Operating Systems 2018F: Assignment 3

Please submit the answers to the following questions via CULearn by 2:30 PM on Friday, November 9, 2018. There are 20 points in 11 questions.

Submit your answers as a single text file named "<username>-comp3000-assign3.txt" (where username is your MyCarletonOne username). The first four lines of this file should be "COMP 3000 Assignment 3", your name, student number, and the date of submission. You may wish to format your answers in Markdown to improve their appearance.

No other formats will be accepted. Submitting in another format will likely result in your assignment not being graded and you receiving no marks for this assignment. In particular do not submit an MS Word or OpenOffice file as your answers document!

Don't forget to include what outside resources you used to complete each of your answers, including other students, man pages, and web resources. You do not need to list help from the instructor, TA, or information found in the textbook.

Questions

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?

Solutions

1. Man pages are not a good source of documentation on kernel APIs because most kernel APIs are, in effect, private - the Linux kernel doesn't maintain a stable internal interface. Any man pages that would be created would need to be continuously updated in order to stay up to date. To figure out how to call kernel routines, you ultimately have to consult the code of the kernel itself.
2. The functions designated by module_init() and module_exit() are guaranteed to be called, with the module_init()-designated function being called when the module is loaded and module_exit()-designated function called when the module is unloaded. These functions run with the same privileges as all kernel code, allowing them to modify anything on the system.
3. USB webcam answers:
   • To find out what modules were loaded, you could run lsmod before and after the webcam was attached and compare the results.
   • You could prevent those module(s) from loading by making their files inaccessible (say, by deleting them). You can find the files in /lib/modules/<kernel-version> (in some subdirectory).
   • If the webcam modules didn't use dynamically allocated memory properly, they could corrupt the kernel and cause the system to crash or otherwise malfunction.
4. It wouldn't change the behavior of /dev/newgetpid; however, it would change the names of the directories created in /sys/class/ and /sys/devices/virtual/.