Solution to Challenge #8 - debugsfs

In this challenge, I was asked to implement a small module which exposes various fields to user-space via debugfs in a dedicated directory called /sys/kernel/debug/eudyptula/. In this directory would be 3 files:

1. id - behaves the same as misc char device from challenge 6 (read-write for all).

2. jiffies - exposes the internal jiffies counter (read-only).

3. foo - a special buffer which can be written into only by root and be read by all.

The testing flow is as follows:

1. Test the Initialization of the Module:

   dmesg -c > /dev/null                                 # clear the current kernel log.
   lsmod                                                # see the module isn't loaded yet.
   mount -t debugfs none /sys/kernel/debug              # mount debugfs
   ls -la /sys/kernel/debug/eudyptula                   # verify there is no such device yet.
   

2. Test the id file:

   modprobe solution_8                                  # load the kernel module.
   dmesg -c                                             # see the loading logs of the module.
   lsmod                                                # see that the module is currently loaded.
   ls -la /sys/kernel/debug/eudyptula/id                # verify that there is a id file under eudyptula.
   cat /sys/kernel/debug/eudyptula/id                   # see roeey777 (i.e. the id) printed out.
   echo -n eudyptula > /sys/kernel/debug/eudyptula/id   # write the "secret" into the device.
   echo $?                                              # expect to see 0, indicating success.
   dmesg -c                                             # verify there is no error logs.
   
   echo -n nonsense > /sys/kernel/debug/eudyptula/id    # write something else than the "secret" into the device.
   echo $?                                              # expect to see 1, indicating failure.
   dmesg -c                                             # verify there is an error log indicating that nonsense isn't the "secret".
   

3. Test the jiffies file:

   ls -la /sys/kernel/debug/eudyptula/jiffies           # verify that there is a jiffies file under eudyptula.
   cat /sys/kernel/debug/eudyptula/jiffies              # see the number of passed jiffies.
   

4. Test the foo file:

   ls -la /sys/kernel/debug/eudyptula/foo               # verify that there is a foo file under eudyptula.
   echo test > /sys/kernel/debug/eudyptula/foo          # write "test\n" into the exposed kernel buffer.
   cat /sys/kernel/debug/eudyptula/foo                  # expect to see test printed out.
   

5. Test the Unloading of the Module:

   modprobe -r solution_8                               # unload the kernel module.
   dmesg -c                                             # see the unloading logs of the module.
   lsmod                                                # see that the module isn't loaded.
   ls -la /sys/kernel/debug/eudyptula                   # verify that there is no such char device.