Android Native库的加载及动态链接的过程

public class MainActivity extends AppCompatActivity {
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  // Example of a call to a native method
  final TextView tv = (TextView) findViewById(R.id.sample_text);
  tv.setOnClickListener(new View.OnClickListener() {
   @Override
   public void onClick(View view) {
    tv.setText(stringFromJNI());
   }
  });
 }
 /**
  * A native method that is implemented by the 'native-lib' native library,
  * which is packaged with this application.
  */
 public native String stringFromJNI();
 // Used to load the 'native-lib' library on application startup.
 // 动态库的装载及链接
 static {
  System.loadLibrary("native-lib");
 }
}

public static void loadLibrary(String libname) {
  这里VMStack.getCallingClassLoader()返回应用的类加载器
  Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), libname);
 }

synchronized void loadLibrary0(ClassLoader loader, String libname) {
  if (libname.indexOf((int)File.separatorChar) != -1) {
   throw new UnsatisfiedLinkError(
 "Directory separator should not appear in library name: " + libname);
  }
  String libraryName = libname;
  if (loader != null) {
   // findLibrary()返回库的全路径名
   String filename = loader.findLibrary(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) + "\"");
   }
   // 装载动态库
   String error = doLoad(filename, loader);
   if (error != null) {
    throw new UnsatisfiedLinkError(error);
   }
   return;
  }
  ......
 }

public String findLibrary(String name) {
  // 调用DexPathList的findLibrary方法
  return pathList.findLibrary(name);
 }

public String findLibrary(String libraryName) {
  // 产生平台相关的库名称这里返回libxxx.so
  String fileName = System.mapLibraryName(libraryName);
  for (Element element : nativeLibraryPathElements) {
   // 查找动态库返回库的全路径名
   String path = element.findNativeLibrary(fileName);

   if (path != null) {
    return path;
   }
  }
  return null;
 }

private String doLoad(String name, ClassLoader loader) {
  ......
  // 获取应用类加载器的Native库搜索路径
  String librarySearchPath = null;
  if (loader != null && loader instanceof BaseDexClassLoader) {
   BaseDexClassLoader dexClassLoader = (BaseDexClassLoader) loader;
   librarySearchPath = dexClassLoader.getLdLibraryPath();
  }
  // nativeLoad should be synchronized so there's only one LD_LIBRARY_PATH in use regardless
  // of how many ClassLoaders are in the system, but dalvik doesn't support synchronized
  // internal natives.
  synchronized (this) {
   return nativeLoad(name, loader, librarySearchPath);
  }
 }

bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
         const std::string& path,
         jobject class_loader,
         jstring library_path,
         std::string* error_msg) {
 ......
 SharedLibrary* library;
 Thread* self = Thread::Current();
 {
 // TODO: move the locking (and more of this logic) into Libraries.
 // 检查动态库是否已经装载过，如果已经装载过(类加载器也匹配)直接返回。
 MutexLock mu(self, *Locks::jni_libraries_lock_);
 library = libraries_- Get(path);
 }
 ......
 // 没有装载过，装载链接动态库
 // 参数patch_str传递的是动态库的全路径，之所以还要传递搜索路径，是因为可能包含它的依赖库
 void* handle = android::OpenNativeLibrary(env,
           runtime_- GetTargetSdkVersion(),
           path_str,
           class_loader,
           library_path);
 ......
 // 查找动态库中的"JNI_OnLoad"函数
 sym = library- FindSymbol("JNI_OnLoad", nullptr);
 if (sym == nullptr) {
 VLOG(jni) << "[No JNI_OnLoad found in \"" << path << "\"]";
 was_successful = true;
 } else {
 // Call JNI_OnLoad. We have to override the current class
 // loader, which will always be "null" since the stuff at the
 // top of the stack is around Runtime.loadLibrary(). (See
 // the comments in the JNI FindClass function.)
 ScopedLocalRef<jobject  old_class_loader(env, env- NewLocalRef(self- GetClassLoaderOverride()));
 self- SetClassLoaderOverride(class_loader);

 VLOG(jni) << "[Calling JNI_OnLoad in \"" << path << "\"]";
 typedef int (*JNI_OnLoadFn)(JavaVM*, void*);
 JNI_OnLoadFn jni_on_load = reinterpret_cast<JNI_OnLoadFn (sym);
 // 调用库的JNI_OnLoad函数注册JNI, 本文暂不讨论
 int version = (*jni_on_load)(this, nullptr);
 ......
 }
 ......
}

void* OpenNativeLibrary(JNIEnv* env,
      int32_t target_sdk_version,
      const char* path,
      jobject class_loader,
      jstring library_path) {
#if defined(__ANDROID__)
 UNUSED(target_sdk_version);
 if (class_loader == nullptr) {
 return dlopen(path, RTLD_NOW);
 }
 std::lock_guard<std::mutex  guard(g_namespaces_mutex);
 // 找到类加载器映射的命名空间(Android应用类加载器创建时创建)
 // 关于命名空间的动态链接请参考http://jackwish.net/namespace-based-dynamic-linking-chn.html
 android_namespace_t* ns = g_namespaces- FindNamespaceByClassLoader(env, class_loader);
 .......
 android_dlextinfo extinfo;
 // 在一个不同的命名空间中装载
 extinfo.flags = ANDROID_DLEXT_USE_NAMESPACE;
 extinfo.library_namespace = ns;
 // RILD_NOW表示重定位在dlopen返回前完成，不会延迟到第一次执行(RTLD_LAZY)
 return android_dlopen_ext(path, RTLD_NOW, &extinfo);
 ......
}

void* android_dlopen_ext(const char* filename, int flags, const android_dlextinfo* extinfo) {
 // __builtin_return_address是编译器的内建函数，__builtin_return_address(0)表示当前函数的返回地址
 void* caller_addr = __builtin_return_address(0);
 return dlopen_ext(filename, flags, extinfo, caller_addr);
}

void* do_dlopen(const char* name, int flags, const android_dlextinfo* extinfo,
     void* caller_addr) {
 // caller_addr在libnativeloader.so中
 // 查找地址所在的动态库(采用遍历查找，可以优化查找)
 soinfo* const caller = find_containing_library(caller_addr);
 // ns为调用库所在命名空间
 android_namespace_t* ns = get_caller_namespace(caller);
 ......
 if (extinfo != nullptr) {
 ......
 // extinfo- flags为ANDROID_DLEXT_USE_NAMESPACE
 if ((extinfo- flags & ANDROID_DLEXT_USE_NAMESPACE) != 0) {
  if (extinfo- library_namespace == nullptr) {
  DL_ERR("ANDROID_DLEXT_USE_NAMESPACE is set but extinfo- library_namespace is null");
  return nullptr;
  }
  // 命名空间使用应用自身类加载器-命名空间
  ns = extinfo- library_namespace;
 }
 }
 ......
 // 在命名空间ns中装载库
 soinfo* si = find_library(ns, translated_name, flags, extinfo, caller);
 ......
}

static bool find_libraries(android_namespace_t* ns,
soinfo* start_with,
const char* const library_names[],
size_t library_names_count, soinfo* soinfos[],
std::vector<soinfo* * ld_preloads,
size_t ld_preloads_count, int rtld_flags,
const android_dlextinfo* extinfo,
bool add_as_children) {
// ns为应用类加载器-命名空间
// 这里start_with为libnativeloader.so的soinfo
// library_names为需要装载的动态库，不包含依赖库
// library_names_count需要装载的动态库的数量，这里为1
// ld_preloads为nullptr
// add_as_children为false
......
// 为需要装载的动态库创建LoadTask添加到load_tasks
// LoadTask用于管理动态库的装载
for (size_t i = 0; i < library_names_count; ++i) {
const char* name = library_names[i];
load_tasks.push_back(LoadTask::create(name, start_with, &readers_map));
}
// Construct global_group.
// 收集命名空间ns中设置了DF_1_GLOBAL(RTLD_GLOBAL:共享库中的符号可被后续装载的库重定位)标志的动态库
soinfo_list_t global_group = make_global_group(ns);
......
// Step 1: expand the list of load_tasks to include
// all DT_NEEDED libraries (do not load them just yet)
// load_tasks以广度优先遍历的顺序存储动态库依赖树
// 例如依赖树: 1
//   / \
//   2 3
//   /  \
//  4  5
// load_tasks: 1- 2- 3- 4- 5
for (size_t i = 0; i<load_tasks.size(); ++i) {
LoadTask* task = load_tasks[i];
soinfo* needed_by = task- get_needed_by();
bool is_dt_needed = needed_by != nullptr && (needed_by != start_with || add_as_children);
task- set_extinfo(is_dt_needed ? nullptr : extinfo);
task- set_dt_needed(is_dt_needed);
// 收集动态库的信息以及它的依赖库
if(!find_library_internal(ns, task, &zip_archive_cache, &load_tasks, rtld_flags)) {
return false;
}
soinfo* si = task- get_soinfo();
if (is_dt_needed) {
// si添加到needed_by的依赖中
needed_by- add_child(si);
}
if (si- is_linked()) {
// 已经链接过的库增加引用计数
si- increment_ref_count();
}
......
if (soinfos_count < library_names_count) {
soinfos[soinfos_count++] = si;
}
}
// Step 2: Load libraries in random order (see b/24047022)
LoadTaskList load_list;
// 需要装载的库放到load_list中
for (auto&& task : load_tasks) {
soinfo* si = task- get_soinfo();
auto pred = [&](const LoadTask* t) {
return t- get_soinfo() == si;
};
if (!si- is_linked() &&
std::find_if(load_list.begin(), load_list.end(), pred) == load_list.end() ) {
load_list.push_back(task);
}
}
// 随机化load_list中库的顺序
shuffle(&load_list);
for (auto&& task : load_list) {
// 装载动态库
if (!task- load()) {
return false;
}
}
// Step 3: pre-link all DT_NEEDED libraries in breadth first order.
// 预链接load_tasks中没有链接过的库，见下文
for (auto&& task : load_tasks) {
soinfo* si = task- get_soinfo();
if (!si- is_linked() && !si- prelink_image()) {
return false;
}
}
// Step 4: Add LD_PRELOADed libraries to the global group for
// future runs. There is no need to explicitly add them to
// the global group for this run because they are going to
// appear in the local group in the correct order.
if (ld_preloads != nullptr) {
for (auto&& si : *ld_preloads) {
si- set_dt_flags_1(si- get_dt_flags_1() | DF_1_GLOBAL);
}
}
// Step 5: link libraries.
// 链接过程，见下文
soinfo_list_t local_group;
// 广度优先遍历添加动态库依赖图到local_group中
walk_dependencies_tree(
(start_with != nullptr && add_as_children) ? &start_with : soinfos,
(start_with != nullptr && add_as_children) ? 1 : soinfos_count,
[&] (soinfo* si) {
local_group.push_back(si);
return true;
});
// We need to increment ref_count in case
// the root of the local group was not linked.
bool was_local_group_root_linked = local_group.front()- is_linked();
bool linked = local_group.visit([&](soinfo* si) {
if (!si- is_linked()) {
if (!si- link_image(global_group, local_group, extinfo)) {
return false;
}
}
return true;
});
if (linked) {
// 设置链接标志
local_group.for_each([](soinfo* si) {
if (!si- is_linked()) {
si- set_linked();
}
});
failure_guard.disable();
}
......
}

static bool find_library_internal(android_namespace_t* ns,
LoadTask* task,
ZipArchiveCache* zip_archive_cache,
LoadTaskList* load_tasks,
int rtld_flags) {
soinfo* candidate;
// 在应用类加载器-命名空间中查找动态库是否已经装载过
if (find_loaded_library_by_soname(ns, task- get_name(), &candidate)) {
task- set_soinfo(candidate);
return true;
}
// 在默认命名空间中查找动态库是否已经装载过
if (ns != &g_default_namespace) {
// check public namespace
candidate = g_public_namespace.find_if([&](soinfo* si) {
return strcmp(task- get_name(), si- get_soname()) == 0;
});
......
}
......
// 装载库
if (load_library(ns, task, zip_archive_cache, load_tasks, rtld_flags)) {
return true;
} else {
// In case we were unable to load the library but there
// is a candidate loaded under the same soname but different
// sdk level - return it anyways.
if (candidate != nullptr) {
task- set_soinfo(candidate);
return true;
}
}
return false;
}

static bool load_library(android_namespace_t* ns,
LoadTask* task,
ZipArchiveCache* zip_archive_cache,
LoadTaskList* load_tasks,
int rtld_flags) {
......
// 打开库文件返回文件描述符
int fd = open_library(ns, zip_archive_cache, name, needed_by, &file_offset, &realpath);
if (fd == -1) {
DL_ERR("library \"%s\" not found", name);
return false;
}
task- set_fd(fd, true);
task- set_file_offset(file_offset);
// 装载库
return load_library(ns, task, load_tasks, rtld_flags, realpath);
}

static bool load_library(android_namespace_t* ns,
LoadTask* task,
LoadTaskList* load_tasks,
int rtld_flags,
const std::string& realpath) {
off64_t file_offset = task- get_file_offset();
const char* name = task- get_name();
const android_dlextinfo* extinfo = task- get_extinfo();
......
// 为动态库创建soinfo，用于记录动态链接信息等
soinfo* si = soinfo_alloc(ns, realpath.c_str(), &file_stat, file_offset, rtld_flags);
if (si == nullptr) {
return false;
}
task- set_soinfo(si);
// Read the ELF header and some of the segments.
// 读取ELF文件头以及一些段信息
if (!task- read(realpath.c_str(), file_stat.st_size)) {
soinfo_free(si);
task- set_soinfo(nullptr);
return false;
}
......
// 查找依赖库，创建LoadTask添加到load_tasks
for_each_dt_needed(task- get_elf_reader(), [&](const char* name) {
load_tasks- push_back(LoadTask::create(name, si, task- get_readers_map()));
});
return true;
}

bool ElfReader::Read(const char* name, int fd, off64_t file_offset, off64_t file_size) {
CHECK(!did_read_);
CHECK(!did_load_);
name_ = name;
fd_ = fd;
file_offset_ = file_offset;
file_size_ = file_size;
if (ReadElfHeader() &&
VerifyElfHeader() &&
ReadProgramHeaders() &&
ReadSectionHeaders() &&
ReadDynamicSection()) {
did_read_ = true;
}
return did_read_;
}
ReadElfHeader() : 读取ELF文件头信息
VerifyElfHeader() : 校验ELF(文件类型等)
ReadProgramHeaders() : 根据ELF文件头信息获取程序头表
ReadSectionHeaders() : 根据ELF文件头信息获取段头表
ReadDynamicSection() : 获取Dynamic Section的信息
装载动态库
动态库的装载在LoadTask的 load() 中实现。
bool load() {
ElfReader& elf_reader = get_elf_reader();
// 映射动态库的可加载Segment到进程的虚拟地址空间中
if (!elf_reader.Load(extinfo_)) {
return false;
}
// 保存装载信息
// 动态库装载的起始地址
si_- base = elf_reader.load_start();
// 可装载的Segment大小之和
si_- size = elf_reader.load_size();
si_- set_mapped_by_caller(elf_reader.is_mapped_by_caller());
// 动态库装载的期望起始地址，通常si_- load_bias = si_- base
si_- load_bias = elf_reader.load_bias();
// 动态库程序头表项数
si_- phnum = elf_reader.phdr_count();
// 动态库程序头表的地址
si_- phdr = elf_reader.loaded_phdr();
return true;
}

bool ElfReader::Load(const android_dlextinfo* extinfo) {
CHECK(did_read_);
CHECK(!did_load_);
if (ReserveAddressSpace(extinfo) &&
LoadSegments() &&
FindPhdr()) {
did_load_ = true;
}
return did_load_;
}

bool soinfo::prelink_image() {
/* Extract dynamic section */
ElfW(Word) dynamic_flags = 0;
// 根据程序头表的地址计算dynamic section的地址
phdr_table_get_dynamic_section(phdr, phnum, load_bias, &dynamic, &dynamic_flags);
......
uint32_t needed_count = 0;
// 解析dynamic section获取动态链接信息
for (ElfW(Dyn)* d = dynamic; d- d_tag != DT_NULL; ++d) {
DEBUG("d = %p, d[0](tag) = %p d[1](val) = %p",
d, reinterpret_cast<void* (d- d_tag), reinterpret_cast<void* (d- d_un.d_val));
switch (d- d_tag) {
......
case DT_STRTAB:
// 动态字符串表的地址
strtab_ = reinterpret_cast<const char* (load_bias + d- d_un.d_ptr);
break;
case DT_STRSZ:
strtab_size_ = d- d_un.d_val;
break;
case DT_SYMTAB:
// 动态符号表的地址
symtab_ = reinterpret_cast<ElfW(Sym)* (load_bias + d- d_un.d_ptr);
break;
......
case DT_JMPREL:
// 需重定位的函数表(.rela.plt)的地址
#if defined(USE_RELA)
plt_rela_ = reinterpret_cast<ElfW(Rela)* (load_bias + d- d_un.d_ptr);
#else
plt_rel_ = reinterpret_cast<ElfW(Rel)* (load_bias + d- d_un.d_ptr);
#endif
break;
......
case DT_RELA:
// 需重定位的数据表(.rela.dyn)的地址
rela_ = reinterpret_cast<ElfW(Rela)* (load_bias + d- d_un.d_ptr);
break;
......
case DT_NEEDED:
// 依赖的动态库
++needed_count;
break;
}
}
......
// Sanity checks.
// 检查动态链接信息
if (relocating_linker && needed_count != 0) {
DL_ERR("linker cannot have DT_NEEDED dependencies on other libraries");
return false;
}
if (nbucket_ == 0 && gnu_nbucket_ == 0) {
DL_ERR("empty/missing DT_HASH/DT_GNU_HASH in \"%s\" "
"(new hash type from the future?)", get_realpath());
return false;
}
if (strtab_ == 0) {
DL_ERR("empty/missing DT_STRTAB in \"%s\"", get_realpath());
return false;
}
if (symtab_ == 0) {
DL_ERR("empty/missing DT_SYMTAB in \"%s\"", get_realpath());
return false;
}
......
}

bool soinfo::link_image(const soinfo_list_t& global_group, const soinfo_list_t& local_group,
const android_dlextinfo* extinfo) {
......
#if defined(USE_RELA)
// rela_为重定位数据表的地址
if (rela_ != nullptr) {
DEBUG("[ relocating %s ]", get_realpath());
// 数据引用重定位
if (!relocate(version_tracker,
plain_reloc_iterator(rela_, rela_count_), global_group, local_group)) {
return false;
}
}
// plt_rela_为重定位函数表的地址
if (plt_rela_ != nullptr) {
DEBUG("[ relocating %s plt ]", get_realpath());
// 函数引用重定位
if (!relocate(version_tracker,
plain_reloc_iterator(plt_rela_, plt_rela_count_), global_group, local_group)) {
return false;
}
}
#else
......
}

template<typename ElfRelIteratorT 
bool soinfo::relocate(const VersionTracker& version_tracker, ElfRelIteratorT&& rel_iterator,
const soinfo_list_t& global_group, const soinfo_list_t& local_group) {
for (size_t idx = 0; rel_iterator.has_next(); ++idx) {
const auto rel = rel_iterator.next();
if (rel == nullptr) {
return false;
}
// rel- r_info的低32位
ElfW(Word) type = ELFW(R_TYPE)(rel- r_info);
// rel- r_info的高32位
ElfW(Word) sym = ELFW(R_SYM)(rel- r_info);
// 重定位地址的存储位置
ElfW(Addr) reloc = static_cast<ElfW(Addr) (rel- r_offset + load_bias);
ElfW(Addr) sym_addr = 0;
const char* sym_name = nullptr;
ElfW(Addr) addend = get_addend(rel, reloc);
......
if (sym != 0) {
// sym为动态符号表项的索引
// symtab_[sym].st_name为符号在动态字符串表的索引
// sysm_name为需重定位的符号名
sym_name = get_string(symtab_[sym].st_name);
const version_info* vi = nullptr;
if (!lookup_version_info(version_tracker, sym, sym_name, &vi)) {
return false;
}
// 查找符号返回符号表项的地址
if (!soinfo_do_lookup(this, sym_name, vi, &lsi, global_group, local_group, &s)) {
return false;
}
if (s == nullptr) {
......
} else {
......
// 根据符号表项计算符号地址
sym_addr = lsi- resolve_symbol_address(s);
......
}
......
}
switch (type) {
// ELF64中R_GENERIC_JUMP_SLOT = R_AARCH64_JUMP_SLOT
case R_GENERIC_JUMP_SLOT:
count_relocation(kRelocAbsolute);
MARK(rel- r_offset);
TRACE_TYPE(RELO, "RELO JMP_SLOT %16p <- %16p %s\n",
reinterpret_cast<void* (reloc),
reinterpret_cast<void* (sym_addr + addend), sym_name);
// 符号地址更新到reloc(GOT表)中
*reinterpret_cast<ElfW(Addr)* (reloc) = (sym_addr + addend);
break;
......
}
}
return true;
}