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.