104 lines
4.3 KiB
Markdown
104 lines
4.3 KiB
Markdown
---
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layout: post
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title: "My first contribution to Linux Kernel: Step by step"
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date: 2021-11-06 00:00:00
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permalink: first-linux-contribution/
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categories: programming
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author: Mahdi
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published: true
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---
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_*This post is under construction. I am in the process of trying to contribute to the Linux Kernel. This post is not finished and will get updated as I go*_
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I use a MacBook Pro (mid-2014) with macOS, so I need to have a virtual machine for running a linux system with my kernel. I will also be doing the coding on this linux virtual machine as building the kernel is easier in a linux system than macOS.
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# Setting up the Virtual Machine (Archlinux)
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I create a virtual machine with Archlinux on my macOS using QEMU:
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1. [Download the Archlinux iso image](https://archlinux.org/download/)
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2. Create a qemu disk
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{% highlight bash %}
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qemu-img create disk.img 15G
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{% endhighlight %}
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3. Start the machine and [install Archlinux](https://wiki.archlinux.org/title/Installation_guide)
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{% highlight bash %}
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qemu-system-x86_64 -cdrom archlinux-2021.11.01-x86_64.iso -boot order=d -drive format=raw,file=disk.img -m 8G
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{% endhighlight %}
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4. Start the machine after installing (note I forward 2222 to 22 so I can SSH/SCP to the virtual machine. I also set 4 CPUs so I can use threads for faster builds in the VM)
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{% highlight bash %}
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qemu-system-x86_64 -boot -drive format=raw,file=disk.img -m 8G -smp cpus=4 -net user,hostfwd=tcp::2222-:22 -net nic
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{% endhighlight %}
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5. Install dependencies for building the kernel
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{% highlight bash %}
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pacman -S gcc git make
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{% endhighlight %}
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6. Clone linux
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{% highlight bash %}
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git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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{% endhighlight %}
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7. Install the necessary dependencies for building the kernel
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{% highlight bash %}
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pacman -S flex base-devel xmlto kmod inetutils bc libelf git cpio perl tar xz
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{% endhighlight %}
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8. Copy configuration of archlinux (optional: also [use modprobed-db to remove unnecessary modules](https://wiki.archlinux.org/title/Kernel/Traditional_compilation#Default_Arch_configuration))
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{% highlight bash %}
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zcat /proc/config.gz > .config
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{% endhighlight %}
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9. Make! The `-j8` parameter specifies the number of threads to be used by the build. My CPU has 8 threads and so I use it all.
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{% highlight bash %}
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make -j8
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{% endhighlight %}
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10. Install the newly built Kernel. I create this as a script file and run it after every build from the root of repository.
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{% highlight bash %}
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make -j8 modules_install
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RELEASE=$(cat include/config/kernel.release)
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cp -v arch/x86_64/boot/bzImage /boot/vmlinuz-linux${RELEASE}
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mkinitcpio -k $RELEASE -g /boot/initramfs-linux${RELEASE}.img
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mkinitcpio -k $RELEASE -s autodetect -g /boot/initramfs-linux-fallback${RELEASE}.img
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grub-mkconfig -o /boot/grub/grub.cfg
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{% endhighlight %}
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- Reboot and choose the new kernel (might be under "Advanced" in the bootloader)
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# Development Environment
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Setup your environment for development. Mine consists of setting up tmux so I can have multiple terminals and neovim.
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In the guest machine:
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{% highlight bash %}
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pacman -S neovim openssh tmux
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echo '[[ -z "$TMUX" ]] && exec tmux' >> /etc/profile
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# also follow https://github.com/junegunn/vim-plug for Neovim
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{% endhighlight %}
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And in the host:
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{% highlight bash %}
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scp -P 2222 ~/.tmux.conf root@localhost:/root
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scp -r -P 2222 ~/.config/nvim root@localhost:/root/.config/
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{% endhighlight %}
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# Debugging
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There is a `pr_debug` function used over the code, in order to enable those logs in `dmesg` for a specific module, you can do this:
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{% highlight bash %}
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echo 8 > /proc/sys/kernel/printk
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echo 'module ip_set +p' > /sys/kernel/debug/dynamic_debug/control
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{% endhighlight %}
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Note that, this works if you have dynamic debug enabled in your `.config`:
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{% highlight bash %}
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CONFIG_DYNAMIC_DEBUG=y
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CONFIG_DYNAMIC_DEBUG_CORE=y
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{% endhighlight %}
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You can then look at `dmesg` while running the code to see those logs:
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{% highlight bash %}
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dmesg
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{% endhighlight %}
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# What did I work on?
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The first issue I was interested in turned out to be an invalid bug: I found that out by investigating the script the user was testing and measuring how much time each part of the script took to find out the main culprit: [bug-214851](https://bugzilla.kernel.org/show_bug.cgi?id=214851). But I learned a lot during this alone, mostly about how to build things quickly, where to look for modules, how to enable debugging for them, etc.
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