Operating Systems 2018F Lecture 20

Sample Questions

1. [1] Does alloc_pages() in the Linux kernel immediately allocate memory, or is memory allocated when it is accessed?
2. [1] Does an anonymous mmap system call allocate memory immediately, or is the memory allocated as it is accessed?
3. [2] What is one key advantage of lazy memory allocation? What is one key disadvantage?
4. [2] Assume we have modified the remember module so it allocates 16K (4 pages) at a time, and assume we have stored in /dev/remember 16K copies of the letter "a".
   • As written, what will a read of /dev/remember return if we use a 4K buffer, starting at the beginning of the file?
   • What will happen if we do a second read, again with a 4K buffer (on the same open file)?
5. [2] When is memory dynamically allocated in the remember module? When is it freed?
6. [2] As written, what happens when multiple write calls are made to /dev/remember? What is remembered? Why?
7. [2] What system call causes getattr() in memoryll.py to be called (on files in a memoryll filesystem)? How did you verify this?
8. [2] What function in memoryll would you have to change to make it impossible to change file timestamps? Why?
9. [2] When you run fusermount via execve, what euid does fusermount's process have? What uid does it have?
10. [2] What is a process's effective uid (euid)? Why is the euid not always equal to its uid?
11. [2] What system calls does memoryll make when performing file operations on behalf of the kernel? Give one concrete example showing how things work. (Hint: you won't be able to strace memoryll as a regular user.)
12. [BONUS 5] Implement mmap for the remember device. Show your implementation and explain how it works.

1. [2] Are man pages a good documentation source when developing kernel modules? Why or why not?
2. [2] When you load a kernel module, what code is guaranteed to run? (Which functions will definitely be called?) With what privileges will this code run?
3. [3] You attach a new USB webcam to your computer that is running Linux. The webcam is recognized and works properly.
   • How could you find out what kernel module(s) were loaded, if any, when you attached the webcam?
   • What is a simple way you could prevent those modules from being loaded in the future?
   • What sort of problems could the webcam drivers (modules) cause if they sometimes use dynamically allocated memory improperly (say, by using a buffer after it had been freed)?
4. [2] What would changing CLASS_NAME in newgetpid.c to "carleton" change in the observable behavior of newgetpid?
5. [2] How do you safely access userspace memory from a kernel module? Give an example of where we do this in a kernel module covered in class.
6. [2] What changes need to be made to newgetpid.c to allow it to respond to write requests? Hint: How does newgetpid.c respond to read requests?
7. [1] If you destroy all of the superblocks in a filesystem, will fsck be able to recover the filesystem? Explain.
8. [1] When would you expect to find files in the lost+found directory?
9. [2] When first connecting to a remote host via ssh, you will normally get a message saying something similar to this: "The authenticity of host 'access.scs.carleton.ca (134.117.29.72)' can't be established. RSA key fingerprint is SHA256:MexEKZF0Os0Vl6VTObN70lRf2DFsGfD8DTQ7FKKqVJ4. Are you sure you want to continue connecting (yes/no)?"
   • Why is this question important?
   • What is the "fingerprint" for?
10. [2] As user student (uid=1000), you run "sshfs <scs-username>@access.scs.carleton.ca:. scs-files", where <scs-username> is your username.
    • Will the files in scs-files have a uid=1000? Why or why not?
    • Assume a file in scs-files is marked as being readable only by the owner. Will you be able to read the contents of this file? Why or why not?
11. [1] How can you make a file that is 20 GB in size (you can read 20 GB of information from it) but takes up essentially no space on disk?