Android 线上性能监控一 基础一 Android 动态链接

bionic/linker/linker_main.cpp
bionic/linker/linker_phdr.cpp
bionic/linker/linker_soinfo.cpp
bionic/linker/linker.cpp

#if defined(__LP64__)
static const char* const kSystemLibDir        = "/system/lib64";
static const char* const kOdmLibDir           = "/odm/lib64";
static const char* const kVendorLibDir        = "/vendor/lib64";
static const char* const kAsanSystemLibDir    = "/data/asan/system/lib64";
static const char* const kAsanOdmLibDir       = "/data/asan/odm/lib64";
static const char* const kAsanVendorLibDir    = "/data/asan/vendor/lib64";
static const char* const kRuntimeApexLibDir   = "/apex/com.android.runtime/lib64";
#else
static const char* const kSystemLibDir        = "/system/lib";
static const char* const kOdmLibDir           = "/odm/lib";
static const char* const kVendorLibDir        = "/vendor/lib";
static const char* const kAsanSystemLibDir    = "/data/asan/system/lib";
static const char* const kAsanOdmLibDir       = "/data/asan/odm/lib";
static const char* const kAsanVendorLibDir    = "/data/asan/vendor/lib";
static const char* const kRuntimeApexLibDir   = "/apex/com.android.runtime/lib";
#endif

static const char* const kAsanLibDirPrefix = "/data/asan";

static const char* const kDefaultLdPaths[] = {
  kSystemLibDir,
  kOdmLibDir,
  kVendorLibDir,
  nullptr
};

bool ElfReader::Read(const char* name, int fd, off64_t file_offset, off64_t file_size) {
  if (did_read_) {
    return true;
  }
  name_ = name;
  fd_ = fd;
  file_offset_ = file_offset;
  file_size_ = file_size;

  if (ReadElfHeader() &&
      VerifyElfHeader() &&
      ReadProgramHeaders() &&
      ReadSectionHeaders() &&
      ReadDynamicSection()) {
    did_read_ = true;
  }

  return did_read_;
}

bool ElfReader::ReadSectionHeaders() {
  shdr_num_ = header_.e_shnum;

  ...

  shdr_table_ = static_cast<const ElfW(Shdr)*>(shdr_fragment_.data());
  return true;
}

bool ElfReader::ReadDynamicSection() {
  // 1. Find .dynamic section (in section headers)
  const ElfW(Shdr)* dynamic_shdr = nullptr;
  for (size_t i = 0; i < shdr_num_; ++i) {
	//读取 .dynamic 表项。
    if (shdr_table_[i].sh_type == SHT_DYNAMIC) {
      dynamic_shdr = &shdr_table_ [i];
      break;
    }
  }
	
  ...

  dynamic_ = static_cast<const ElfW(Dyn)*>(dynamic_fragment_.data());

  ...
  return true;
}

bool soinfo::prelink_image() {
  /* Extract dynamic section */
  ElfW(Word) dynamic_flags = 0;
  phdr_table_get_dynamic_section(phdr, phnum, load_bias, &dynamic, &dynamic_flags);

  for (ElfW(Dyn)* d = dynamic; d->d_tag != DT_NULL; ++d) {
    switch (d->d_tag) {
      ...

      case DT_JMPREL:
#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;

      ...
#if defined(USE_RELA)
      case DT_RELA:
        rela_ = reinterpret_cast<ElfW(Rela)*>(load_bias + d->d_un.d_ptr);
        break;

      ...

      case DT_NEEDED:
        ++needed_count;
        break;
	   ...
      break;
    }
  }

  ...
  return true;
}

plt_rela_ = reinterpret_cast<ElfW(Rela)*>(load_bias + d->d_un.d_ptr);

if (plt_rela_ != nullptr) {
	DEBUG("[ relocating %s plt rela ]", get_realpath());
	if (!relocate(version_tracker,
		plain_reloc_iterator(plt_rela_, plt_rela_count_), global_group, 			local_group)) {
		return false;
	}
}

bool soinfo::relocate(const VersionTracker& version_tracker, ElfRelIteratorT&& rel_iterator,
                      const soinfo_list_t& global_group, const soinfo_list_t& local_group) {
  const size_t tls_tp_base = __libc_shared_globals()->static_tls_layout.offset_thread_pointer();
  std::vector<std::pair<TlsDescriptor*, size_t>> deferred_tlsdesc_relocs;

  for (size_t idx = 0; rel_iterator.has_next(); ++idx) {
    const auto rel = rel_iterator.next();
    if (rel == nullptr) {
      return false;
    }

	//解析出 .rela.plt 当前项的 type 和 sym
    ElfW(Word) type = ELFW(R_TYPE)(rel->r_info);
    ElfW(Word) sym = ELFW(R_SYM)(rel->r_info);
	//计算出当前 rela.plt 表项地址
	//.rela.plt 表项中记录着 .got 表项的地址
    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);

    DEBUG("Processing \"%s\" relocation at index %zd", get_realpath(), idx);
    if (type == R_GENERIC_NONE) {
      continue;
    }

    const ElfW(Sym)* s = nullptr;
    soinfo* lsi = nullptr;

    if (sym == 0) {
      if (is_tls_reloc(type)) {
        lsi = this;
      }
    } else if (ELF_ST_BIND(symtab_[sym].st_info) == STB_LOCAL && is_tls_reloc(type)) {
      s = &symtab_[sym];
      sym_name = get_string(s->st_name);
      DL_ERR("unexpected TLS reference to local symbol \"%s\": "
             "sym type %d, rel type %u (idx %zu of \"%s\")",
             sym_name, ELF_ST_TYPE(s->st_info), type, idx, get_realpath());
      return false;
    } else {
	  //在符号表中找出特定符号
      sym_name = get_string(symtab_[sym].st_name);
	  
	  // 根据 sym_name 找出包含当前符号的库和在其中的地址
	  if (!soinfo_do_lookup(this, sym_name, vi, &lsi, global_group, local_group, &s)) {
        return false;
      }

      ...

	  // 在包含当前符号的库中解析出符号地址
	  // Android 不支持延迟绑定因此这个函数直接返回地址
	  sym_addr = lsi->resolve_symbol_address(s);
	  ...
    }
	
	switch (type) {
      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);
		//将当前符号的真实地址写到 .got 表项中
        *reinterpret_cast<ElfW(Addr)*>(reloc) = (sym_addr + addend);
        break;
	...
  return true;
}

Relocation section '.rela.plt' at offset 0x458 contains 4 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000001fd8  000300000402 R_AARCH64_JUMP_SL 0000000000000000 printf@LIBC + 0
000000001fe0  000400000402 R_AARCH64_JUMP_SL 0000000000000000 __cxa_finalize@LIBC + 0
000000001fe8  000500000402 R_AARCH64_JUMP_SL 0000000000000000 __register_atfork@LIBC + 0
000000001ff0  000600000402 R_AARCH64_JUMP_SL 0000000000000000 __cxa_atexit@LIBC + 0

The decoding of unwind sections for machine type AArch64 is not currently supported.

Symbol table '.dynsym' contains 15 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000520     0 SECTION LOCAL  DEFAULT    9 
     2: 0000000000001e08     0 SECTION LOCAL  DEFAULT   13 
     3: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@LIBC (2)
     4: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __cxa_finalize@LIBC (2)
     5: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __register_atfork@LIBC (2)
     6: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __cxa_atexit@LIBC (2)
     7: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS _bss_end__
     8: 0000000000000568    28 FUNC    GLOBAL DEFAULT    9 say_hello
     9: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    10: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    11: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    12: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_end__
    13: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS __end__
    14: 0000000000002000     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start__

//aarch64-linux-android-objdump -d libtest.so
0000000000000568 <say_hello>:
 568:	a9bf7bfd 	stp	x29, x30, [sp,#-16]!
 56c:	910003fd 	mov	x29, sp
 570:	90000000 	adrp	x0, 0 <printf@plt-0x4e0>
 574:	91161000 	add	x0, x0, #0x584
 578:	97ffffda 	bl	4e0 <printf@plt>
 57c:	a8c17bfd 	ldp	x29, x30, [sp],#16
 580:	d65f03c0 	ret