Port Android relocation packer to chromium build

third_party/android_platform/relocation_packer/src/elf_file.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Implementation notes:
+//
+// We need to remove a piece from the ELF shared library.  However, we also
+// want to avoid fixing DWARF cfi data and relative relocation addresses.
+// So after packing we shift offets and starting address of the RX segment
+// while preserving code/data vaddrs location.
+// This requires some fixups for symtab/hash/gnu_hash dynamic section addresses.
+
+#include "elf_file.h"
+
+#include <stdlib.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "debug.h"
+#include "elf_traits.h"
+#include "libelf.h"
+#include "packer.h"
+
+namespace relocation_packer {
+
+// Out-of-band dynamic tags used to indicate the offset and size of the
+// android packed relocations section.
+static constexpr int32_t DT_ANDROID_REL = DT_LOOS + 2;
+static constexpr int32_t DT_ANDROID_RELSZ = DT_LOOS + 3;
+
+static constexpr int32_t DT_ANDROID_RELA = DT_LOOS + 4;
+static constexpr int32_t DT_ANDROID_RELASZ = DT_LOOS + 5;
+
+static constexpr uint32_t SHT_ANDROID_REL = SHT_LOOS + 1;
+static constexpr uint32_t SHT_ANDROID_RELA = SHT_LOOS + 2;
+
+// Alignment to preserve, in bytes.  This must be at least as large as the
+// largest d_align and sh_addralign values found in the loaded file.
+// Out of caution for RELRO page alignment, we preserve to a complete target
+// page.  See http://www.airs.com/blog/archives/189.
+static constexpr size_t kPreserveAlignment = 4096;
+
+// Get section data.  Checks that the section has exactly one data entry,
+// so that the section size and the data size are the same.  True in
+// practice for all sections we resize when packing or unpacking.  Done
+// by ensuring that a call to elf_getdata(section, data) returns NULL as
+// the next data entry.
+static Elf_Data* GetSectionData(Elf_Scn* section) {
+  Elf_Data* data = elf_getdata(section, NULL);
+  CHECK(data && elf_getdata(section, data) == NULL);
+  return data;
+}
+
+// Rewrite section data.  Allocates new data and makes it the data element's
+// buffer.  Relies on program exit to free allocated data.
+static void RewriteSectionData(Elf_Scn* section,
+                               const void* section_data,
+                               size_t size) {
+  Elf_Data* data = GetSectionData(section);
+  CHECK(size == data->d_size);
+  uint8_t* area = new uint8_t[size];
+  memcpy(area, section_data, size);
+  data->d_buf = area;
+}
+
+// Verbose ELF header logging.
+template <typename Ehdr>
+static void VerboseLogElfHeader(const Ehdr* elf_header) {
+  VLOG(1) << "e_phoff = " << elf_header->e_phoff;
+  VLOG(1) << "e_shoff = " << elf_header->e_shoff;
+  VLOG(1) << "e_ehsize = " << elf_header->e_ehsize;
+  VLOG(1) << "e_phentsize = " << elf_header->e_phentsize;
+  VLOG(1) << "e_phnum = " << elf_header->e_phnum;
+  VLOG(1) << "e_shnum = " << elf_header->e_shnum;
+  VLOG(1) << "e_shstrndx = " << elf_header->e_shstrndx;
+}
+
+// Verbose ELF program header logging.
+template <typename Phdr>
+static void VerboseLogProgramHeader(size_t program_header_index,
+                             const Phdr* program_header) {
+  std::string type;
+  switch (program_header->p_type) {
+    case PT_NULL: type = "NULL"; break;
+    case PT_LOAD: type = "LOAD"; break;
+    case PT_DYNAMIC: type = "DYNAMIC"; break;
+    case PT_INTERP: type = "INTERP"; break;
+    case PT_PHDR: type = "PHDR"; break;
+    case PT_GNU_RELRO: type = "GNU_RELRO"; break;
+    case PT_GNU_STACK: type = "GNU_STACK"; break;
+    case PT_ARM_EXIDX: type = "EXIDX"; break;
+    default: type = "(OTHER)"; break;
+  }
+  VLOG(1) << "phdr[" << program_header_index << "] : " << type;
+  VLOG(1) << "  p_offset = " << program_header->p_offset;
+  VLOG(1) << "  p_vaddr = " << program_header->p_vaddr;
+  VLOG(1) << "  p_paddr = " << program_header->p_paddr;
+  VLOG(1) << "  p_filesz = " << program_header->p_filesz;
+  VLOG(1) << "  p_memsz = " << program_header->p_memsz;
+  VLOG(1) << "  p_flags = " << program_header->p_flags;
+  VLOG(1) << "  p_align = " << program_header->p_align;
+}
+
+// Verbose ELF section header logging.
+template <typename Shdr>
+static void VerboseLogSectionHeader(const std::string& section_name,
+                             const Shdr* section_header) {
+  VLOG(1) << "section " << section_name;
+  VLOG(1) << "  sh_addr = " << section_header->sh_addr;
+  VLOG(1) << "  sh_offset = " << section_header->sh_offset;
+  VLOG(1) << "  sh_size = " << section_header->sh_size;
+  VLOG(1) << "  sh_entsize = " << section_header->sh_entsize;
+  VLOG(1) << "  sh_addralign = " << section_header->sh_addralign;
+}
+
+// Verbose ELF section data logging.
+static void VerboseLogSectionData(const Elf_Data* data) {
+  VLOG(1) << "  data";
+  VLOG(1) << "    d_buf = " << data->d_buf;
+  VLOG(1) << "    d_off = " << data->d_off;
+  VLOG(1) << "    d_size = " << data->d_size;
+  VLOG(1) << "    d_align = " << data->d_align;
+}
+
+// Load the complete ELF file into a memory image in libelf, and identify
+// the .rel.dyn or .rela.dyn, .dynamic, and .android.rel.dyn or
+// .android.rela.dyn sections.  No-op if the ELF file has already been loaded.
+template <typename ELF>
+bool ElfFile<ELF>::Load() {
+  if (elf_)
+    return true;
+
+  Elf* elf = elf_begin(fd_, ELF_C_RDWR, NULL);
+  CHECK(elf);
+
+  if (elf_kind(elf) != ELF_K_ELF) {
+    LOG(ERROR) << "File not in ELF format";
+    return false;
+  }
+
+  auto elf_header = ELF::getehdr(elf);
+  if (!elf_header) {
+    LOG(ERROR) << "Failed to load ELF header: " << elf_errmsg(elf_errno());
+    return false;
+  }
+
+  if (elf_header->e_type != ET_DYN) {
+    LOG(ERROR) << "ELF file is not a shared object";
+    return false;
+  }
+
+  // Require that our endianness matches that of the target, and that both
+  // are little-endian.  Safe for all current build/target combinations.
+  const int endian = elf_header->e_ident[EI_DATA];
+  CHECK(endian == ELFDATA2LSB);
+  CHECK(__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__);
+
+  const int file_class = elf_header->e_ident[EI_CLASS];
+  VLOG(1) << "endian = " << endian << ", file class = " << file_class;
+  VerboseLogElfHeader(elf_header);
+
+  auto elf_program_header = ELF::getphdr(elf);
+  CHECK(elf_program_header != nullptr);
+
+  const typename ELF::Phdr* dynamic_program_header = NULL;
+  for (size_t i = 0; i < elf_header->e_phnum; ++i) {
+    auto program_header = &elf_program_header[i];
+    VerboseLogProgramHeader(i, program_header);
+
+    if (program_header->p_type == PT_DYNAMIC) {
+      CHECK(dynamic_program_header == NULL);
+      dynamic_program_header = program_header;
+    }
+  }
+  CHECK(dynamic_program_header != nullptr);
+
+  size_t string_index;
+  elf_getshdrstrndx(elf, &string_index);
+
+  // Notes of the dynamic relocations, packed relocations, and .dynamic
+  // sections.  Found while iterating sections, and later stored in class
+  // attributes.
+  Elf_Scn* found_relocations_section = nullptr;
+  Elf_Scn* found_dynamic_section = nullptr;
+
+  // Notes of relocation section types seen.  We require one or the other of
+  // these; both is unsupported.
+  bool has_rel_relocations = false;
+  bool has_rela_relocations = false;
+
+  Elf_Scn* section = NULL;
+  while ((section = elf_nextscn(elf, section)) != nullptr) {
+    auto section_header = ELF::getshdr(section);
+    std::string name = elf_strptr(elf, string_index, section_header->sh_name);
+    VerboseLogSectionHeader(name, section_header);
+
+    // Note relocation section types.
+    if (section_header->sh_type == SHT_REL || section_header->sh_type == SHT_ANDROID_REL) {
+      has_rel_relocations = true;
+    }
+    if (section_header->sh_type == SHT_RELA || section_header->sh_type == SHT_ANDROID_RELA) {
+      has_rela_relocations = true;
+    }
+
+    // Note special sections as we encounter them.
+    if ((name == ".rel.dyn" || name == ".rela.dyn") &&
+        section_header->sh_size > 0) {
+      found_relocations_section = section;
+    }
+
+    if (section_header->sh_offset == dynamic_program_header->p_offset) {
+      found_dynamic_section = section;
+    }
+
+    // Ensure we preserve alignment, repeated later for the data block(s).
+    CHECK(section_header->sh_addralign <= kPreserveAlignment);
+
+    Elf_Data* data = NULL;
+    while ((data = elf_getdata(section, data)) != NULL) {
+      CHECK(data->d_align <= kPreserveAlignment);
+      VerboseLogSectionData(data);
+    }
+  }
+
+  // Loading failed if we did not find the required special sections.
+  if (!found_relocations_section) {
+    LOG(ERROR) << "Missing or empty .rel.dyn or .rela.dyn section";
+    return false;
+  }
+  if (!found_dynamic_section) {
+    LOG(ERROR) << "Missing .dynamic section";
+    return false;
+  }
+
+  // Loading failed if we could not identify the relocations type.
+  if (!has_rel_relocations && !has_rela_relocations) {
+    LOG(ERROR) << "No relocations sections found";
+    return false;
+  }
+  if (has_rel_relocations && has_rela_relocations) {
+    LOG(ERROR) << "Multiple relocations sections with different types found, "
+               << "not currently supported";
+    return false;
+  }
+
+  elf_ = elf;
+  relocations_section_ = found_relocations_section;
+  dynamic_section_ = found_dynamic_section;
+  relocations_type_ = has_rel_relocations ? REL : RELA;
+  return true;
+}
+
+// Helper for ResizeSection().  Adjust the main ELF header for the hole.
+template <typename ELF>
+static void AdjustElfHeaderForHole(typename ELF::Ehdr* elf_header,
+                                   typename ELF::Off hole_start,
+                                   ssize_t hole_size) {
+  if (elf_header->e_phoff > hole_start) {
+    elf_header->e_phoff += hole_size;
+    VLOG(1) << "e_phoff adjusted to " << elf_header->e_phoff;
+  }
+  if (elf_header->e_shoff > hole_start) {
+    elf_header->e_shoff += hole_size;
+    VLOG(1) << "e_shoff adjusted to " << elf_header->e_shoff;
+  }
+}
+
+// Helper for ResizeSection().  Adjust all section headers for the hole.
+template <typename ELF>
+static void AdjustSectionHeadersForHole(Elf* elf,
+                                        typename ELF::Off hole_start,
+                                        ssize_t hole_size) {
+  size_t string_index;
+  elf_getshdrstrndx(elf, &string_index);
+
+  Elf_Scn* section = NULL;
+  while ((section = elf_nextscn(elf, section)) != NULL) {
+    auto section_header = ELF::getshdr(section);
+    std::string name = elf_strptr(elf, string_index, section_header->sh_name);
+
+    if (section_header->sh_offset > hole_start) {
+      section_header->sh_offset += hole_size;
+      VLOG(1) << "section " << name
+              << " sh_offset adjusted to " << section_header->sh_offset;
+    } else {
+      section_header->sh_addr -= hole_size;
+      VLOG(1) << "section " << name
+              << " sh_addr adjusted to " << section_header->sh_addr;
+    }
+  }
+}
+
+// Helper for ResizeSection().  Adjust the offsets of any program headers
+// that have offsets currently beyond the hole start.
+template <typename ELF>
+static void AdjustProgramHeaderOffsets(typename ELF::Phdr* program_headers,
+                                       size_t count,
+                                       typename ELF::Off hole_start,
+                                       ssize_t hole_size) {
+  for (size_t i = 0; i < count; ++i) {
+    typename ELF::Phdr* program_header = &program_headers[i];
+
+    if (program_header->p_offset > hole_start) {
+      // The hole start is past this segment, so adjust offset.
+      program_header->p_offset += hole_size;
+      VLOG(1) << "phdr[" << i
+              << "] p_offset adjusted to "<< program_header->p_offset;
+    } else {
+      program_header->p_vaddr -= hole_size;
+      program_header->p_paddr -= hole_size;
+      VLOG(1) << "phdr[" << i
+              << "] p_vaddr adjusted to "<< program_header->p_vaddr
+              << "; p_paddr adjusted to "<< program_header->p_paddr;
+    }
+  }
+}
+
+// Helper for ResizeSection().  Find the first loadable segment in the
+// file.  We expect it to map from file offset zero.
+template <typename ELF>
+static typename ELF::Phdr* FindLoadSegmentForHole(typename ELF::Phdr* program_headers,
+                                                  size_t count,
+                                                  typename ELF::Off hole_start) {
+  for (size_t i = 0; i < count; ++i) {
+    typename ELF::Phdr* program_header = &program_headers[i];
+
+    if (program_header->p_type == PT_LOAD &&
+        program_header->p_offset <= hole_start &&
+        (program_header->p_offset + program_header->p_filesz) >= hole_start ) {
+      return program_header;
+    }
+  }
+  LOG(FATAL) << "Cannot locate a LOAD segment with hole_start=0x" << std::hex << hole_start;
+  NOTREACHED();
+
+  return nullptr;
+}
+
+// Helper for ResizeSection().  Rewrite program headers.
+template <typename ELF>
+static void RewriteProgramHeadersForHole(Elf* elf,
+                                         typename ELF::Off hole_start,
+                                         ssize_t hole_size) {
+  const typename ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  CHECK(elf_header);
+
+  typename ELF::Phdr* elf_program_header = ELF::getphdr(elf);
+  CHECK(elf_program_header);
+
+  const size_t program_header_count = elf_header->e_phnum;
+
+  // Locate the segment that we can overwrite to form the new LOAD entry,
+  // and the segment that we are going to split into two parts.
+  typename ELF::Phdr* target_load_header =
+      FindLoadSegmentForHole<ELF>(elf_program_header, program_header_count, hole_start);
+
+  VLOG(1) << "phdr[" << target_load_header - elf_program_header << "] adjust";
+  // Adjust PT_LOAD program header memsz and filesz
+  target_load_header->p_filesz += hole_size;
+  target_load_header->p_memsz += hole_size;
+
+  // Adjust the offsets and p_vaddrs
+  AdjustProgramHeaderOffsets<ELF>(elf_program_header,
+                                  program_header_count,
+                                  hole_start,
+                                  hole_size);
+}
+
+// Helper for ResizeSection().  Locate and return the dynamic section.
+template <typename ELF>
+static Elf_Scn* GetDynamicSection(Elf* elf) {
+  const typename ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  CHECK(elf_header);
+
+  const typename ELF::Phdr* elf_program_header = ELF::getphdr(elf);
+  CHECK(elf_program_header);
+
+  // Find the program header that describes the dynamic section.
+  const typename ELF::Phdr* dynamic_program_header = NULL;
+  for (size_t i = 0; i < elf_header->e_phnum; ++i) {
+    const typename ELF::Phdr* program_header = &elf_program_header[i];
+
+    if (program_header->p_type == PT_DYNAMIC) {
+      dynamic_program_header = program_header;
+    }
+  }
+  CHECK(dynamic_program_header);
+
+  // Now find the section with the same offset as this program header.
+  Elf_Scn* dynamic_section = NULL;
+  Elf_Scn* section = NULL;
+  while ((section = elf_nextscn(elf, section)) != NULL) {
+    typename ELF::Shdr* section_header = ELF::getshdr(section);
+
+    if (section_header->sh_offset == dynamic_program_header->p_offset) {
+      dynamic_section = section;
+    }
+  }
+  CHECK(dynamic_section != NULL);
+
+  return dynamic_section;
+}
+
+// Helper for ResizeSection().  Adjust the .dynamic section for the hole.
+template <typename ELF>
+void ElfFile<ELF>::AdjustDynamicSectionForHole(Elf_Scn* dynamic_section,
+                                               typename ELF::Off hole_start,
+                                               ssize_t hole_size,
+                                               relocations_type_t relocations_type) {
+  CHECK(relocations_type != NONE);
+  Elf_Data* data = GetSectionData(dynamic_section);
+
+  auto dynamic_base = reinterpret_cast<typename ELF::Dyn*>(data->d_buf);
+  std::vector<typename ELF::Dyn> dynamics(
+      dynamic_base,
+      dynamic_base + data->d_size / sizeof(dynamics[0]));
+
+  if (hole_size > 0) { // expanding
+    hole_start += hole_size;
+  }
+
+  for (size_t i = 0; i < dynamics.size(); ++i) {
+    typename ELF::Dyn* dynamic = &dynamics[i];
+    const typename ELF::Sword tag = dynamic->d_tag;
+
+    // Any tags that hold offsets are adjustment candidates.
+    const bool is_adjustable = (tag == DT_PLTGOT ||
+                                tag == DT_HASH ||
+                                tag == DT_GNU_HASH ||
+                                tag == DT_STRTAB ||
+                                tag == DT_SYMTAB ||
+                                tag == DT_RELA ||
+                                tag == DT_INIT ||
+                                tag == DT_FINI ||
+                                tag == DT_REL ||
+                                tag == DT_JMPREL ||
+                                tag == DT_INIT_ARRAY ||
+                                tag == DT_FINI_ARRAY ||
+                                tag == DT_ANDROID_REL||
+                                tag == DT_ANDROID_RELA);
+
+    if (is_adjustable && dynamic->d_un.d_ptr <= hole_start) {
+      dynamic->d_un.d_ptr -= hole_size;
+      VLOG(1) << "dynamic[" << i << "] " << dynamic->d_tag
+              << " d_ptr adjusted to " << dynamic->d_un.d_ptr;
+    }
+
+    // DT_RELSZ or DT_RELASZ indicate the overall size of relocations.
+    // Only one will be present.  Adjust by hole size.
+    if (tag == DT_RELSZ || tag == DT_RELASZ || tag == DT_ANDROID_RELSZ || tag == DT_ANDROID_RELASZ) {
+      dynamic->d_un.d_val += hole_size;
+      VLOG(1) << "dynamic[" << i << "] " << dynamic->d_tag
+              << " d_val adjusted to " << dynamic->d_un.d_val;
+    }
+
+    // Ignore DT_RELCOUNT and DT_RELACOUNT: (1) nobody uses them and
+    // technically (2) the relative relocation count is not changed.
+
+    // DT_RELENT and DT_RELAENT don't change, ignore them as well.
+  }
+
+  void* section_data = &dynamics[0];
+  size_t bytes = dynamics.size() * sizeof(dynamics[0]);
+  RewriteSectionData(dynamic_section, section_data, bytes);
+}
+
+// Resize a section.  If the new size is larger than the current size, open
+// up a hole by increasing file offsets that come after the hole.  If smaller
+// than the current size, remove the hole by decreasing those offsets.
+template <typename ELF>
+void ElfFile<ELF>::ResizeSection(Elf* elf, Elf_Scn* section, size_t new_size,
+                                 typename ELF::Word new_sh_type,
+                                 relocations_type_t relocations_type) {
+
+  size_t string_index;
+  elf_getshdrstrndx(elf, &string_index);
+  auto section_header = ELF::getshdr(section);
+  std::string name = elf_strptr(elf, string_index, section_header->sh_name);
+
+  if (section_header->sh_size == new_size) {
+    return;
+  }
+
+  // Require that the section size and the data size are the same.  True
+  // in practice for all sections we resize when packing or unpacking.
+  Elf_Data* data = GetSectionData(section);
+  CHECK(data->d_off == 0 && data->d_size == section_header->sh_size);
+
+  // Require that the section is not zero-length (that is, has allocated
+  // data that we can validly expand).
+  CHECK(data->d_size && data->d_buf);
+
+  const auto hole_start = section_header->sh_offset;
+  const ssize_t hole_size = new_size - data->d_size;
+
+  VLOG_IF(1, (hole_size > 0)) << "expand section (" << name << ") size: " <<
+      data->d_size << " -> " << (data->d_size + hole_size);
+  VLOG_IF(1, (hole_size < 0)) << "shrink section (" << name << ") size: " <<
+      data->d_size << " -> " << (data->d_size + hole_size);
+
+  // libelf overrides sh_entsize for known sh_types, so it does not matter what we set
+  // for SHT_REL/SHT_RELA.
+  typename ELF::Xword new_entsize =
+      (new_sh_type == SHT_ANDROID_REL || new_sh_type == SHT_ANDROID_RELA) ? 1 : 0;
+
+  VLOG(1) << "Update section (" << name << ") entry size: " <<
+      section_header->sh_entsize << " -> " << new_entsize;
+
+  // Resize the data and the section header.
+  data->d_size += hole_size;
+  section_header->sh_size += hole_size;
+  section_header->sh_entsize = new_entsize;
+  section_header->sh_type = new_sh_type;
+
+  // Add the hole size to all offsets in the ELF file that are after the
+  // start of the hole.  If the hole size is positive we are expanding the
+  // section to create a new hole; if negative, we are closing up a hole.
+
+  // Start with the main ELF header.
+  typename ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  AdjustElfHeaderForHole<ELF>(elf_header, hole_start, hole_size);
+
+  // Adjust all section headers.
+  AdjustSectionHeadersForHole<ELF>(elf, hole_start, hole_size);
+
+  // Rewrite the program headers to either split or coalesce segments,
+  // and adjust dynamic entries to match.
+  RewriteProgramHeadersForHole<ELF>(elf, hole_start, hole_size);
+
+  Elf_Scn* dynamic_section = GetDynamicSection<ELF>(elf);
+  AdjustDynamicSectionForHole(dynamic_section, hole_start, hole_size, relocations_type);
+}
+
+// Find the first slot in a dynamics array with the given tag.  The array
+// always ends with a free (unused) element, and which we exclude from the
+// search.  Returns dynamics->size() if not found.
+template <typename ELF>
+static size_t FindDynamicEntry(typename ELF::Sword tag,
+                               std::vector<typename ELF::Dyn>* dynamics) {
+  // Loop until the penultimate entry.  We exclude the end sentinel.
+  for (size_t i = 0; i < dynamics->size() - 1; ++i) {
+    if (dynamics->at(i).d_tag == tag) {
+      return i;
+    }
+  }
+
+  // The tag was not found.
+  return dynamics->size();
+}
+
+// Replace dynamic entry.
+template <typename ELF>
+static void ReplaceDynamicEntry(typename ELF::Sword tag,
+                                const typename ELF::Dyn& dyn,
+                                std::vector<typename ELF::Dyn>* dynamics) {
+  const size_t slot = FindDynamicEntry<ELF>(tag, dynamics);
+  if (slot == dynamics->size()) {
+    LOG(FATAL) << "Dynamic slot is not found for tag=" << tag;
+  }
+
+  // Replace this entry with the one supplied.
+  dynamics->at(slot) = dyn;
+  VLOG(1) << "dynamic[" << slot << "] overwritten with " << dyn.d_tag;
+}
+
+// Remove relative entries from dynamic relocations and write as packed
+// data into android packed relocations.
+template <typename ELF>
+bool ElfFile<ELF>::PackRelocations() {
+  // Load the ELF file into libelf.
+  if (!Load()) {
+    LOG(ERROR) << "Failed to load as ELF";
+    return false;
+  }
+
+  // Retrieve the current dynamic relocations section data.
+  Elf_Data* data = GetSectionData(relocations_section_);
+  // we always pack rela, because packed format is pretty much the same
+  std::vector<typename ELF::Rela> relocations;
+
+  if (relocations_type_ == REL) {
+    // Convert data to a vector of relocations.
+    const typename ELF::Rel* relocations_base = reinterpret_cast<typename ELF::Rel*>(data->d_buf);
+    ConvertRelArrayToRelaVector(relocations_base,
+        data->d_size / sizeof(typename ELF::Rel), &relocations);
+    LOG(INFO) << "Relocations   : REL";
+  } else if (relocations_type_ == RELA) {
+    // Convert data to a vector of relocations with addends.
+    const typename ELF::Rela* relocations_base = reinterpret_cast<typename ELF::Rela*>(data->d_buf);
+    relocations = std::vector<typename ELF::Rela>(
+        relocations_base,
+        relocations_base + data->d_size / sizeof(relocations[0]));
+
+    LOG(INFO) << "Relocations   : RELA";
+  } else {
+    NOTREACHED();
+  }
+
+  return PackTypedRelocations(&relocations);
+}
+
+// Helper for PackRelocations().  Rel type is one of ELF::Rel or ELF::Rela.
+template <typename ELF>
+bool ElfFile<ELF>::PackTypedRelocations(std::vector<typename ELF::Rela>* relocations) {
+  typedef typename ELF::Rela Rela;
+
+  // If no relocations then we have nothing packable.  Perhaps
+  // the shared object has already been packed?
+  if (relocations->empty()) {
+    LOG(ERROR) << "No relocations found (already packed?)";
+    return false;
+  }
+
+  const size_t rel_size =
+      relocations_type_ == RELA ? sizeof(typename ELF::Rela) : sizeof(typename ELF::Rel);
+  const size_t initial_bytes = relocations->size() * rel_size;
+
+  LOG(INFO) << "Unpacked                   : " << initial_bytes << " bytes";
+  std::vector<uint8_t> packed;
+  RelocationPacker<ELF> packer;
+
+  // Pack relocations: dry run to estimate memory savings.
+  packer.PackRelocations(*relocations, &packed);
+  const size_t packed_bytes_estimate = packed.size() * sizeof(packed[0]);
+  LOG(INFO) << "Packed         (no padding): " << packed_bytes_estimate << " bytes";
+
+  if (packed.empty()) {
+    LOG(INFO) << "Too few relocations to pack";
+    return false;
+  }
+
+  // Pre-calculate the size of the hole we will close up when we rewrite
+  // dynamic relocations.  We have to adjust relocation addresses to
+  // account for this.
+  typename ELF::Shdr* section_header = ELF::getshdr(relocations_section_);
+  ssize_t hole_size = initial_bytes - packed_bytes_estimate;
+
+  // hole_size needs to be page_aligned.
+  hole_size -= hole_size % kPreserveAlignment;
+
+  LOG(INFO) << "Compaction                 : " << hole_size << " bytes";
+
+  // Adjusting for alignment may have removed any packing benefit.
+  if (hole_size == 0) {
+    LOG(INFO) << "Too few relocations to pack after alignment";
+    return false;
+  }
+
+  if (hole_size <= 0) {
+    LOG(INFO) << "Packing relocations saves no space";
+    return false;
+  }
+
+  size_t data_padding_bytes = is_padding_relocations_ ?
+      initial_bytes - packed_bytes_estimate :
+      initial_bytes - hole_size - packed_bytes_estimate;
+
+  // pad data
+  std::vector<uint8_t> padding(data_padding_bytes, 0);
+  packed.insert(packed.end(), padding.begin(), padding.end());
+
+  const void* packed_data = &packed[0];
+
+  // Run a loopback self-test as a check that packing is lossless.
+  std::vector<Rela> unpacked;
+  packer.UnpackRelocations(packed, &unpacked);
+  CHECK(unpacked.size() == relocations->size());
+  CHECK(!memcmp(&unpacked[0],
+                &relocations->at(0),
+                unpacked.size() * sizeof(unpacked[0])));
+
+  // Rewrite the current dynamic relocations section with packed one then shrink it to size.
+  const size_t bytes = packed.size() * sizeof(packed[0]);
+  ResizeSection(elf_, relocations_section_, bytes,
+      relocations_type_ == REL ? SHT_ANDROID_REL : SHT_ANDROID_RELA, relocations_type_);
+  RewriteSectionData(relocations_section_, packed_data, bytes);
+
+  // TODO (dimitry): fix string table and replace .rel.dyn/plt with .android.rel.dyn/plt
+
+  // Rewrite .dynamic and rename relocation tags describing the packed android
+  // relocations.
+  Elf_Data* data = GetSectionData(dynamic_section_);
+  const typename ELF::Dyn* dynamic_base = reinterpret_cast<typename ELF::Dyn*>(data->d_buf);
+  std::vector<typename ELF::Dyn> dynamics(
+      dynamic_base,
+      dynamic_base + data->d_size / sizeof(dynamics[0]));
+  section_header = ELF::getshdr(relocations_section_);
+  {
+    typename ELF::Dyn dyn;
+    dyn.d_tag = relocations_type_ == REL ? DT_ANDROID_REL : DT_ANDROID_RELA;
+    dyn.d_un.d_ptr = section_header->sh_addr;
+    ReplaceDynamicEntry<ELF>(relocations_type_ == REL ? DT_REL : DT_RELA, dyn, &dynamics);
+  }
+  {
+    typename ELF::Dyn dyn;
+    dyn.d_tag = relocations_type_ == REL ? DT_ANDROID_RELSZ : DT_ANDROID_RELASZ;
+    dyn.d_un.d_val = section_header->sh_size;
+    ReplaceDynamicEntry<ELF>(relocations_type_ == REL ? DT_RELSZ : DT_RELASZ, dyn, &dynamics);
+  }
+
+  const void* dynamics_data = &dynamics[0];
+  const size_t dynamics_bytes = dynamics.size() * sizeof(dynamics[0]);
+  RewriteSectionData(dynamic_section_, dynamics_data, dynamics_bytes);
+
+  Flush();
+  return true;
+}
+
+// Find packed relative relocations in the packed android relocations
+// section, unpack them, and rewrite the dynamic relocations section to
+// contain unpacked data.
+template <typename ELF>
+bool ElfFile<ELF>::UnpackRelocations() {
+  // Load the ELF file into libelf.
+  if (!Load()) {
+    LOG(ERROR) << "Failed to load as ELF";
+    return false;
+  }
+
+  typename ELF::Shdr* section_header = ELF::getshdr(relocations_section_);
+  // Retrieve the current packed android relocations section data.
+  Elf_Data* data = GetSectionData(relocations_section_);
+
+  // Convert data to a vector of bytes.
+  const uint8_t* packed_base = reinterpret_cast<uint8_t*>(data->d_buf);
+  std::vector<uint8_t> packed(
+      packed_base,
+      packed_base + data->d_size / sizeof(packed[0]));
+
+  if ((section_header->sh_type == SHT_ANDROID_RELA || section_header->sh_type == SHT_ANDROID_REL) &&
+      packed.size() > 3 &&
+      packed[0] == 'A' &&
+      packed[1] == 'P' &&
+      (packed[2] == 'U' || packed[2] == 'S') &&
+      packed[3] == '2') {
+    LOG(INFO) << "Relocations   : " << (relocations_type_ == REL ? "REL" : "RELA");
+  } else {
+    LOG(ERROR) << "Packed relocations not found (not packed?)";
+    return false;
+  }
+
+  return UnpackTypedRelocations(packed);
+}
+
+// Helper for UnpackRelocations().  Rel type is one of ELF::Rel or ELF::Rela.
+template <typename ELF>
+bool ElfFile<ELF>::UnpackTypedRelocations(const std::vector<uint8_t>& packed) {
+  // Unpack the data to re-materialize the relative relocations.
+  const size_t packed_bytes = packed.size() * sizeof(packed[0]);
+  LOG(INFO) << "Packed           : " << packed_bytes << " bytes";
+  std::vector<typename ELF::Rela> unpacked_relocations;
+  RelocationPacker<ELF> packer;
+  packer.UnpackRelocations(packed, &unpacked_relocations);
+
+  const size_t relocation_entry_size =
+      relocations_type_ == REL ? sizeof(typename ELF::Rel) : sizeof(typename ELF::Rela);
+  const size_t unpacked_bytes = unpacked_relocations.size() * relocation_entry_size;
+  LOG(INFO) << "Unpacked         : " << unpacked_bytes << " bytes";
+
+  // Retrieve the current dynamic relocations section data.
+  Elf_Data* data = GetSectionData(relocations_section_);
+
+  LOG(INFO) << "Relocations      : " << unpacked_relocations.size() << " entries";
+
+  // If we found the same number of null relocation entries in the dynamic
+  // relocations section as we hold as unpacked relative relocations, then
+  // this is a padded file.
+
+  const bool is_padded = packed_bytes == unpacked_bytes;
+
+  // Unless padded, pre-apply relative relocations to account for the
+  // hole, and pre-adjust all relocation offsets accordingly.
+  typename ELF::Shdr* section_header = ELF::getshdr(relocations_section_);
+
+  if (!is_padded) {
+    LOG(INFO) << "Expansion     : " << unpacked_bytes - packed_bytes << " bytes";
+  }
+
+  // Rewrite the current dynamic relocations section with unpacked version of
+  // relocations.
+  const void* section_data = nullptr;
+  std::vector<typename ELF::Rel> unpacked_rel_relocations;
+  if (relocations_type_ == RELA) {
+    section_data = &unpacked_relocations[0];
+  } else if (relocations_type_ == REL) {
+    ConvertRelaVectorToRelVector(unpacked_relocations, &unpacked_rel_relocations);
+    section_data = &unpacked_rel_relocations[0];
+  } else {
+    NOTREACHED();
+  }
+
+  ResizeSection(elf_, relocations_section_, unpacked_bytes,
+      relocations_type_ == REL ? SHT_REL : SHT_RELA, relocations_type_);
+  RewriteSectionData(relocations_section_, section_data, unpacked_bytes);
+
+  // Rewrite .dynamic to remove two tags describing packed android relocations.
+  data = GetSectionData(dynamic_section_);
+  const typename ELF::Dyn* dynamic_base = reinterpret_cast<typename ELF::Dyn*>(data->d_buf);
+  std::vector<typename ELF::Dyn> dynamics(
+      dynamic_base,
+      dynamic_base + data->d_size / sizeof(dynamics[0]));
+  {
+    typename ELF::Dyn dyn;
+    dyn.d_tag = relocations_type_ == REL ? DT_REL : DT_RELA;
+    dyn.d_un.d_ptr = section_header->sh_addr;
+    ReplaceDynamicEntry<ELF>(relocations_type_ == REL ? DT_ANDROID_REL : DT_ANDROID_RELA,
+        dyn, &dynamics);
+  }
+
+  {
+    typename ELF::Dyn dyn;
+    dyn.d_tag = relocations_type_ == REL ? DT_RELSZ : DT_RELASZ;
+    dyn.d_un.d_val = section_header->sh_size;
+    ReplaceDynamicEntry<ELF>(relocations_type_ == REL ? DT_ANDROID_RELSZ : DT_ANDROID_RELASZ,
+        dyn, &dynamics);
+  }
+
+  const void* dynamics_data = &dynamics[0];
+  const size_t dynamics_bytes = dynamics.size() * sizeof(dynamics[0]);
+  RewriteSectionData(dynamic_section_, dynamics_data, dynamics_bytes);
+
+  Flush();
+  return true;
+}
+
+// Flush rewritten shared object file data.
+template <typename ELF>
+void ElfFile<ELF>::Flush() {
+  // Flag all ELF data held in memory as needing to be written back to the
+  // file, and tell libelf that we have controlled the file layout.
+  elf_flagelf(elf_, ELF_C_SET, ELF_F_DIRTY);
+  elf_flagelf(elf_, ELF_C_SET, ELF_F_LAYOUT);
+
+  // Write ELF data back to disk.
+  const off_t file_bytes = elf_update(elf_, ELF_C_WRITE);
+  if (file_bytes == -1) {
+    LOG(ERROR) << "elf_update failed: " << elf_errmsg(elf_errno());
+  }
+
+  CHECK(file_bytes > 0);
+  VLOG(1) << "elf_update returned: " << file_bytes;
+
+  // Clean up libelf, and truncate the output file to the number of bytes
+  // written by elf_update().
+  elf_end(elf_);
+  elf_ = NULL;
+  const int truncate = ftruncate(fd_, file_bytes);
+  CHECK(truncate == 0);
+}
+
+template <typename ELF>
+void ElfFile<ELF>::ConvertRelArrayToRelaVector(const typename ELF::Rel* rel_array,
+                                               size_t rel_array_size,
+                                               std::vector<typename ELF::Rela>* rela_vector) {
+  for (size_t i = 0; i<rel_array_size; ++i) {
+    typename ELF::Rela rela;
+    rela.r_offset = rel_array[i].r_offset;
+    rela.r_info = rel_array[i].r_info;
+    rela.r_addend = 0;
+    rela_vector->push_back(rela);
+  }
+}
+
+template <typename ELF>
+void ElfFile<ELF>::ConvertRelaVectorToRelVector(const std::vector<typename ELF::Rela>& rela_vector,
+                                                std::vector<typename ELF::Rel>* rel_vector) {
+  for (auto rela : rela_vector) {
+    typename ELF::Rel rel;
+    rel.r_offset = rela.r_offset;
+    rel.r_info = rela.r_info;
+    CHECK(rela.r_addend == 0);
+    rel_vector->push_back(rel);
+  }
+}
+
+template class ElfFile<ELF32_traits>;
+template class ElfFile<ELF64_traits>;
+
+}  // namespace relocation_packer