SubHook is a simple library for hooking arbitrary functions at run time. It's written in C and also provides an optional C++ wrapper API.

The library was originally developed to intercept a bunch of API calls in the SA-MP server, which is a Windows/Linux 32-bit app, in order to extend a plugin that I wrote for it. Since then, it has been adapted to better support x86_64, macOS, and more common use-cases, thanks to the contributors.

Installation

Easy method:

1. Copy the source and header files to your project and include subhook.c in your build.
2. On Windows only: Define SUBHOOK_STATIC before including subhook.h.

With CMake:

1. Copy the subhook repo to your project tree.

2. Call add_subdirectory(path/to/subhook) in your CMakeLists.txt.

3. Optional: configure how the library is built by setting these varaible prior to add_subdirectory(...):

   • SUBHOOK_STATIC - Build as static library (OFF by default)
   • SUBHOOK_INSTALL - Enable installation and packaging of targets/files with CPack (OFF by default)
   • SUBHOOK_TESTS - Enable tests (ON by default)
   • SUBHOOK_FORCE_32BIT - Configure for compiling 32-bit binaries on 64-bit systems (default is OFF)

Use of CMake is not mandatory, the library can be built without it (no extra build configuration is required).

Examples

In the following examples foo is some function or a function pointer that takes a single argument of type int and uses the same calling convention as my_foo (depends on compiler).

Basic usage

#include <stdio.h>
#include <subhook.h>

subhook_t foo_hook;

void my_foo(int x) {
  /* Remove the hook so that you can call the original function. */
  subhook_remove(foo_hook);

  printf("foo(%d) called\n", x);
  foo(x);

  /* Install the hook back to intercept further calls. */
  subhook_install(foo_hook);
}

int main() {
  /* Create a hook that will redirect all foo() calls to to my_foo(). */
  foo_hook = subhook_new((void *)foo, (void *)my_foo, 0);

  /* Install it. */
  subhook_install(foo_hook);

  foo(123);

  /* Remove the hook and free memory when you're done. */
  subhook_remove(foo_hook);
  subhook_free(foo_hook);
}

Trampolines

Using trampolines allows you to jump to the original code without removing and re-installing hooks every time your function gets called.

typedef void (*foo_func)(int x);

void my_foo(int x) {
  printf("foo(%d) called\n", x);

  /* Call foo() via trampoline. */
  ((foo_func)subhook_get_trampoline(foo_hook))(x);
}

int main() {
   /* Same code as in the previous example. */
}

Please note that subhook has a very simple length disassmebler engine (LDE) that works only with most common prologue instructions like push, mov, call, etc. When it encounters an unknown instruction subhook_get_trampoline() will return NULL. You can delegate instruction decoding to a custom disassembler of your choice via subhook_set_disasm_handler().

C++

#include <iostream>
#include <subhook.h>

subhook::Hook foo_hook;
subhook::Hook foo_hook_tr;

typedef void (*foo_func)(int x);

void my_foo(int x) {
  // ScopedHookRemove removes the specified hook and automatically re-installs
  // it when the object goes out of scope (thanks to C++ destructors).
  subhook::ScopedHookRemove remove(&foo_hook);

  std::cout << "foo(" << x << ") called" << std::endl;
  foo(x + 1);
}

void my_foo_tr(int x) {
  std::cout << "foo(" << x << ") called" << std::endl;

  // Call the original function via trampoline.
  ((foo_func)foo_hook_tr.GetTrampoline())(x + 1);
}

int main() {
  foo_hook.Install((void *)foo, (void *)my_foo);
  foo_hook_tr.Install((void *)foo, (void *)my_foo_tr);
}

Known issues/limitations

• subhook_get_trampoline() may return NULL because only a small subset of x86 instructions is supported by the disassembler in this library (just common prologue instructions). As a workaround you can plug in a more advanced instruction length decoder using subhook_set_disasm_handler().

• If a target function (the function you are hooking) is less than N bytes in length, for example if it's a short 2-byte jump to a nearby location (sometimes compilers generate code like this), then you will not be able to hook it.

  N is 5 by default: 1 byte for jmp opcode + 4 bytes for offset. But if you enable the use of 64-bit offsets in 64-bit mode N becomes 14 (see the definition of subhook_jmp64).

  On x64_64, another cause could be that the function contains instructions referencing memory that is too far away from the trampline code buffer's address trampoline_addr, such as cmp dword ptr [some_32bit_addr], rax (i.e. RIP-relative addressing) where the offset between some_32bit_addr and trampoline_addr cannot fit into 32 bits, and therefore we cannot update the memory address referenced in the original code (we need to do that because because it's relative).

• Some systems protect executable code form being modified at runtime, which will not allow you to install hooks, or don't allow to mark heap-allocated memory as executable, which prevents the use of trampolines.

  For example, on Fedora you can have such problems because of SELinux (though you can disable it or exclude your files).

License

Licensed under the 2-clause BSD license.