runtime/oat_file.cc

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "oat_file.h"

#include <dlfcn.h>
#include <string.h>
#include <type_traits>
#include <unistd.h>

#include <cstdlib>
#ifndef __APPLE__
#include <link.h>  // for dl_iterate_phdr.
#endif
#include <sstream>

// dlopen_ext support from bionic.
#ifdef __ANDROID__
#include "android/dlext.h"
#endif

#include "art_method-inl.h"
#include "base/bit_vector.h"
#include "base/stl_util.h"
#include "base/systrace.h"
#include "base/unix_file/fd_file.h"
#include "elf_file.h"
#include "elf_utils.h"
#include "oat.h"
#include "mem_map.h"
#include "mirror/class.h"
#include "mirror/object-inl.h"
#include "oat_file-inl.h"
#include "oat_file_manager.h"
#include "os.h"
#include "runtime.h"
#include "type_lookup_table.h"
#include "utils.h"
#include "utils/dex_cache_arrays_layout-inl.h"
#include "vmap_table.h"

namespace art {

// Whether OatFile::Open will try dlopen. Fallback is our own ELF loader.
static constexpr bool kUseDlopen = true;

// Whether OatFile::Open will try dlopen on the host. On the host we're not linking against
// bionic, so cannot take advantage of the support for changed semantics (loading the same soname
// multiple times). However, if/when we switch the above, we likely want to switch this, too,
// to get test coverage of the code paths.
static constexpr bool kUseDlopenOnHost = true;

// For debugging, Open will print DlOpen error message if set to true.
static constexpr bool kPrintDlOpenErrorMessage = false;

// Note for OatFileBase and descendents:
//
// These are used in OatFile::Open to try all our loaders.
//
// The process is simple:
//
// 1) Allocate an instance through the standard constructor (location, executable)
// 2) Load() to try to open the file.
// 3) ComputeFields() to populate the OatFile fields like begin_, using FindDynamicSymbolAddress.
// 4) PreSetup() for any steps that should be done before the final setup.
// 5) Setup() to complete the procedure.

class OatFileBase : public OatFile {
 public:
  virtual ~OatFileBase() {}

  template <typename kOatFileBaseSubType>
  static OatFileBase* OpenOatFile(const std::string& elf_filename,
                                  const std::string& location,
                                  uint8_t* requested_base,
                                  uint8_t* oat_file_begin,
                                  bool writable,
                                  bool executable,
                                  bool low_4gb,
                                  const char* abs_dex_location,
                                  std::string* error_msg);

 protected:
  OatFileBase(const std::string& filename, bool executable) : OatFile(filename, executable) {}

  virtual const uint8_t* FindDynamicSymbolAddress(const std::string& symbol_name,
                                                  std::string* error_msg) const = 0;

  virtual bool Load(const std::string& elf_filename,
                    uint8_t* oat_file_begin,
                    bool writable,
                    bool executable,
                    bool low_4gb,
                    std::string* error_msg) = 0;

  bool ComputeFields(uint8_t* requested_base,
                     const std::string& file_path,
                     std::string* error_msg);

  virtual void PreSetup(const std::string& elf_filename) = 0;

  bool Setup(const char* abs_dex_location, std::string* error_msg);

  // Setters exposed for ElfOatFile.

  void SetBegin(const uint8_t* begin) {
    begin_ = begin;
  }

  void SetEnd(const uint8_t* end) {
    end_ = end;
  }

 private:
  DISALLOW_COPY_AND_ASSIGN(OatFileBase);
};

template <typename kOatFileBaseSubType>
OatFileBase* OatFileBase::OpenOatFile(const std::string& elf_filename,
                                      const std::string& location,
                                      uint8_t* requested_base,
                                      uint8_t* oat_file_begin,
                                      bool writable,
                                      bool executable,
                                      bool low_4gb,
                                      const char* abs_dex_location,
                                      std::string* error_msg) {
  std::unique_ptr<OatFileBase> ret(new kOatFileBaseSubType(location, executable));
  if (!ret->Load(elf_filename,
                 oat_file_begin,
                 writable,
                 executable,
                 low_4gb,
                 error_msg)) {
    return nullptr;
  }

  if (!ret->ComputeFields(requested_base, elf_filename, error_msg)) {
    return nullptr;
  }
  ret->PreSetup(elf_filename);

  if (!ret->Setup(abs_dex_location, error_msg)) {
    return nullptr;
  }

  return ret.release();
}

bool OatFileBase::ComputeFields(uint8_t* requested_base,
                                const std::string& file_path,
                                std::string* error_msg) {
  std::string symbol_error_msg;
  begin_ = FindDynamicSymbolAddress("oatdata", &symbol_error_msg);
  if (begin_ == nullptr) {
    *error_msg = StringPrintf("Failed to find oatdata symbol in '%s' %s",
                              file_path.c_str(),
                              symbol_error_msg.c_str());
    return false;
  }
  if (requested_base != nullptr && begin_ != requested_base) {
    // Host can fail this check. Do not dump there to avoid polluting the output.
    if (kIsTargetBuild) {
      PrintFileToLog("/proc/self/maps", LogSeverity::WARNING);
    }
    *error_msg = StringPrintf("Failed to find oatdata symbol at expected address: "
        "oatdata=%p != expected=%p. See process maps in the log.",
        begin_, requested_base);
    return false;
  }
  end_ = FindDynamicSymbolAddress("oatlastword", &symbol_error_msg);
  if (end_ == nullptr) {
    *error_msg = StringPrintf("Failed to find oatlastword symbol in '%s' %s",
                              file_path.c_str(),
                              symbol_error_msg.c_str());
    return false;
  }
  // Readjust to be non-inclusive upper bound.
  end_ += sizeof(uint32_t);

  bss_begin_ = const_cast<uint8_t*>(FindDynamicSymbolAddress("oatbss", &symbol_error_msg));
  if (bss_begin_ == nullptr) {
    // No .bss section.
    bss_end_ = nullptr;
  } else {
    bss_end_ = const_cast<uint8_t*>(FindDynamicSymbolAddress("oatbsslastword", &symbol_error_msg));
    if (bss_end_ == nullptr) {
      *error_msg = StringPrintf("Failed to find oatbasslastword symbol in '%s'", file_path.c_str());
      return false;
    }
    // Readjust to be non-inclusive upper bound.
    bss_end_ += sizeof(uint32_t);
  }

  return true;
}

// Read an unaligned entry from the OatDexFile data in OatFile and advance the read
// position by the number of bytes read, i.e. sizeof(T).
// Return true on success, false if the read would go beyond the end of the OatFile.
template <typename T>
inline static bool ReadOatDexFileData(const OatFile& oat_file,
                                      /*inout*/const uint8_t** oat,
                                      /*out*/T* value) {
  DCHECK(oat != nullptr);
  DCHECK(value != nullptr);
  DCHECK_LE(*oat, oat_file.End());
  if (UNLIKELY(static_cast<size_t>(oat_file.End() - *oat) < sizeof(T))) {
    return false;
  }
  static_assert(std::is_trivial<T>::value, "T must be a trivial type");
  typedef __attribute__((__aligned__(1))) T unaligned_type;
  *value = *reinterpret_cast<const unaligned_type*>(*oat);
  *oat += sizeof(T);
  return true;
}

bool OatFileBase::Setup(const char* abs_dex_location, std::string* error_msg) {
  if (!GetOatHeader().IsValid()) {
    std::string cause = GetOatHeader().GetValidationErrorMessage();
    *error_msg = StringPrintf("Invalid oat header for '%s': %s",
                              GetLocation().c_str(),
                              cause.c_str());
    return false;
  }
  const uint8_t* oat = Begin();
  oat += sizeof(OatHeader);
  if (oat > End()) {
    *error_msg = StringPrintf("In oat file '%s' found truncated OatHeader", GetLocation().c_str());
    return false;
  }

  oat += GetOatHeader().GetKeyValueStoreSize();
  if (oat > End()) {
    *error_msg = StringPrintf("In oat file '%s' found truncated variable-size data: "
                                  "%p + %zu + %u <= %p",
                              GetLocation().c_str(),
                              Begin(),
                              sizeof(OatHeader),
                              GetOatHeader().GetKeyValueStoreSize(),
                              End());
    return false;
  }

  size_t pointer_size = GetInstructionSetPointerSize(GetOatHeader().GetInstructionSet());
  uint8_t* dex_cache_arrays = bss_begin_;
  uint32_t dex_file_count = GetOatHeader().GetDexFileCount();
  oat_dex_files_storage_.reserve(dex_file_count);
  for (size_t i = 0; i < dex_file_count; i++) {
    uint32_t dex_file_location_size;
    if (UNLIKELY(!ReadOatDexFileData(*this, &oat, &dex_file_location_size))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu truncated after dex file "
                                    "location size",
                                GetLocation().c_str(),
                                i);
      return false;
    }
    if (UNLIKELY(dex_file_location_size == 0U)) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu with empty location name",
                                GetLocation().c_str(),
                                i);
      return false;
    }
    if (UNLIKELY(static_cast<size_t>(End() - oat) < dex_file_location_size)) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu with truncated dex file "
                                    "location",
                                GetLocation().c_str(),
                                i);
      return false;
    }
    const char* dex_file_location_data = reinterpret_cast<const char*>(oat);
    oat += dex_file_location_size;

    std::string dex_file_location = ResolveRelativeEncodedDexLocation(
        abs_dex_location,
        std::string(dex_file_location_data, dex_file_location_size));

    uint32_t dex_file_checksum;
    if (UNLIKELY(!ReadOatDexFileData(*this, &oat, &dex_file_checksum))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' truncated after "
                                    "dex file checksum",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str());
      return false;
    }

    uint32_t dex_file_offset;
    if (UNLIKELY(!ReadOatDexFileData(*this, &oat, &dex_file_offset))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' truncated "
                                    "after dex file offsets",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str());
      return false;
    }
    if (UNLIKELY(dex_file_offset == 0U)) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with zero dex "
                                    "file offset",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str());
      return false;
    }
    if (UNLIKELY(dex_file_offset > Size())) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with dex file "
                                    "offset %u > %zu",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                dex_file_offset,
                                Size());
      return false;
    }
    if (UNLIKELY(Size() - dex_file_offset < sizeof(DexFile::Header))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with dex file "
                                    "offset %u of %zu but the size of dex file header is %zu",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                dex_file_offset,
                                Size(),
                                sizeof(DexFile::Header));
      return false;
    }

    const uint8_t* dex_file_pointer = Begin() + dex_file_offset;
    if (UNLIKELY(!DexFile::IsMagicValid(dex_file_pointer))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with invalid "
                                    "dex file magic '%s'",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                dex_file_pointer);
      return false;
    }
    if (UNLIKELY(!DexFile::IsVersionValid(dex_file_pointer))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with invalid "
                                    "dex file version '%s'",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                dex_file_pointer);
      return false;
    }
    const DexFile::Header* header = reinterpret_cast<const DexFile::Header*>(dex_file_pointer);
    if (Size() - dex_file_offset < header->file_size_) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with dex file "
                                    "offset %u and size %u truncated at %zu",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                dex_file_offset,
                                header->file_size_,
                                Size());
      return false;
    }

    uint32_t class_offsets_offset;
    if (UNLIKELY(!ReadOatDexFileData(*this, &oat, &class_offsets_offset))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' truncated "
                                    "after class offsets offset",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str());
      return false;
    }
    if (UNLIKELY(class_offsets_offset > Size()) ||
        UNLIKELY((Size() - class_offsets_offset) / sizeof(uint32_t) < header->class_defs_size_)) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with truncated "
                                    "class offsets, offset %u of %zu, class defs %u",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                class_offsets_offset,
                                Size(),
                                header->class_defs_size_);
      return false;
    }
    if (UNLIKELY(!IsAligned<alignof(uint32_t)>(class_offsets_offset))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with unaligned "
                                    "class offsets, offset %u",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                class_offsets_offset);
      return false;
    }
    const uint32_t* class_offsets_pointer =
        reinterpret_cast<const uint32_t*>(Begin() + class_offsets_offset);

    uint32_t lookup_table_offset;
    if (UNLIKELY(!ReadOatDexFileData(*this, &oat, &lookup_table_offset))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zd for '%s' truncated "
                                    "after lookup table offset",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str());
      return false;
    }
    const uint8_t* lookup_table_data = lookup_table_offset != 0u
        ? Begin() + lookup_table_offset
        : nullptr;
    if (lookup_table_offset != 0u &&
        (UNLIKELY(lookup_table_offset > Size()) ||
            UNLIKELY(Size() - lookup_table_offset <
                     TypeLookupTable::RawDataLength(header->class_defs_size_)))) {
      *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with truncated "
                                    "type lookup table, offset %u of %zu, class defs %u",
                                GetLocation().c_str(),
                                i,
                                dex_file_location.c_str(),
                                lookup_table_offset,
                                Size(),
                                header->class_defs_size_);
      return false;
    }

    uint8_t* current_dex_cache_arrays = nullptr;
    if (dex_cache_arrays != nullptr) {
      DexCacheArraysLayout layout(pointer_size, *header);
      if (layout.Size() != 0u) {
        if (static_cast<size_t>(bss_end_ - dex_cache_arrays) < layout.Size()) {
          *error_msg = StringPrintf("In oat file '%s' found OatDexFile #%zu for '%s' with "
                                        "truncated dex cache arrays, %zu < %zu.",
                                    GetLocation().c_str(),
                                    i,
                                    dex_file_location.c_str(),
                                    static_cast<size_t>(bss_end_ - dex_cache_arrays),
                                    layout.Size());
          return false;
        }
        current_dex_cache_arrays = dex_cache_arrays;
        dex_cache_arrays += layout.Size();
      }
    }

    std::string canonical_location = DexFile::GetDexCanonicalLocation(dex_file_location.c_str());

    // Create the OatDexFile and add it to the owning container.
    OatDexFile* oat_dex_file = new OatDexFile(this,
                                              dex_file_location,
                                              canonical_location,
                                              dex_file_checksum,
                                              dex_file_pointer,
                                              lookup_table_data,
                                              class_offsets_pointer,
                                              current_dex_cache_arrays);
    oat_dex_files_storage_.push_back(oat_dex_file);

    // Add the location and canonical location (if different) to the oat_dex_files_ table.
    StringPiece key(oat_dex_file->GetDexFileLocation());
    oat_dex_files_.Put(key, oat_dex_file);
    if (canonical_location != dex_file_location) {
      StringPiece canonical_key(oat_dex_file->GetCanonicalDexFileLocation());
      oat_dex_files_.Put(canonical_key, oat_dex_file);
    }
  }

  if (dex_cache_arrays != bss_end_) {
    // We expect the bss section to be either empty (dex_cache_arrays and bss_end_
    // both null) or contain just the dex cache arrays and nothing else.
    *error_msg = StringPrintf("In oat file '%s' found unexpected bss size bigger by %zu bytes.",
                              GetLocation().c_str(),
                              static_cast<size_t>(bss_end_ - dex_cache_arrays));
    return false;
  }
  return true;
}

////////////////////////
// OatFile via dlopen //
////////////////////////

static bool RegisterOatFileLocation(const std::string& location) {
  if (!kIsTargetBuild) {
    Runtime* const runtime = Runtime::Current();
    if (runtime != nullptr && !runtime->IsAotCompiler()) {
      return runtime->GetOatFileManager().RegisterOatFileLocation(location);
    }
    return false;
  }
  return true;
}

static void UnregisterOatFileLocation(const std::string& location) {
  if (!kIsTargetBuild) {
    Runtime* const runtime = Runtime::Current();
    if (runtime != nullptr && !runtime->IsAotCompiler()) {
      runtime->GetOatFileManager().UnRegisterOatFileLocation(location);
    }
  }
}

class DlOpenOatFile FINAL : public OatFileBase {
 public:
  DlOpenOatFile(const std::string& filename, bool executable)
      : OatFileBase(filename, executable),
        dlopen_handle_(nullptr),
        first_oat_(RegisterOatFileLocation(filename)) {
  }

  ~DlOpenOatFile() {
    if (dlopen_handle_ != nullptr) {
      dlclose(dlopen_handle_);
    }
    UnregisterOatFileLocation(GetLocation());
  }

 protected:
  const uint8_t* FindDynamicSymbolAddress(const std::string& symbol_name,
                                          std::string* error_msg) const OVERRIDE {
    const uint8_t* ptr =
        reinterpret_cast<const uint8_t*>(dlsym(dlopen_handle_, symbol_name.c_str()));
    if (ptr == nullptr) {
      *error_msg = dlerror();
    }
    return ptr;
  }

  bool Load(const std::string& elf_filename,
            uint8_t* oat_file_begin,
            bool writable,
            bool executable,
            bool low_4gb,
            std::string* error_msg) OVERRIDE;

  // Ask the linker where it mmaped the file and notify our mmap wrapper of the regions.
  void PreSetup(const std::string& elf_filename) OVERRIDE;

 private:
  bool Dlopen(const std::string& elf_filename,
              uint8_t* oat_file_begin,
              std::string* error_msg);

  // dlopen handle during runtime.
  void* dlopen_handle_;  // TODO: Unique_ptr with custom deleter.

  // Dummy memory map objects corresponding to the regions mapped by dlopen.
  std::vector<std::unique_ptr<MemMap>> dlopen_mmaps_;

  // Track the registration status (= was this the first oat file) for the location.
  const bool first_oat_;

  DISALLOW_COPY_AND_ASSIGN(DlOpenOatFile);
};

bool DlOpenOatFile::Load(const std::string& elf_filename,
                         uint8_t* oat_file_begin,
                         bool writable,
                         bool executable,
                         bool low_4gb,
                         std::string* error_msg) {
  // Use dlopen only when flagged to do so, and when it's OK to load things executable.
  // TODO: Also try when not executable? The issue here could be re-mapping as writable (as
  //       !executable is a sign that we may want to patch), which may not be allowed for
  //       various reasons.
  if (!kUseDlopen) {
    *error_msg = "DlOpen is disabled.";
    return false;
  }
  if (low_4gb) {
    *error_msg = "DlOpen does not support low 4gb loading.";
    return false;
  }
  if (writable) {
    *error_msg = "DlOpen does not support writable loading.";
    return false;
  }
  if (!executable) {
    *error_msg = "DlOpen does not support non-executable loading.";
    return false;
  }

  // dlopen always returns the same library if it is already opened on the host. For this reason
  // we only use dlopen if we are the target or we do not already have the dex file opened. Having
  // the same library loaded multiple times at different addresses is required for class unloading
  // and for having dex caches arrays in the .bss section.
  if (!kIsTargetBuild) {
    if (!kUseDlopenOnHost) {
      *error_msg = "DlOpen disabled for host.";
      return false;
    }
    // For RAII, tracking multiple loads is done in the constructor and destructor. The result is
    // stored in the first_oat_ flag.
    if (!first_oat_) {
      *error_msg = "Loading oat files multiple times with dlopen not supported on host.";
      return false;
    }
  }

  bool success = Dlopen(elf_filename, oat_file_begin, error_msg);
  DCHECK(dlopen_handle_ != nullptr || !success);

  return success;
}

bool DlOpenOatFile::Dlopen(const std::string& elf_filename,
                           uint8_t* oat_file_begin,
                           std::string* error_msg) {
#ifdef __APPLE__
  // The dl_iterate_phdr syscall is missing.  There is similar API on OSX,
  // but let's fallback to the custom loading code for the time being.
  UNUSED(elf_filename, oat_file_begin);
  *error_msg = "Dlopen unsupported on Mac.";
  return false;
#else
  {
    UniqueCPtr<char> absolute_path(realpath(elf_filename.c_str(), nullptr));
    if (absolute_path == nullptr) {
      *error_msg = StringPrintf("Failed to find absolute path for '%s'", elf_filename.c_str());
      return false;
    }
#ifdef __ANDROID__
    android_dlextinfo extinfo;
    extinfo.flags = ANDROID_DLEXT_FORCE_LOAD |                  // Force-load, don't reuse handle
                                                                //   (open oat files multiple
                                                                //    times).
                    ANDROID_DLEXT_FORCE_FIXED_VADDR;            // Take a non-zero vaddr as absolute
                                                                //   (non-pic boot image).
    if (oat_file_begin != nullptr) {                            //
      extinfo.flags |= ANDROID_DLEXT_LOAD_AT_FIXED_ADDRESS;     // Use the requested addr if
      extinfo.reserved_addr = oat_file_begin;                   // vaddr = 0.
    }                                                           //   (pic boot image).
    dlopen_handle_ = android_dlopen_ext(absolute_path.get(), RTLD_NOW, &extinfo);
#else
    dlopen_handle_ = dlopen(absolute_path.get(), RTLD_NOW);
    UNUSED(oat_file_begin);
#endif
  }
  if (dlopen_handle_ == nullptr) {
    *error_msg = StringPrintf("Failed to dlopen '%s': %s", elf_filename.c_str(), dlerror());
    return false;
  }
  return true;
#endif
}

void DlOpenOatFile::PreSetup(const std::string& elf_filename) {
#ifdef __APPLE__
  UNUSED(elf_filename);
  LOG(FATAL) << "Should not reach here.";
  UNREACHABLE();
#else
  struct dl_iterate_context {
    static int callback(struct dl_phdr_info *info, size_t /* size */, void *data) {
      auto* context = reinterpret_cast<dl_iterate_context*>(data);
      // See whether this callback corresponds to the file which we have just loaded.
      bool contains_begin = false;
      for (int i = 0; i < info->dlpi_phnum; i++) {
        if (info->dlpi_phdr[i].p_type == PT_LOAD) {
          uint8_t* vaddr = reinterpret_cast<uint8_t*>(info->dlpi_addr +
              info->dlpi_phdr[i].p_vaddr);
          size_t memsz = info->dlpi_phdr[i].p_memsz;
          if (vaddr <= context->begin_ && context->begin_ < vaddr + memsz) {
            contains_begin = true;
            break;
          }
        }
      }
      // Add dummy mmaps for this file.
      if (contains_begin) {
        for (int i = 0; i < info->dlpi_phnum; i++) {
          if (info->dlpi_phdr[i].p_type == PT_LOAD) {
            uint8_t* vaddr = reinterpret_cast<uint8_t*>(info->dlpi_addr +
                info->dlpi_phdr[i].p_vaddr);
            size_t memsz = info->dlpi_phdr[i].p_memsz;
            MemMap* mmap = MemMap::MapDummy(info->dlpi_name, vaddr, memsz);
            context->dlopen_mmaps_->push_back(std::unique_ptr<MemMap>(mmap));
          }
        }
        return 1;  // Stop iteration and return 1 from dl_iterate_phdr.
      }
      return 0;  // Continue iteration and return 0 from dl_iterate_phdr when finished.
    }
    const uint8_t* const begin_;
    std::vector<std::unique_ptr<MemMap>>* const dlopen_mmaps_;
  } context = { Begin(), &dlopen_mmaps_ };

  if (dl_iterate_phdr(dl_iterate_context::callback, &context) == 0) {
    PrintFileToLog("/proc/self/maps", LogSeverity::WARNING);
    LOG(ERROR) << "File " << elf_filename << " loaded with dlopen but cannot find its mmaps.";
  }
#endif
}

////////////////////////////////////////////////
// OatFile via our own ElfFile implementation //
////////////////////////////////////////////////

class ElfOatFile FINAL : public OatFileBase {
 public:
  ElfOatFile(const std::string& filename, bool executable) : OatFileBase(filename, executable) {}

  static ElfOatFile* OpenElfFile(File* file,
                                 const std::string& location,
                                 uint8_t* requested_base,
                                 uint8_t* oat_file_begin,  // Override base if not null
                                 bool writable,
                                 bool executable,
                                 bool low_4gb,
                                 const char* abs_dex_location,
                                 std::string* error_msg);

  bool InitializeFromElfFile(ElfFile* elf_file,
                             const char* abs_dex_location,
                             std::string* error_msg);

 protected:
  const uint8_t* FindDynamicSymbolAddress(const std::string& symbol_name,
                                          std::string* error_msg) const OVERRIDE {
    const uint8_t* ptr = elf_file_->FindDynamicSymbolAddress(symbol_name);
    if (ptr == nullptr) {
      *error_msg = "(Internal implementation could not find symbol)";
    }
    return ptr;
  }

  bool Load(const std::string& elf_filename,
            uint8_t* oat_file_begin,  // Override where the file is loaded to if not null
            bool writable,
            bool executable,
            bool low_4gb,
            std::string* error_msg) OVERRIDE;

  void PreSetup(const std::string& elf_filename ATTRIBUTE_UNUSED) OVERRIDE {
  }

 private:
  bool ElfFileOpen(File* file,
                   uint8_t* oat_file_begin,  // Override where the file is loaded to if not null
                   bool writable,
                   bool executable,
                   bool low_4gb,
                   std::string* error_msg);

 private:
  // Backing memory map for oat file during cross compilation.
  std::unique_ptr<ElfFile> elf_file_;

  DISALLOW_COPY_AND_ASSIGN(ElfOatFile);
};

ElfOatFile* ElfOatFile::OpenElfFile(File* file,
                                    const std::string& location,
                                    uint8_t* requested_base,
                                    uint8_t* oat_file_begin,  // Override base if not null
                                    bool writable,
                                    bool executable,
                                    bool low_4gb,
                                    const char* abs_dex_location,
                                    std::string* error_msg) {
  ScopedTrace trace("Open elf file " + location);
  std::unique_ptr<ElfOatFile> oat_file(new ElfOatFile(location, executable));
  bool success = oat_file->ElfFileOpen(file,
                                       oat_file_begin,
                                       writable,
                                       low_4gb,
                                       executable,
                                       error_msg);
  if (!success) {
    CHECK(!error_msg->empty());
    return nullptr;
  }

  // Complete the setup.
  if (!oat_file->ComputeFields(requested_base, file->GetPath(), error_msg)) {
    return nullptr;
  }

  if (!oat_file->Setup(abs_dex_location, error_msg)) {
    return nullptr;
  }

  return oat_file.release();
}

bool ElfOatFile::InitializeFromElfFile(ElfFile* elf_file,
                                       const char* abs_dex_location,
                                       std::string* error_msg) {
  ScopedTrace trace(__PRETTY_FUNCTION__);
  if (IsExecutable()) {
    *error_msg = "Cannot initialize from elf file in executable mode.";
    return false;
  }
  elf_file_.reset(elf_file);
  uint64_t offset, size;
  bool has_section = elf_file->GetSectionOffsetAndSize(".rodata", &offset, &size);
  CHECK(has_section);
  SetBegin(elf_file->Begin() + offset);
  SetEnd(elf_file->Begin() + size + offset);
  // Ignore the optional .bss section when opening non-executable.
  return Setup(abs_dex_location, error_msg);
}

bool ElfOatFile::Load(const std::string& elf_filename,
                      uint8_t* oat_file_begin,  // Override where the file is loaded to if not null
                      bool writable,
                      bool executable,
                      bool low_4gb,
                      std::string* error_msg) {
  ScopedTrace trace(__PRETTY_FUNCTION__);
  std::unique_ptr<File> file(OS::OpenFileForReading(elf_filename.c_str()));
  if (file == nullptr) {
    *error_msg = StringPrintf("Failed to open oat filename for reading: %s", strerror(errno));
    return false;
  }
  return ElfOatFile::ElfFileOpen(file.get(),
                                 oat_file_begin,
                                 writable,
                                 executable,
                                 low_4gb,
                                 error_msg);
}

bool ElfOatFile::ElfFileOpen(File* file,
                             uint8_t* oat_file_begin,
                             bool writable,
                             bool executable,
                             bool low_4gb,
                             std::string* error_msg) {
  ScopedTrace trace(__PRETTY_FUNCTION__);
  // TODO: rename requested_base to oat_data_begin
  elf_file_.reset(ElfFile::Open(file,
                                writable,
                                /*program_header_only*/true,
                                low_4gb,
                                error_msg,
                                oat_file_begin));
  if (elf_file_ == nullptr) {
    DCHECK(!error_msg->empty());
    return false;
  }
  bool loaded = elf_file_->Load(executable, low_4gb, error_msg);
  DCHECK(loaded || !error_msg->empty());
  return loaded;
}

//////////////////////////
// General OatFile code //
//////////////////////////

std::string OatFile::ResolveRelativeEncodedDexLocation(
      const char* abs_dex_location, const std::string& rel_dex_location) {
  if (abs_dex_location != nullptr && rel_dex_location[0] != '/') {
    // Strip :classes<N>.dex used for secondary multidex files.
    std::string base = DexFile::GetBaseLocation(rel_dex_location);
    std::string multidex_suffix = DexFile::GetMultiDexSuffix(rel_dex_location);

    // Check if the base is a suffix of the provided abs_dex_location.
    std::string target_suffix = "/" + base;
    std::string abs_location(abs_dex_location);
    if (abs_location.size() > target_suffix.size()) {
      size_t pos = abs_location.size() - target_suffix.size();
      if (abs_location.compare(pos, std::string::npos, target_suffix) == 0) {
        return abs_location + multidex_suffix;
      }
    }
  }
  return rel_dex_location;
}

static void CheckLocation(const std::string& location) {
  CHECK(!location.empty());
}

OatFile* OatFile::OpenWithElfFile(ElfFile* elf_file,
                                  const std::string& location,
                                  const char* abs_dex_location,
                                  std::string* error_msg) {
  std::unique_ptr<ElfOatFile> oat_file(new ElfOatFile(location, false /* executable */));
  return oat_file->InitializeFromElfFile(elf_file, abs_dex_location, error_msg)
      ? oat_file.release()
      : nullptr;
}

OatFile* OatFile::Open(const std::string& filename,
                       const std::string& location,
                       uint8_t* requested_base,
                       uint8_t* oat_file_begin,
                       bool executable,
                       bool low_4gb,
                       const char* abs_dex_location,
                       std::string* error_msg) {
  ScopedTrace trace("Open oat file " + location);
  CHECK(!filename.empty()) << location;
  CheckLocation(location);
  std::unique_ptr<OatFile> ret;

  // Try dlopen first, as it is required for native debuggability. This will fail fast if dlopen is
  // disabled.
  OatFile* with_dlopen = OatFileBase::OpenOatFile<DlOpenOatFile>(filename,
                                                                 location,
                                                                 requested_base,
                                                                 oat_file_begin,
                                                                 false,
                                                                 executable,
                                                                 low_4gb,
                                                                 abs_dex_location,
                                                                 error_msg);
  if (with_dlopen != nullptr) {
    return with_dlopen;
  }
  if (kPrintDlOpenErrorMessage) {
    LOG(ERROR) << "Failed to dlopen: " << filename << " with error " << *error_msg;
  }
  // If we aren't trying to execute, we just use our own ElfFile loader for a couple reasons:
  //
  // On target, dlopen may fail when compiling due to selinux restrictions on installd.
  //
  // We use our own ELF loader for Quick to deal with legacy apps that
  // open a generated dex file by name, remove the file, then open
  // another generated dex file with the same name. http://b/10614658
  //
  // On host, dlopen is expected to fail when cross compiling, so fall back to OpenElfFile.
  //
  //
  // Another independent reason is the absolute placement of boot.oat. dlopen on the host usually
  // does honor the virtual address encoded in the ELF file only for ET_EXEC files, not ET_DYN.
  OatFile* with_internal = OatFileBase::OpenOatFile<ElfOatFile>(filename,
                                                                location,
                                                                requested_base,
                                                                oat_file_begin,
                                                                false,
                                                                executable,
                                                                low_4gb,
                                                                abs_dex_location,
                                                                error_msg);
  return with_internal;
}

OatFile* OatFile::OpenWritable(File* file,
                               const std::string& location,
                               const char* abs_dex_location,
                               std::string* error_msg) {
  CheckLocation(location);
  return ElfOatFile::OpenElfFile(file,
                                 location,
                                 nullptr,
                                 nullptr,
                                 true,
                                 false,
                                 /*low_4gb*/false,
                                 abs_dex_location,
                                 error_msg);
}

OatFile* OatFile::OpenReadable(File* file,
                               const std::string& location,
                               const char* abs_dex_location,
                               std::string* error_msg) {
  CheckLocation(location);
  return ElfOatFile::OpenElfFile(file,
                                 location,
                                 nullptr,
                                 nullptr,
                                 false,
                                 false,
                                 /*low_4gb*/false,
                                 abs_dex_location,
                                 error_msg);
}

OatFile::OatFile(const std::string& location, bool is_executable)
    : location_(location),
      begin_(nullptr),
      end_(nullptr),
      bss_begin_(nullptr),
      bss_end_(nullptr),
      is_executable_(is_executable),
      secondary_lookup_lock_("OatFile secondary lookup lock", kOatFileSecondaryLookupLock) {
  CHECK(!location_.empty());
}

OatFile::~OatFile() {
  STLDeleteElements(&oat_dex_files_storage_);
}

const OatHeader& OatFile::GetOatHeader() const {
  return *reinterpret_cast<const OatHeader*>(Begin());
}

const uint8_t* OatFile::Begin() const {
  CHECK(begin_ != nullptr);
  return begin_;
}

const uint8_t* OatFile::End() const {
  CHECK(end_ != nullptr);
  return end_;
}

const uint8_t* OatFile::BssBegin() const {
  return bss_begin_;
}

const uint8_t* OatFile::BssEnd() const {
  return bss_end_;
}

const OatFile::OatDexFile* OatFile::GetOatDexFile(const char* dex_location,
                                                  const uint32_t* dex_location_checksum,
                                                  bool warn_if_not_found) const {
  // NOTE: We assume here that the canonical location for a given dex_location never
  // changes. If it does (i.e. some symlink used by the filename changes) we may return
  // an incorrect OatDexFile. As long as we have a checksum to check, we shall return
  // an identical file or fail; otherwise we may see some unpredictable failures.

  // TODO: Additional analysis of usage patterns to see if this can be simplified
  // without any performance loss, for example by not doing the first lock-free lookup.

  const OatFile::OatDexFile* oat_dex_file = nullptr;
  StringPiece key(dex_location);
  // Try to find the key cheaply in the oat_dex_files_ map which holds dex locations
  // directly mentioned in the oat file and doesn't require locking.
  auto primary_it = oat_dex_files_.find(key);
  if (primary_it != oat_dex_files_.end()) {
    oat_dex_file = primary_it->second;
    DCHECK(oat_dex_file != nullptr);
  } else {
    // This dex_location is not one of the dex locations directly mentioned in the
    // oat file. The correct lookup is via the canonical location but first see in
    // the secondary_oat_dex_files_ whether we've looked up this location before.
    MutexLock mu(Thread::Current(), secondary_lookup_lock_);
    auto secondary_lb = secondary_oat_dex_files_.lower_bound(key);
    if (secondary_lb != secondary_oat_dex_files_.end() && key == secondary_lb->first) {
      oat_dex_file = secondary_lb->second;  // May be null.
    } else {
      // We haven't seen this dex_location before, we must check the canonical location.
      std::string dex_canonical_location = DexFile::GetDexCanonicalLocation(dex_location);
      if (dex_canonical_location != dex_location) {
        StringPiece canonical_key(dex_canonical_location);
        auto canonical_it = oat_dex_files_.find(canonical_key);
        if (canonical_it != oat_dex_files_.end()) {
          oat_dex_file = canonical_it->second;
        }  // else keep null.
      }  // else keep null.

      // Copy the key to the string_cache_ and store the result in secondary map.
      string_cache_.emplace_back(key.data(), key.length());
      StringPiece key_copy(string_cache_.back());
      secondary_oat_dex_files_.PutBefore(secondary_lb, key_copy, oat_dex_file);
    }
  }
  if (oat_dex_file != nullptr &&
      (dex_location_checksum == nullptr ||
       oat_dex_file->GetDexFileLocationChecksum() == *dex_location_checksum)) {
    return oat_dex_file;
  }

  if (warn_if_not_found) {
    std::string dex_canonical_location = DexFile::GetDexCanonicalLocation(dex_location);
    std::string checksum("<unspecified>");
    if (dex_location_checksum != nullptr) {
      checksum = StringPrintf("0x%08x", *dex_location_checksum);
    }
    LOG(WARNING) << "Failed to find OatDexFile for DexFile " << dex_location
                 << " ( canonical path " << dex_canonical_location << ")"
                 << " with checksum " << checksum << " in OatFile " << GetLocation();
    if (kIsDebugBuild) {
      for (const OatDexFile* odf : oat_dex_files_storage_) {
        LOG(WARNING) << "OatFile " << GetLocation()
                     << " contains OatDexFile " << odf->GetDexFileLocation()
                     << " (canonical path " << odf->GetCanonicalDexFileLocation() << ")"
                     << " with checksum 0x" << std::hex << odf->GetDexFileLocationChecksum();
      }
    }
  }

  return nullptr;
}

OatFile::OatDexFile::OatDexFile(const OatFile* oat_file,
                                const std::string& dex_file_location,
                                const std::string& canonical_dex_file_location,
                                uint32_t dex_file_location_checksum,
                                const uint8_t* dex_file_pointer,
                                const uint8_t* lookup_table_data,
                                const uint32_t* oat_class_offsets_pointer,
                                uint8_t* dex_cache_arrays)
    : oat_file_(oat_file),
      dex_file_location_(dex_file_location),
      canonical_dex_file_location_(canonical_dex_file_location),
      dex_file_location_checksum_(dex_file_location_checksum),
      dex_file_pointer_(dex_file_pointer),
      lookup_table_data_(lookup_table_data),
      oat_class_offsets_pointer_(oat_class_offsets_pointer),
      dex_cache_arrays_(dex_cache_arrays) {}

OatFile::OatDexFile::~OatDexFile() {}

size_t OatFile::OatDexFile::FileSize() const {
  return reinterpret_cast<const DexFile::Header*>(dex_file_pointer_)->file_size_;
}

std::unique_ptr<const DexFile> OatFile::OatDexFile::OpenDexFile(std::string* error_msg) const {
  ScopedTrace trace(__PRETTY_FUNCTION__);
  return DexFile::Open(dex_file_pointer_,
                       FileSize(),
                       dex_file_location_,
                       dex_file_location_checksum_,
                       this,
                       false /* verify */,
                       error_msg);
}

uint32_t OatFile::OatDexFile::GetOatClassOffset(uint16_t class_def_index) const {
  return oat_class_offsets_pointer_[class_def_index];
}

OatFile::OatClass OatFile::OatDexFile::GetOatClass(uint16_t class_def_index) const {
  uint32_t oat_class_offset = GetOatClassOffset(class_def_index);

  const uint8_t* oat_class_pointer = oat_file_->Begin() + oat_class_offset;
  CHECK_LT(oat_class_pointer, oat_file_->End()) << oat_file_->GetLocation();

  const uint8_t* status_pointer = oat_class_pointer;
  CHECK_LT(status_pointer, oat_file_->End()) << oat_file_->GetLocation();
  mirror::Class::Status status =
      static_cast<mirror::Class::Status>(*reinterpret_cast<const int16_t*>(status_pointer));
  CHECK_LT(status, mirror::Class::kStatusMax);

  const uint8_t* type_pointer = status_pointer + sizeof(uint16_t);
  CHECK_LT(type_pointer, oat_file_->End()) << oat_file_->GetLocation();
  OatClassType type = static_cast<OatClassType>(*reinterpret_cast<const uint16_t*>(type_pointer));
  CHECK_LT(type, kOatClassMax);

  const uint8_t* after_type_pointer = type_pointer + sizeof(int16_t);
  CHECK_LE(after_type_pointer, oat_file_->End()) << oat_file_->GetLocation();

  uint32_t bitmap_size = 0;
  const uint8_t* bitmap_pointer = nullptr;
  const uint8_t* methods_pointer = nullptr;
  if (type != kOatClassNoneCompiled) {
    if (type == kOatClassSomeCompiled) {
      bitmap_size = static_cast<uint32_t>(*reinterpret_cast<const uint32_t*>(after_type_pointer));
      bitmap_pointer = after_type_pointer + sizeof(bitmap_size);
      CHECK_LE(bitmap_pointer, oat_file_->End()) << oat_file_->GetLocation();
      methods_pointer = bitmap_pointer + bitmap_size;
    } else {
      methods_pointer = after_type_pointer;
    }
    CHECK_LE(methods_pointer, oat_file_->End()) << oat_file_->GetLocation();
  }

  return OatFile::OatClass(oat_file_,
                           status,
                           type,
                           bitmap_size,
                           reinterpret_cast<const uint32_t*>(bitmap_pointer),
                           reinterpret_cast<const OatMethodOffsets*>(methods_pointer));
}

OatFile::OatClass::OatClass(const OatFile* oat_file,
                            mirror::Class::Status status,
                            OatClassType type,
                            uint32_t bitmap_size,
                            const uint32_t* bitmap_pointer,
                            const OatMethodOffsets* methods_pointer)
    : oat_file_(oat_file), status_(status), type_(type),
      bitmap_(bitmap_pointer), methods_pointer_(methods_pointer) {
    switch (type_) {
      case kOatClassAllCompiled: {
        CHECK_EQ(0U, bitmap_size);
        CHECK(bitmap_pointer == nullptr);
        CHECK(methods_pointer != nullptr);
        break;
      }
      case kOatClassSomeCompiled: {
        CHECK_NE(0U, bitmap_size);
        CHECK(bitmap_pointer != nullptr);
        CHECK(methods_pointer != nullptr);
        break;
      }
      case kOatClassNoneCompiled: {
        CHECK_EQ(0U, bitmap_size);
        CHECK(bitmap_pointer == nullptr);
        CHECK(methods_pointer_ == nullptr);
        break;
      }
      case kOatClassMax: {
        LOG(FATAL) << "Invalid OatClassType " << type_;
        break;
      }
    }
}

uint32_t OatFile::OatClass::GetOatMethodOffsetsOffset(uint32_t method_index) const {
  const OatMethodOffsets* oat_method_offsets = GetOatMethodOffsets(method_index);
  if (oat_method_offsets == nullptr) {
    return 0u;
  }
  return reinterpret_cast<const uint8_t*>(oat_method_offsets) - oat_file_->Begin();
}

const OatMethodOffsets* OatFile::OatClass::GetOatMethodOffsets(uint32_t method_index) const {
  // NOTE: We don't keep the number of methods and cannot do a bounds check for method_index.
  if (methods_pointer_ == nullptr) {
    CHECK_EQ(kOatClassNoneCompiled, type_);
    return nullptr;
  }
  size_t methods_pointer_index;
  if (bitmap_ == nullptr) {
    CHECK_EQ(kOatClassAllCompiled, type_);
    methods_pointer_index = method_index;
  } else {
    CHECK_EQ(kOatClassSomeCompiled, type_);
    if (!BitVector::IsBitSet(bitmap_, method_index)) {
      return nullptr;
    }
    size_t num_set_bits = BitVector::NumSetBits(bitmap_, method_index);
    methods_pointer_index = num_set_bits;
  }
  const OatMethodOffsets& oat_method_offsets = methods_pointer_[methods_pointer_index];
  return &oat_method_offsets;
}

const OatFile::OatMethod OatFile::OatClass::GetOatMethod(uint32_t method_index) const {
  const OatMethodOffsets* oat_method_offsets = GetOatMethodOffsets(method_index);
  if (oat_method_offsets == nullptr) {
    return OatMethod(nullptr, 0);
  }
  if (oat_file_->IsExecutable() ||
      Runtime::Current() == nullptr ||        // This case applies for oatdump.
      Runtime::Current()->IsAotCompiler()) {
    return OatMethod(oat_file_->Begin(), oat_method_offsets->code_offset_);
  }
  // We aren't allowed to use the compiled code. We just force it down the interpreted / jit
  // version.
  return OatMethod(oat_file_->Begin(), 0);
}

void OatFile::OatMethod::LinkMethod(ArtMethod* method) const {
  CHECK(method != nullptr);
  method->SetEntryPointFromQuickCompiledCode(GetQuickCode());
}

bool OatFile::IsPic() const {
  return GetOatHeader().IsPic();
  // TODO: Check against oat_patches. b/18144996
}

bool OatFile::IsDebuggable() const {
  return GetOatHeader().IsDebuggable();
}

bool OatFile::IsExtractOnly() const {
  return GetOatHeader().IsExtractOnly();
}

bool OatFile::IsProfileGuideCompiled() const {
  return GetOatHeader().IsProfileGuideCompiled();
}

static constexpr char kDexClassPathEncodingSeparator = '*';

std::string OatFile::EncodeDexFileDependencies(const std::vector<const DexFile*>& dex_files) {
  std::ostringstream out;

  for (const DexFile* dex_file : dex_files) {
    out << dex_file->GetLocation().c_str();
    out << kDexClassPathEncodingSeparator;
    out << dex_file->GetLocationChecksum();
    out << kDexClassPathEncodingSeparator;
  }

  return out.str();
}

bool OatFile::CheckStaticDexFileDependencies(const char* dex_dependencies, std::string* msg) {
  if (dex_dependencies == nullptr || dex_dependencies[0] == 0) {
    // No dependencies.
    return true;
  }

  // Assumption: this is not performance-critical. So it's OK to do this with a std::string and
  //             Split() instead of manual parsing of the combined char*.
  std::vector<std::string> split;
  Split(dex_dependencies, kDexClassPathEncodingSeparator, &split);
  if (split.size() % 2 != 0) {
    // Expected pairs of location and checksum.
    *msg = StringPrintf("Odd number of elements in dependency list %s", dex_dependencies);
    return false;
  }

  for (auto it = split.begin(), end = split.end(); it != end; it += 2) {
    std::string& location = *it;
    std::string& checksum = *(it + 1);
    int64_t converted = strtoll(checksum.c_str(), nullptr, 10);
    if (converted == 0) {
      // Conversion error.
      *msg = StringPrintf("Conversion error for %s", checksum.c_str());
      return false;
    }

    uint32_t dex_checksum;
    std::string error_msg;
    if (DexFile::GetChecksum(DexFile::GetDexCanonicalLocation(location.c_str()).c_str(),
                             &dex_checksum,
                             &error_msg)) {
      if (converted != dex_checksum) {
        *msg = StringPrintf("Checksums don't match for %s: %" PRId64 " vs %u",
                            location.c_str(), converted, dex_checksum);
        return false;
      }
    } else {
      // Problem retrieving checksum.
      // TODO: odex files?
      *msg = StringPrintf("Could not retrieve checksum for %s: %s", location.c_str(),
                          error_msg.c_str());
      return false;
    }
  }

  return true;
}

bool OatFile::GetDexLocationsFromDependencies(const char* dex_dependencies,
                                              std::vector<std::string>* locations) {
  DCHECK(locations != nullptr);
  if (dex_dependencies == nullptr || dex_dependencies[0] == 0) {
    return true;
  }

  // Assumption: this is not performance-critical. So it's OK to do this with a std::string and
  //             Split() instead of manual parsing of the combined char*.
  std::vector<std::string> split;
  Split(dex_dependencies, kDexClassPathEncodingSeparator, &split);
  if (split.size() % 2 != 0) {
    // Expected pairs of location and checksum.
    return false;
  }

  for (auto it = split.begin(), end = split.end(); it != end; it += 2) {
    locations->push_back(*it);
  }

  return true;
}

}  // namespace art