite the file
• Execute : Load file into memory and execute
• Append : Write new information at end of file
• Delete : Delete the file and free its space
• List : List the name and attributes of the file
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11. Protection Mechanisms
• Different mechanisms proposed
• Select mechanism depending on the application
Ex : A small computer used only by a few
members of a research group may not need the
same protection as a large corporate computer
used for research, finance and personnel
operations
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12. Access Lists and Groups
• A common approach to protection is provide
access based on user identity
• Associate with each file and directory an access list
• Access list specifies user name and types of
access allowed for each user
• Access to user is provided as per the access list
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13. Problem with access lists
• Length of the list
• If every user need to be given read access to a file,
we must list all users with read access
a) constructing such a list is tedious
b) The directory entry needs to be of
variable size, resulting in complicated
space management
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14. Classification of users
To condense the length of access list many
systems recognize three classifications of users
in connection with each file
• Owner : The user who created the file
• Group : A set of users sharing the file
and need similar access
• Universe : All other users in the system
constitute the universe
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15. Representing different users in UNIX
• Unix defines three fields of three bits each
rwx, where
r controls read access
w controls write access
x controls execution
• A separate field is kept for the file owner,
group users and other users
• 9 bits per file are needed
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16. A sample directory listing in Unix
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17. Consider the first field in the above listing
-rwxrw-r--
• The first field describes the file’s protection
• A d as the first character indicates that the file is a
directory ; a – indicates an ordinary file
• 2nd,3rd & 4th bits owner’s access permissions
• 5th,6th & 7th bits group users’ permissions
• 8th,9th & 10th bits others’ permissions
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18. Other Protection Approaches
• Associate a password with each file
• Access to the file is controlled by the password
• Effective scheme in limiting access to a file if
passwords chosen randomly
passwords changed often
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19. Disadvantages of password protection
• If each file has a separate password, the no. of
passwords a user needs to remember becomes
large
• If all files have same password, then once it is
discovered all files are accessible
• To deal with this problem, some systems allow a
user to associate a password with a subdirectory
rather than with a single file
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20. Summary
In this class, you have learnt:
• File protection
• Need for controlled access to files
• Types of access
• Access Lists
• User classification
• Password protection
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21. Frequently Asked Questions
1. Explain the concept of file protection
2. List the types of access to files
3. Explain access lists and groups
4. Explain disadvantages of password protection
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22. Any questions ?
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23. Quiz
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24. 1. File systems can be damaged by
a) Hardware problems
b) Power failures
c) Dirt
d) All of the above
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25. 2. Which of following is desirable with regard to access
to files?
a) Free access without protection
b) No access at all
c) Controlled access
d) None of the above
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26. 3. Which is true about password protection ?
a) Passwords should be chosen randomly
b) Passwords must never be changed
c) Password should be changed often
d) Both a & c
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27. State whether the following are true or false
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28. State true or false
Reliability of files can be provided by duplicate
copies
True
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29. State true or false
The problem with access lists is their length
True
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30. State true or false
Owner is the user who created the file
True
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31. State true or false
Unauthorized access to files is desirable
False
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32. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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