Besides dependency resolution failures, other reasons for a kexts failure to load include incorrect file permissions, a flawed bundle structure, a missing CFBundleIdentifier property in the kexts Info.plist file, and a missing or syntactically invalid Info.plist file. We can use the kextstat command to check whether our kext is currently loaded in the kernel.Note that Apple urges third-party programmers to avoid programming in the kernel unless absolutely necessary.
Kernel Extension List Driver To ResideA legitimate reason for a device driver to reside in the kernel is if it handles a primary interrupt or if its primary client is kernel resident.A Generic Kernel Extension Let us create a trivial kextone that implements only the start and stop entry points.
Once we can compile and load the kext, we will extend it to implement a couple of sysctl entries. Kernel Extension List Code Project InstantiatedWe begin with an Xcode project instantiated from the template for generic kernel extensions. ![]() We will use the kernel version of the printf() function to print messages in our kext. The key contents of the kexts Info.plist file are as follows:.. CFBundleExecutable DummySysctl CFBundleIdentifier com.osxbook.kext.DummySysctl. OSBundleLibraries com.apple.kpi.bsd 8.0.0 com.apple.kpi.libkern 8.0.0 At least certain values in the stock Info.plist file generated by Xcode must be modified or added before the compiled kext can be successfully loaded. The CFBundleIdentifier key should be set to a reverse-DNS-style name for the kext. The OSBundleLibraries key is used to enumerate a kexts dependencies. This keys value is a dictionaryspecifically, an OSDictionary that has been serialized into the XML property list format. Beginning with Mac OS X 10.4, a kext can declare dependencies either on new-style kernel programming interfaces (KPIs) or on compatibility interfaces. KPI dependencies are specified through com.apple.kpi. The former start from version 8.0.0 (Mac OS X 10.4 and newer), whereas the latter end at version 7.9.9 (Mac OS X 10.3 and older). The kextstat command can be used to list the interfaces available in the current kernelthe interfaces correspond to the fake kexts that represent built-in kernel components such as Mach, BSD, libkern, and the IO Kit. A kext can also declare a dependency on a specific version of the entire kernel by using the com.apple.kernel identifier. Although this approach would provide the kext access to all available kernel interfaces, including internal ones, it is not recommended because Apple does not guarantee binary compatibility across kernel versions to such kexts. Besides core kernel components, kexts can depend on various IO Kit families by using identifiers such as com.apple.iokit.IOGraphicsFamily, com.apple.iokit.IONetworkingFamily, com.apple.iokit.IOPCIFamily, and com.apple.iokit.IOStorageFamily. Figure 1013 shows the source for the kext. The start function is called when the kext is loaded, and the stop function is called when it is unloaded. Xcode inserts skeletal implementations of these functions in the automatically generated C file for the Generic Kernel Extension project template. Figure 1013. Source for the DummySysctl kernel extension DummySysctl.c include kernreturnt DummySysctlstart(kmodinfot ki, void d) printf(DummySysctlstartn); return KERNSUCCESS; kernreturnt DummySysctlstop(kmodinfot ki, void d) printf(DummySysctlstopn); return KERNSUCCESS; Let us now compile the kext. The benefits of Xcode are most evident in the compilation stage, since a manual compilation would need to specify the appropriate combination of compiler arguments, environment variables, linker arguments, and so on. Note that it is possible to initiate an Xcode build from the command line using the xcodebuild program. On successful compilation, the target kext bundle is created in a subdirectory of the build directory within the Xcode project directory. BUILD SUCCEEDED ls buildDebug DummySysctl.kext Since loading a kext requires the kext bundles contents to have root and wheel as the owner and group, respectively, a typical compile-test-debug cycle would involve copying the kext bundle from the build directory to a temporary locationsay, to tmp and using the chown command on the copy. As we noted earlier, besides ownership, the modifiability of objects within the bundle also mattersthe bundles contents must not be writable by any user except root. Kernel Extension List Series Of TestsIt only performs a series of tests on the kext and determines whether it is loadable. ![]() Besides dependency resolution failures, other reasons for a kexts failure to load include incorrect file permissions, a flawed bundle structure, a missing CFBundleIdentifier property in the kexts Info.plist file, and a missing or syntactically invalid Info.plist file. We can use the kextstat command to check whether our kext is currently loaded in the kernel.
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