Import relocation packer from chromium repo

Bug: 18051137
Change-Id: Ia67fa11da8247e3f86f70a8ce99e6695f2c05423

1 files changed, 1283 insertions, 0 deletions

diff --git a/tools/relocation_packer/src/elf_file.cc b/tools/relocation_packer/src/elf_file.cc
new file mode 100644
index 0000000..3ffccec
--- /dev/null
+++ b/tools/relocation_packer/src/elf_file.cc
@@ -0,0 +1,1283 @@
+// 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 ensure that code and data loads at the same addresses as before
+// packing, so that tools like breakpad can still match up addresses found
+// in any crash dumps with data extracted from the pre-packed version of
+// the shared library.
+//
+// Arranging this means that we have to split one of the LOAD segments into
+// two.  Unfortunately, the program headers are located at the very start
+// of the shared library file, so expanding the program header section
+// would cause a lot of consequent changes to files offsets that we don't
+// really want to have to handle.
+//
+// Luckily, though, there is a segment that is always present and always
+// unused on Android; the GNU_STACK segment.  What we do is to steal that
+// and repurpose it to be one of the split LOAD segments.  We then have to
+// sort LOAD segments by offset to keep the crazy linker happy.
+//
+// All of this takes place in SplitProgramHeadersForHole(), used on packing,
+// and is unraveled on unpacking in CoalesceProgramHeadersForHole().  See
+// commentary on those functions for an example of this segment stealing
+// in action.
+
+#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 {
+
+// Stub identifier written to 'null out' packed data, "NULL".
+static const uint32_t kStubIdentifier = 0x4c4c554eu;
+
+// Out-of-band dynamic tags used to indicate the offset and size of the
+// android packed relocations section.
+static const ELF::Sword DT_ANDROID_REL_OFFSET = DT_LOOS;
+static const ELF::Sword DT_ANDROID_REL_SIZE = DT_LOOS + 1;
+
+// 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 const size_t kPreserveAlignment = 4096;
+
+namespace {
+
+// 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.
+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.
+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.
+void VerboseLogElfHeader(const ELF::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.
+void VerboseLogProgramHeader(size_t program_header_index,
+                             const ELF::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.
+void VerboseLogSectionHeader(const std::string& section_name,
+                             const ELF::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_addralign = " << section_header->sh_addralign;
+}
+
+// Verbose ELF section data logging.
+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;
+}
+
+}  // namespace
+
+// 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.
+bool ElfFile::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;
+  }
+
+  ELF::Ehdr* 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_machine != ELF::kMachine) {
+    LOG(ERROR) << "ELF file architecture is not " << ELF::Machine();
+    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__);
+
+  // Also require that the file class is as expected.
+  const int file_class = elf_header->e_ident[EI_CLASS];
+  CHECK(file_class == ELF::kFileClass);
+
+  VLOG(1) << "endian = " << endian << ", file class = " << file_class;
+  VerboseLogElfHeader(elf_header);
+
+  const ELF::Phdr* elf_program_header = ELF::getphdr(elf);
+  CHECK(elf_program_header);
+
+  const ELF::Phdr* dynamic_program_header = NULL;
+  for (size_t i = 0; i < elf_header->e_phnum; ++i) {
+    const ELF::Phdr* 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 != NULL);
+
+  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 = NULL;
+  Elf_Scn* found_android_relocations_section = NULL;
+  Elf_Scn* found_dynamic_section = NULL;
+
+  // 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)) != NULL) {
+    const ELF::Shdr* 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) {
+      has_rel_relocations = true;
+    }
+    if (section_header->sh_type == SHT_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 ((name == ".android.rel.dyn" || name == ".android.rela.dyn") &&
+        section_header->sh_size > 0) {
+      found_android_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_android_relocations_section) {
+    LOG(ERROR) << "Missing or empty .android.rel.dyn or .android.rela.dyn "
+               << "section (to fix, run with --help and follow the "
+               << "pre-packing instructions)";
+    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;
+  android_relocations_section_ = found_android_relocations_section;
+  relocations_type_ = has_rel_relocations ? REL : RELA;
+  return true;
+}
+
+namespace {
+
+// Helper for ResizeSection().  Adjust the main ELF header for the hole.
+void AdjustElfHeaderForHole(ELF::Ehdr* elf_header,
+                            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.
+void AdjustSectionHeadersForHole(Elf* elf,
+                                 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) {
+    ELF::Shdr* 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;
+    }
+  }
+}
+
+// Helper for ResizeSection().  Adjust the offsets of any program headers
+// that have offsets currently beyond the hole start.
+void AdjustProgramHeaderOffsets(ELF::Phdr* program_headers,
+                                size_t count,
+                                ELF::Phdr* ignored_1,
+                                ELF::Phdr* ignored_2,
+                                ELF::Off hole_start,
+                                ssize_t hole_size) {
+  for (size_t i = 0; i < count; ++i) {
+    ELF::Phdr* program_header = &program_headers[i];
+
+    if (program_header == ignored_1 || program_header == ignored_2)
+      continue;
+
+    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;
+    }
+  }
+}
+
+// Helper for ResizeSection().  Find the first loadable segment in the
+// file.  We expect it to map from file offset zero.
+ELF::Phdr* FindFirstLoadSegment(ELF::Phdr* program_headers,
+                                size_t count) {
+  ELF::Phdr* first_loadable_segment = NULL;
+
+  for (size_t i = 0; i < count; ++i) {
+    ELF::Phdr* program_header = &program_headers[i];
+
+    if (program_header->p_type == PT_LOAD &&
+        program_header->p_offset == 0 &&
+        program_header->p_vaddr == 0 &&
+        program_header->p_paddr == 0) {
+      first_loadable_segment = program_header;
+    }
+  }
+  LOG_IF(FATAL, !first_loadable_segment)
+      << "Cannot locate a LOAD segment with address and offset zero";
+
+  return first_loadable_segment;
+}
+
+// Helper for ResizeSection().  Find the PT_GNU_STACK segment, and check
+// that it contains what we expect so we can restore it on unpack if needed.
+ELF::Phdr* FindUnusedGnuStackSegment(ELF::Phdr* program_headers,
+                                     size_t count) {
+  ELF::Phdr* unused_segment = NULL;
+
+  for (size_t i = 0; i < count; ++i) {
+    ELF::Phdr* program_header = &program_headers[i];
+
+    if (program_header->p_type == PT_GNU_STACK &&
+        program_header->p_offset == 0 &&
+        program_header->p_vaddr == 0 &&
+        program_header->p_paddr == 0 &&
+        program_header->p_filesz == 0 &&
+        program_header->p_memsz == 0 &&
+        program_header->p_flags == (PF_R | PF_W) &&
+        program_header->p_align == ELF::kGnuStackSegmentAlignment) {
+      unused_segment = program_header;
+    }
+  }
+  LOG_IF(FATAL, !unused_segment)
+      << "Cannot locate the expected GNU_STACK segment";
+
+  return unused_segment;
+}
+
+// Helper for ResizeSection().  Find the segment that was the first loadable
+// one before we split it into two.  This is the one into which we coalesce
+// the split segments on unpacking.
+ELF::Phdr* FindOriginalFirstLoadSegment(ELF::Phdr* program_headers,
+                                        size_t count) {
+  const ELF::Phdr* first_loadable_segment =
+      FindFirstLoadSegment(program_headers, count);
+
+  ELF::Phdr* original_first_loadable_segment = NULL;
+
+  for (size_t i = 0; i < count; ++i) {
+    ELF::Phdr* program_header = &program_headers[i];
+
+    // The original first loadable segment is the one that follows on from
+    // the one we wrote on split to be the current first loadable segment.
+    if (program_header->p_type == PT_LOAD &&
+        program_header->p_offset == first_loadable_segment->p_filesz) {
+      original_first_loadable_segment = program_header;
+    }
+  }
+  LOG_IF(FATAL, !original_first_loadable_segment)
+      << "Cannot locate the LOAD segment that follows a LOAD at offset zero";
+
+  return original_first_loadable_segment;
+}
+
+// Helper for ResizeSection().  Find the segment that contains the hole.
+Elf_Scn* FindSectionContainingHole(Elf* elf,
+                                   ELF::Off hole_start,
+                                   ssize_t hole_size) {
+  Elf_Scn* section = NULL;
+  Elf_Scn* last_unholed_section = NULL;
+
+  while ((section = elf_nextscn(elf, section)) != NULL) {
+    const ELF::Shdr* section_header = ELF::getshdr(section);
+
+    // Because we get here after section headers have been adjusted for the
+    // hole, we need to 'undo' that adjustment to give a view of the original
+    // sections layout.
+    ELF::Off offset = section_header->sh_offset;
+    if (section_header->sh_offset >= hole_start) {
+      offset -= hole_size;
+    }
+
+    if (offset <= hole_start) {
+      last_unholed_section = section;
+    }
+  }
+  LOG_IF(FATAL, !last_unholed_section)
+      << "Cannot identify the section before the one containing the hole";
+
+  // The section containing the hole is the one after the last one found
+  // by the loop above.
+  Elf_Scn* holed_section = elf_nextscn(elf, last_unholed_section);
+  LOG_IF(FATAL, !holed_section)
+      << "Cannot identify the section containing the hole";
+
+  return holed_section;
+}
+
+// Helper for ResizeSection().  Find the last section contained in a segment.
+Elf_Scn* FindLastSectionInSegment(Elf* elf,
+                                  ELF::Phdr* program_header,
+                                  ELF::Off hole_start,
+                                  ssize_t hole_size) {
+  const ELF::Off segment_end =
+      program_header->p_offset + program_header->p_filesz;
+
+  Elf_Scn* section = NULL;
+  Elf_Scn* last_section = NULL;
+
+  while ((section = elf_nextscn(elf, section)) != NULL) {
+    const ELF::Shdr* section_header = ELF::getshdr(section);
+
+    // As above, 'undo' any section offset adjustment to give a view of the
+    // original sections layout.
+    ELF::Off offset = section_header->sh_offset;
+    if (section_header->sh_offset >= hole_start) {
+      offset -= hole_size;
+    }
+
+    if (offset < segment_end) {
+      last_section = section;
+    }
+  }
+  LOG_IF(FATAL, !last_section)
+      << "Cannot identify the last section in the given segment";
+
+  return last_section;
+}
+
+// Helper for ResizeSection().  Order loadable segments by their offsets.
+// The crazy linker contains assumptions about loadable segment ordering,
+// and it is better if we do not break them.
+void SortOrderSensitiveProgramHeaders(ELF::Phdr* program_headers,
+                                      size_t count) {
+  std::vector<ELF::Phdr*> orderable;
+
+  // Collect together orderable program headers.  These are all the LOAD
+  // segments, and any GNU_STACK that may be present (removed on packing,
+  // but replaced on unpacking).
+  for (size_t i = 0; i < count; ++i) {
+    ELF::Phdr* program_header = &program_headers[i];
+
+    if (program_header->p_type == PT_LOAD ||
+        program_header->p_type == PT_GNU_STACK) {
+      orderable.push_back(program_header);
+    }
+  }
+
+  // Order these program headers so that any PT_GNU_STACK is last, and
+  // the LOAD segments that precede it appear in offset order.  Uses
+  // insertion sort.
+  for (size_t i = 1; i < orderable.size(); ++i) {
+    for (size_t j = i; j > 0; --j) {
+      ELF::Phdr* first = orderable[j - 1];
+      ELF::Phdr* second = orderable[j];
+
+      if (!(first->p_type == PT_GNU_STACK ||
+            first->p_offset > second->p_offset)) {
+        break;
+      }
+      std::swap(*first, *second);
+    }
+  }
+}
+
+// Helper for ResizeSection().  The GNU_STACK program header is unused in
+// Android, so we can repurpose it here.  Before packing, the program header
+// table contains something like:
+//
+//   Type      Offset    VirtAddr   PhysAddr   FileSiz   MemSiz    Flg Align
+//   LOAD      0x000000  0x00000000 0x00000000 0x1efc818 0x1efc818 R E 0x1000
+//   LOAD      0x1efd008 0x01efe008 0x01efe008 0x17ec3c  0x1a0324  RW  0x1000
+//   DYNAMIC   0x205ec50 0x0205fc50 0x0205fc50 0x00108   0x00108   RW  0x4
+//   GNU_STACK 0x000000  0x00000000 0x00000000 0x00000   0x00000   RW  0
+//
+// The hole in the file is in the first of these.  In order to preserve all
+// load addresses, what we do is to turn the GNU_STACK into a new LOAD entry
+// that maps segments up to where we created the hole, adjust the first LOAD
+// entry so that it maps segments after that, adjust any other program
+// headers whose offset is after the hole start, and finally order the LOAD
+// segments by offset, to give:
+//
+//   Type      Offset    VirtAddr   PhysAddr   FileSiz   MemSiz    Flg Align
+//   LOAD      0x000000  0x00000000 0x00000000 0x14ea4   0x14ea4   R E 0x1000
+//   LOAD      0x014ea4  0x00212ea4 0x00212ea4 0x1cea164 0x1cea164 R E 0x1000
+//   DYNAMIC   0x1e60c50 0x0205fc50 0x0205fc50 0x00108   0x00108   RW  0x4
+//   LOAD      0x1cff008 0x01efe008 0x01efe008 0x17ec3c  0x1a0324  RW  0x1000
+//
+// We work out the split points by finding the .rel.dyn or .rela.dyn section
+// that contains the hole, and by finding the last section in a given segment.
+//
+// To unpack, we reverse the above to leave the file as it was originally.
+void SplitProgramHeadersForHole(Elf* elf,
+                                ELF::Off hole_start,
+                                ssize_t hole_size) {
+  CHECK(hole_size < 0);
+  const ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  CHECK(elf_header);
+
+  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.
+  ELF::Phdr* spliced_header =
+      FindUnusedGnuStackSegment(elf_program_header, program_header_count);
+  ELF::Phdr* split_header =
+      FindFirstLoadSegment(elf_program_header, program_header_count);
+
+  VLOG(1) << "phdr[" << split_header - elf_program_header << "] split";
+  VLOG(1) << "phdr[" << spliced_header - elf_program_header << "] new LOAD";
+
+  // Find the section that contains the hole.  We split on the section that
+  // follows it.
+  Elf_Scn* holed_section =
+      FindSectionContainingHole(elf, hole_start, hole_size);
+
+  size_t string_index;
+  elf_getshdrstrndx(elf, &string_index);
+
+  ELF::Shdr* section_header = ELF::getshdr(holed_section);
+  std::string name = elf_strptr(elf, string_index, section_header->sh_name);
+  VLOG(1) << "section " << name << " split after";
+
+  // Find the last section in the segment we are splitting.
+  Elf_Scn* last_section =
+      FindLastSectionInSegment(elf, split_header, hole_start, hole_size);
+
+  section_header = ELF::getshdr(last_section);
+  name = elf_strptr(elf, string_index, section_header->sh_name);
+  VLOG(1) << "section " << name << " split end";
+
+  // Split on the section following the holed one, and up to (but not
+  // including) the section following the last one in the split segment.
+  Elf_Scn* split_section = elf_nextscn(elf, holed_section);
+  LOG_IF(FATAL, !split_section)
+      << "No section follows the section that contains the hole";
+  Elf_Scn* end_section = elf_nextscn(elf, last_section);
+  LOG_IF(FATAL, !end_section)
+      << "No section follows the last section in the segment being split";
+
+  // Split the first portion of split_header into spliced_header.
+  const ELF::Shdr* split_section_header = ELF::getshdr(split_section);
+  spliced_header->p_type = split_header->p_type;
+  spliced_header->p_offset = split_header->p_offset;
+  spliced_header->p_vaddr = split_header->p_vaddr;
+  spliced_header->p_paddr = split_header->p_paddr;
+  CHECK(split_header->p_filesz == split_header->p_memsz);
+  spliced_header->p_filesz = split_section_header->sh_offset;
+  spliced_header->p_memsz = split_section_header->sh_offset;
+  spliced_header->p_flags = split_header->p_flags;
+  spliced_header->p_align = split_header->p_align;
+
+  // Now rewrite split_header to remove the part we spliced from it.
+  const ELF::Shdr* end_section_header = ELF::getshdr(end_section);
+  split_header->p_offset = spliced_header->p_filesz;
+  CHECK(split_header->p_vaddr == split_header->p_paddr);
+  split_header->p_vaddr = split_section_header->sh_addr;
+  split_header->p_paddr = split_section_header->sh_addr;
+  CHECK(split_header->p_filesz == split_header->p_memsz);
+  split_header->p_filesz =
+      end_section_header->sh_offset - spliced_header->p_filesz;
+  split_header->p_memsz =
+      end_section_header->sh_offset - spliced_header->p_filesz;
+
+  // Adjust the offsets of all program headers that are not one of the pair
+  // we just created by splitting.
+  AdjustProgramHeaderOffsets(elf_program_header,
+                             program_header_count,
+                             spliced_header,
+                             split_header,
+                             hole_start,
+                             hole_size);
+
+  // Finally, order loadable segments by offset/address.  The crazy linker
+  // contains assumptions about loadable segment ordering.
+  SortOrderSensitiveProgramHeaders(elf_program_header,
+                                   program_header_count);
+}
+
+// Helper for ResizeSection().  Undo the work of SplitProgramHeadersForHole().
+void CoalesceProgramHeadersForHole(Elf* elf,
+                                   ELF::Off hole_start,
+                                   ssize_t hole_size) {
+  CHECK(hole_size > 0);
+  const ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  CHECK(elf_header);
+
+  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 overwrote to form the new LOAD entry, and
+  // the segment that we split into two parts on packing.
+  ELF::Phdr* spliced_header =
+      FindFirstLoadSegment(elf_program_header, program_header_count);
+  ELF::Phdr* split_header =
+      FindOriginalFirstLoadSegment(elf_program_header, program_header_count);
+
+  VLOG(1) << "phdr[" << spliced_header - elf_program_header << "] stack";
+  VLOG(1) << "phdr[" << split_header - elf_program_header << "] coalesce";
+
+  // Find the last section in the second segment we are coalescing.
+  Elf_Scn* last_section =
+      FindLastSectionInSegment(elf, split_header, hole_start, hole_size);
+
+  size_t string_index;
+  elf_getshdrstrndx(elf, &string_index);
+
+  const ELF::Shdr* section_header = ELF::getshdr(last_section);
+  std::string name = elf_strptr(elf, string_index, section_header->sh_name);
+  VLOG(1) << "section " << name << " coalesced";
+
+  // Rewrite the coalesced segment into split_header.
+  const ELF::Shdr* last_section_header = ELF::getshdr(last_section);
+  split_header->p_offset = spliced_header->p_offset;
+  CHECK(split_header->p_vaddr == split_header->p_paddr);
+  split_header->p_vaddr = spliced_header->p_vaddr;
+  split_header->p_paddr = spliced_header->p_vaddr;
+  CHECK(split_header->p_filesz == split_header->p_memsz);
+  split_header->p_filesz =
+      last_section_header->sh_offset + last_section_header->sh_size;
+  split_header->p_memsz =
+      last_section_header->sh_offset + last_section_header->sh_size;
+
+  // Reconstruct the original GNU_STACK segment into spliced_header.
+  spliced_header->p_type = PT_GNU_STACK;
+  spliced_header->p_offset = 0;
+  spliced_header->p_vaddr = 0;
+  spliced_header->p_paddr = 0;
+  spliced_header->p_filesz = 0;
+  spliced_header->p_memsz = 0;
+  spliced_header->p_flags = PF_R | PF_W;
+  spliced_header->p_align = ELF::kGnuStackSegmentAlignment;
+
+  // Adjust the offsets of all program headers that are not one of the pair
+  // we just coalesced.
+  AdjustProgramHeaderOffsets(elf_program_header,
+                             program_header_count,
+                             spliced_header,
+                             split_header,
+                             hole_start,
+                             hole_size);
+
+  // Finally, order loadable segments by offset/address.  The crazy linker
+  // contains assumptions about loadable segment ordering.
+  SortOrderSensitiveProgramHeaders(elf_program_header,
+                                   program_header_count);
+}
+
+// Helper for ResizeSection().  Rewrite program headers.
+void RewriteProgramHeadersForHole(Elf* elf,
+                                  ELF::Off hole_start,
+                                  ssize_t hole_size) {
+  // If hole_size is negative then we are removing a piece of the file, and
+  // we want to split program headers so that we keep the same addresses
+  // for text and data.  If positive, then we are putting that piece of the
+  // file back in, so we coalesce the previously split program headers.
+  if (hole_size < 0)
+    SplitProgramHeadersForHole(elf, hole_start, hole_size);
+  else if (hole_size > 0)
+    CoalesceProgramHeadersForHole(elf, hole_start, hole_size);
+}
+
+// Helper for ResizeSection().  Locate and return the dynamic section.
+Elf_Scn* GetDynamicSection(Elf* elf) {
+  const ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  CHECK(elf_header);
+
+  const ELF::Phdr* elf_program_header = ELF::getphdr(elf);
+  CHECK(elf_program_header);
+
+  // Find the program header that describes the dynamic section.
+  const ELF::Phdr* dynamic_program_header = NULL;
+  for (size_t i = 0; i < elf_header->e_phnum; ++i) {
+    const 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) {
+    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 Rel>
+void AdjustDynamicSectionForHole(Elf_Scn* dynamic_section,
+                                 ELF::Off hole_start,
+                                 ssize_t hole_size) {
+  Elf_Data* data = GetSectionData(dynamic_section);
+
+  const ELF::Dyn* dynamic_base = reinterpret_cast<ELF::Dyn*>(data->d_buf);
+  std::vector<ELF::Dyn> dynamics(
+      dynamic_base,
+      dynamic_base + data->d_size / sizeof(dynamics[0]));
+
+  for (size_t i = 0; i < dynamics.size(); ++i) {
+    ELF::Dyn* dynamic = &dynamics[i];
+    const ELF::Sword tag = dynamic->d_tag;
+
+    // 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) {
+      dynamic->d_un.d_val += hole_size;
+      VLOG(1) << "dynamic[" << i << "] " << dynamic->d_tag
+              << " d_val adjusted to " << dynamic->d_un.d_val;
+    }
+
+    // DT_RELCOUNT or DT_RELACOUNT hold the count of relative relocations.
+    // Only one will be present.  Packing reduces it to the alignment
+    // padding, if any; unpacking restores it to its former value.  The
+    // crazy linker does not use it, but we update it anyway.
+    if (tag == DT_RELCOUNT || tag == DT_RELACOUNT) {
+      // Cast sizeof to a signed type to avoid the division result being
+      // promoted into an unsigned size_t.
+      const ssize_t sizeof_rel = static_cast<ssize_t>(sizeof(Rel));
+      dynamic->d_un.d_val += hole_size / sizeof_rel;
+      VLOG(1) << "dynamic[" << i << "] " << dynamic->d_tag
+              << " d_val adjusted to " << dynamic->d_un.d_val;
+    }
+
+    // DT_RELENT and DT_RELAENT do not change, but make sure they are what
+    // we expect.  Only one will be present.
+    if (tag == DT_RELENT || tag == DT_RELAENT) {
+      CHECK(dynamic->d_un.d_val == sizeof(Rel));
+    }
+  }
+
+  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 Rel>
+void ResizeSection(Elf* elf, Elf_Scn* section, size_t new_size) {
+  ELF::Shdr* section_header = ELF::getshdr(section);
+  if (section_header->sh_size == new_size)
+    return;
+
+  // Note if we are resizing the real dyn relocations.
+  size_t string_index;
+  elf_getshdrstrndx(elf, &string_index);
+  const std::string section_name =
+      elf_strptr(elf, string_index, section_header->sh_name);
+  const bool is_relocations_resize =
+      (section_name == ".rel.dyn" || section_name == ".rela.dyn");
+
+  // 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 ELF::Off hole_start = section_header->sh_offset;
+  const ssize_t hole_size = new_size - data->d_size;
+
+  VLOG_IF(1, (hole_size > 0)) << "expand section size = " << data->d_size;
+  VLOG_IF(1, (hole_size < 0)) << "shrink section size = " << data->d_size;
+
+  // Resize the data and the section header.
+  data->d_size += hole_size;
+  section_header->sh_size += hole_size;
+
+  // 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.
+  ELF::Ehdr* elf_header = ELF::getehdr(elf);
+  AdjustElfHeaderForHole(elf_header, hole_start, hole_size);
+
+  // Adjust all section headers.
+  AdjustSectionHeadersForHole(elf, hole_start, hole_size);
+
+  // If resizing the dynamic relocations, rewrite the program headers to
+  // either split or coalesce segments, and adjust dynamic entries to match.
+  if (is_relocations_resize) {
+    RewriteProgramHeadersForHole(elf, hole_start, hole_size);
+
+    Elf_Scn* dynamic_section = GetDynamicSection(elf);
+    AdjustDynamicSectionForHole<Rel>(dynamic_section, hole_start, hole_size);
+  }
+}
+
+// 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.
+size_t FindDynamicEntry(ELF::Sword tag,
+                        std::vector<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 the first free (unused) slot in a dynamics vector with the given
+// value.  The vector always ends with a free (unused) element, so the slot
+// found cannot be the last one in the vector.
+void AddDynamicEntry(const ELF::Dyn& dyn,
+                     std::vector<ELF::Dyn>* dynamics) {
+  const size_t slot = FindDynamicEntry(DT_NULL, dynamics);
+  if (slot == dynamics->size()) {
+    LOG(FATAL) << "No spare dynamic array slots found "
+               << "(to fix, increase gold's --spare-dynamic-tags value)";
+  }
+
+  // Replace this entry with the one supplied.
+  dynamics->at(slot) = dyn;
+  VLOG(1) << "dynamic[" << slot << "] overwritten with " << dyn.d_tag;
+}
+
+// Remove the element in the dynamics vector that matches the given tag with
+// unused slot data.  Shuffle the following elements up, and ensure that the
+// last is the null sentinel.
+void RemoveDynamicEntry(ELF::Sword tag,
+                        std::vector<ELF::Dyn>* dynamics) {
+  const size_t slot = FindDynamicEntry(tag, dynamics);
+  CHECK(slot != dynamics->size());
+
+  // Remove this entry by shuffling up everything that follows.
+  for (size_t i = slot; i < dynamics->size() - 1; ++i) {
+    dynamics->at(i) = dynamics->at(i + 1);
+    VLOG(1) << "dynamic[" << i
+            << "] overwritten with dynamic[" << i + 1 << "]";
+  }
+
+  // Ensure that the end sentinel is still present.
+  CHECK(dynamics->at(dynamics->size() - 1).d_tag == DT_NULL);
+}
+
+// Construct a null relocation without addend.
+void NullRelocation(ELF::Rel* relocation) {
+  relocation->r_offset = 0;
+  relocation->r_info = ELF_R_INFO(0, ELF::kNoRelocationCode);
+}
+
+// Construct a null relocation with addend.
+void NullRelocation(ELF::Rela* relocation) {
+  relocation->r_offset = 0;
+  relocation->r_info = ELF_R_INFO(0, ELF::kNoRelocationCode);
+  relocation->r_addend = 0;
+}
+
+// Pad relocations with the given number of null entries.  Generates its
+// null entry with the appropriate NullRelocation() invocation.
+template <typename Rel>
+void PadRelocations(size_t count, std::vector<Rel>* relocations) {
+  Rel null_relocation;
+  NullRelocation(&null_relocation);
+  std::vector<Rel> padding(count, null_relocation);
+  relocations->insert(relocations->end(), padding.begin(), padding.end());
+}
+
+}  // namespace
+
+// Remove relative entries from dynamic relocations and write as packed
+// data into android packed relocations.
+bool ElfFile::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_);
+
+  if (relocations_type_ == REL) {
+    // Convert data to a vector of relocations.
+    const ELF::Rel* relocations_base = reinterpret_cast<ELF::Rel*>(data->d_buf);
+    std::vector<ELF::Rel> relocations(
+        relocations_base,
+        relocations_base + data->d_size / sizeof(relocations[0]));
+
+    LOG(INFO) << "Relocations   : REL";
+    return PackTypedRelocations<ELF::Rel>(relocations);
+  }
+
+  if (relocations_type_ == RELA) {
+    // Convert data to a vector of relocations with addends.
+    const ELF::Rela* relocations_base =
+        reinterpret_cast<ELF::Rela*>(data->d_buf);
+    std::vector<ELF::Rela> relocations(
+        relocations_base,
+        relocations_base + data->d_size / sizeof(relocations[0]));
+
+    LOG(INFO) << "Relocations   : RELA";
+    return PackTypedRelocations<ELF::Rela>(relocations);
+  }
+
+  NOTREACHED();
+  return false;
+}
+
+// Helper for PackRelocations().  Rel type is one of ELF::Rel or ELF::Rela.
+template <typename Rel>
+bool ElfFile::PackTypedRelocations(const std::vector<Rel>& relocations) {
+  // Filter relocations into those that are relative and others.
+  std::vector<Rel> relative_relocations;
+  std::vector<Rel> other_relocations;
+
+  for (size_t i = 0; i < relocations.size(); ++i) {
+    const Rel& relocation = relocations[i];
+    if (ELF_R_TYPE(relocation.r_info) == ELF::kRelativeRelocationCode) {
+      CHECK(ELF_R_SYM(relocation.r_info) == 0);
+      relative_relocations.push_back(relocation);
+    } else {
+      other_relocations.push_back(relocation);
+    }
+  }
+  LOG(INFO) << "Relative      : " << relative_relocations.size() << " entries";
+  LOG(INFO) << "Other         : " << other_relocations.size() << " entries";
+  LOG(INFO) << "Total         : " << relocations.size() << " entries";
+
+  // If no relative relocations then we have nothing packable.  Perhaps
+  // the shared object has already been packed?
+  if (relative_relocations.empty()) {
+    LOG(ERROR) << "No relative relocations found (already packed?)";
+    return false;
+  }
+
+  // If not padding fully, apply only enough padding to preserve alignment.
+  // Otherwise, pad so that we do not shrink the relocations section at all.
+  if (!is_padding_relocations_) {
+    // Calculate the size of the hole we will close up when we rewrite
+    // dynamic relocations.
+    ssize_t hole_size =
+        relative_relocations.size() * sizeof(relative_relocations[0]);
+    const ssize_t unaligned_hole_size = hole_size;
+
+    // Adjust the actual hole size to preserve alignment.  We always adjust
+    // by a whole number of NONE-type relocations.
+    while (hole_size % kPreserveAlignment)
+      hole_size -= sizeof(relative_relocations[0]);
+    LOG(INFO) << "Compaction    : " << hole_size << " bytes";
+
+    // Adjusting for alignment may have removed any packing benefit.
+    if (hole_size == 0) {
+      LOG(INFO) << "Too few relative relocations to pack after alignment";
+      return false;
+    }
+
+    // Find the padding needed in other_relocations to preserve alignment.
+    // Ensure that we never completely empty the real relocations section.
+    size_t padding_bytes = unaligned_hole_size - hole_size;
+    if (padding_bytes == 0 && other_relocations.size() == 0) {
+      do {
+        padding_bytes += sizeof(relative_relocations[0]);
+      } while (padding_bytes % kPreserveAlignment);
+    }
+    CHECK(padding_bytes % sizeof(other_relocations[0]) == 0);
+    const size_t padding = padding_bytes / sizeof(other_relocations[0]);
+
+    // Padding may have removed any packing benefit.
+    if (padding >= relative_relocations.size()) {
+      LOG(INFO) << "Too few relative relocations to pack after padding";
+      return false;
+    }
+
+    // Add null relocations to other_relocations to preserve alignment.
+    PadRelocations<Rel>(padding, &other_relocations);
+    LOG(INFO) << "Alignment pad : " << padding << " relocations";
+  } else {
+    // If padding, add NONE-type relocations to other_relocations to make it
+    // the same size as the the original relocations we read in.  This makes
+    // the ResizeSection() below a no-op.
+    const size_t padding = relocations.size() - other_relocations.size();
+    PadRelocations<Rel>(padding, &other_relocations);
+  }
+
+  // Pack relative relocations.
+  const size_t initial_bytes =
+      relative_relocations.size() * sizeof(relative_relocations[0]);
+  LOG(INFO) << "Unpacked relative: " << initial_bytes << " bytes";
+  std::vector<uint8_t> packed;
+  RelocationPacker packer;
+  packer.PackRelativeRelocations(relative_relocations, &packed);
+  const void* packed_data = &packed[0];
+  const size_t packed_bytes = packed.size() * sizeof(packed[0]);
+  LOG(INFO) << "Packed   relative: " << packed_bytes << " bytes";
+
+  // If we have insufficient relative relocations to form a run then
+  // packing fails.
+  if (packed.empty()) {
+    LOG(INFO) << "Too few relative relocations to pack";
+    return false;
+  }
+
+  // Run a loopback self-test as a check that packing is lossless.
+  std::vector<Rel> unpacked;
+  packer.UnpackRelativeRelocations(packed, &unpacked);
+  CHECK(unpacked.size() == relative_relocations.size());
+  CHECK(!memcmp(&unpacked[0],
+                &relative_relocations[0],
+                unpacked.size() * sizeof(unpacked[0])));
+
+  // Make sure packing saved some space.
+  if (packed_bytes >= initial_bytes) {
+    LOG(INFO) << "Packing relative relocations saves no space";
+    return false;
+  }
+
+  // Rewrite the current dynamic relocations section to be only the ARM
+  // non-relative relocations, then shrink it to size.
+  const void* section_data = &other_relocations[0];
+  const size_t bytes = other_relocations.size() * sizeof(other_relocations[0]);
+  ResizeSection<Rel>(elf_, relocations_section_, bytes);
+  RewriteSectionData(relocations_section_, section_data, bytes);
+
+  // Rewrite the current packed android relocations section to hold the packed
+  // relative relocations.
+  ResizeSection<Rel>(elf_, android_relocations_section_, packed_bytes);
+  RewriteSectionData(android_relocations_section_, packed_data, packed_bytes);
+
+  // Rewrite .dynamic to include two new tags describing the packed android
+  // relocations.
+  Elf_Data* data = GetSectionData(dynamic_section_);
+  const ELF::Dyn* dynamic_base = reinterpret_cast<ELF::Dyn*>(data->d_buf);
+  std::vector<ELF::Dyn> dynamics(
+      dynamic_base,
+      dynamic_base + data->d_size / sizeof(dynamics[0]));
+  // Use two of the spare slots to describe the packed section.
+  ELF::Shdr* section_header = ELF::getshdr(android_relocations_section_);
+  {
+    ELF::Dyn dyn;
+    dyn.d_tag = DT_ANDROID_REL_OFFSET;
+    dyn.d_un.d_ptr = section_header->sh_offset;
+    AddDynamicEntry(dyn, &dynamics);
+  }
+  {
+    ELF::Dyn dyn;
+    dyn.d_tag = DT_ANDROID_REL_SIZE;
+    dyn.d_un.d_val = section_header->sh_size;
+    AddDynamicEntry(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.
+bool ElfFile::UnpackRelocations() {
+  // Load the ELF file into libelf.
+  if (!Load()) {
+    LOG(ERROR) << "Failed to load as ELF";
+    return false;
+  }
+
+  // Retrieve the current packed android relocations section data.
+  Elf_Data* data = GetSectionData(android_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 (packed.size() > 3 &&
+      packed[0] == 'A' &&
+      packed[1] == 'P' &&
+      packed[2] == 'R' &&
+      packed[3] == '1') {
+    // Signature is APR1, unpack relocations.
+    CHECK(relocations_type_ == REL);
+    LOG(INFO) << "Relocations   : REL";
+    return UnpackTypedRelocations<ELF::Rel>(packed);
+  }
+
+  if (packed.size() > 3 &&
+      packed[0] == 'A' &&
+      packed[1] == 'P' &&
+      packed[2] == 'A' &&
+      packed[3] == '1') {
+    // Signature is APA1, unpack relocations with addends.
+    CHECK(relocations_type_ == RELA);
+    LOG(INFO) << "Relocations   : RELA";
+    return UnpackTypedRelocations<ELF::Rela>(packed);
+  }
+
+  LOG(ERROR) << "Packed relative relocations not found (not packed?)";
+  return false;
+}
+
+// Helper for UnpackRelocations().  Rel type is one of ELF::Rel or ELF::Rela.
+template <typename Rel>
+bool ElfFile::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   relative: " << packed_bytes << " bytes";
+  std::vector<Rel> relative_relocations;
+  RelocationPacker packer;
+  packer.UnpackRelativeRelocations(packed, &relative_relocations);
+  const size_t unpacked_bytes =
+      relative_relocations.size() * sizeof(relative_relocations[0]);
+  LOG(INFO) << "Unpacked relative: " << unpacked_bytes << " bytes";
+
+  // Retrieve the current dynamic relocations section data.
+  Elf_Data* data = GetSectionData(relocations_section_);
+
+  // Interpret data as relocations.
+  const Rel* relocations_base = reinterpret_cast<Rel*>(data->d_buf);
+  std::vector<Rel> relocations(
+      relocations_base,
+      relocations_base + data->d_size / sizeof(relocations[0]));
+
+  std::vector<Rel> other_relocations;
+  size_t padding = 0;
+
+  // Filter relocations to locate any that are NONE-type.  These will occur
+  // if padding was turned on for packing.
+  for (size_t i = 0; i < relocations.size(); ++i) {
+    const Rel& relocation = relocations[i];
+    if (ELF_R_TYPE(relocation.r_info) != ELF::kNoRelocationCode) {
+      other_relocations.push_back(relocation);
+    } else {
+      ++padding;
+    }
+  }
+  LOG(INFO) << "Relative      : " << relative_relocations.size() << " entries";
+  LOG(INFO) << "Other         : " << other_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 = padding == relative_relocations.size();
+
+  // Unless padded, report by how much we expand the file.
+  if (!is_padded) {
+    // Calculate the size of the hole we will open up when we rewrite
+    // dynamic relocations.
+    ssize_t hole_size =
+        relative_relocations.size() * sizeof(relative_relocations[0]);
+
+    // Adjust the hole size for the padding added to preserve alignment.
+    hole_size -= padding * sizeof(other_relocations[0]);
+    LOG(INFO) << "Expansion     : " << hole_size << " bytes";
+  }
+
+  // Rewrite the current dynamic relocations section to be the relative
+  // relocations followed by other relocations.  This is the usual order in
+  // which we find them after linking, so this action will normally put the
+  // entire dynamic relocations section back to its pre-split-and-packed state.
+  relocations.assign(relative_relocations.begin(), relative_relocations.end());
+  relocations.insert(relocations.end(),
+                     other_relocations.begin(), other_relocations.end());
+  const void* section_data = &relocations[0];
+  const size_t bytes = relocations.size() * sizeof(relocations[0]);
+  LOG(INFO) << "Total         : " << relocations.size() << " entries";
+  ResizeSection<Rel>(elf_, relocations_section_, bytes);
+  RewriteSectionData(relocations_section_, section_data, bytes);
+
+  // Nearly empty the current packed android relocations section.  Leaves a
+  // four-byte stub so that some data remains allocated to the section.
+  // This is a convenience which allows us to re-pack this file again without
+  // having to remove the section and then add a new small one with objcopy.
+  // The way we resize sections relies on there being some data in a section.
+  ResizeSection<Rel>(
+      elf_, android_relocations_section_, sizeof(kStubIdentifier));
+  RewriteSectionData(
+      android_relocations_section_, &kStubIdentifier, sizeof(kStubIdentifier));
+
+  // Rewrite .dynamic to remove two tags describing packed android relocations.
+  data = GetSectionData(dynamic_section_);
+  const ELF::Dyn* dynamic_base = reinterpret_cast<ELF::Dyn*>(data->d_buf);
+  std::vector<ELF::Dyn> dynamics(
+      dynamic_base,
+      dynamic_base + data->d_size / sizeof(dynamics[0]));
+  RemoveDynamicEntry(DT_ANDROID_REL_OFFSET, &dynamics);
+  RemoveDynamicEntry(DT_ANDROID_REL_SIZE, &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.
+void ElfFile::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);
+  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);
+}
+
+}  // namespace relocation_packer