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Linux kernel development
================================================================================
Introduction
--------------------------------------------------------------------------------
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 university. It was a long time ago and, as I already said, I didn't write low-level code at all. Some time ago I became interested in such things. I understood that I can write programs, but didn't actually 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:
![github](http://s2.postimg.org/jjb3s4frt/stars.png)
It seems that people are interested in the insides of the Linux kernel. Besides this, in all the time that I have been writing `linux-insides`, I have received many questions from different people about how to begin contributing to the Linux kernel. Generally people are interested in contributing to open source projects and the Linux kernel is not an exception:
![google-linux](http://s4.postimg.org/yg9z5zx0d/google_linux.png)
So, it seems that people are interested in the Linux kernel development process. I thought it would be strange if a book about the Linux kernel would not contain a part describing 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. But if you are interested how to start with Linux kernel development, this part is for you.
Let's start.
How to start with Linux kernel
---------------------------------------------------------------------------------
First of all, let's see how to get, build, and run the Linux kernel. You can run your custom build of the Linux kernel in two ways:
* Run the Linux kernel on a virtual machine;
* Run the Linux kernel on real hardware.
I'll provide descriptions for both methods. Before we start doing anything with the Linux kernel, we need to get it. There are a couple of ways to do this depending 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 to 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:
```
$ sudo add-apt-repository ppa:kernel-ppa/ppa
$ sudo apt-get update
```
After this execute this command:
```
$ apt-cache showpkg linux-headers
```
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:
```
$ sudo apt-get install linux-headers-${version} linux-headers-${version}-generic linux-image-${version}-generic --fix-missing
```
and reboot your system. After the reboot you will see the new kernel in the [grub](https://en.wikipedia.org/wiki/GNU_GRUB) menu.
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`:
```
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
```
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:
```
$ git clone git@github.com:torvalds/linux.git
```
I use my own [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:
```
$ git checkout master
$ git pull upstream master
```
Note that the remote name of the main repository is `upstream`. To add a new remote with the main Linux repository you can execute:
```
git remote add upstream git@github.com:torvalds/linux.git
```
After this you will have two remotes:
```
~/dev/linux (master) $ git remote -v
origin git@github.com:0xAX/linux.git (fetch)
origin git@github.com:0xAX/linux.git (push)
upstream https://github.com/torvalds/linux.git (fetch)
upstream https://github.com/torvalds/linux.git (push)
```
One is of your fork (`origin`) and the second is for the main repository (`upstream`).
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:
```
$ sudo cp /boot/config-$(uname -r) ~/dev/linux/.config
```
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:
```
$ cat /proc/config.gz | gunzip > ~/dev/linux/.config
```
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:
![menuconfig](http://s21.postimg.org/zcz48p7yf/menucnonfig.png)
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:
```
$ make ARCH=arm64 defconfig
```
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:
![nconfig](http://s29.postimg.org/hpghikp4n/nconfig.png)
And even `randconfig` to generate random Linux kernel configuration file. I will not write about how to configure the Linux kernel or which options to enable 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, we now have 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 to 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)
```
To increase the speed of kernel compilation you can pass `-jN` command line argument to `make`, 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 configuration 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 that we have compiled the kernel, we can either install it on our computer or just run it in an emulator.
Installing Linux kernel
--------------------------------------------------------------------------------
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:
```
$ sudo make headers_install
$ sudo make modules_install
```
and directly the kernel itself:
```
$ sudo make install
```
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 different 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:
```shell
#!/bin/bash
source "term-colors"
DISTRIBUTIVE=$(cat /etc/*-release | grep NAME | head -1 | sed -n -e 's/NAME\=//p')
echo -e "Distributive: ${Green}${DISTRIBUTIVE}${Color_Off}"
if [[ "$DISTRIBUTIVE" == "Fedora" ]] ;
then
su -c 'grub2-mkconfig -o /boot/grub2/grub.cfg'
else
sudo update-grub
fi
echo "${Green}Done.${Color_Off}"
```
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 -
$ cd busybox-1.23.2/
$ make menuconfig
$ make -j4
```
`busybox` is an executable file - `/bin/busybox` that contains a set of standard tools like [coreutils](https://en.wikipedia.org/wiki/GNU_Core_Utilities). In the `busysbox` menu we need to enable: `Build BusyBox as a static binary (no shared libs)` option:
![busysbox menu](http://s18.postimg.org/sj92uoweh/busybox.png)
We can find this menu in the:
```
Busybox Settings
--> Build Options
```
After this we exit from the `busysbox` configuration menu and execute following commands for building and installation of it:
```
$ make -j4
$ sudo make install
```
Now that `busybox` is installed, we can begin building our `initrd`. To do this, we go to the previous `initrd` directory and:
```
$ cd ..
$ mkdir -p initramfs
$ cd initramfs
$ mkdir -pv {bin,sbin,etc,proc,sys,usr/{bin,sbin}}
$ cp -av ../busybox-1.23.2/_install/* .
```
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:
```shell
#!/bin/sh
mount -t proc none /proc
mount -t sysfs none /sys
exec /bin/sh
```
Now we can create an archive that will be our `initrd`:
```
$ find . -print0 | cpio --null -ov --format=newc | gzip -9 > ~/dev/initrd_x86_64.gz
```
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:
```
$ qemu-system-x86_64 -snapshot -m 8GB -serial stdio -kernel ~/dev/linux/arch/x86_64/boot/bzImage -initrd ~/dev/initrd_x86_64.gz -append "root=/dev/sda1 ignore_loglevel"
```
![qemu](http://s22.postimg.org/b8ttyigup/qemu.png)
From now we can run the Linux kernel in the virtual machine and this means that we can begin to change and test the kernel.
Consider using [ivandaviov/minimal](https://github.com/ivandavidov/minimal) to automate the process of generating initrd.
Getting started with the Linux Kernel Development
---------------------------------------------------------------------------------
The main point of this paragraph is to answer two questions: What to do and what not to do before sending 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 pull the latest updates from 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 look in the driver for the [Digi International EPCA PCI](https://github.com/torvalds/linux/tree/master/drivers/staging/dgap) based devices, we will see the `dgap_sindex` function on line 295:
```C
static char *dgap_sindex(char *string, char *group)
{
char *ptr;
if (!string || !group)
return NULL;
for (; *string; string++) {
for (ptr = group; *ptr; ptr++) {
if (*ptr == *string)
return string;
}
}
return NULL;
}
```
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 the [lib/string.c](https://github.com/torvalds/linux/blob/master/lib/string.c#L473) source code file contains the implementation of the `strpbrk` function that does the same thing as `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:
```
Device Drivers
--> Staging drivers
----> Digi EPCA PCI products
```
![dgap menu](http://s4.postimg.org/d3pozpge5/digi.png)
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 opened 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:
```
[PATCH] staging/dgap: Use strpbrk() instead of dgap_sindex()
```
After short description usually we have an empty line and full description of the commit. In our case it will be:
```
The <linux/string.h> provides strpbrk() function that does the same that the
dgap_sindex(). Let's use already defined function instead of writing custom.
```
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.
After we have committed changes time to generate patch. We can do it with the `format-patch` command:
```
$ git format-patch master
0001-staging-dgap-Use-strpbrk-instead-of-dgap_sindex.patch
```
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:
```
$ git format-patch master --stdout > dgap-patch-1.patch
```
The last step after we have generated our patch is to send it to the Linux kernel mailing list. Of course, you can use any email client, `git` provides a special command for this: `git send-email`. Before you send your patch, you need to know where to send it. Yes, you can just send it to the Linux kernel mailing list address which is `linux-kernel@vger.kernel.org`, but it is very likely that the patch will be ignored, because of the large flow of messages. The better choice would be to send the patch to the maintainers of the subsystem where you have made changes. To find the names of these maintainers use the `get_maintainer.pl` script. All you need to do is pass the file or directory where you wrote code.
```
$ ./scripts/get_maintainer.pl -f drivers/staging/dgap/dgap.c
Lidza Louina <lidza.louina@gmail.com> (maintainer:DIGI EPCA PCI PRODUCTS)
Mark Hounschell <markh@compro.net> (maintainer:DIGI EPCA PCI PRODUCTS)
Daeseok Youn <daeseok.youn@gmail.com> (maintainer:DIGI EPCA PCI PRODUCTS)
Greg Kroah-Hartman <gregkh@linuxfoundation.org> (supporter:STAGING SUBSYSTEM)
driverdev-devel@linuxdriverproject.org (open list:DIGI EPCA PCI PRODUCTS)
devel@driverdev.osuosl.org (open list:STAGING SUBSYSTEM)
linux-kernel@vger.kernel.org (open list)
```
You will see the set of the names and related emails. Now we can send our patch with:
```
$ git send-email --to "Lidza Louina <lidza.louina@gmail.com>" \
--cc "Mark Hounschell <markh@compro.net>" \
--cc "Daeseok Youn <daeseok.youn@gmail.com>" \
--cc "Greg Kroah-Hartman <gregkh@linuxfoundation.org>" \
--cc "driverdev-devel@linuxdriverproject.org" \
--cc "devel@driverdev.osuosl.org" \
--cc "linux-kernel@vger.kernel.org"
```
That's all. The patch is sent and now you only have to wait for feedback from the Linux kernel developers. After you send a patch and a maintainer accepts 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 the maintainer will send a pull request to Linus and you will see your patch in the mainline repository.
That's all.
Some advice
--------------------------------------------------------------------------------
In the end of this part I want to give you some advice that will describe what to do and what not to do during development of the Linux kernel:
* Think, Think, Think. And think again before you decide to send a patch.
* 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:
```
$ ./scripts/checkpatch.pl -f drivers/staging/dgap/dgap.c
WARNING: Block comments use * on subsequent lines
#94: FILE: drivers/staging/dgap/dgap.c:94:
+/*
+ SUPPORTED PRODUCTS
CHECK: spaces preferred around that '|' (ctx:VxV)
#143: FILE: drivers/staging/dgap/dgap.c:143:
+ { PPCM, PCI_DEV_XEM_NAME, 64, (T_PCXM|T_PCLITE|T_PCIBUS) },
```
Also you can see problematic places with the help of the `git diff`:
![git diff](http://oi60.tinypic.com/2u91rgn.jpg)
* [Linus doesn't accept github pull requests](https://github.com/torvalds/linux/pull/17#issuecomment-5654674)
* 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:
```
|--> cover letter
|----> patch_1
|----> patch_2
```
You need to pass `message-id` as an argument of the `--in-reply-to` option that you can find in the output of the `git send-email`:
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).
* Do not be surprised if you do not get an immediate answer after you send your patch. Maintainers can be very busy.
* 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:
```
[PATCH v2] staging/dgap: Use strpbrk() instead of dgap_sindex()
```
Also it must contain a changelog that describes all 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.
Happy Hacking!
Conclusion
--------------------------------------------------------------------------------
I hope this will help others join the Linux kernel community!
If you have any questions or suggestions, write me at [email](kuleshovmail@gmail.com) or ping [me](https://twitter.com/0xAX) on twitter.
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.
Links
--------------------------------------------------------------------------------
* [blog posts about assembly programming for x86_64](http://0xax.github.io/categories/assembly/)
* [Assembler](https://en.wikipedia.org/wiki/Assembly_language#Assembler)
* [distro](https://en.wikipedia.org/wiki/Linux_distribution)
* [package manager](https://en.wikipedia.org/wiki/Package_manager)
* [grub](https://en.wikipedia.org/wiki/GNU_GRUB)
* [kernel.org](https://kernel.org/)
* [version control system](https://en.wikipedia.org/wiki/Version_control)
* [arm64](https://en.wikipedia.org/wiki/ARM_architecture#AArch64_features)
* [bzImage](https://en.wikipedia.org/wiki/Vmlinux#bzImage)
* [qemu](https://en.wikipedia.org/wiki/QEMU)
* [initrd](https://en.wikipedia.org/wiki/Initrd)
* [busybox](https://en.wikipedia.org/wiki/BusyBox)
* [coreutils](https://en.wikipedia.org/wiki/GNU_Core_Utilities)
* [procfs](https://en.wikipedia.org/wiki/Procfs)
* [sysfs](https://en.wikipedia.org/wiki/Sysfs)
* [Linux kernel mail listing archive](https://lkml.org/)
* [Linux kernel coding style guide](https://github.com/torvalds/linux/blob/master/Documentation/CodingStyle)
* [How to Get Your Change Into the Linux Kernel](https://github.com/torvalds/linux/blob/master/Documentation/SubmittingPatches)
* [Linux Kernel Newbies](http://kernelnewbies.org/)
* [plain text](https://en.wikipedia.org/wiki/Plain_text)

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@@ -0,0 +1,638 @@
Introduction
---------------
During the writing of the [linux-insides](http://0xax.gitbooks.io/linux-insides/content/) book I have received many emails with questions related to the [linker](https://en.wikipedia.org/wiki/Linker_%28computing%29) script and linker-related subjects. So I've decided to write this to cover some aspects of the linker and the linking of object files.
If we open the `Linker` page on Wikipedia, we will see following definition:
>In computer science, a linker or link editor is a computer program that takes one or more object files generated by a compiler and combines them into a single executable file, library file, or another object file.
If you've written at least one program on C in your life, you will have seen files with the `*.o` extension. These files are [object files](https://en.wikipedia.org/wiki/Object_file). Object files are blocks of machine code and data with placeholder addresses that reference data and functions in other object files or libraries, as well as a list of its own functions and data. The main purpose of the linker is collect/handle the code and data of each object file, turning it into the final executable file or library. In this post we will try to go through all aspects of this process. Let's start.
Linking process
---------------
Let's create a simple project with the following structure:
```
*-linkers
*--main.c
*--lib.c
*--lib.h
```
Our `main.c` source code file contains:
```C
#include <stdio.h>
#include "lib.h"
int main(int argc, char **argv) {
printf("factorial of 5 is: %d\n", factorial(5));
return 0;
}
```
The `lib.c` file contains:
```C
int factorial(int base) {
int res,i = 1;
if (base == 0) {
return 1;
}
while (i <= base) {
res *= i;
i++;
}
return res;
}
```
And the `lib.h` file contains:
```C
#ifndef LIB_H
#define LIB_H
int factorial(int base);
#endif
```
Now let's compile only the `main.c` source code file with:
```
$ gcc -c main.c
```
If we look inside the outputted object file with the `nm` util, we will see the
following output:
```
$ nm -A main.o
main.o: U factorial
main.o:0000000000000000 T main
main.o: U printf
```
The `nm` util allows us to see the list of symbols from the given object file. It consists of three columns: the first is the name of the given object file and the address of any resolved symbols. The second column contains a character that represents the status of the given symbol. In this case the `U` means `undefined` and the `T` denotes that the symbols are placed in the `.text` section of the object. The `nm` utility shows us here that we have three symbols in the `main.c` source code file:
* `factorial` - the factorial function defined in the `lib.c` source code file. It is marked as `undefined` here because we compiled only the `main.c` source code file, and it does not know anything about code from the `lib.c` file for now;
* `main` - the main function;
* `printf` - the function from the [glibc](https://en.wikipedia.org/wiki/GNU_C_Library) library. `main.c` does not know anything about it for now either.
What can we understand from the output of `nm` so far? The `main.o` object file contains the local symbol `main` at address `0000000000000000` (it will be filled with correct address after is is linked), and two unresolved symbols. We can see all of this information in the disassembly output of the `main.o` object file:
```
$ objdump -S main.o
main.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 10 sub $0x10,%rsp
8: 89 7d fc mov %edi,-0x4(%rbp)
b: 48 89 75 f0 mov %rsi,-0x10(%rbp)
f: bf 05 00 00 00 mov $0x5,%edi
14: e8 00 00 00 00 callq 19 <main+0x19>
19: 89 c6 mov %eax,%esi
1b: bf 00 00 00 00 mov $0x0,%edi
20: b8 00 00 00 00 mov $0x0,%eax
25: e8 00 00 00 00 callq 2a <main+0x2a>
2a: b8 00 00 00 00 mov $0x0,%eax
2f: c9 leaveq
30: c3 retq
```
Here we are interested only in the two `callq` operations. The two `callq` operations contain `linker stubs`, or the function name and offset from it to the next instruction. These stubs will be updated to the real addresses of the functions. We can see these functions' names with in the following `objdump` output:
```
$ objdump -S -r main.o
...
14: e8 00 00 00 00 callq 19 <main+0x19>
15: R_X86_64_PC32 factorial-0x4
19: 89 c6 mov %eax,%esi
...
25: e8 00 00 00 00 callq 2a <main+0x2a>
26: R_X86_64_PC32 printf-0x4
2a: b8 00 00 00 00 mov $0x0,%eax
...
```
The `-r` or `--reloc ` flags of the `objdump` util print the `relocation` entries of the file. Now let's look in more detail at the relocation process.
Relocation
------------
Relocation is the process of connecting symbolic references with symbolic definitions. Let's look at the previous snippet from the `objdump` output:
```
14: e8 00 00 00 00 callq 19 <main+0x19>
15: R_X86_64_PC32 factorial-0x4
19: 89 c6 mov %eax,%esi
```
Note the `e8 00 00 00 00` on the first line. The `e8` is the [opcode](https://en.wikipedia.org/wiki/Opcode) of the `call`, and the remainder of the line is a relative offset. So the `e8 00 00 00 00` contains a one-byte operation code followed by a four-byte address. Note that the `00 00 00 00` is 4-bytes. Why only 4-bytes if an address can be 8-bytes in a `x86_64` (64-bit) machine? Actually we compiled the `main.c` source code file with the `-mcmodel=small`! From the `gcc` man page:
```
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
```
Of course we didn't pass this option to the `gcc` when we compiled the `main.c`, but it is the default. We know that our program will be linked in the lower 2 GB of the address space from the `gcc` manual extract above. Four bytes is therefore enough for this. So we have opcode of the `call` instruction and an unknown address. When we compile `main.c` with all its dependencies to an executable file, and then look at the factorial call we see:
```
$ gcc main.c lib.c -o factorial | objdump -S factorial | grep factorial
factorial: file format elf64-x86-64
...
...
0000000000400506 <main>:
40051a: e8 18 00 00 00 callq 400537 <factorial>
...
...
0000000000400537 <factorial>:
400550: 75 07 jne 400559 <factorial+0x22>
400557: eb 1b jmp 400574 <factorial+0x3d>
400559: eb 0e jmp 400569 <factorial+0x32>
40056f: 7e ea jle 40055b <factorial+0x24>
...
...
```
As we can see in the previous output, the address of the `main` function is `0x0000000000400506`. Why it does not start from `0x0`? You may already know that standard C programs are linked with the `glibc` C standard library (assuming the `-nostdlib` was not passed to the `gcc`). The compiled code for a program includes constructor functions to initialize data in the program when the program is started. These functions need to be called before the program is started, or in another words before the `main` function is called. To make the initialization and termination functions work, the compiler must output something in the assembler code to cause those functions to be called at the appropriate time. Execution of this program will start from the code placed in the special `.init` section. We can see this in the beginning of the objdump output:
```
objdump -S factorial | less
factorial: file format elf64-x86-64
Disassembly of section .init:
00000000004003a8 <_init>:
4003a8: 48 83 ec 08 sub $0x8,%rsp
4003ac: 48 8b 05 a5 05 20 00 mov 0x2005a5(%rip),%rax # 600958 <_DYNAMIC+0x1d0>
```
Not that it starts at the `0x00000000004003a8` address relative to the `glibc` code. We can check it also in the [ELF](https://en.wikipedia.org/wiki/Executable_and_Linkable_Format) output by running `readelf`:
```
$ readelf -d factorial | grep \(INIT\)
0x000000000000000c (INIT) 0x4003a8
```
So, the address of the `main` function is `0000000000400506` and is offset from the `.init` section. As we can see from the output, the address of the `factorial` function is `0x0000000000400537` and binary code for the call of the `factorial` function now is `e8 18 00 00 00`. We already know that `e8` is opcode for the `call` instruction, the next `18 00 00 00` (note that address represented as little endian for `x86_64`, so it is `00 00 00 18`) is the offset from the `callq` to the `factorial` function:
```python
>>> hex(0x40051a + 0x18 + 0x5) == hex(0x400537)
True
```
So we add `0x18` and `0x5` to the address of the `call` instruction. The offset is measured from the address of the following instruction. Our call instruction is 5-bytes long (`e8 18 00 00 00`) and the `0x18` is the offset of the call after the `factorial` function. A compiler generally creates each object file with the program addresses starting at zero. But if a program is created from multiple object files, these will overlap.
What we have seen in this section is the `relocation` process. This process assigns load addresses to the various parts of the program, adjusting the code and data in the program to reflect the assigned addresses.
Ok, now that we know a little about linkers and relocation it is time to learn more about linkers by linking our object files.
GNU linker
-----------------
As you can understand from the title, I will use [GNU linker](https://en.wikipedia.org/wiki/GNU_linker) or just `ld` in this post. Of course we can use `gcc` to link our `factorial` project:
```
$ gcc main.c lib.o -o factorial
```
and after it we will get executable file - `factorial` as a result:
```
./factorial
factorial of 5 is: 120
```
But `gcc` does not link object files. Instead it uses `collect2` which is just wrapper for the `GNU ld` linker:
```
~$ /usr/lib/gcc/x86_64-linux-gnu/4.9/collect2 --version
collect2 version 4.9.3
/usr/bin/ld --version
GNU ld (GNU Binutils for Debian) 2.25
...
...
...
```
Ok, we can use gcc and it will produce executable file of our program for us. But let's look how to use `GNU ld` linker for the same purpose. First of all let's try to link these object files with the following example:
```
ld main.o lib.o -o factorial
```
Try to do it and you will get following error:
```
$ ld main.o lib.o -o factorial
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000b0
main.o: In function `main':
main.c:(.text+0x26): undefined reference to `printf'
```
Here we can see two problems:
* Linker can't find `_start` symbol;
* Linker does not know anything about `printf` function.
First of all let's try to understand what is this `_start` entry symbol that appears to be required for our program to run? When I started to learn programming I learned that the `main` function is the entry point of the program. I think you learned this too :) But it actually isn't the entry point, it's `_start` instead. The `_start` symbol is defined in the `crt1.o` object file. We can find it with the following command:
```
$ objdump -S /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_start>:
0: 31 ed xor %ebp,%ebp
2: 49 89 d1 mov %rdx,%r9
...
...
...
```
We pass this object file to the `ld` command as its first argument (see above). Now let's try to link it and will look on result:
```
ld /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
main.o lib.o -o factorial
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o: In function `_start':
/tmp/buildd/glibc-2.19/csu/../sysdeps/x86_64/start.S:115: undefined reference to `__libc_csu_fini'
/tmp/buildd/glibc-2.19/csu/../sysdeps/x86_64/start.S:116: undefined reference to `__libc_csu_init'
/tmp/buildd/glibc-2.19/csu/../sysdeps/x86_64/start.S:122: undefined reference to `__libc_start_main'
main.o: In function `main':
main.c:(.text+0x26): undefined reference to `printf'
```
Unfortunately we will see even more errors. We can see here old error about undefined `printf` and yet another three undefined references:
* `__libc_csu_fini`
* `__libc_csu_init`
* `__libc_start_main`
The `_start` symbol is defined in the [sysdeps/x86_64/start.S](https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_64/start.S;h=0d27a38e9c02835ce17d1c9287aa01be222e72eb;hb=HEAD) assembly file in the `glibc` source code. We can find following assembly code lines there:
```assembly
mov $__libc_csu_fini, %R8_LP
mov $__libc_csu_init, %RCX_LP
...
call __libc_start_main
```
Here we pass address of the entry point to the `.init` and `.fini` section that contain code that starts to execute when the program is ran and the code that executes when program terminates. And in the end we see the call of the `main` function from our program. These three symbols are defined in the [csu/elf-init.c](https://sourceware.org/git/?p=glibc.git;a=blob;f=csu/elf-init.c;hb=1d4bbc54bd4f7d85d774871341b49f4357af1fb7) source code file. The following two object files:
* `crtn.o`;
* `crti.o`.
define the function prologs/epilogs for the .init and .fini sections (with the `_init` and `_fini` symbols respectively).
The `crtn.o` object file contains these `.init` and `.fini` sections:
```
$ objdump -S /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o
0000000000000000 <.init>:
0: 48 83 c4 08 add $0x8,%rsp
4: c3 retq
Disassembly of section .fini:
0000000000000000 <.fini>:
0: 48 83 c4 08 add $0x8,%rsp
4: c3 retq
```
And the `crti.o` object file contains the `_init` and `_fini` symbols. Let's try to link again with these two object files:
```
$ ld \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o main.o lib.o \
-o factorial
```
And anyway we will get the same errors. Now we need to pass `-lc` option to the `ld`. This option will search for the standard library in the paths present in the `$LD_LIBRARY_PATH` environment variable. Let's try to link again wit the `-lc` option:
```
$ ld \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o main.o lib.o -lc \
-o factorial
```
Finally we get an executable file, but if we try to run it, we will get strange results:
```
$ ./factorial
bash: ./factorial: No such file or directory
```
What's the problem here? Let's look on the executable file with the [readelf](https://sourceware.org/binutils/docs/binutils/readelf.html) util:
```
$ readelf -l factorial
Elf file type is EXEC (Executable file)
Entry point 0x4003c0
There are 7 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x0000000000000188 0x0000000000000188 R E 8
INTERP 0x00000000000001c8 0x00000000004001c8 0x00000000004001c8
0x000000000000001c 0x000000000000001c R 1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x0000000000000610 0x0000000000000610 R E 200000
LOAD 0x0000000000000610 0x0000000000600610 0x0000000000600610
0x00000000000001cc 0x00000000000001cc RW 200000
DYNAMIC 0x0000000000000610 0x0000000000600610 0x0000000000600610
0x0000000000000190 0x0000000000000190 RW 8
NOTE 0x00000000000001e4 0x00000000004001e4 0x00000000004001e4
0x0000000000000020 0x0000000000000020 R 4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 10
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame
03 .dynamic .got .got.plt .data
04 .dynamic
05 .note.ABI-tag
06
```
Note on the strange line:
```
INTERP 0x00000000000001c8 0x00000000004001c8 0x00000000004001c8
0x000000000000001c 0x000000000000001c R 1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
```
The `.interp` section in the `elf` file holds the path name of a program interpreter or in another words the `.interp` section simply contains an `ascii` string that is the name of the dynamic linker. The dynamic linker is the part of Linux that loads and links shared libraries needed by an executable when it is executed, by copying the content of libraries from disk to RAM. As we can see in the output of the `readelf` command it is placed in the `/lib64/ld-linux-x86-64.so.2` file for the `x86_64` architecture. Now let's add the `-dynamic-linker` option with the path of `ld-linux-x86-64.so.2` to the `ld` call and will see the following results:
```
$ gcc -c main.c lib.c
$ ld \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o main.o lib.o \
-dynamic-linker /lib64/ld-linux-x86-64.so.2 \
-lc -o factorial
```
Now we can run it as normal executable file:
```
$ ./factorial
factorial of 5 is: 120
```
It works! With the first line we compile the `main.c` and the `lib.c` source code files to object files. We will get the `main.o` and the `lib.o` after execution of the `gcc`:
```
$ file lib.o main.o
lib.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
main.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
```
and after this we link object files of our program with the needed system object files and libraries. We just saw a simple example of how to compile and link a C program with the `gcc` compiler and `GNU ld` linker. In this example we have used a couple command line options of the `GNU linker`, but it supports much more command line options than `-o`, `-dynamic-linker`, etc... Moreover `GNU ld` has its own language that allows to control the linking process. In the next two paragraphs we will look into it.
Useful command line options of the GNU linker
----------------------------------------------
As I already wrote and as you can see in the manual of the `GNU linker`, it has big set of the command line options. We've seen a couple of options in this post: `-o <output>` - that tells `ld` to produce an output file called `output` as the result of linking, `-l<name>` that adds the archive or object file specified by the name, `-dynamic-linker` that specifies the name of the dynamic linker. Of course `ld` supports much more command line options, let's look at some of them.
The first useful command line option is `@file`. In this case the `file` specifies filename where command line options will be read. For example we can create file with the name `linker.ld`, put there our command line arguments from the previous example and execute it with:
```
$ ld @linker.ld
```
The next command line option is `-b` or `--format`. This command line option specifies format of the input object files `ELF`, `DJGPP/COFF` and etc. There is a command line option for the same purpose but for the output file: `--oformat=output-format`.
The next command line option is `--defsym`. Full format of this command line option is the `--defsym=symbol=expression`. It allows to create global symbol in the output file containing the absolute address given by expression. We can find following case where this command line option can be useful: in the Linux kernel source code and more precisely in the Makefile that is related to the kernel decompression for the ARM architecture - [arch/arm/boot/compressed/Makefile](https://github.com/torvalds/linux/blob/master/arch/arm/boot/compressed/Makefile), we can find following definition:
```
LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(KBSS_SZ)
```
As we already know, it defines the `_kernel_bss_size` symbol with the size of the `.bss` section in the output file. This symbol will be used in the first [assembly file](https://github.com/torvalds/linux/blob/master/arch/arm/boot/compressed/head.S) that will be executed during kernel decompressing:
```assembly
ldr r5, =_kernel_bss_size
```
The next command line options is the `-shared` that allows us to create shared library. The `-M` or `-map <filename>` command line option prints the linking map with the information about symbols. In our case:
```
$ ld -M @linker.ld
...
...
...
.text 0x00000000004003c0 0x112
*(.text.unlikely .text.*_unlikely .text.unlikely.*)
*(.text.exit .text.exit.*)
*(.text.startup .text.startup.*)
*(.text.hot .text.hot.*)
*(.text .stub .text.* .gnu.linkonce.t.*)
.text 0x00000000004003c0 0x2a /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o
...
...
...
.text 0x00000000004003ea 0x31 main.o
0x00000000004003ea main
.text 0x000000000040041b 0x3f lib.o
0x000000000040041b factorial
```
Of course the `GNU linker` support standard command line options: `--help` and `--version` that print common help of the usage of the `ld` and its version. That's all about command line options of the `GNU linker`. Of course it is not the full set of command line options supported by the `ld` util. You can find the complete documentation of the `ld` util in the manual.
Control Language linker
----------------------------------------------
As I wrote previously, `ld` has support for its own language. It accepts Linker Command Language files written in a superset of AT&T's Link Editor Command Language syntax, to provide explicit and total control over the linking process. Let's look on its details.
With the linker language we can control:
* input files;
* output files;
* file formats
* addresses of sections;
* etc...
Commands written in the linker control language are usually placed in a file called linker script. We can pass it to `ld` with the `-T` command line option. The main command in a linker script is the `SECTIONS` command. Each linker script must contain this command and it determines the `map` of the output file. The special variable `.` contains current position of the output. Let's write a simple assembly program and we will look at how we can use a linker script to control linking of this program. We will take a hello world program for this example:
```assembly
section .data
msg db "hello, world!",`\n`
section .text
global _start
_start:
mov rax, 1
mov rdi, 1
mov rsi, msg
mov rdx, 14
syscall
mov rax, 60
mov rdi, 0
syscall
```
We can compile and link it with the following commands:
```
$ nasm -f elf64 -o hello.o hello.asm
$ ld -o hello hello.o
```
Our program consists from two sections: `.text` contains code of the program and `.data` contains initialized variables. Let's write simple linker script and try to link our `hello.asm` assembly file with it. Our script is:
```
/*
* Linker script for the factorial
*/
OUTPUT(hello)
OUTPUT_FORMAT("elf64-x86-64")
INPUT(hello.o)
SECTIONS
{
. = 0x200000;
.text : {
*(.text)
}
. = 0x400000;
.data : {
*(.data)
}
}
```
On the first three lines you can see a comment written in `C` style. After it the `OUTPUT` and the `OUTPUT_FORMAT` commands specify the name of our executable file and its format. The next command, `INPUT`, specifies the input file to the `ld` linker. Then, we can see the main `SECTIONS` command, which, as I already wrote, must be present in every linker script. The `SECTIONS` command represents the set and order of the sections which will be in the output file. At the beginning of the `SECTIONS` command we can see following line `. = 0x200000`. I already wrote above that `.` command points to the current position of the output. This line says that the code should be loaded at address `0x200000` and the line `. = 0x400000` says that data section should be loaded at address `0x400000`. The second line after the `. = 0x200000` defines `.text` as an output section. We can see `*(.text)` expression inside it. The `*` symbol is wildcard that matches any file name. In other words, the `*(.text)` expression says all `.text` input sections in all input files. We can rewrite it as `hello.o(.text)` for our example. After the following location counter `. = 0x400000`, we can see definition of the data section.
We can compile and link it with the:
```
$ nasm -f elf64 -o hello.o hello.S && ld -T linker.script && ./hello
hello, world!
```
If we will look inside it with the `objdump` util, we can see that `.text` section starts from the address `0x200000` and the `.data` sections starts from the address `0x400000`:
```
$ objdump -D hello
Disassembly of section .text:
0000000000200000 <_start>:
200000: b8 01 00 00 00 mov $0x1,%eax
...
Disassembly of section .data:
0000000000400000 <msg>:
400000: 68 65 6c 6c 6f pushq $0x6f6c6c65
...
```
Apart from the commands we have already seen, there are a few others. The first is the `ASSERT(exp, message)` that ensures that given expression is not zero. If it is zero, then exit the linker with an error code and print the given error message. If you've read about Linux kernel booting process in the [linux-insides](http://0xax.gitbooks.io/linux-insides/content/) book, you may know that the setup header of the Linux kernel has offset `0x1f1`. In the linker script of the Linux kernel we can find a check for this:
```
. = ASSERT(hdr == 0x1f1, "The setup header has the wrong offset!");
```
The `INCLUDE filename` command allows to include external linker script symbols in the current one. In a linker script we can assign a value to a symbol. `ld` supports a couple of assignment operators:
* symbol = expression ;
* symbol += expression ;
* symbol -= expression ;
* symbol *= expression ;
* symbol /= expression ;
* symbol <<= expression ;
* symbol >>= expression ;
* symbol &= expression ;
* symbol |= expression ;
As you can note all operators are C assignment operators. For example we can use it in our linker script as:
```
START_ADDRESS = 0x200000;
DATA_OFFSET = 0x200000;
SECTIONS
{
. = START_ADDRESS;
.text : {
*(.text)
}
. = START_ADDRESS + DATA_OFFSET;
.data : {
*(.data)
}
}
```
As you already may noted the syntax for expressions in the linker script language is identical to that of C expressions. Besides this the control language of the linking supports following builtin functions:
* `ABSOLUTE` - returns absolute value of the given expression;
* `ADDR` - takes the section and returns its address;
* `ALIGN` - returns the value of the location counter (`.` operator) that aligned by the boundary of the next expression after the given expression;
* `DEFINED` - returns `1` if the given symbol placed in the global symbol table and `0` in other way;
* `MAX` and `MIN` - return maximum and minimum of the two given expressions;
* `NEXT` - returns the next unallocated address that is a multiple of the give expression;
* `SIZEOF` - returns the size in bytes of the given named section.
That's all.
Conclusion
-----------------
This is the end of the post about linkers. We learned many things about linkers in this post, such as what is a linker and why it is needed, how to use it, etc..
If you have any questions or suggestions, write me an [email](kuleshovmail@gmail.com) or ping [me](https://twitter.com/0xAX) on twitter.
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.
Links
-----------------
* [Book about Linux kernel insides](http://0xax.gitbooks.io/linux-insides/content/)
* [linker](https://en.wikipedia.org/wiki/Linker_%28computing%29)
* [object files](https://en.wikipedia.org/wiki/Object_file)
* [glibc](https://en.wikipedia.org/wiki/GNU_C_Library)
* [opcode](https://en.wikipedia.org/wiki/Opcode)
* [ELF](https://en.wikipedia.org/wiki/Executable_and_Linkable_Format)
* [GNU linker](https://en.wikipedia.org/wiki/GNU_linker)
* [My posts about assembly programming for x86_64](http://0xax.github.io/categories/assembly/)
* [readelf](https://sourceware.org/binutils/docs/binutils/readelf.html)

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@@ -85,7 +85,7 @@ Linux Insides
|├ 11.0|[@mudongliang](https://github.com/mudongliang)|已完成|
|├ 11.1|[@mudongliang](https://github.com/mudongliang)|已完成|
|├ 11.2|[@mudongliang](https://github.com/mudongliang)|已完成|
|└ 10.3||未开始|
|└ 11.3||未开始|
| 12. Initial ram disk||未开始|
| 13. Misc||正在进行|
|├ 13.0|[@mudongliang](https://github.com/mudongliang)|已完成|