Access control matrix in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the structure of an access control matrix

Which of the following best describes the structure of an access control matrix in operating systems?

A A hierarchical tree showing the inheritance of permissions from parent to child objects.

B A list of all users and their passwords stored in a secure file.

C A table where rows represent subjects, columns represent objects, and each cell lists the access rights the subject has over the object.

D A log file recording all access attempts to system resources.

💡 Hint

Think about how permissions are organized between users and resources.

An access control matrix is a conceptual model where each row corresponds to a subject (like a user or process), each column corresponds to an object (like a file or device), and each cell contains the rights the subject has over that object.

📋 Factual

intermediate

Access control matrix size implications

What is a major practical limitation of implementing a full access control matrix in real operating systems?

A It requires too much memory because the matrix can be very large with many subjects and objects.

B It cannot represent different types of access rights.

C It does not allow dynamic changes to permissions.

D It only works for single-user systems.

💡 Hint

Consider what happens when there are many users and many resources.

The access control matrix can become huge because it must store permissions for every subject-object pair, which uses a lot of memory and is inefficient to manage.

🚀 Application

advanced

Determining effective permissions from an access control matrix

Given the following access control matrix snippet:

       File1   File2   Printer
UserA   rw      r       -
UserB   r       rw      w
UserC   -       r       rw

What are the permissions UserB has on File2 and Printer?

A Read only on File2; no access on Printer

B No access on File2; write only on Printer

C Write only on File2; read and write on Printer

D Read and write on File2; write on Printer

💡 Hint

Look at the row for UserB and the columns for File2 and Printer.

UserB has 'rw' (read and write) permissions on File2 and 'w' (write) permission on Printer according to the matrix.

🔍 Analysis

advanced

Comparing access control matrix with ACL and capability list

Which statement correctly compares access control matrices with Access Control Lists (ACLs) and capability lists?

A An access control matrix can be implemented by storing ACLs for each object or capability lists for each subject, but ACLs focus on objects and capability lists focus on subjects.

B ACLs and capability lists are unrelated to access control matrices and serve different purposes.

C Access control matrices are only theoretical and cannot be implemented using ACLs or capability lists.

D Capability lists store permissions per object, while ACLs store permissions per subject, opposite to the matrix.

💡 Hint

Think about how permissions can be organized by objects or by subjects.

Access control matrices are conceptual. ACLs list permissions per object (columns), while capability lists list permissions per subject (rows). Both are practical ways to implement the matrix.

❓ Reasoning

expert

Impact of dynamic changes in access control matrix

In a system using an access control matrix, what is the likely effect of frequently adding and removing subjects and objects on the matrix's management?

A It reduces memory usage because unused rows and columns are automatically deleted.

B It increases complexity and overhead because the matrix must be updated for every change, making real-time management difficult.

C It has no effect since the matrix is static and does not change after system initialization.

D It simplifies management because the matrix automatically adjusts without manual updates.

💡 Hint

Consider what happens when the number of users and resources changes often.

Frequent changes require updating the matrix structure and entries, which is costly and complex, especially for large systems.

Practice

1. What does an access control matrix primarily represent in operating systems?

easy

A. Which users have what permissions on which resources

B. The order in which processes are executed

C. The memory allocation for each program

D. The list of installed software on a system

Solution

1. Step 1: Understand the purpose of access control matrix

   An access control matrix is a table that shows permissions users have on resources.

2. Step 2: Match the description to the options

   Only Which users have what permissions on which resources correctly describes permissions of users on resources.

3. Final Answer:

   Which users have what permissions on which resources -> Option A

4. Quick Check:

   Access control matrix = user permissions on resources [OK]

Hint: Think: Who can do what on which resource? [OK]

Common Mistakes:

• Confusing access control matrix with process scheduling
• Thinking it shows memory or software lists
• Mixing permissions with system hardware details

2. Which of the following correctly represents a row in an access control matrix?

easy

A. Process1 -> Memory address 0x1000

B. User1: read, write

C. File size = 1024 bytes

D. CPU usage = 50%

Solution

1. Step 1: Identify what a row in access control matrix shows

   A row lists a user and their permissions on various resources.

2. Step 2: Check which option matches this format

   User1: read, write shows a user with permissions, matching the row concept.

3. Final Answer:

   User1: read, write -> Option B

4. Quick Check:

   Row = user permissions list [OK]

Hint: Rows list users and their permissions [OK]

Common Mistakes:

• Confusing resource details with user permissions
• Mixing process or CPU info with access control
• Choosing options unrelated to permissions

3. Given the access control matrix below:

Users\Files | File1 | File2
---------|--------|-------
Alice | read | write
Bob | write | execute

What permission does Bob have on File2?

medium

A. execute

B. write

C. read

D. no permission

Solution

1. Step 1: Locate Bob's row in the matrix

   Bob's permissions are listed in the second row.

2. Step 2: Find the permission under File2 column for Bob

   Bob has 'execute' permission on File2.

3. Final Answer:

   execute -> Option A

4. Quick Check:

   Bob on File2 = execute [OK]

Hint: Find user row, then resource column [OK]

Common Mistakes:

• Mixing up rows and columns
• Choosing Alice's permissions instead of Bob's
• Assuming default permissions if not listed

4. Identify the error in this access control matrix snippet:

Users\Resources | FileA | FileB
--------------|--------|-------
Carol | read | readwriteexecute

What is wrong with this matrix?

medium

A. The matrix must list users vertically, not horizontally

B. Carol cannot have read permission

C. Multiple permissions should be separated, not combined in one cell

D. FileB should not have any permissions listed

Solution

1. Step 1: Check how permissions are listed in each cell

   Permissions should be clearly separated, usually by commas or listed distinctly.

2. Step 2: Identify if combining multiple permissions in one cell is acceptable

   Combining multiple permissions in one cell without clear separation can cause confusion.

3. Final Answer:

   Multiple permissions should be separated, not combined in one cell -> Option C

4. Quick Check:

   Permissions must be clearly separated [OK]

Hint: Permissions must be clearly separated in matrix cells [OK]

Common Mistakes:

• Thinking users must be listed horizontally
• Assuming permissions can't be combined at all
• Ignoring formatting clarity in matrix

5. You have an access control matrix with users and resources. If you want to quickly check which users can write to a resource named Report.doc, what is the best approach?

hard

A. Look across the row for Report.doc and note users with write permission

B. Check the entire matrix for any write permission

C. Check the diagonal cells only for write permission

D. Look down the column for Report.doc and note users with write permission

Solution

1. Step 1: Understand matrix layout

   Rows represent users, columns represent resources.

2. Step 2: To find users who can write to Report.doc, check the column for that resource

   Looking down the column shows which users have write permission.

3. Final Answer:

   Look down the column for Report.doc and note users with write permission -> Option D

4. Quick Check:

   Users with write on resource = column check [OK]

Hint: Check down the resource column for permissions [OK]

Common Mistakes:

• Looking across a resource row instead of column
• Checking only diagonal cells
• Scanning entire matrix unnecessarily