Kernmodule

A Kernmodule, or kernel module, is a piece of code that can be loaded into an operating system's kernel to extend its functionality without rebooting. Kernel modules run in kernel space and have direct access to hardware and core kernel services, making them powerful but also potential sources of instability or security risk if not well designed.

Loading and unloading: In Linux, modules are loaded with tools such as insmod or modprobe and can be removed with rmmod. The system keeps track of loaded modules, and dependencies between modules are resolved by modprobe using information from the kernel's module directory, typically under /lib/modules/$(uname -r)/kernel. The command lsmod shows loaded modules, and modinfo displays metadata.

Structure and metadata: A module provides initialization and cleanup functions, registered via module_init and module_exit. Macros such as MODULE_LICENSE, MODULE_AUTHOR, and MODULE_DESCRIPTION embed metadata. Modules may export symbols with EXPORT_SYMBOL to be used by other modules. They compile into a file with .ko extension and are loaded by the kernel at runtime.

Uses and limitations: Common kernel modules include device drivers, new filesystems, or networking protocols. They enable features without rebooting, but loaded code runs with kernel privileges and must maintain compatibility with the running kernel; mismatches can cause crashes or taint the kernel. Licensing, particularly GPL-compatible licenses, affects compatibility with some kernel features.

Development and safety: Kernel developers build modules against kernel headers for the target kernel version. Testing in controlled environments is essential. Tools such as modinfo, lsmod, and dmesg help diagnose issues and verify stable operation.