Static Function Hooking in Objective-C Using LLVM Pass

This article introduces a method for static function hooking in Objective-C using LLVM Pass. Traditional hooking methods like method swizzle, message forwarding, libffi, and fishhook can only hook function calls before or after execution, but cannot insert hooks within the function logic. The article proposes using LLVM Pass to modify the Intermediate Representation (IR) file during compilation to insert stub functions at specific locations.

The article covers the following topics: common Objective-C hooking methods, the concept of LLVM Pass, the compilation process, LLVM IR description, preparation work (downloading LLVM, writing custom Pass, CMake configuration), customizing Clang commands, and execution verification. The author provides detailed steps for implementing this technique, including modifying LLVM source code, creating custom Pass, and configuring Clang to use the custom Pass.

The article concludes with instructions on how to apply this technique in Xcode, including setting up custom Clang, modifying build settings, and using the technique in practice. The author also provides references for further learning about LLVM and Clang.