theread.me/_posts/2021-11-06-linux-contribution.md

layout	title	date	permalink	categories	author	published
post	My first contribution to Linux Kernel: Step by step	2021-11-06 00:00:00	first-linux-contribution/	programming	Mahdi	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
2. Create a qemu disk {% highlight bash %} qemu-img create disk.img 15G {% endhighlight %}
3. Start the machine and install Archlinux {% 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 {% highlight bash %} git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.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) {% 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 %}

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:

• Understanding a Kernel Oops!
• How to read, understand, analyze and debug a linux kernel panic
• Kernel Debugging

Reading The Call Trace

For example, I wanted to be able to understand the call trace of this Kernel Bug: bug-207773

The call trace section starts with:

[226832.533889] Call Trace:
[226832.534377]  <IRQ>
[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 %}

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. 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.

Next, I found bug-