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Kernel Watchpoint Module

How to use:

insmod watchpoint-mod.ko target_addr=0x12345678

What i can do better?

  • Muxex. Why? For more safe code, bc if 2 processes try to write new adress in sysfs causes a crash probably.
  • Use only 1 breakpoint not 2. Why? Optimization
  • Maded in v2_optimization branch

This project is a simple Linux kernel module that lets you "watch" a memory address. When something in the system reads from or writes to that address, this module will print a debug message to the kernel log dmesg.

This guide is for those new to both Yocto and kernel dmesg development, like me ;) This is all the pain I went through doing this. Happy reading.

1 Part: What Are We Doing?

We are developing a kernel module that monitors memory and provides callbacks.

2 Part: Setting Up a Yocto Environment (From Scratch)

https://docs.yoctoproject.org/brief-yoctoprojectqs/index.html

3 Part: Adding Our Kernel Module to Yocto

Now we need to create our own "layer" to hold the recipe for our module. A layer is just a way to keep our custom stuff organized and separate from the official Yocto code.

Create a New Layer: From inside your poky/build directory, use the built-in tool to create a new layer for us.

bitbake-layers create-layer ../meta-custom

Add the Layer to the Build: Now, tell Yocto to pay attention to our new layer.

bitbake-layers add-layer ../meta-custom

Create the Recipe Directory: A recipe is a file that tells Yocto how to build something. We need to create a folder structure for our recipe inside our meta-custom layer.

mkdir -p ../meta-custom/recipes-kernel/watchpoint-mod/

3.1 Copy the Files: Place the files from this project into the directory you just created.

  • Copy watchpoint-mod.c to ../meta-custom/recipes-kernel/watchpoint-mod/watchpoint-mod.c

  • Copy watchpoint-mod.bb to ../meta-custom/recipes-kernel/watchpoint-mod/watchpoint-mod.bb

  • Tell Yocto to Build the Module: We need to add our module's name to the main image configuration file. Open conf/local.conf with a text editor (e.g., nano conf/local.conf ) and add this line to the end of the file:

IMAGE_INSTALL:append = " watchpoint-mod"

(Note the space at the beginning! It's important.)

3.2 How to make sure you add layer:

bitbake-layers show-layers

You should see:

layer                 path                                                                    priority
========================================================================================================
...
meta-custom           /home/koccc/poky/meta-custom                                            6

4 Part: Building and Running in QEMU

Build the Image: This is the magic step when you wait! The bitbake command will read all the recipes and build a complete, bootable Linux image from source code. This will take a long time (possibly hours) the first time you run it, as it has to download(more 90GB) and compile everything(more threads CPU better).

bitbake core-image-minimal

build

Run the Virtual Machine (QEMU): Once the build is finished, run this command to start a virtual machine with your new custom Linux OS.

runqemu qemux86-64 slirp nodesktop

The login is root with no password.

5 Part: How to watch if module loaded

Your module (watchpoint-mod.ko) should be find by command in QEMU:

find / -name "watchpoint-mod.ko"

If find shows a path:

  • Update module database:

depmod -a

  • Load module:

modprobe watchpoint-mod

modules

If no you anwser, you do something wrong in Part 3

ALL DONE! Cheers for you!

6 Part: helpful sites

https://sysprog21.github.io/lkmpg/#introduction - How to code Kernel Module

https://docs.yoctoproject.org/brief-yoctoprojectqs/index.html - How to Yocto. Yes, just Yocto.

https://www.youtube.com/@techabyte8/playlists - How to Yocto for beginners