fake-linker

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fake-linker is a framework that provides features such as Linker modification, PLT Hook, and Java Native Hook for android applications. Its implementation principle involves dynamically searching for and modifying Linker data within the process. It offers PLT hook based on a LD_PRELOAD-like mode and various interfaces for operating on soinfo and namespace. For a detailed analysis of the principle, please refer to Android Dynamic Modification of Linker to Implement LD_PRELOAD Global Library PLT Hook.

Supports Android 5.0 ~ Android 14+ devices with x86, x86_64, arm, and arm64 instruction sets. It also supports HarmonyOS 2.x and 3.x versions, with versions beyond 3.x untested.

Project Description

We now offer the new v3.0.0+ version with the following major updates:

1. Removed the need for different libfakelinker.so files for different Android Api Levels. The internal function table now automatically adapts to different Android versions.
2. Introduced a stable C/C++ FakeLinker function table. Future versions guarantee backward compatibility and will not change the function pointer offsets of FakeLinker. The current version number can be obtained via FakeLinker.get_fakelinker_version.
3. Added support for the fake-linker static library, allowing customization of fake-linker and usage of interfaces such as elf_reader.h, jni_helper.h, maps_util.h individually. This enables fake-linker to act as a hook module and reduces the number of so files.
4. Optional Java FakeLinker API. The new version no longer requires the Java API mandatorily; it can be deleted as needed. You can also customize the class name for dynamic registration of the Java API.

Below is the description for the sub-projects:

1. The library project is the core implementation of fake-linker, providing both static and dynamic libraries for fakelinker. Note that when linking the fakelinker dynamic library, only the fake_linker.h header file can be used. Other header files require linking to the fakelinker_static static library for support.
2. The emulator-testapp project is used to test the functionality of fake-linker in an emulator. It supports the houdini architecture, meaning that despite using dynamic translation to support the arm architecture, it can actually load x86/x64 libraries. Therefore, it effectively uses the x86/x64 architecture of fake-linker to provide functionality.
3. The fakelinker-test project is for normal apk testing on x86, x86_64, arm, arm64 architectures. It includes androidTest and executable test programs. androidTest can be run via the UI in Android Studio by importing the project and running FakelinkerGTest, or by executing the command ./gradlew :fakelinker-test:connectedDebugAndroidTest to connect to an Android device for testing. The executable test programs can be pushed to a device via adb shell and executed after adding executable permissions, using the path fake-linker\fakelinker-test\src\main\cpp\build\Debug\xxxx\fakelinker_static_test.
4. The Stub project only provides private api interfaces and is not packaged into the apk.

Build and Integrate

1. Source Code Build

   Refer to the local.properties.sample for configuration parameters, then rename it to local.properties. After that, you can import the project into Android Studio or compile it directly using the gradle command line. For reference, see the workflow.

   Note: The new version adds the private APIs BaseDexClassLoader.getLdLibraryPath and BaseDexClassLoader.addNativePath. Incremental builds might cause errors.

   ...fake-linker\library\src\main\java\com\sanfengandroid\fakelinker\FakeLinker.java:153: error: cannot find symbol
      ((BaseDexClassLoader) loader).addNativePath(paths);

   You can clean the project or use the gradlew command with the --rerun-tasks parameter to clear the build cache and then compile again. Once the library project code is compiled successfully, subsequent incremental builds should not be affected.

2. Using the Built Library

   Download the latest Release version aar or the latest test version Action artifact to extract aar file as a library and add it to your project dependencies, enabling the prefab feature.

   // build.gradle
   
   android {
       buildFeatures {
           prefab true
       }
   }
   
   dependencies {
       implementation fileTree(dir: 'libs', include: ['*.jar', '*.aar'])
   }

   Then use the following in CMakeLists.txt:

   find_package(FakeLinker REQUIRED CONFIG)
   
   ...
   
   target_link_libraries(
       your_target
       PRIVATE
       # Dynamic library dependency
       FakeLinker::fakelinker
       # Static library dependency
       # FakeLinker::fakelinker_static
   )

3. CMake Integration

   Use FetchContent to directly download and build from a remote source by adding the following code to your CMakeLists.txt:

   include(FetchContent)
   FetchContent_Declare(
   fakelinker
   GIT_REPOSITORY https://github.com/sanfengAndroid/fake-linker.git
   GIT_TAG main
   SOURCE_SUBDIR library/src/main/cpp
   )
   
   FetchContent_MakeAvailable(fakelinker)

   Then, add the fakelinker module dependency in your CMakeLists.txt:

   target_link_libraries(
   your_target
   PRIVATE
   # Dynamic library dependency
   fakelinker
   # Static library dependency
   # fakelinker_static
   )

   Note: This method only uses the native library of fake-linker and does not use the Java code FakeLinker and ErrorCode. You can manually copy these files to your project or use only the native interfaces.

Hook Module Development

Both the aar package import and CMake integration methods have already imported the fakelinker dependency. Therefore, you can directly link the fakelinker dynamic library in your CMakeLists.txt:

target_link_libraries(
  your_target
  PRIVATE
  # Dynamic library dependency
  fakelinker
  # Static library dependency
  # fakelinker_static
)

Since the module already depends on fakelinker, there are two ways to load it. One method is to use the old version's Java layer to call the initialization method under FakeLinker. The second method is to directly use System.loadLibrary(hookModule). It relies on libfakelinker.so, which will automatically load and initialize fakelinker.

Method One: Old Version Java Invocation

Initialization of fakelinker and Hook Module

Essentially, this involves loading libfakelinker.so and the hook module. However, due to the different methods used, the loading process varies. When libfakelinker.so is initialized, it will automatically load the hook module and call back the fakelinker_module_init method. Note: If the initialization of fake-linker fails, it will not load or call back the hook module.

1. Project Self-Usage

   You can directly call FakeLinker.initFakeLinker to load it from the apk.

2. Usage in Xposed Modules

   The LSPosed framework has already handled the native library search paths internally. Therefore, you only need to configure the following settings in the build.gradle file to disable so compression (which is enabled by default when minSdk >= 23), and then call FakeLinker.initFakeLinker to load from the Xposed module APK.

   android {
       packagingOptions {
           jniLibs {
               useLegacyPackaging true
           }
       }
   }

   For non-LSPosed frameworks or lower versions, you need to set the native library search path and then load it. See method 3 for details.

3. Dynamically Setting the Native Library Search Path

   Version v3.1.0+ provides the interfaces FakeLinker.addHookApkNativePath or FakeLinker.addNativeLibraryPath to change the native library search path of the classloader. The default classloader is the one associated with the FakeLinker class. Since the loading call is from FakeLinker, you can keep the default. You can also call it to change the search path of other classloaders. Set the search paths for fakelinker and the hook module, then call FakeLinker.initFakeLinker to initialize.

4. Manually Installing Libraries to a Specified Location

   Install the fake-linker and Hook modules to a path accessible by the application, such as /data/local/tmp, and then load it directly using the absolute path. It is not recommended if you can set the native library search path and then load.

Writing the Hook Module

1. Include the fake_linker.h header file and implement the fakelinker_module_init method. This method needs to be exported; otherwise, fakelinker cannot call it back.

   #include <fake_linker.h>
   
   C_API API_PUBLIC void fakelinker_module_init(JNIEnv *env, SoinfoPtr fake_soinfo, const FakeLinker *fake_linker) {
       // Set global so here, relocate already loaded so, etc.
   }

Method Two: Native Invocation

Since the hook module depends on libfakelinker.so, you can directly call System.loadLibrary("hook-module") to automatically initialize fakelinker. This method does not require any Java code related to FakeLinker since all functionalities are merely wrappers around the internal native FakeLinker structure. This usage differs from Method One.

Initialization of Hook Module and fakelinker

Directly load the hook module:

try {
    System.loadLibrary("hook-module");
} catch (UnsatisfiedLinkError e) {
    // Handle the error
}

At this point, fakelinker is automatically initialized and the native methods under the com/sanfengandroid/fakelinker/FakeLinker class are registered by default. If they do not exist, they will not be registered.

This method no longer requires the fakelinker_module_init function. Instead, you can directly obtain the const FakeLinker* pointer through get_fakelinker and ensure that fake_linker->is_init_success() returns true before using it.

// hook_module.cpp

C_API API_PUBLIC jint JNI_OnLoad(JavaVM *vm, void *reserved) {
    const FakeLinker *fake_linker = get_fakelinker();
    // Obtain the FakeLinker pointer and check if initialization is successful
    JNIEnv *env;
    if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_6) != JNI_OK) {
        async_safe_fatal("JNI environment error");
    }
    // Initialize fake-linker
    init_fakelinker(env, static_cast<FakeLinkerMode>(FakeLinkerMode::kFMSoinfo | FakeLinkerMode::kFMNativeHook | FakeLinkerMode::kFMJavaRegister), nullptr);

    if (fake_linker->is_init_success()) {
        // Get own soinfo
        int error;
        SoinfoPtr thiz = fake_linker->soinfo_find(SoinfoFindType::kSTAddress, nullptr, &error);
        if (thiz) {
            // Similar to Method One, set global SO, relocate already loaded SOs, etc.
            ...
        }

        // Call init_fakelinker to check if native hook initialization is successful
        if (init_fakelinker(env, FakeLinkerMode::kFMNativeHook, nullptr) == 0) {
            LOGI("native hook init success");
        }
        // Register custom Java API
        if (init_fakelinker(env, FakeLinkerMode::kFMJavaRegister, "java/to/your/class") == 0) {
            LOGI("register custom java api success");
        }
    } else {
        // Error handling, cannot use fake_linker related methods
    }
    return JNI_VERSION_1_6;
}

Using the fakelinker Static Library

Starting from version v3.1.0, the fakelinker_static static library is available, which allows you to customize fakelinker or use some of its provided native APIs. Static linking does not include JNI_OnLoad, so you need to initialize it manually. You can call init_fakelinker to initialize the required functionalities as needed. Please ensure that the initialization is successful before using the corresponding functions. The usage is similar to Method Two mentioned above. Common code is as follows:

#include <fake_linker.h>

C_API API_PUBLIC jint JNI_OnLoad(JavaVM *vm, void *reserved) {
    const FakeLinker *fake_linker = get_fakelinker();
    // Obtain the FakeLinker pointer and check if initialization is successful
    JNIEnv *env;
    if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_6) != JNI_OK) {
        async_safe_fatal("JNI environment error");
    }
    if (init_fakelinker(env, static_cast<FakeLinkerMode>(FakeLinkerMode::kFMSoinfo | FakeLinkerMode::kFMNativeHook | FakeLinkerMode::kFMJavaRegister), nullptr) == 0) {
        // Initialization successful
    }
}

Another common use of static library linking is for the fake-linker to act both as the fake-linker framework and the hook module. After initialization, it can be set as the global so to take effect.

Additional Notes

1. Unlike directly setting the LD_PRELOAD environment variable, which typically cannot intercept the dlsym method since intercepting it would require implementing symbol lookup manually (with higher versions also imposing caller address restrictions), this module provides a dlsym call through the intermediary fake-linker module. Thus, the hook module can intercept dlsym and offer more Linker-related functionalities.

Precautions

1. When using frameworks like Xposed to hook system processes, please ensure you have backup and removal measures in place to avoid system process crashes that might prevent the device from booting.
2. Depending on the module loading timing, relocate already loaded modules. Typically, when loading the hook module, some system libraries have already been loaded, such as libjavacore.so, libnativehelper.so, libnativeloader.so, libart.so, libopenjdk.so, etc. To make these libraries effective, you need to manually call methods like FakeLinker.call_manual_relocation_by_names in native code.
3. Set the hook module as a global library so that subsequently loaded so files will automatically take effect.
4. Before using any fake-linker related functionalities, ensure you call init_fakelinker to check if the corresponding functionality is successfully initialized. init_fakelinker can be called multiple times without causing reinitialization.

Usage Examples

1. Relocate already loaded so files to make the hook module effective

   void fakelinker_module_init(JNIEnv *env, SoinfoPtr fake_soinfo, const FakeLinker *fake_linker) {
       const char* loaded_libs[] = {
           "libart.so",
           "libopenjdk.so",
           "libnativehelper.so",
           "libjavacore.so",
       };
       fake_linker->call_manual_relocation_by_names(fake_soinfo, 4, loaded_libs);
   }

2. Set the hook module as a global library

   C_API API_PUBLIC jint JNI_OnLoad(JavaVM *vm, void *reserved) {
       const FakeLinker *fake_linker = get_fakelinker();
       // Obtain the FakeLinker pointer and check if initialization is successful
       JNIEnv *env;
       if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_6) != JNI_OK) {
           async_safe_fatal("JNI environment error");
       }
       if (init_fakelinker(env, static_cast<FakeLinkerMode>(FakeLinkerMode::kFMSoinfo | FakeLinkerMode::kFMNativeHook | FakeLinkerMode::kFMJavaRegister), nullptr) == 0) {
           // Initialization successful
           if (SoinfoPtr thiz = fake_linker->soinfo_find(SoinfoFindType::kSTAddress, nullptr, nullptr)) {
               // Set the hook module as a global library; subsequent `so` loads will trigger the hook
               if (fake_linker->soinfo_add_to_global(thiz)) {
                   LOG("soinfo add to global success");
               }
           }
       }
       return JNI_VERSION_1_6;
   }
   
   // After the necessary hooks have taken effect, you can remove the global so setting.
   // This will not affect already loaded libraries.
   fake_linker->soinfo_remove_global(thiz);

3. For more usage examples, refer to the method pointers in the FakeLinker struct. We promise to maintain API compatibility starting from version v3.1.0.