Operating Systems 2019W: Assignment 4

For this assignment please work on the questions below. While you will be able to submit your answers, by default they will not be graded. Instead, an approximately 20 question multiple choice quiz (based on the questions below) will be made available by April 8 and will be due by 4 PM on April 10th. The grade for this quiz will substitute for Assignment 4.

In class on April 10th solutions for the assignment questions below and the Assignment 4 Quiz will be given and discussed.

If you feel your answers to the quiz do not demonstrate what you actually learned, you can make an appointment with Prof. Somayaji to grade your submitted Assignment 4 answers (assuming you submitted them by April 10th) before April 19th.

Questions

1. [2] When you hit Ctrl-C while running the bc command, does the Ctrl-C generate a signal? How do you know?
2. [2] Does bashreadline report commands given to bash via shell scripts? How do you know?
3. [2] How can you modify 3000pc so the producer stops on the third time it fills its queue? Give your code as a diff -c versus the original 3000pc.
4. [2] Under what conditions would 3000pc never send a signal (from the producer to the consumer or vice versa)? How could you modify 3000pc so this occurance was more likely?
5. [2] Implement and critique your own version of sem_wait() and sem_post() in pure C with no system calls. Do you think your version is free of race conditions? Explain.
6. [2] Change 3000pc so that uses a shared memory mapped file rather than simply shared memory to communicate between the producer and consumer. How big is the file? Does its size change?
7. [2] How difficult is it for a userspace process to generate truly random numbers on its own, without the assistance of the kernel? Explain.
8. [2] Does the ssh command use random numbers? How can you demonstrate this?
9. [2] Can a setuid root process give up its privileges and run as a regular user? Is this change necessarily permanant?
10. [2] What are the main factors that influence whether the attacker wins the race with 3000log-write? Explain what it means for the attacker to win.

Multiple choice questions

1. What Linux distribution does Anil use?
   1. CentOS
   2. Arch
   3. Ubuntu
   4. Debian
   5. Gentoo
2. Which of the following operations are permitted for an unprivileged user?
   1. Making a hard link to a file owned by root
   2. Making a symbolic link to a file owned by root
   3. Making a character device file (with mknod)
   4. Mounting a USB stick on /mnt
   5. None of the above
3. Suppose you are running bc in a Linux terminal and press CTRL-C during its execution. Which of the following is TRUE?
   1. The kernel sends SIGSTOP to the terminal and causes bc to pause.
   2. Highlighted text gets copied to the clipboard on Linux.
   3. bc sends SIGTERM to itself and exits.
   4. bc sends SIGINT to itself and exits.
   5. None of the above
4. Bashreadline can be used to observe
   1. commands given to bash when entered in a terminal
   2. commands given to bash from sourced scripts
   3. commands given to any program on the system via a terminal
   4. A and B
   5. A, B, and C
5. The call b.attach_uretprobe() in bashreadline needs root access in order to succeed. Why does it need such privileges?
   1. It is directly modifying files owned by root.
   2. It is binding a privileged UNIX domain socket.
   3. It is accessing memory in an unsafe manner.
   4. It is loading code into the kernel.
   5. All of the above
6. When the producer receives a SIGUSR1 signal in 3000pc, it immediately responds by
   1. printing a message to standard out
   2. changes the prod_waiting flag
   3. wakes up the producer
   4. All of the above
   5. None of the above
7. Increasing the queue size in 3000pc will
   1. make the consumer send more signals
   2. make the producer sleep less frequently
   3. make the producer produce faster
   4. make the consumer consume faster
   5. All of the above
8. An implementation of a semaphore tests the value of the semaphore and changes it if the semaphore is available. What makes it hard to write a good semaphore implementation?
   1. semaphore operations should be fast
   2. semaphores work on shared data
   3. testing and setting a variable are normally separate instructions
   4. CPU cores have separate caches
   5. All of the above
9. Which of the following is true about "shared" in 3000pc?
   1. It is fixed sized
   2. Its size varies depending upon how many words are stored in it
   3. It is allocated in the standard process heap
   4. It is never cached
   5. All of the above
10. Which of the following could be used to generate truly random numbers?
    1. Seeding rand() in C with the current time
    2. Gathering input from a webcam
    3. Using an algorithm that generates numbers using a cryptographic hash
    4. Calculating digits of a transcendental number such as pi
    5. None of the above
11. The ssh command on Linux opens all of these files EXCEPT
    1. /etc/ld.so.cache
    2. /etc/services
    3. /dev/random
    4. /dev/urandom
    5. /etc/passwd
12. With what uid and euid values is it possible for a process to (successfully) call seteuid? Note that seteuid is a privileged system call that regular user processes cannot invoke.
    1. A process is running with uid=0 and euid=0
    2. A process is running with uid=1000 and euid=0
    3. A process is running with uid=1000 and euid=1000
    4. A process is running with uid=0 and euid=1000
    5. All of the above
13. A process's execution can be paused, allowing another process to run on the CPU, when it
    1. makes a system call
    2. copies a string
    3. reads a file
    4. All of the above
    5. None of the above
14. Increasing a process's priority (lowering its niceness value)
    1. increases the likelihood of it running when it needs CPU time
    2. gives it a guaranteed share of CPU time
    3. makes it send signal faster
    4. All of the above
    5. None of the above
15. Assuming that the attacker has already set up the TOCTTOU attack in advance, when must the attacker's code run relative to the code in safe_write() in 3000log-write?
    1. Before the call to access()
    2. After the call to access(), before the open()
    3. After the call to open(), before the write()
    4. After the call to write()
    5. None of the above