What is XNU?

XNU kernel is part of the Darwin operating system for use in macOS and iOS operating systems. XNU is an acronym for X is Not Unix. XNU is a hybrid kernel combining the Mach kernel developed at Carnegie Mellon University with components from FreeBSD and a C++ API for writing drivers called IOKit. XNU runs on x86_64 for both single processor and multi-processor configurations.

XNU Source Tree

• config - configurations for exported apis for supported architecture and platform
• SETUP - Basic set of tools used for configuring the kernel, versioning and kextsymbol management.
• EXTERNAL_HEADERS - Headers sourced from other projects to avoid dependency cycles when building. These headers should be regularly synced when source is updated.
• libkern - C++ IOKit library code for handling of drivers and kexts.
• libsa - kernel bootstrap code for startup
• libsyscall - syscall library interface for userspace programs
• libkdd - source for user library for parsing kernel data like kernel chunked data.
• makedefs - top level rules and defines for kernel build.
• osfmk - Mach kernel based subsystems
• pexpert - Platform specific code like interrupt handling, atomics etc.
• security - Mandatory Access Check policy interfaces and related implementation.
• bsd - BSD subsystems code
• tools - A set of utilities for testing, debugging and profiling kernel.

How to build XNU

Building DEVELOPMENT kernel

The xnu make system can build kernel based on KERNEL_CONFIGS & ARCH_CONFIGS variables as arguments. Here is the syntax:

make SDKROOT=<sdkroot> ARCH_CONFIGS=<arch> KERNEL_CONFIGS=<variant>

Where:

• <sdkroot>: path to macOS SDK on disk. (defaults to /)
• <variant>: can be debug, development, release, profile and configures compilation flags and asserts throughout kernel code.
• <arch> : can be valid arch to build for. (E.g. X86_64)

To build a kernel for the same architecture as running OS, just type

$ make
$ make SDKROOT=macosx.internal

Additionally, there is support for configuring architectures through ARCH_CONFIGS and kernel configurations with KERNEL_CONFIGS.

$ make SDKROOT=macosx.internal ARCH_CONFIGS=X86_64 KERNEL_CONFIGS=DEVELOPMENT
$ make SDKROOT=macosx.internal ARCH_CONFIGS=X86_64 KERNEL_CONFIGS="RELEASE DEVELOPMENT DEBUG"

Note:

• By default, architecture is set to the build machine architecture, and the default kernel config is set to build for DEVELOPMENT.

This will also create a bootable image, kernel.[config], and a kernel binary with symbols, kernel.[config].unstripped.

To intall the kernel into a DSTROOT, use the install_kernels target:

$ make install_kernels DSTROOT=/tmp/xnu-dst

Hint: For a more satisfying kernel debugging experience, with access to all local variables and arguments, but without all the extra check of the DEBUG kernel, add something like: CFLAGS_DEVELOPMENTARM64="-O0 -g -DKERNEL_STACK_MULTIPLIER=2" CXXFLAGS_DEVELOPMENTARM64="-O0 -g -DKERNEL_STACK_MULTIPLIER=2" to your make command. Replace DEVELOPMENT and ARM64 with the appropriate build and platform.

• To build with RELEASE kernel configuration

  make KERNEL_CONFIGS=RELEASE SDKROOT=/path/to/SDK
  

Building FAT kernel binary

Define architectures in your environment or when running a make command.

$ make ARCH_CONFIGS="X86_64" exporthdrs all

Other makefile options

• $ make MAKEJOBS=-j8 # this will use 8 processes during the build. The default is 2x the number of active CPUS.
• $ make -j8 # the standard command-line option is also accepted
• $ make -w # trace recursive make invocations. Useful in combination with VERBOSE=YES
• $ make BUILD_LTO=0 # build without LLVM Link Time Optimization
• $ make BOUND_CHECKS=1 # enable -fbound-attributes for this build
• $ make REMOTEBUILD=user@remotehost # perform build on remote host
• $ make BUILD_JSON_COMPILATION_DATABASE=1 # Build Clang JSON Compilation Database

The XNU build system can optionally output color-formatted build output. To enable this, you can either set the XNU_LOGCOLORS environment variable to y, or you can pass LOGCOLORS=y to the make command.

Debug information formats

By default, a DWARF debug information repository is created during the install phase; this is a "bundle" named kernel.development.<variant>.dSYM To select the older STABS debug information format (where debug information is embedded in the kernel.development.unstripped image), set the BUILD_STABS environment variable.

$ export BUILD_STABS=1
$ make

Building KernelCaches

To test the xnu kernel, you need to build a kernelcache that links the kexts and kernel together into a single bootable image. To build a kernelcache you can use the following mechanisms:

• Using automatic kernelcache generation with kextd. The kextd daemon keeps watching for changing in /System/Library/Extensions directory. So you can setup new kernel as

  $ cp BUILD/obj/DEVELOPMENT/X86_64/kernel.development /System/Library/Kernels/
  $ touch /System/Library/Extensions
  $ ps -e | grep kextd
  

• Manually invoking kextcache to build new kernelcache.

  $ kextcache -q -z -a x86_64 -l -n -c /var/tmp/kernelcache.test -K /var/tmp/kernel.test /System/Library/Extensions
  

Running KernelCache on Target machine

The development kernel and iBoot supports configuring boot arguments so that we can safely boot into test kernel and, if things go wrong, safely fall back to previously used kernelcache. Following are the steps to get such a setup:

1. Create kernel cache using the kextcache command as /kernelcache.test

2. Copy exiting boot configurations to alternate file

   $ cp /Library/Preferences/SystemConfiguration/com.apple.Boot.plist /next_boot.plist
   

3. Update the kernelcache and boot-args for your setup

   $ plutil -insert "Kernel Cache" -string "kernelcache.test" /next_boot.plist
   $ plutil -replace "Kernel Flags" -string "debug=0x144 -v kernelsuffix=test " /next_boot.plist
   

4. Copy the new config to /Library/Preferences/SystemConfiguration/

   $ cp /next_boot.plist /Library/Preferences/SystemConfiguration/boot.plist
   

5. Bless the volume with new configs.

   $ sudo -n bless  --mount / --setBoot --nextonly --options "config=boot"
   

   The --nextonly flag specifies that use the boot.plist configs only for one boot. So if the kernel panic's you can easily power reboot and recover back to original kernel.

Creating tags and cscope

Set up your build environment and from the top directory, run:

$ make tags     # this will build ctags and etags on a case-sensitive volume, only ctags on case-insensitive
$ make TAGS     # this will build etags
$ make cscope   # this will build cscope database

How to install a new header file from XNU

To install IOKit headers, see additional comments in iokit/IOKit/Makefile.

XNU installs header files at the following locations -

a. $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers
b. $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders
c. $(DSTROOT)/usr/include/
d. $(DSTROOT)/usr/local/include/
e. $(DSTROOT)/System/DriverKit/usr/include/
f. $(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders

Kernel.framework is used by kernel extensions.
The System.framework, /usr/include and /usr/local/include are used by user level applications.
/System/DriverKit/usr/include is used by userspace drivers.
The header files in framework's PrivateHeaders are only available for ** Apple Internal Development **.

The directory containing the header file should have a Makefile that creates the list of files that should be installed at different locations. If you are adding the first header file in a directory, you will need to create Makefile similar to xnu/bsd/sys/Makefile.

Add your header file to the correct file list depending on where you want to install it. The default locations where the header files are installed from each file list are -

a. `DATAFILES` : To make header file available in user level -
   `$(DSTROOT)/usr/include`
   `$(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders`

b. `DRIVERKIT_DATAFILES` : To make header file available to DriverKit userspace drivers -
   `$(DSTROOT)/System/DriverKit/usr/include`

c. `PRIVATE_DATAFILES` : To make header file available to Apple internal in
   user level -
   `$(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders`

d. `EMBEDDED_PRIVATE_DATAFILES` : To make header file available in user
   level for macOS as `EXTRA_DATAFILES`, but Apple internal in user level
   for embedded OSes as `EXTRA_PRIVATE_DATAFILES` -
   `$(DSTROOT)/usr/include` (`EXTRA_DATAFILES`)
   `$(DSTROOT)/usr/local/include` (`EXTRA_PRIVATE_DATAFILES`)

d. `KERNELFILES` : To make header file available in kernel level -
   `$(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers`
   `$(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders`

e. `PRIVATE_KERNELFILES` : To make header file available to Apple internal
   for kernel extensions -
   `$(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders`

f. `MODULEMAPFILES` : To make module map file available in user level -
   `$(DSTROOT)/usr/include`

g. `PRIVATE_MODULEMAPFILES` : To make module map file available to Apple
   internal in user level -
   `$(DSTROOT)/usr/local/include`

The Makefile combines the file lists mentioned above into different install lists which are used by build system to install the header files. There are two types of install lists: machine-dependent and machine-independent. These lists are indicated by the presence of MD and MI in the build setting, respectively. If your header is architecture-specific, then you should use a machine-dependent install list (e.g. INSTALL_MD_LIST). If your header should be installed for all architectures, then you should use a machine-independent install list (e.g. INSTALL_MI_LIST).

If the install list that you are interested does not exist, create it by adding the appropriate file lists. The default install lists, its member file lists and their default location are described below -

a. `INSTALL_MI_LIST`, `INSTALL_MODULEMAP_MI_LIST` : Installs header and module map
   files to a location that is available to everyone in user level.
   Locations -
       $(DSTROOT)/usr/include
   Definition -
       INSTALL_MI_LIST = ${DATAFILES}
       INSTALL_MODULEMAP_MI_LIST = ${MODULEMAPFILES}

b. `INSTALL_DRIVERKIT_MI_LIST` : Installs header file to a location that is
    available to DriverKit userspace drivers.
    Locations -
       $(DSTROOT)/System/DriverKit/usr/include
   Definition -
       INSTALL_DRIVERKIT_MI_LIST = ${DRIVERKIT_DATAFILES}

c.  `INSTALL_MI_LCL_LIST`, `INSTALL_MODULEMAP_MI_LCL_LIST` : Installs header and
   module map files to a location that is available for Apple internal in user level.
   Locations -
       $(DSTROOT)/usr/local/include
   Definition -
       INSTALL_MI_LCL_LIST =
       INSTALL_MODULEMAP_MI_LCL_LIST = ${PRIVATE_MODULEMAPFILES}

d.  `INSTALL_SF_MI_LCL_LIST` : Installs header file to a location that is available
   for Apple internal in user level.
   Locations -
       $(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders
   Definition -
       INSTALL_SF_MI_LCL_LIST = ${DATAFILES} ${PRIVATE_DATAFILES}

e. `INSTALL_KF_MI_LIST` : Installs header file to location that is available
   to everyone for kernel extensions.
   Locations -
        $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers
   Definition -
        INSTALL_KF_MI_LIST = ${KERNELFILES}

f. `INSTALL_KF_MI_LCL_LIST` : Installs header file to location that is
   available for Apple internal for kernel extensions.
   Locations -
        $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders
   Definition -
        INSTALL_KF_MI_LCL_LIST = ${KERNELFILES} ${PRIVATE_KERNELFILES}

g. `EXPORT_MI_LIST` : Exports header file to all of xnu (bsd/, osfmk/, etc.)
   for compilation only. Does not install anything into the SDK.
   Definition -
        EXPORT_MI_LIST = ${KERNELFILES} ${PRIVATE_KERNELFILES}

If you want to install the header file in a sub-directory of the paths described in (1), specify the directory name using two variables INSTALL_MI_DIR and EXPORT_MI_DIR as follows -

INSTALL_MI_DIR = dirname
EXPORT_MI_DIR = dirname

If you want to install the module map file in a sub-directory, specify the directory name using the variable INSTALL_MODULEMAP_MI_DIR as follows -

INSTALL_MODULEMAP_MI_DIR = dirname

A single header file can exist at different locations using the steps mentioned above. However it might not be desirable to make all the code in the header file available at all the locations. For example, you want to export a function only to kernel level but not user level.

You can use C language's pre-processor directive (#ifdef, #endif, #ifndef) to control the text generated before a header file is installed. The kernel only includes the code if the conditional macro is TRUE and strips out code for FALSE conditions from the header file.

Some pre-defined macros and their descriptions are -

a. `PRIVATE` : If defined, enclosed definitions are considered System
Private Interfaces. These are visible within xnu and
exposed in user/kernel headers installed within the AppleInternal
"PrivateHeaders" sections of the System and Kernel frameworks.
b. `KERNEL_PRIVATE` : If defined, enclosed code is available to all of xnu
kernel and Apple internal kernel extensions and omitted from user
headers.
c. `BSD_KERNEL_PRIVATE` : If defined, enclosed code is visible exclusively
within the xnu/bsd module.
d. `MACH_KERNEL_PRIVATE`: If defined, enclosed code is visible exclusively
within the xnu/osfmk module.
e. `XNU_KERNEL_PRIVATE`: If defined, enclosed code is visible exclusively
within xnu.
f. `KERNEL` :  If defined, enclosed code is available within xnu and kernel
   extensions and is not visible in user level header files.  Only the
   header files installed in following paths will have the code -

        $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers
        $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders
g. `DRIVERKIT`: If defined, enclosed code is visible exclusively in the
DriverKit SDK headers used by userspace drivers.

Module map file name convention

In the simple case, a subdirectory of usr/include or usr/local/include can be represented by a standalone module. Where this is the case, set INSTALL_MODULEMAP_MI_DIR to INSTALL_MI_DIR and install a module.modulemap file there. module.modulemap is used even for private modules in usr/local/include; module.private.modulemap is not used. Caveat: in order to stay in the simple case, the module name needs to be exactly the same as the directory name. If that's not possible, then the following method will need to be applied.

xnu contributes to the modules defined in CoreOSModuleMaps by installing module map files that are sourced from usr/include/module.modulemap and usr/local/include/module.modulemap. The naming convention for the xnu module map files are as follows.

a. Ideally the module map file covers an entire directory. A module map
   file covering `usr/include/a/b/c` would be named `a_b_c.modulemap`.
   `usr/local/include/a/b/c` would be `a_b_c_private.modulemap`.
b. Some headers are special and require their own module. In that case,
   the module map file would be named after the module it defines.
   A module map file defining the module `One.Two.Three` would be named
   `one_two_three.modulemap`.

Conditional compilation

xnu offers the following mechanisms for conditionally compiling code:

a. *CPU Characteristics* If the code you are guarding has specific
characterstics that will vary only based on the CPU architecture being
targeted, use this option. Prefer checking for features of the
architecture (e.g. `__LP64__`, `__LITTLE_ENDIAN__`, etc.).
b. *New Features* If the code you are guarding, when taken together,
implements a feature, you should define a new feature in `config/MASTER`
and use the resulting `CONFIG` preprocessor token (e.g. for a feature
named `config_virtual_memory`, check for `#if CONFIG_VIRTUAL_MEMORY`).
This practice ensures that existing features may be brought to other
platforms by simply changing a feature switch.
c. *Existing Features* You can use existing features if your code is
strongly tied to them (e.g. use `SECURE_KERNEL` if your code implements
new functionality that is exclusively relevant to the trusted kernel and
updates the definition/understanding of what being a trusted kernel means).

It is recommended that you avoid compiling based on the target platform. xnu does not define the platform macros from TargetConditionals.h (TARGET_OS_OSX, TARGET_OS_IOS, etc.).

Debugging xnu

By default, the kernel reboots in the event of a panic. This behavior can be overriden by the debug boot-arg -- debug=0x14e will cause a panic to wait for a debugger to attach. To boot a kernel so it can be debugged by an attached machine, override the kdp_match_name boot-arg with the appropriate ifconfig interface. Ethernet, Thunderbolt, and serial debugging are supported, depending on the hardware.

Use LLDB to debug the kernel:

; xcrun -sdk macosx lldb <path-to-unstripped-kernel>
(lldb) gdb-remote [<host-ip>:]<port>

The debug info for the kernel (dSYM) comes with a set of macros to support kernel debugging. To load these macros automatically when attaching to the kernel, add the following to ~/.lldbinit:

settings set target.load-script-from-symbol-file true

tools/lldbmacros contains the source for these commands. See the README in that directory for their usage, or use the built-in LLDB help with:

(lldb) help showcurrentstacks