什么共享库包含android_init_namespaces？

Question

当我为android共享库编写自己的动态加载器时，我遇到了这样的问题:在库libnativeloader.so中有这样的plt条目android_init_namespaces@plt，但没有符号android_init_namespaces。我在仿生学中找到了这样的代码：

LIBC {
  global:
    android_dlopen_ext;
    dl_iterate_phdr;
    dladdr;
    dlclose;
    dlerror;
    dlopen;
    dlsym;
  local:
    *;
};

LIBC_N {
  global:
    android_init_namespaces;
    android_create_namespace;
    dlvsym;
} LIBC;

LIBC_PLATFORM {
 global:
    android_get_application_target_sdk_version;
    android_set_application_target_sdk_version;
    android_get_LD_LIBRARY_PATH;
    android_update_LD_LIBRARY_PATH;
} LIBC_N;

在这种情况下，这个符号似乎是在libc.so中。但是在libc.so中没有这样的符号。有人可以帮助解决这个问题吗？

Answer 1

我猜你说的是Android N。下面的一切都是基于这个假设。

这些符号不是来自任何共享库，它们是在链接器中实现的，并通过源代码中手工制作的soinfo导出。

static ElfW(Sym) g_libdl_symtab[] = {
  // Total length of libdl_info.strtab, including trailing 0.
  // This is actually the STH_UNDEF entry. Technically, it's
  // supposed to have st_name == 0, but instead, it points to an index
  // in the strtab with a \0 to make iterating through the symtab easier.
  ELFW(SYM_INITIALIZER)(sizeof(ANDROID_LIBDL_STRTAB) - 1, nullptr, 0),
  ELFW(SYM_INITIALIZER)(  0, &dlopen, 1),
  ELFW(SYM_INITIALIZER)(  7, &dlclose, 1),
  ELFW(SYM_INITIALIZER)( 15, &dlsym, 1),
  ELFW(SYM_INITIALIZER)( 21, &dlerror, 1),
  ELFW(SYM_INITIALIZER)( 29, &dladdr, 1),
  ELFW(SYM_INITIALIZER)( 36, &android_update_LD_LIBRARY_PATH, 1),
  ELFW(SYM_INITIALIZER)( 67, &android_get_LD_LIBRARY_PATH, 1),
  ELFW(SYM_INITIALIZER)( 95, &dl_iterate_phdr, 1),
  ELFW(SYM_INITIALIZER)(111, &android_dlopen_ext, 1),
  ELFW(SYM_INITIALIZER)(130, &android_set_application_target_sdk_version, 1),
  ELFW(SYM_INITIALIZER)(173, &android_get_application_target_sdk_version, 1),
  ELFW(SYM_INITIALIZER)(216, &android_init_namespaces, 1),
  ELFW(SYM_INITIALIZER)(240, &android_create_namespace, 1),
  ELFW(SYM_INITIALIZER)(265, &dlvsym, 1),
  ELFW(SYM_INITIALIZER)(272, &android_dlwarning, 1),
#if defined(__arm__)
  ELFW(SYM_INITIALIZER)(290, &dl_unwind_find_exidx, 1),
#endif
};

// This is used by the dynamic linker. Every process gets these symbols for free.
soinfo* get_libdl_info() {
  if (__libdl_info == nullptr) {
    __libdl_info = new (__libdl_info_buf) soinfo(&g_default_namespace, "libdl.so", nullptr, 0, RTLD_GLOBAL);
    __libdl_info->flags_ |= FLAG_LINKED;
    __libdl_info->strtab_ = ANDROID_LIBDL_STRTAB;
    __libdl_info->symtab_ = g_libdl_symtab;
    __libdl_info->nbucket_ = sizeof(g_libdl_buckets)/sizeof(unsigned);
    __libdl_info->nchain_ = sizeof(g_libdl_chains)/sizeof(unsigned);
    __libdl_info->bucket_ = g_libdl_buckets;
    __libdl_info->chain_ = g_libdl_chains;
    __libdl_info->ref_count_ = 1;
    __libdl_info->strtab_size_ = sizeof(ANDROID_LIBDL_STRTAB);
    __libdl_info->local_group_root_ = __libdl_info;
    __libdl_info->soname_ = "libdl.so";
    __libdl_info->target_sdk_version_ = __ANDROID_API__;
    __libdl_info->generate_handle();
#if defined(__work_around_b_24465209__)
    strlcpy(__libdl_info->old_name_, __libdl_info->soname_, sizeof(__libdl_info->old_name_));
#endif
  }

有关更多详细信息，请查看https://android.googlesource.com/platform/bionic/+/android-7.0.0_r1/linker/dlfcn.cpp。