FreeBSD/Linux Kernel Cross Reference
sys/README.md

     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=0  # disable -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
   =========================================
   
   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/IOKit.framework/Headers
       g. $(DSTROOT)/System/Library/Frameworks/IOKit.framework/PrivateHeaders
       h. $(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. \
   `IOKit.framework` is used by IOKit userspace clients. \
   `/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_IF_MI_LIST` : Installs header file to location that is available
          to everyone for IOKit userspace clients.
          Locations -
               $(DSTROOT)/System/Library/Frameworks/IOKit.framework/Headers
          Definition -
               INSTALL_IF_MI_LIST = ${DATAFILES}
   
       e. `INSTALL_IF_MI_LCL_LIST` : Installs header file to location that is
          available to Apple internal for IOKit userspace clients.
          Locations -
               $(DSTROOT)/System/Library/Frameworks/IOKit.framework/PrivateHeaders
          Definition -
               INSTALL_IF_MI_LCL_LIST = ${DATAFILES} ${PRIVATE_DATAFILES}
   
       f.  `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}
   
       g. `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}
   
       h. `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}
   
       i. `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
   

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