As you already may know, I've started a series of [blog posts](http://0xax.github.io/categories/assembly/) about assembler programming for `x86_64` architecture in the last year. I have never written a line of low-level code before this moment, except for a couple of toy `Hello World` examples in the university. It was already a long time ago and as I already said I didn't write low-level code at all. Some time ago I was interested in such things or in other words I understood that I can write programs, but actually I didn't understand how my program is arranged.
After writing some assembler code I began to understand how my program looks after compilation, **approximately**. But anyway, I didn't understand many other things. For example: what occurs when the `syscall` instruction is executed in my assembler, what occurs when the `printf` function starts to work or how can my program talk with other computers via network. [Assembler](https://en.wikipedia.org/wiki/Assembly_language#Assembler) programming language didn't give me answers to my questions and I decided to go deeper in my research. I started to learn from the source code of the Linux kernel and tried to understand the things that I'm interested in. The source code of the Linux kernel didn't give me the answers to **all** of my questions, but now my knowledge about the Linux kernel and the processes around it is much better.
I'm writing this part nine and a half months after I've started to learn from the source code of the Linux kernel and published the first [part](https://0xax.gitbooks.io/linux-insides/content/Booting/linux-bootstrap-1.html) of this book. Now it contains forty parts and it is not the end. I decided to write this series about the Linux kernel mostly for myself. As you know the Linux kernel is very huge piece of code and it is easy to forget what does this or that part of the Linux kernel mean and how does it implement something. But soon the [linux-insides](https://github.com/0xAX/linux-insides) repo became popular and after nine months it has `9096` stars:
It seems that people are interested in the internals of the Linux kernel. Besides this, in all that time that I'm writing `linux-inside`, I have received many questions from different people like: how to start with the Linux kernel, what do I need to start contribute to the Linux kernel and and others like these. Generally people are interested contribute to open source project for different reasons and the Linux kernel is not exception:
So, seems that people are interested about Linux kernel development process. I thought it will be strange if the book about the Linux kernel will not contain a part that will describe how to take a part in the Linux kernel development and that's why I decided to write it. You will not find information about why you should be interested in contributing to the Linux kernel in this part. I see many benefits to learn source code of the Linux kernel. I don't know how about you, that's why I have no answer on this question. But if you are interested how to start with Linux kernel development, this part is for you.
I'll provide descriptions for both methods. Before we will start to do something with the Linux kernel, we need to get it. There are a couple of ways how to do it. All depends on your purpose. If you just want to update the current version of the Linux kernel on your computer, you can use the instructions specific for your Linux [distro](https://en.wikipedia.org/wiki/Linux_distribution).
In the first case you just need to download new version of the Linux kernel with the [package manager](https://en.wikipedia.org/wiki/Package_manager). For example, to upgrade the version of the Linux kernel to `4.1` for [Ubuntu (Vivid Vervet)](http://releases.ubuntu.com/15.04/), you will just need to execute the following commands:
and choose the version of the Linux kernel in which you are interested. In the end execute the next command and replace `${version}` with the version that you chose in the output of the previous command:
In the other way if you are interested in the Linux kernel development, you will need to get the source code of the Linux kernel. You can find it on the [kernel.org](https://kernel.org/) website and download an archive with the Linux kernel source code. Actually the Linux kernel development process is fully built around `git` [version control system](https://en.wikipedia.org/wiki/Version_control). So you can get it with `git` from the `kernel.org`:
I don't know how about you, but I prefer `github`. There is a [mirror](https://github.com/torvalds/linux) of the Linux kernel mainline repository, so you can clone it with:
Actually I'm using my [fork](https://github.com/0xAX/linux) for development and when I want to pull updates from the main repository I just execute the following command:
Now that we have a local copy of the Linux kernel source code, we need to configure and build it. The Linux kernel can be configured in different ways. The simplest way is to just copy the configuration file of the already installed kernel that is located in the `/boot` directory:
If your current Linux kernel was built with the support for access to the `/proc/config.gz` file, you can copy your actual kernel configuration file with this command:
If you are not satisfied with the standard kernel configuration that is provided by the maintainers of your distro, you can configure the Linux kernel manually. There are a couple of ways to do it. The Linux kernel root [Makefile](https://github.com/torvalds/linux/blob/master/Makefile) provides a set of targets that allows you to configure it. For example `menuconfig` provides a menu-driven interface for the kernel configuration:
The `defconfig` argument generates the default kernel configuration file for the current architecture, for example [x86_64 defconfig](https://github.com/torvalds/linux/blob/master/arch/x86/configs/x86_64_defconfig). You can pass the `ARCH` command line argument to `make` to build `defconfig` for the given architecture:
The `allnoconfig`, `allyesconfig` and `allmodconfig` arguments allow you to generate a new configuration file where all options will be disabled, enabled and enabled as modules respectively. The `nconfig` command line arguments that provides `ncurses` based program with menu to configure Linux kernel:
And even `randconfig` to generate random Linux kernel configuration file. I will not write how to configure the Linux kernel, which options to enable and what not, because it makes no sense to do so for two reasons: First of all I do not know your hardware and second, if you know your hardware, the only remaining task is to find out how to use programs for kernel configuration, and all of them are pretty simple to use.
Ok, for this moment we got the source code of the Linux kernel and configured it. The next step is the compilation of the Linux kernel. The simplest way to compile Linux kernel is just execute:
```
$ make
scripts/kconfig/conf --silentoldconfig Kconfig
#
# configuration written to .config
#
CHK include/config/kernel.release
UPD include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
...
...
...
OBJCOPY arch/x86/boot/vmlinux.bin
AS arch/x86/boot/header.o
LD arch/x86/boot/setup.elf
OBJCOPY arch/x86/boot/setup.bin
BUILD arch/x86/boot/bzImage
Setup is 15740 bytes (padded to 15872 bytes).
System is 4342 kB
CRC 82703414
Kernel: arch/x86/boot/bzImage is ready (#73)
```
command. To increase the speed of kernel compilation you can pass `-jN` command line argument to the `make` util, where `N` specifies the number of commands to run simultaneously:
```
$ make -j8
```
If you want to build Linux kernel for an architecture that differs from your current, the simplest way to do it pass two arguments:
*`ARCH` command line argument and the name of the target architecture;
*`CROSS_COMPILER` command line argument and the cross-compiler tool prefix;
For example if we want to compile the Linux kernel for the [arm64](https://en.wikipedia.org/wiki/ARM_architecture#AArch64_features) with default kernel cnofiguration file, we need to execute following command:
```
$ make -j4 ARCH=arm64 CROSS_COMPILER=aarch64-linux-gnu- defconfig
$ make -j4 ARCH=arm64 CROSS_COMPILER=aarch64-linux-gnu-
As result of compilation we can see the compressed kernel - `arch/x86/boot/bzImage`. Now we have compiled kernel and we can either install it on our computer or just run it in an emulator.
As I already wrote we will consider two ways how to launch new kernel: In the first case we can install and run the new version of the Linux kernel on the real hardware and the second is launch the Linux kernel on a virtual machine. In the previous paragraph we saw how to build the Linux kernel from source code and as a result we have got compressed image:
```
...
...
...
Kernel: arch/x86/boot/bzImage is ready (#73)
```
After we have got the [bzImage](https://en.wikipedia.org/wiki/Vmlinux#bzImage) we need to install `headers`, `modules` of the new Linux kernel with the:
From this moment we have installed new version of the Linux kernel and now we must tell the `bootloader` about it. Of course we can add it manually by the editing of the `/boot/grub2/grub.cfg` configuration file, but I prefer to use a script for this purpose. I'm using two differnet Linux distros: Fedora and Ubuntu. There are two different ways to update the [grub](https://en.wikipedia.org/wiki/GNU_GRUB) configuration file. I'm using following script for this purpose:
This is the last step of the new Linux kernel installation and after this you can reboot your computer and select new version of the kernel during boot.
The second case is to launch new Linux kernel in the virtual machine. I prefer [qemu](https://en.wikipedia.org/wiki/QEMU). First of all we need to build initial ramdisk - [initrd](https://en.wikipedia.org/wiki/Initrd) for this. The `initrd` is a temporary root file system that is used by the Linux kernel during initialization process while other filesystems are not mounted. We can build `initrd` with the following commands:
First of all we need to download [busybox](https://en.wikipedia.org/wiki/BusyBox) and run `menuconfig` for its configuration:
```shell
$ mkdir initrd
$ cd initrd
$ curl http://busybox.net/downloads/busybox-1.23.2.tar.bz2 | tar xjf -
The `busybox` is an executable file - `/bin/busybox` that contains a set of standard tools like [coreutils](https://en.wikipedia.org/wiki/GNU_Core_Utilities) and etc. In the `busysbox` menu we need to enable: `Build BusyBox as a static binary (no shared libs)` option:
copy `busybox` fields to the `bin`, `sbin` and other directories. Now we need to create executable `init` file that will be executed as a first process in the system. My `init` file just mounts [procfs](https://en.wikipedia.org/wiki/Procfs) and [sysfs](https://en.wikipedia.org/wiki/Sysfs) filesystems and executed shell:
We can now run our kernel in the virtual machine. As I already wrote I prefer [qemu](https://en.wikipedia.org/wiki/QEMU) for this. We can run our kernel with the following command:
The main point of this paragraph is answer on two questions: What to do and what not to do before you will send your first patch to the Linux kernel. Please, do not confuse this `to do` with `todo`. I have no answer what you can fix in the Linux kernel. I just want to tell you my workflow during experimenting with the Linux kernel source code.
First of all I'm trying to pull last updates from the Linus's repo with the following commands:
```
$ git checkout master
$ git pull upstream master
```
After this my local repository with the Linux kernel source code is synced with the [mainline](https://github.com/torvalds/linux) repository. Now we can make some changes in the source code. As I already wrote, I have no advice for you where you can start and what `TODO` in the Linux kernel. But the best place for newbies is `staging` tree. In other words the set of drivers from the [drivers/staging](https://github.com/torvalds/linux/tree/master/drivers/staging). The maintainer of the `staging` tree is [Greg Kroah-Hartman](https://en.wikipedia.org/wiki/Greg_Kroah-Hartman) and the `staging` tree is that place where your trivial patch can be accepted. Let's look on a simple example that describes how to generate patch, check it and send to the [Linux kernel mail listing](https://lkml.org/).
If we will look on the driver for the [Digi International EPCA PCI](https://github.com/torvalds/linux/tree/master/drivers/staging/dgap) based devices, we will see `dgap_sindex` function:
on the `295` line. This function looks for a match of any character in the group, and returns that position. During research of source code of the Linux kernel, I have noted that [lib/string.c](https://github.com/torvalds/linux/blob/master/lib/string.c#L473) source code file contains implementation of the `strpbrk` function that does the same that `dgap_sinidex`. It is not a good idea to use a custom implementation of a function that already exists. So we can remove the `dgap_sindex` function from the [drivers/staging/dgap/dgap.c](https://github.com/torvalds/linux/blob/master/drivers/staging/dgap/dgap.c) source code file and use the `strpbrk` instead.
First of all let's create new `git` branch based on the current master that synced with the Linux kernel mainline repo:
```
$ git checkout -b "dgap-remove-dgap_sindex"
```
And now we can replace the `dgap_sindex` with the `strpbrk`. After we did all changes we need to recompile the Linux kernel or just [dgap](https://github.com/torvalds/linux/tree/master/drivers/staging/dgap) directory. Do not forget to enable this driver in the kernel configuration. You can find it in the:
Now is time to make commit. I'm using following combination for this:
```
$ git add .
$ git commit -s -v
```
After the last command an editor will be openned that will be chosen from `$GIT_EDITOR` or `$EDITOR` environment variable. The `-s` command line argument will add `Signed-off-by` line by the committer at the end of the commit log message. You can find this line in the end of each commit message, for example - [00cc1633](https://github.com/torvalds/linux/commit/00cc1633816de8c95f337608a1ea64e228faf771). The main point of this line is the tracking of who did a change. The `-v` option show unified diff between the HEAD commit and what would be committed at the bottom of the commit message. It is not necessary, but very useful sometimes. A couple of words about commit message. Actually a commit message consists from two parts:
The first part is on the first line and contains short description of changes. It starts from the `[PATCH]` prefix followed by a subsystem, driver or architecture name and after `:` symbol short description. In our case it will be something like this:
And the `Sign-off-by` line in the end of the commit message. Note that each line of a commit message must no be longer than `80` symbols and commit message must describe your changes in details. Do not just write a commit message like: `Custom function removed`, you need to describe what you did and why. The patch reviewers must know what they review. Besides this commit messages in this view are very helpful. Each time when we can't understand something, we can use [git blame](http://git-scm.com/docs/git-blame) to read description of changes.
We've passed name of the branch (`master` in this case) to the `format-patch` command that will generate a patch with the last changes that are in the `dgap-remove-dgap_sindex` branch and not are in the `master` branch. As you can note, the `format-patch` command generates file that contains last changes and has name that is based on the commit short description. If you want to generate a patch with the custom name, you can use `--stdout` option:
The last step after we have generated our patch is just to send it to the Linux kernel mail listing. Of course you can use any email client, but the `Git` provides special command for this: `git send-email`. Before you will send your patch, you need to know where to send it. Yes, you can send it just to the Linux kernel mail listing address which is `linux-kernel@vger.kernel.org`, but there is a high probability that the patch will be ignored, because as you may already know there is the large flow of messages on the Linux kernel mail listing. The better way will be send to a maintainer of subsystem where you have made changes. We can find maintainer and other related guys who has touched the code with the `get_maintainer.pl` script. All of you need is just pass file or directory where you wrote a code. Go to the root directory with source code of the Linux kernel and execute it:
That's all. The patch is sent and now only have to wait feedback from the Linux kernel developers. After you will sent a patch and a maintainer accepted it, you will find it in the maintainer's repository (for example [patch](https://git.kernel.org/cgit/linux/kernel/git/gregkh/staging.git/commit/?h=staging-testing&id=b9f7f1d0846f15585b8af64435b6b706b25a5c0b) that you saw in this part) and after some time a maintainer will send pull request to Linus and you will see your patch in the mainline repository.
* Each time when you have changed something in the Linux kernel source code - compile it. After any changes. Again and again. Nobody likes changes that don't even compile.
* The Linux kernel has a coding style [guide](https://github.com/torvalds/linux/blob/master/Documentation/CodingStyle) and you need to comply with it. There is great script which can help to check your changes. This script is - [scripts/checkpatch.pl](https://github.com/torvalds/linux/blob/master/scripts/checkpatch.pl). Just pass source code file with changes to it and you will see:
* If your change consists from some different and unrelated changes, you need to split the changes via separate commits. The `git format-patch` command will generate patches for each commit and the subject of each patch will contain a `vN` prefix where the `N` is the number of the patch. If you are planning to send a series of patches it will be helpful to pass the `--cover-letter` option to the `git format-patch` command. This will generate an additional file that will contain the cover letter that you can use to describe what your patchset changes. It is also a good idea to use the `--in-reply-to` option in the `git send-email` command. This option allows you to send your patch series in reply to your cover message. The structure of the your patch will look like this for a maintainer:
It's important that your email be in the [plain text](https://en.wikipedia.org/wiki/Plain_text) format. Generally, `send-email` and `format-patch` are very useful during development, so look at the documentation for the commands and you'll find some useful options such as: [git send-email](http://git-scm.com/docs/git-send-email) and [git format-patch](http://git-scm.com/docs/git-format-patch).
* The [scripts](https://github.com/torvalds/linux/tree/master/scripts) directory contains many different useful scripts that are related to Linux kernel development. We already saw two scripts from this directory: the `checkpatch.pl` and the `get_maintainer.pl` scripts. Outside of those scripts, you can find the [stackusage](https://github.com/torvalds/linux/blob/master/scripts/stackusage) script that will print usage of the stack, [extract-vmlinux](https://github.com/torvalds/linux/blob/master/scripts/extract-vmlinux) for extracting an uncompressed kernel image, and many others. Outside of the `scripts` directory you can find some very useful [scripts](https://github.com/lorenzo-stoakes/kernel-scripts) by [Lorenzo Stoakes](https://twitter.com/ljsloz) for kernel development.
* Subscribe to the Linux kernel mailing list. There are a large number of letters every day on `lkml`, but it is very useful to read them and understand things such as the current state of the Linux kernel. Other than `lkml` there are [set](http://vger.kernel.org/vger-lists.html) mailing listings which are related to the different Linux kernel subsystems.
* If your patch is not accepted the first time and you receive feedback from Linux kernel developers, make your changes and resend the patch with the `[PATCH vN]` prefix (where `N` is the number of patch version). For example:
Also it must contain changelog that will describe all changes changes from previous patch versions. Of course, this is not an exhaustive list of requirements for Linux kernel development, but some of the most important items were addressed.
Please note that English is not my first language, and I am really sorry for any inconvenience. If you find any mistakes please let me know via email or send a PR.
