/ Miscellaneous

A Procedure Linkage Table hook library for Android native ELF

A Procedure Linkage Table hook library for Android native ELF

xhook

xhook is a PLT (Procedure Linkage Table) hook library for Android native ELF (executable and shared libraries).

xhook has been keeping optimized for stability and compatibility.

Features

  • Support Android 4.0+ (API level 14+).
  • Support armeabi, armeabi-v7a and arm64-v8a.
  • Support ELF HASH and GNU HASH indexed symbols.
  • Support SLEB128 encoded relocation info.
  • Do NOT need root permission.
  • Do not depends on any third-party shared libraries.
  • Pure C code. Small library size.

Build

You need google NDK for building xhook.
See: https://developer.android.com/ndk/downloads/index.html

The latest version of xhook is developed and debugged with the NDK version r16b.

  • build the libraries (libxhook.so and other libraries for test)
./build_libs.sh
  • install the libraries to test project's libs path
./install_libs.sh
  • clean the libraries
./clean_libs.sh

Demo

cd ./xhookwrapper/
./gradlew assembleDebug
adb install ./app/build/outputs/apk/debug/app-debug.apk

API

External API header file: libxhook/jni/xhook.h

1. Register hook info

int xhook_register(const char  *pathname_regex_str,  
                   const char  *symbol,  
                   void        *new_func,  
                   void       **old_func);

In current process's memory space, in every loaded ELF which pathname matches regular expression pathname_regex_str, every PLT entries to symbol will be replaced with new_func. The original one will be saved in old_func.

The new_func must have the same function declaration as the original one.

Return zero if successful, non-zero otherwise.

The regular expression for pathname_regex_str only support POSIX BRE.

2. Do hook

int xhook_refresh(int async);

Do the real hook operations according to the registered hook info.

Pass 1 to async for asynchronous hook. Pass 0 to async for synchronous hook.

Return zero if successful, non-zero otherwise.

xhook will keep a global cache for saving the last ELF loading info from /proc/self/maps. This cache will also be updated in xhook_refresh. With this cache, xhook_refresh can determine which ELF is newly loaded. We only need to do hook in these newly loaded ELF.

3. Clear cache

void xhook_clear();

Clear all cache owned by xhook, reset all global flags to default value.

If you confirm that all PLT entries you want have been hooked, you could call this function to save some memory.

4. Enable/Disable debug info

void xhook_enable_debug(int flag);

Pass 1 to flag for enable debug info. Pass 0 to flag for disable. (disabled by default)

Debug info will be sent to logcat with tag xhook.

5. Enable/Disable SFP (segmentation fault protection)

void xhook_enable_sigsegv_protection(int flag);

Pass 1 to flag for enable SFP. Pass 0 to flag for disable. (enabled by default)

xhook is NOT a compliant business layer library. We have to calculate the value of some pointers directly. Reading or writing the memory pointed to by these pointers will cause a segmentation fault in some unusual situations and environment. The APP crash rate increased which caused by xhook is about one ten-millionth (0.0000001) according to our test. (The increased crash rate is also related to the ELFs and symbols you need to hook). Finally, we have to use some trick to prevent this harmless crashing. We called it SFP (segmentation fault protection) which consists of: sigaction(), SIGSEGV, siglongjmp() and sigsetjmp().

You should always enable SFP for release-APP, this will prevent your app from crashing. On the other hand, you should always disable SFP for debug-APP, so you can't miss any common coding mistakes that should be fixed.

Examples

//detect memory leaks
xhook_register(".*\\.so$", "malloc",  my_malloc,  NULL);
xhook_register(".*\\.so$", "calloc",  my_calloc,  NULL);
xhook_register(".*\\.so$", "realloc", my_realloc, NULL);
xhook_register(".*\\.so$", "free",    my_free,    NULL);

//inspect sockets lifecycle
xhook_register(".*\\.so$", "getaddrinfo", my_getaddrinfo, NULL);
xhook_register(".*\\.so$", "socket",      my_socket,      NULL);
xhook_register(".*\\.so$", "setsockopt"   my_setsockopt,  NULL);
xhook_register(".*\\.so$", "bind",        my_bind,        NULL);
xhook_register(".*\\.so$", "listen",      my_listen,      NULL);
xhook_register(".*\\.so$", "connect",     my_connect,     NULL);
xhook_register(".*\\.so$", "shutdown",    my_shutdown,    NULL);
xhook_register(".*\\.so$", "close",       my_close,       NULL);

//filter off and save some android log to local file
xhook_register(".*\\.so$", "__android_log_write",  my_log_write,  NULL);
xhook_register(".*\\.so$", "__android_log_print",  my_log_print,  NULL);
xhook_register(".*\\.so$", "__android_log_vprint", my_log_vprint, NULL);
xhook_register(".*\\.so$", "__android_log_assert", my_log_assert, NULL);

//tracking
xhook_register("^/system/.*$", "mmap",   my_mmap,   NULL);
xhook_register("^/vendor/.*$", "munmap", my_munmap, NULL);

//defense to some injection attacks
xhook_register(".*com\\.hacker.*\\.so$", "malloc",  my_malloc_always_return_NULL, NULL);
xhook_register(".*/libhacker\\.so$",     "connect", my_connect_with_recorder,     NULL);

//fix some system bug
xhook_register(".*some_vendor.*/libvictim\\.so$", "bad_func", my_nice_func, NULL);

//hook now!
xhook_refresh(1);

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