media/Linux_kernel_module_cheat_demo.srt at master · cirosantilli/media · GitHub

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Linux kernel module cheat. This project is the perfect emulation setup to study and develop the Linux kernel,

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kernel modules, as well as x86 and ARM assembly. One major feature of this project is that everything

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is built from source. This means that you can inspect and GDB step debug

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everything to perfectly understand how your system is working. To get started,

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you can check out the quickstart instructions right here on the README.

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First, let's clone the repository. You don't need to do a recursive clone.

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Let's go into it.

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Make sure Docker is installed. I already have it.

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Let's set up the Python Virtual Environment.

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Activate it.

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Now we'll run some basic setup.

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The next step is to make sure that we are running on Ubuntu 20.04. This is because the

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Buildroot build only works on that version. In my case, I'm running on Ubuntu already,

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so I'm just going to use Docker. But if you are running on Windows or macOS,

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you could just use a virtual machine for Ubuntu 20.04. To do that, I'm going to create my Docker

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container with this little helper. Then I'm going to go into it.

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Let's just check that we are on Ubuntu 20.04.

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And now we're ready to build. So this command here will download and build

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everything that we need, including QEMU, Linux kernel, and all the userland.

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So because it builds so much, it will take a few minutes.

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Once the build has finished, we are now ready to run the simulation.

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The run command boots Linux inside of QEMU. We are now left inside a userland shell inside

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the emulator. And we can run regular userland commands like ls and pwd. These are provided

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by the Buildroot userland packages. We can also try to insmod our test kernel module hello.ko.

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And we see that it was successfully insmodded. Now let's quit the simulation with Ctrl A

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followed by X. Because everything was built from source, it is very easy to modify any

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part of our system. For example, let's try and add a hello message to the Linux kernel boot.

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So we're searching for the start_kernel function, which is a bit like the main.

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Then let's add a printk saying "hello, Linux kernel module cheat".

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Let's save and exit. Then we'll build Linux again.

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And now let's run the simulation again.

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And here we can see there is our little test message.

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Hello, Linux kernel module cheat. Because we have the source code available,

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one thing we can do is GDB step debug pretty much anything.

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So let's try and GDB step debug the Linux kernel. First, let's open a new terminal window.

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And let's go inside Docker there as well. Back to the first terminal. Let's run,

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but with the --gdb-wait flag added. So this makes QEMU weights for GDB to connect

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before running the simulation. Now we go back to tab number two.

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And let's do ./run-gdb and ask it to stop at the start_kernel function.

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So here we see that GDB has put a breakpoint on start_kernel,

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and then it has automatically continued the simulation. We're now left right at the start

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kernel function. Let's do a next GDB command. Now we're over the printk that we had just added

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and so on. We can also just list the source code as normal.

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If you go back to the first terminal, we see that it is empty because the simulation is paused.

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So let's do a continue on GDB. And back on the first terminal, Linux kernel finished booting.

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And GDB is just hanging. If we do a control C for example, it will stop wherever the simulation

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had been running at the current instruction here, the idle function, and we can continue

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debugging as desired. So in summary, the Linux kernel module cheat aims to be an extremely clean way

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to study systems programming and to make it very, very fun and easy to do.

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Thank you for watching and make sure to give it a try.