so加载Linker跟NameSpace机制详解

System.loadLibrary(xxxso);

linker_namespaces.h 
struct android_namespace_link_t 
private:
        std::string name_; namespace名称
        bool is_isolated_; 是否隔离（大部分是true)
        std::vector<std::string> ld_library_paths_; 链接路径
        std::vector<std::string> default_library_paths_;默认可访问路径
        std::vector<std::string> permitted_paths_;已允许访问路径
        ....

private synchronized void loadLibrary0(ClassLoader loader, Class<?> callerClass, String libname) {
    文件名校验
    if (libname.indexOf((int)File.separatorChar) != -1) {
        throw new UnsatisfiedLinkError(
"Directory separator should not appear in library name: " + libname);
    }
    String libraryName = libname;
    // Android-note: BootClassLoader doesn't implement findLibrary(). Http://b/111850480
    // Android's class.getClassLoader() can return BootClassLoader where the RI would
    // have returned null; therefore we treat BootClassLoader the same as null here.
    if (loader != null && !(loader instanceof BootClassLoader)) {
        String filename = loader.findLibrary(libraryName);
        if (filename == null &&
                (loader.getClass() == PathClassLoader.class ||
                 loader.getClass() == DelegateLastClassLoader.class)) {
            // Don't give up even if we failed to find the library in the native lib paths.
            // The underlying dynamic linker might be able to find the lib in one of the linker
            // namespaces associated with the current linker namespace. In order to give the
            // dynamic linker a chance, proceed to load the library with its soname, which
            // is the fileName.
            // Note that we do this only for PathClassLoader  and DelegateLastClassLoader to
            // minimize the scope of this behavioral change as much as possible, which might
            // cause problem like b/143649498. These two class loaders are the only
            // platfORM-provided class loaders that can load apps. See the classLoader attribute
            // of the application tag in app manifest.
            filename = System.mapLibraryName(libraryName);
        }
        if (filename == null) {
            // It's not necessarily true that the ClassLoader used
            // System.mapLibraryName, but the default setup does, and it's
            // misleading to say we didn't find "libMyLibrary.so" when we
            // actually searched for "liblibMyLibrary.so.so".
            throw new UnsatisfiedLinkError(loader + " couldn't find "" +
                                           System.mapLibraryName(libraryName) + """);
        }
        String error = nativeLoad(filename, loader);
        if (error != null) {
            throw new UnsatisfiedLinkError(error);
        }
        return;
    }
    // We know some apps use mLibPaths directly, potentially assuming it's not null.
    // Initialize it here to make sure apps see a non-null value.
    getLibPaths();
    String filename = System.mapLibraryName(libraryName);
    //最终调用nativeLoad
    String error = nativeLoad(filename, loader, callerClass);
    if (error != null) {
        throw new UnsatisfiedLinkError(error);
    }
}

LoadNativeLibrary 非常长，我们截取部分
bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
                                  const std::string& path,
jobject class_loader,
        jclass caller_class,
std::string* error_msg) {
会判断是否已经加载过当前so，同时也要加锁，因为存在多线程加载的情况
SharedLibrary* library;
Thread* self = Thread::Current();
{
// TODO: move the locking (and more of this logic) into Libraries.
MutexLock mu(self, *Locks::jni_libraries_lock_);
library = libraries_->Get(path);
}
调用OpenNativeLibrary加载
void* handle = android::OpenNativeLibrary(
        env,
        runtime_->GetTargetSdkVersion(),
        path_str,
        class_loader,
        (caller_location.empty() ? nullptr : caller_location.c_str()),
        library_path.get(),
        &needs_native_bridge,
        &nativeloader_error_msg);

void* OpenNativeLibrary(JNIEnv* env, int32_t target_sdk_version, const char* path,
jobject class_loader, const char* caller_location, jstring library_path,
bool* needs_native_bridge, char** error_msg) {
    #if defined(ART_TARGET_ANDROID)
    UNUSED(target_sdk_version);
    if (class_loader == nullptr) {
        *needs_native_bridge = false;
        if (caller_location != nullptr) {
            android_namespace_t* boot_namespace = FindExportedNamespace(caller_location);
            if (boot_namespace != nullptr) {
                const android_dlextinfo dlextinfo = {
                    .flags = ANDROID_DLEXT_USE_NAMESPACE,
                    .library_namespace = boot_namespace,
                };
                //最终调用android_dlopen_ext打开
                void* handle = android_dlopen_ext(path, RTLD_NOW, &dlextinfo);
                if (handle == nullptr) {
                    *error_msg = strdup(dlerror());
                }
                return handle;
            }
        }
        // Check if the library is in NATIVELOADER_DEFAULT_NAMESPACE_LIBS and should
        // be loaded from the kNativeloaderExtraLibs namespace.
        {
            Result<void*> handle = TryLoadNativeloaderExtraLib(path);
            if (!handle.ok()) {
                *error_msg = strdup(handle.error().message().c_str());
                return nullptr;
            }
            if (handle.value() != nullptr) {
                return handle.value();
            }
        }
        // Fall back to the system namespace. This happens for preloaded JNI
        // libraries in the zyGote.
        // TODO(b/185833744): Investigate if this should fall back to the app main
        // namespace (aka anonymous namespace) instead.
        void* handle = OpenSystemLibrary(path, RTLD_NOW);
        if (handle == nullptr) {
            *error_msg = strdup(dlerror());
        }
        return handle;
    }
    std::lock_guard<std::mutex> guard(g_namespaces_mutex);
    NativeLoaderNamespace* ns;
    //涉及到了namespace，如果当前classloader没有，则创建，但是这属于异常情况
    if ((ns = g_namespaces->FindNamespaceByClassLoader(env, class_loader)) == nullptr) {
        // This is the case where the classloader was not created by ApplicationLoaders
        // In this case we create an isolated not-shared namespace for it.
        Result<NativeLoaderNamespace*> isolated_ns =
        CreateClassLoaderNamespaceLocked(env,
            target_sdk_version,
            class_loader,
            false,
            nullptr,
            library_path,
            nullptr,
            nullptr);
        if (!isolated_ns.ok()) {
            *error_msg = strdup(isolated_ns.error().message().c_str());
            return nullptr;
        } else {
            ns = *isolated_ns;
        }
    }
    return OpenNativeLibraryInNamespace(ns, path, needs_native_bridge, error_msg);

frameworks/base/core/java/android/app/LoadedApk.java 
if (!mIncludeCode) {
    if (mDefaultClassLoader == null) {
        StrictMode.ThreadPolicy oldPolicy = allowThreadDiskReads();
        mDefaultClassLoader = ApplicationLoaders.getDefault().getClassLoader(
            "" , mApplicationInfo.targetSdkVersion, isBundledApp,
            librarySearchPath, libraryPermittedPath, mBaseClassLoader,
            null );
        setThreadPolicy(oldPolicy);
        mAppComponentFactory = AppComponentFactory.DEFAULT;
    }
    if (mClassLoader == null) {
        mClassLoader = mAppComponentFactory.instantiateClassLoader(mDefaultClassLoader,
            new ApplicationInfo(mApplicationInfo));
    }
    return;
}

public static ClassLoader createClassLoader(String dexPath,
                                            String librarySearchPath, String libraryPermittedPath, ClassLoader parent,
                                            int targetSdkVersion, boolean isNamespaceShared, String classLoaderName,
                                            List<ClassLoader> sharedLibraries, List<String> nativeSharedLibraries,
                                            List<ClassLoader> sharedLibrariesAfter) {
    final ClassLoader classLoader = createClassLoader(dexPath, librarySearchPath, parent,
            classLoaderName, sharedLibraries, sharedLibrariesAfter);
    String sonameList = "";
    if (nativeSharedLibraries != null) {
        sonameList = String.join(":", nativeSharedLibraries);
    }
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "createClassloaderNamespace");
    //这里就讲上述的属性传入，创建了一个属于该classloader的namespace
    String errorMessage = createClassloaderNamespace(classLoader,
            targetSdkVersion,
            librarySearchPath,
            libraryPermittedPath,
            isNamespaceShared,
            dexPath,
            sonameList);
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    if (errorMessage != null) {
        throw new UnsatisfiedLinkError("Unable to create namespace for the classloader " +
                classLoader + ": " + errorMessage);
    }
    return classLoader;
}

void* android_dlopen_ext(const char* filename, int flag, const android_dlextinfo* extinfo) {
        const void* caller_addr = __builtin_return_address(0);
        return __loader_android_dlopen_ext(filename, flag, extinfo, caller_addr);
        }

void* do_dlopen(const char* name, int flags,
        const android_dlextinfo* extinfo,
        const void* caller_addr) {
        std::string trace_prefix = std::string("dlopen: ") + (name == nullptr ? "(nullptr)" : name);
        ScopedTrace trace(trace_prefix.c_str());
        ScopedTrace loading_trace((trace_prefix + " - loading and linking").c_str());
        soinfo* const caller = find_containing_library(caller_addr);
        // 找到调用者，属于哪个namespace
        android_namespace_t* ns = get_caller_namespace(caller);
        ... 
        ProtectedDataGuard guard;
        之后就是在namespace的加载列表找library的过程了
        soinfo* si = find_library(ns, translated_name, flags, extinfo, caller);
       ....
        return nullptr;
        }