IOCTL Linux Driver

Simple example on how to create a IOCTL driver for Linux thanks to LDD3. This can be used for simple testing purposes:

• Access to a special register from kernel mode to get the result in user mode. For example Arm register from CP15 processor.
• Access to some memory mapped region
• ...

This repo contains the folders below:

• dev/: driver source code:

  • ioctl_interface.c is the source code of the driver. The function that should be modified to add more IOCTL
  • ioctl_dev.h is the header file
  • ioctl.h contains the list of IOCTL codes. This list is also included in the application
  • Makefile to build the driver. If the driver is cross-compiled, the variable KDEV should be adjusted

• app/: application source code:

  • ioctl_app.c shows an example to access the driver and get the returned value
  • Makefile to build the application

• load_module.sh: Load the module

  • Unload and load the module. This probably needs sudo permissions!

Run

Use make, it will build the .ko module, the application and load the module.

Debug

Look for printk output in: /dev/kmsg or dmesg. Look for printk_ratelimit to limit the rate of a specific printk if it occurs too much See cat /proc/sys/kernel/printk to know the current, default, min, boot values for debug Note that printk can be used everywhere (even in interrupt) but care about having functions that have unexpected time to execute.

strace can be used to trace the interactions between user and kernel space. Use strace ./app/ioctl_app for example

Use dynamic debug pr_debug and enables them using:

echo 8 > /proc/sys/kernel/printk
echo 'file ioctl.c +p' > /sys/kernel/debug/dynamic_debug/control