1What is XNU? 2=========== 3 4XNU 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. 5XNU 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. 6XNU runs on x86_64 for both single processor and multi-processor configurations. 7 8XNU Source Tree 9=============== 10 11 * `config` - configurations for exported apis for supported architecture and platform 12 * `SETUP` - Basic set of tools used for configuring the kernel, versioning and kextsymbol management. 13 * `EXTERNAL_HEADERS` - Headers sourced from other projects to avoid dependency cycles when building. These headers should be regularly synced when source is updated. 14 * `libkern` - C++ IOKit library code for handling of drivers and kexts. 15 * `libsa` - kernel bootstrap code for startup 16 * `libsyscall` - syscall library interface for userspace programs 17 * `libkdd` - source for user library for parsing kernel data like kernel chunked data. 18 * `makedefs` - top level rules and defines for kernel build. 19 * `osfmk` - Mach kernel based subsystems 20 * `pexpert` - Platform specific code like interrupt handling, atomics etc. 21 * `security` - Mandatory Access Check policy interfaces and related implementation. 22 * `bsd` - BSD subsystems code 23 * `tools` - A set of utilities for testing, debugging and profiling kernel. 24 25How to build XNU 26================ 27 28Building `DEVELOPMENT` kernel 29----------------------------- 30 31The xnu make system can build kernel based on `KERNEL_CONFIGS` & `ARCH_CONFIGS` variables as arguments. 32Here is the syntax: 33 34 make SDKROOT=<sdkroot> ARCH_CONFIGS=<arch> KERNEL_CONFIGS=<variant> 35 36Where: 37 38 * \<sdkroot>: path to macOS SDK on disk. (defaults to `/`) 39 * \<variant>: can be `debug`, `development`, `release`, `profile` and configures compilation flags and asserts throughout kernel code. 40 * \<arch> : can be valid arch to build for. (E.g. `X86_64`) 41 42To build a kernel for the same architecture as running OS, just type 43 44 $ make 45 $ make SDKROOT=macosx.internal 46 47Additionally, there is support for configuring architectures through `ARCH_CONFIGS` and kernel configurations with `KERNEL_CONFIGS`. 48 49 $ make SDKROOT=macosx.internal ARCH_CONFIGS=X86_64 KERNEL_CONFIGS=DEVELOPMENT 50 $ make SDKROOT=macosx.internal ARCH_CONFIGS=X86_64 KERNEL_CONFIGS="RELEASE DEVELOPMENT DEBUG" 51 52 53Note: 54 * By default, architecture is set to the build machine architecture, and the default kernel 55 config is set to build for DEVELOPMENT. 56 57 58This will also create a bootable image, kernel.[config], and a kernel binary 59with symbols, kernel.[config].unstripped. 60 61To intall the kernel into a DSTROOT, use the `install_kernels` target: 62 63 $ make install_kernels DSTROOT=/tmp/xnu-dst 64 65Hint: 66For a more satisfying kernel debugging experience, with access to all 67local variables and arguments, but without all the extra check of the 68DEBUG kernel, add something like: 69 CFLAGS_DEVELOPMENTARM64="-O0 -g -DKERNEL_STACK_MULTIPLIER=2" 70 CXXFLAGS_DEVELOPMENTARM64="-O0 -g -DKERNEL_STACK_MULTIPLIER=2" 71to your make command. 72Replace DEVELOPMENT and ARM64 with the appropriate build and platform. 73 74 75 * To build with RELEASE kernel configuration 76 77 make KERNEL_CONFIGS=RELEASE SDKROOT=/path/to/SDK 78 79 80Building FAT kernel binary 81-------------------------- 82 83Define architectures in your environment or when running a make command. 84 85 $ make ARCH_CONFIGS="X86_64" exporthdrs all 86 87Other makefile options 88---------------------- 89 90 * $ make MAKEJOBS=-j8 # this will use 8 processes during the build. The default is 2x the number of active CPUS. 91 * $ make -j8 # the standard command-line option is also accepted 92 * $ make -w # trace recursive make invocations. Useful in combination with VERBOSE=YES 93 * $ make BUILD_LTO=0 # build without LLVM Link Time Optimization 94 * $ make BOUND_CHECKS=1 # enable -fbound-attributes for this build 95 * $ make REMOTEBUILD=user@remotehost # perform build on remote host 96 * $ make BUILD_JSON_COMPILATION_DATABASE=1 # Build Clang JSON Compilation Database 97 98The XNU build system can optionally output color-formatted build output. To enable this, you can either 99set the `XNU_LOGCOLORS` environment variable to `y`, or you can pass `LOGCOLORS=y` to the make command. 100 101 102Debug information formats 103========================= 104 105By default, a DWARF debug information repository is created during the install phase; this is a "bundle" named kernel.development.\<variant>.dSYM 106To select the older STABS debug information format (where debug information is embedded in the kernel.development.unstripped image), set the BUILD_STABS environment variable. 107 108 $ export BUILD_STABS=1 109 $ make 110 111 112Building KernelCaches 113===================== 114 115To test the xnu kernel, you need to build a kernelcache that links the kexts and 116kernel together into a single bootable image. 117To build a kernelcache you can use the following mechanisms: 118 119 * Using automatic kernelcache generation with `kextd`. 120 The kextd daemon keeps watching for changing in `/System/Library/Extensions` directory. 121 So you can setup new kernel as 122 123 $ cp BUILD/obj/DEVELOPMENT/X86_64/kernel.development /System/Library/Kernels/ 124 $ touch /System/Library/Extensions 125 $ ps -e | grep kextd 126 127 * Manually invoking `kextcache` to build new kernelcache. 128 129 $ kextcache -q -z -a x86_64 -l -n -c /var/tmp/kernelcache.test -K /var/tmp/kernel.test /System/Library/Extensions 130 131 132 133Running KernelCache on Target machine 134===================================== 135 136The 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. 137Following are the steps to get such a setup: 138 139 1. Create kernel cache using the kextcache command as `/kernelcache.test` 140 2. Copy exiting boot configurations to alternate file 141 142 $ cp /Library/Preferences/SystemConfiguration/com.apple.Boot.plist /next_boot.plist 143 144 3. Update the kernelcache and boot-args for your setup 145 146 $ plutil -insert "Kernel Cache" -string "kernelcache.test" /next_boot.plist 147 $ plutil -replace "Kernel Flags" -string "debug=0x144 -v kernelsuffix=test " /next_boot.plist 148 149 4. Copy the new config to `/Library/Preferences/SystemConfiguration/` 150 151 $ cp /next_boot.plist /Library/Preferences/SystemConfiguration/boot.plist 152 153 5. Bless the volume with new configs. 154 155 $ sudo -n bless --mount / --setBoot --nextonly --options "config=boot" 156 157 The `--nextonly` flag specifies that use the `boot.plist` configs only for one boot. 158 So if the kernel panic's you can easily power reboot and recover back to original kernel. 159 160 161 162 163Creating tags and cscope 164======================== 165 166Set up your build environment and from the top directory, run: 167 168 $ make tags # this will build ctags and etags on a case-sensitive volume, only ctags on case-insensitive 169 $ make TAGS # this will build etags 170 $ make cscope # this will build cscope database 171 172 173How to install a new header file from XNU 174========================================= 175 176To install IOKit headers, see additional comments in [iokit/IOKit/Makefile](). 177 178XNU installs header files at the following locations - 179 180 a. $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers 181 b. $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders 182 c. $(DSTROOT)/usr/include/ 183 d. $(DSTROOT)/usr/local/include/ 184 e. $(DSTROOT)/System/DriverKit/usr/include/ 185 f. $(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders 186 187`Kernel.framework` is used by kernel extensions.\ 188The `System.framework`, `/usr/include` and `/usr/local/include` are used by user level applications. \ 189`/System/DriverKit/usr/include` is used by userspace drivers. \ 190The header files in framework's `PrivateHeaders` are only available for ** Apple Internal Development **. 191 192The directory containing the header file should have a Makefile that 193creates the list of files that should be installed at different locations. 194If you are adding the first header file in a directory, you will need to 195create Makefile similar to `xnu/bsd/sys/Makefile`. 196 197Add your header file to the correct file list depending on where you want 198to install it. The default locations where the header files are installed 199from each file list are - 200 201 a. `DATAFILES` : To make header file available in user level - 202 `$(DSTROOT)/usr/include` 203 `$(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders` 204 205 b. `DRIVERKIT_DATAFILES` : To make header file available to DriverKit userspace drivers - 206 `$(DSTROOT)/System/DriverKit/usr/include` 207 208 c. `PRIVATE_DATAFILES` : To make header file available to Apple internal in 209 user level - 210 `$(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders` 211 212 d. `EMBEDDED_PRIVATE_DATAFILES` : To make header file available in user 213 level for macOS as `EXTRA_DATAFILES`, but Apple internal in user level 214 for embedded OSes as `EXTRA_PRIVATE_DATAFILES` - 215 `$(DSTROOT)/usr/include` (`EXTRA_DATAFILES`) 216 `$(DSTROOT)/usr/local/include` (`EXTRA_PRIVATE_DATAFILES`) 217 218 d. `KERNELFILES` : To make header file available in kernel level - 219 `$(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers` 220 `$(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders` 221 222 e. `PRIVATE_KERNELFILES` : To make header file available to Apple internal 223 for kernel extensions - 224 `$(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders` 225 226 f. `MODULEMAPFILES` : To make module map file available in user level - 227 `$(DSTROOT)/usr/include` 228 229 g. `PRIVATE_MODULEMAPFILES` : To make module map file available to Apple 230 internal in user level - 231 `$(DSTROOT)/usr/local/include` 232 233The Makefile combines the file lists mentioned above into different 234install lists which are used by build system to install the header files. There 235are two types of install lists: machine-dependent and machine-independent. 236These lists are indicated by the presence of `MD` and `MI` in the build 237setting, respectively. If your header is architecture-specific, then you should 238use a machine-dependent install list (e.g. `INSTALL_MD_LIST`). If your header 239should be installed for all architectures, then you should use a 240machine-independent install list (e.g. `INSTALL_MI_LIST`). 241 242If the install list that you are interested does not exist, create it 243by adding the appropriate file lists. The default install lists, its 244member file lists and their default location are described below - 245 246 a. `INSTALL_MI_LIST`, `INSTALL_MODULEMAP_MI_LIST` : Installs header and module map 247 files to a location that is available to everyone in user level. 248 Locations - 249 $(DSTROOT)/usr/include 250 Definition - 251 INSTALL_MI_LIST = ${DATAFILES} 252 INSTALL_MODULEMAP_MI_LIST = ${MODULEMAPFILES} 253 254 b. `INSTALL_DRIVERKIT_MI_LIST` : Installs header file to a location that is 255 available to DriverKit userspace drivers. 256 Locations - 257 $(DSTROOT)/System/DriverKit/usr/include 258 Definition - 259 INSTALL_DRIVERKIT_MI_LIST = ${DRIVERKIT_DATAFILES} 260 261 c. `INSTALL_MI_LCL_LIST`, `INSTALL_MODULEMAP_MI_LCL_LIST` : Installs header and 262 module map files to a location that is available for Apple internal in user level. 263 Locations - 264 $(DSTROOT)/usr/local/include 265 Definition - 266 INSTALL_MI_LCL_LIST = 267 INSTALL_MODULEMAP_MI_LCL_LIST = ${PRIVATE_MODULEMAPFILES} 268 269 d. `INSTALL_SF_MI_LCL_LIST` : Installs header file to a location that is available 270 for Apple internal in user level. 271 Locations - 272 $(DSTROOT)/System/Library/Frameworks/System.framework/PrivateHeaders 273 Definition - 274 INSTALL_SF_MI_LCL_LIST = ${DATAFILES} ${PRIVATE_DATAFILES} 275 276 e. `INSTALL_KF_MI_LIST` : Installs header file to location that is available 277 to everyone for kernel extensions. 278 Locations - 279 $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers 280 Definition - 281 INSTALL_KF_MI_LIST = ${KERNELFILES} 282 283 f. `INSTALL_KF_MI_LCL_LIST` : Installs header file to location that is 284 available for Apple internal for kernel extensions. 285 Locations - 286 $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders 287 Definition - 288 INSTALL_KF_MI_LCL_LIST = ${KERNELFILES} ${PRIVATE_KERNELFILES} 289 290 g. `EXPORT_MI_LIST` : Exports header file to all of xnu (bsd/, osfmk/, etc.) 291 for compilation only. Does not install anything into the SDK. 292 Definition - 293 EXPORT_MI_LIST = ${KERNELFILES} ${PRIVATE_KERNELFILES} 294 295If you want to install the header file in a sub-directory of the paths 296described in (1), specify the directory name using two variables 297`INSTALL_MI_DIR` and `EXPORT_MI_DIR` as follows - 298 299 INSTALL_MI_DIR = dirname 300 EXPORT_MI_DIR = dirname 301 302If you want to install the module map file in a sub-directory, specify the 303directory name using the variable `INSTALL_MODULEMAP_MI_DIR` as follows - 304 305 INSTALL_MODULEMAP_MI_DIR = dirname 306 307A single header file can exist at different locations using the steps 308mentioned above. However it might not be desirable to make all the code 309in the header file available at all the locations. For example, you 310want to export a function only to kernel level but not user level. 311 312 You can use C language's pre-processor directive (#ifdef, #endif, #ifndef) 313 to control the text generated before a header file is installed. The kernel 314 only includes the code if the conditional macro is TRUE and strips out 315 code for FALSE conditions from the header file. 316 317 Some pre-defined macros and their descriptions are - 318 319 a. `PRIVATE` : If defined, enclosed definitions are considered System 320 Private Interfaces. These are visible within xnu and 321 exposed in user/kernel headers installed within the AppleInternal 322 "PrivateHeaders" sections of the System and Kernel frameworks. 323 b. `KERNEL_PRIVATE` : If defined, enclosed code is available to all of xnu 324 kernel and Apple internal kernel extensions and omitted from user 325 headers. 326 c. `BSD_KERNEL_PRIVATE` : If defined, enclosed code is visible exclusively 327 within the xnu/bsd module. 328 d. `MACH_KERNEL_PRIVATE`: If defined, enclosed code is visible exclusively 329 within the xnu/osfmk module. 330 e. `XNU_KERNEL_PRIVATE`: If defined, enclosed code is visible exclusively 331 within xnu. 332 f. `KERNEL` : If defined, enclosed code is available within xnu and kernel 333 extensions and is not visible in user level header files. Only the 334 header files installed in following paths will have the code - 335 336 $(DSTROOT)/System/Library/Frameworks/Kernel.framework/Headers 337 $(DSTROOT)/System/Library/Frameworks/Kernel.framework/PrivateHeaders 338 g. `DRIVERKIT`: If defined, enclosed code is visible exclusively in the 339 DriverKit SDK headers used by userspace drivers. 340 341Module map file name convention 342=============================== 343 344In the simple case, a subdirectory of `usr/include` or `usr/local/include` 345can be represented by a standalone module. Where this is the case, set 346`INSTALL_MODULEMAP_MI_DIR` to `INSTALL_MI_DIR` and install a `module.modulemap` 347file there. `module.modulemap` is used even for private modules in 348`usr/local/include`; `module.private.modulemap` is not used. Caveat: in order 349to stay in the simple case, the module name needs to be exactly the same as 350the directory name. If that's not possible, then the following method will 351need to be applied. 352 353`xnu` contributes to the modules defined in CoreOSModuleMaps by installing 354module map files that are sourced from `usr/include/module.modulemap` and 355`usr/local/include/module.modulemap`. The naming convention for the `xnu` 356module map files are as follows. 357 358 a. Ideally the module map file covers an entire directory. A module map 359 file covering `usr/include/a/b/c` would be named `a_b_c.modulemap`. 360 `usr/local/include/a/b/c` would be `a_b_c_private.modulemap`. 361 b. Some headers are special and require their own module. In that case, 362 the module map file would be named after the module it defines. 363 A module map file defining the module `One.Two.Three` would be named 364 `one_two_three.modulemap`. 365 366Conditional compilation 367======================= 368 369`xnu` offers the following mechanisms for conditionally compiling code: 370 371 a. *CPU Characteristics* If the code you are guarding has specific 372 characterstics that will vary only based on the CPU architecture being 373 targeted, use this option. Prefer checking for features of the 374 architecture (e.g. `__LP64__`, `__LITTLE_ENDIAN__`, etc.). 375 b. *New Features* If the code you are guarding, when taken together, 376 implements a feature, you should define a new feature in `config/MASTER` 377 and use the resulting `CONFIG` preprocessor token (e.g. for a feature 378 named `config_virtual_memory`, check for `#if CONFIG_VIRTUAL_MEMORY`). 379 This practice ensures that existing features may be brought to other 380 platforms by simply changing a feature switch. 381 c. *Existing Features* You can use existing features if your code is 382 strongly tied to them (e.g. use `SECURE_KERNEL` if your code implements 383 new functionality that is exclusively relevant to the trusted kernel and 384 updates the definition/understanding of what being a trusted kernel means). 385 386It is recommended that you avoid compiling based on the target platform. `xnu` 387does not define the platform macros from `TargetConditionals.h` 388(`TARGET_OS_OSX`, `TARGET_OS_IOS`, etc.). 389 390 391Debugging xnu 392============= 393 394By default, the kernel reboots in the event of a panic. 395This behavior can be overriden by the `debug` boot-arg -- `debug=0x14e` will cause a panic to wait for a debugger to attach. 396To boot a kernel so it can be debugged by an attached machine, override the `kdp_match_name` boot-arg with the appropriate `ifconfig` interface. 397Ethernet, Thunderbolt, and serial debugging are supported, depending on the hardware. 398 399Use LLDB to debug the kernel: 400 401 ; xcrun -sdk macosx lldb <path-to-unstripped-kernel> 402 (lldb) gdb-remote [<host-ip>:]<port> 403 404The debug info for the kernel (dSYM) comes with a set of macros to support kernel debugging. 405To load these macros automatically when attaching to the kernel, add the following to `~/.lldbinit`: 406 407 settings set target.load-script-from-symbol-file true 408 409`tools/lldbmacros` contains the source for these commands. 410See the README in that directory for their usage, or use the built-in LLDB help with: 411 412 (lldb) help showcurrentstacks 413