![]() And now you have to write a driver for that board. Let’s say you have an embedded system where the board is already running a Linux kernel of some different version. This is what we did, or install the Linux- headers-of the target Linux kernel. Download the kernel from your distributor and build it by yourself. There are two ways to obtain a prebuilt kernel version. That’s why the thumb rule is “Have a precompiled Linux kernel source tree on your machine and build your module against that”. The module load may not be successful, and even if it is successful, you will encounter runtime issues with the symbols. You cannot compile your module against one Linux kernel version and load it into the system, which is running kernel of a different version. ![]() The reason is modules are linked against object files found in the kernel source tree. We have downloaded the kernel source tree and built it. There are some important note I would like to mention, so please remember that when you are building an Out of tree module, you need to have a complete and pre-compiled kernel source tree on your system. If you need more information about building a module using “kbuild,” you can go through the kernel documentation at this path. This ensures that as the developer changes the kernel configuration, his custom driver is automatically rebuilt with the current kernel configuration. We will be building our module against that prebuilt kernel source. Basically, we have already installed the kernel source. To build external modules, you must have a prebuilt kernel source available that contains the configuration and header files used in the build. ![]() You have to invoke the kernel build system, and the kernel build system will take care of using the right compiler switches to the compiler. You use a compiler called GCC, and you use various compiler flags, you use various linker flags to compile your program, So the same procedure we have to follow here, but while building a kernel module, you need not worry about what kind of compiler switches or compiler arguments you have to use the compiler. When you compile a user-level program, what do you do? Modules must use “kernel build system” to stay compatible with changes in the build infrastructure and to pick up the right flags to GCC. Doesn’t matter whether it is a static module or dynamic module, modules are built using “kbuild”, that is, the kernel build. Modules are built using “kbuild” which is the build system used by the Linux kernel. Later will also explore In tree building. Let’s explore building a kernel module as Out of tree. That is, a taint flag will be set in the kernel. Hence when you load an out of tree kernel module, kernel throws a warning message saying it got tainted. When you write a module separately (which is not approved and may be buggy), build and link it against the running kernel, then it’s called as out of the tree module. That is approved by the kernel developers and maintainers. The modules which are already part of the Linux kernel are called In tree modules. You will see that when you load that module to the kernel.īasically, out of tree means outside of the Linux kernel source tree. This is just a warning from the kernel, and we can safely ignore this warning. That is, kernel issues a warning saying Out of tree module has been loaded. Loading Out of tree module taints the kernel. The module that we just wrote, our hello world module, it is an Out of tree module. That means, they are internal to the linux kernel tree. All those modules are called as In tree modules. In tree, modules means internal to the linux kernel tree.įor example, in the earlier articles, you actually ran a command modules, make modules to build all dynamically loadable kernel modules of the Linux kernel. One is In tree module, and another one is an Out of tree module. Let’s say you want to build a kernel module as a dynamically loadable kernel module. We will be writing and using dynamically loadable kernel modules for most of the exercises. One is statically linked against the kernel image, or it can be build as dynamically loadable kernel Module. In this article, let’s learn how to build a kernel module.
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