Solution to Challenge #9 - sysfs

Note

In the implementation of this module Greg Kroah-Hartman’s blog was taken into considitation when it came to the race-condition between the publication of UEVENT_ADD of the struct kobject and the actual addition of files to sysfs.

This is the reason for creating the attributes groups and a custom struct ktype which uses them instead of simply using sysfs_create_group which is far easier to use.

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

id - behaves the same as misc char device from challenge 6 (and the id file from challenge 8).
jiffies - exposes the internal jiffies counter (read-only).
foo - a special buffer which can be written into only by root and be read by all.

Basically it’s the same challenge as challenge 8 but with sysfs instead of debugfs.

The testing flow is as follows:

Test the Initialization of the Module:

dmesg -c > /dev/null                           # clear the current kernel log.
lsmod                                          # see the module isn't loaded yet.
ls -la /sys/kernel/eudyptula                   # verify there is no such device yet.

Test the id file:

modprobe solution_9                            # 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/eudyptula/id                # verify that there is a id file under eudyptula.
cat /sys/kernel/eudyptula/id                   # see roeey777 (i.e. the id) printed out.
echo -n eudyptula > /sys/kernel/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/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".

Test the jiffies file:

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

Test the foo file:

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

Test the Unloading of the Module:

modprobe -r solution_9                         # 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/eudyptula                   # verify that there is no such char device.