--- layout: post title: "My first contribution to Linux Kernel: Step by step" date: 2021-11-06 00:00:00 permalink: first-linux-contribution/ categories: programming author: Mahdi published: true --- _*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*_ 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. # Setting up the Virtual Machine (Archlinux) I create a virtual machine with Archlinux on my macOS using QEMU: 1. [Download the Archlinux iso image](https://archlinux.org/download/) 2. Create a qemu disk {% highlight bash %} qemu-img create disk.img 15G {% endhighlight %} 3. Start the machine and [install Archlinux](https://wiki.archlinux.org/title/Installation_guide) {% highlight bash %} qemu-system-x86_64 -cdrom archlinux-2021.11.01-x86_64.iso -boot order=d -drive format=raw,file=disk.img -m 8G {% endhighlight %} 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) {% highlight bash %} qemu-system-x86_64 -boot -drive format=raw,file=disk.img -m 8G -smp cpus=4 -net user,hostfwd=tcp::2222-:22 -net nic {% endhighlight %} 5. Install dependencies for building the kernel {% highlight bash %} pacman -S gcc git make {% endhighlight %} 6. Clone linux (note that you may want to clone another repository other than the stable if you are working on a specific part. For example I had to clone the `netdev` group's tree). [List of different trees](https://git.kernel.org/). {% highlight bash %} git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git # netdev group git clone git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git {% endhighlight %} 7. Install the necessary dependencies for building the kernel {% highlight bash %} pacman -S flex base-devel xmlto kmod inetutils bc libelf git cpio perl tar xz {% endhighlight %} 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)) {% highlight bash %} zcat /proc/config.gz > .config {% endhighlight %} 9. Make sure you enable debugging configurations {% highlight bash %} CONFIG_FRAME_POINTER=y CONFIG_KGDB=y CONFIG_KGDB_SERIAL_CONSOLE=y CONFIG_DEBUG_INFO=y {% endhighlight %} 10. 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. {% highlight bash %} make -j8 {% endhighlight %} 11. Install the newly built Kernel. I create this as a script file and run it after every build from the root of repository. {% highlight bash %} make -j8 modules_install RELEASE=$(cat include/config/kernel.release) cp -v arch/x86_64/boot/bzImage /boot/vmlinuz-linux${RELEASE} mkinitcpio -k $RELEASE -g /boot/initramfs-linux${RELEASE}.img mkinitcpio -k $RELEASE -s autodetect -g /boot/initramfs-linux-fallback${RELEASE}.img grub-mkconfig -o /boot/grub/grub.cfg {% endhighlight %} - Reboot and choose the new kernel (might be under "Advanced" in the bootloader) # Development Environment Setup your environment for development. Mine consists of setting up tmux so I can have multiple terminals and neovim. In the guest machine: {% highlight bash %} pacman -S neovim openssh tmux echo '[[ -z "$TMUX" ]] && exec tmux' >> /etc/profile # also follow https://github.com/junegunn/vim-plug for Neovim {% endhighlight %} And in the host: {% highlight bash %} scp -P 2222 ~/.tmux.conf root@localhost:/root scp -r -P 2222 ~/.config/nvim root@localhost:/root/.config/ {% endhighlight %} One thing I found necessary, due to limited storage, is a script to cleanup each linux version after I'm done with them, since they create a couple of files in different places. I call this `cleanup-linux.sh`: {% highlight bash %} VERSION=$1 rm /boot/vmlinuz-linux${VERSION} rm /boot/initramfs-linux${VERSION}.img rm /boot/initramfs-linux-fallback${VERSION}.img rm -r /usr/lib/modules/${VERSION} {% endhighlight %} # Debugging 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: {% highlight bash %} echo 8 > /proc/sys/kernel/printk echo 'module ip_set +p' > /sys/kernel/debug/dynamic_debug/control {% endhighlight %} Note that, this works if you have dynamic debug enabled in your `.config`: {% highlight bash %} CONFIG_DYNAMIC_DEBUG=y CONFIG_DYNAMIC_DEBUG_CORE=y {% endhighlight %} You can then look at `dmesg` while running the code to see those logs: {% highlight bash %} dmesg {% endhighlight %} ## Kernel Oops, Bug and Panic If you get a Kernel Oops, Kernel Bug or similar, here are some good resources on how to read and understand the output: - [Bug Hunting](https://www.kernel.org/doc/html/v5.0/admin-guide/bug-hunting.html?highlight=dmesg) - [Understanding a Kernel Oops!](https://www.opensourceforu.com/2011/01/understanding-a-kernel-oops/) - [How to read, understand, analyze and debug a linux kernel panic](https://stackoverflow.com/questions/13468286/how-to-read-understand-analyze-and-debug-a-linux-kernel-panic) - [Kernel Debugging](https://docs.freebsd.org/en/books/developers-handbook/kerneldebug/) ### Reading The Call Trace For example, I wanted to be able to understand the call trace of this Kernel Bug: [bug-207773](https://bugzilla.kernel.org/show_bug.cgi?id=207773) The call trace section starts with: ``` [226832.533889] Call Trace: [226832.534377] [226832.534776] recent_entry_update+0x52/0xa0 [xt_recent] [226832.535690] recent_mt+0x167/0x328 [xt_recent] [226832.536488] ? set_match_v4+0x96/0xb0 [xt_set] [226832.537407] ipt_do_table+0x24f/0x610 [ip_tables] [226832.538277] ? ipt_do_table+0x33e/0x610 [ip_tables] [226832.539146] ? l4proto_manip_pkt+0xde/0x440 [nf_nat] [226832.540049] ? ip_route_input_rcu+0x40/0x280 [226832.540831] nf_hook_slow+0x40/0xb0 [226832.541477] ip_forward+0x424/0x450 [226832.542116] ? ip_defrag.cold+0x37/0x37 [226832.542814] ip_rcv+0x9c/0xb0 ``` The way I did it was to run `gdb` on the `vmlinux` file in the root of the repository after build, and then load the symbol files of each module that is relevant: {% highlight gdb %} gdb vmlinux (gdb) add-symbol-file vmlinux.o (gdb) add-symbol-file net/ipv4/netfilter/ip_tables.o (gdb) list *(ipt_do_table+0x24f) (gdb) list *(nf_hook_slow+0x40) {% endhighlight %} # Creating your patch Here are some good guidelines on how to prepare and send your patch: - [The perfect patch](https://www.ozlabs.org/~akpm/stuff/tpp.txt) - [Linux kernel patch submission checklist](https://www.kernel.org/doc/html/v4.10/process/submit-checklist.html) There might be a file called `MAINTAINERS` in the root of the repository you cloned, it may include some notes and tips on how to make the life of maintainers easier by following some guidelines, read it! # What did I work on? 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. I wanted to work on the network modules, so I looked around there. In order to understand how networking module of linux works, I read these resources: - [Tracing the Way of Data in a TCP Connec- tion through the Linux Kernel](https://archive.org/details/linux_kernel_data_flow_short_paper/page/n1/mode/1up?view=theater)