In this article, we introduce driver development for macOS, including the basics of implementing kernel extensions (kexts). We discuss typical tasks requiring kernel extensions, tools, and environments for creating them, and some aspects of this process.
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1. Overview
1.1 What is the macOS kernel?
The kernel is the central part of the operating system, providing applications with coordinated access to computer resources: processor, memory, external hardware, external input/output devices, and so on. Also, the kernel typically provides file system and network protocol services.
As a fundamental element of the operating system, the kernel represents the lowest level of abstraction for applications to access the system resources they need to operate. The kernel usually provides such access to the executable processes of applications using mechanisms for interprocess communication and application access to operating system calls.
The macOS kernel is XNU: a hybrid core developed by Apple and used in the macOS family.
1.2 Kernel extensions: .kext
A kernel extension (kext) is a special application bundle that extends the functionality of the macOS kernel.
A kext is the minimum unit of executable code that can be loaded and used in the kernel.
1.3 Why might I need a custom kernel extension?
When developing for macOS, usually there's no need for creating a kernel extension. The functionality available in user mode is sufficient for most tasks.
![Download Download](https://video.informer.com/frames/mac/f234200203224205.frame.jpg)
Another restriction of creating a kext is that the code of the kext itself should be close to ideal. This is indicated in Apple's official documentation.
The reason is simple enough: the worst-case scenario for an application is a crash and emergency exit. But if a kernel module fails, the worst-case scenario is a crash of the entire operating system and a reboot of the device. If a kext is loaded at system startup and contains an error, it will crash the system each time it starts, further complicating system recovery.
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However, despite possible inconveniences and dangers, there are tasks that can’t be implemented without a kernel extension:
- supporting a certain type of file system (including creating a new one)
- providing quick access for a large number of concurrent applications to the same resource provided by the kext code
- intercepting and substituting an API system call for the purposes of intercepting traffic, I/O, and other system calls as well as for hiding files and processes
- writing a device driver (for network controllers, graphics/audio devices, etc.)
2. Inside a kernel extension
2.1 Kext bundle structure
A kext, like any other macOS application, is a bundle, only with the .kext extension. A bundle is a special folder that encapsulates application resources (in this case, kext resources).
A kext bundle must contain two files:
- a compiled binary file with executable code
- an Info.plist file containing information about the kernel extension: name, version, identifier, dependencies from kernel libraries, etc.
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Sometimes, the bundle.kext contains additional files (resources, plugins, etc.):
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- device firmware
- resources (including those localized for use by user mode applications)
- plugins, including other kexts
![Kext Kext](https://support.shinywhitebox.com/hc/article_attachments/360037621492/IMG_5308.jpg)
2.2 Enter/exit routines
Depending on the type of extension, a kext can be written in C or C ++ and has its own peculiarities when loading to and unloading to/from the kernel:
Type | Generic kernel extension | IOKit driver |
Programming language | Usually C | C++ |
Implementation | C functions registered as callbacks with relevant subsystems | Subclasses of one or more I/O Kit driver classes, such as IOGraphicsDevice |
Entry points | Start and stop functions with C linkage | C++ static constructors and destructors |
Loading behavior | Must be loaded explicitly | Loaded automatically by IOKit when needed |
Unloading behavior | Must be unloaded explicitly | Unloaded automatically by IOKit after a fixed interval when no longer needed |
Since this article is devoted to usual kexts, let's take a closer look at loading and unloading kernel extensions. Hexen xbox boot disc.
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In the kernel extension code, you must implement entry points — functions that are called when a kext is loaded to and unloaded from the kernel.
These functions can have arbitrary names that must be specified in the project file:
These functions have fixed prototypes: