Reverse Engineering MacOS
HookCase is a tool for debugging and reverse engineering applications on macOS (aka OS X), and the operating system itself. It re-implements and extends Apple’s DYLD_INSERT_LIBRARIES functionality. It can be used to hook any method defined in any module’s symbol table, whether or not that method is exported. In a single operation, it can be applied to a parent process and all its child processes, whether or not the child processes inherit their parent’s environment. So HookCase is considerably more powerful than DYLD_INSERT_LIBRARIES. It also doesn’t have the restrictions Apple has placed on DYLD_INSERT_LIBRARIES. So, for example, HookCase can be used with applications that have entitlements.
HookCase runs on OS X 10.9 (Mavericks) through macOS 10.12 (Sierra).
Apple’s DYLD_INSERT_LIBRARIES environment variable is used to load an “interpose library” into a process before any of its other modules. The interpose library contains “hooks” for methods implemented elsewhere. Once the process is up and running, each hook is called instead of the original method that it hooks. Then the hook can call the original method (if that’s how it’s been written). Typically a hook logs calls to its original method (including its parameters and return value, plus maybe a stack trace). It can also be used to alter the behavior of the original method, or to replace it entirely.
DYLD_INSERT_LIBRARIES‘s hooks are what we call “interpose hooks”. They’re implemented by changing pointers in tables used to dynamically link methods called from other modules. So they can only be used to hook “cross-module” calls to exported methods.
HookCase supports interpose hooks. But it also supports another, more powerful kind of hook that we call “patch hooks”. These hook calls to any method defined in any module’s symbol table, including ones that come from the same module. And they can be used with non-exported (aka private) methods — ones not intended for use by external modules.
Patch hooks are so-called because we set them up by “patching” the beginning of an original method with a software interrupt instruction (int 0x20). HookCase’s kernel extension handles the interrupt to implement the hook. This is analogous to what a debugger does when it sets a breakpoint (though it uses int 3 instead of int 0x20). Software interrupts are mostly not used on BSD-style operating systems like macOS and OS X, so we have plenty to choose among. For now we’re using those in the range 0x20-0x23.
Whatever their disadvantages, interpose hooks are very performant. They’re implemented by changing a pointer, so they impose no performance penalty whatsoever (aside from the cost of whatever additional code runs inside the hook). Patch hooks can be substantially less performant — if we have to unset the breakpoint on every call to the hook, then reset it afterwards (and protect these operations from race conditions). But this isn’t needed for methods that start with a standard C/C++ prologue in machine code (which is most of them). So most patch hooks run with only a very small performance penalty (that of a single software interrupt).
HookCase is compatible with DYLD_INSERT_LIBRARIES, and doesn’t stomp on any of the changes it may have been used to make. So a DYLD_INTERPOSE_LIBRARIES hook will always override the “same” HookCase interpose hook. This is because Apple often uses DYLD_INSERT_LIBRARIES internally, in ways it doesn’t document. HookCase would likely break Apple functionality if it could override Apple’s hooks. But this doesn’t apply to patch hooks. Since Apple doesn’t use them, we don’t need to worry about overriding any that Apple may have set. If an interpose hook doesn’t seem to work, try a patch hook instead. (Unless you write them to do so, neither interpose hooks nor patch hooks inherently change the behavior of the methods they hook.)
HookCase is compatible with lldb and gdb: Any process with HookCase’s interpose or patch hooks can run inside these debuggers. But you may encounter trouble if you set a breakpoint and a patch hook on the same method, or try to step through code that contains a patch hook.
Apple’s support for DYLD_INSERT_LIBRARIES is implemented in /usr/lib/dyld. A (shared) copy of this module gets loaded into the image of every new process before it starts running. dyld‘s man page calls it the “dynamic link editor”, and it’s what runs first (starting from _dyld_start in dyld‘s src/dyldStartup.s) as a new process starts up. Once dyldhas finished its work, it jumps to the new process’s main() method.
HookCase is a kernel extension (HookCase.kext), which can’t call user-mode code directly. So we can’t just replace dyldwith code in HookCase. But we can read and alter the memory image of any running process. And we can “set up” user-mode calls to take place after we’ve returned to user mode from kernel mode. So we can hook methods in dyld, and “call” code in it indirectly. The best time to intervene is after dyld has finished most of its initialization, but before any of the new process’s “own” code has run (including its C++ initializers). This is when dyld‘s dyld::InitializeMainExecutable()method runs. So we hook that method as a process starts up, perform our own initialization, then allow the original dyld::InitializeMainExecutable() method to run (which, among other things, runs the process’s C++ initializers).
For more information, the best place to start is the long series of comments in HookCase.cpp before the definition of C_64_REDZONE_LEN.
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