sdk/runtime/vm/elf.cc

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include "vm/elf.h"

#include "platform/elf.h"
#include "vm/cpu.h"
#include "vm/dwarf.h"
#include "vm/hash_map.h"
#include "vm/image_snapshot.h"
#include "vm/thread.h"
#include "vm/zone_text_buffer.h"

namespace dart {

// A wrapper around StreamingWriteStream that provides methods useful for
// writing ELF files (e.g., using ELF definitions of data sizes).
class ElfWriteStream : public ValueObject {
 public:
  explicit ElfWriteStream(StreamingWriteStream* stream)
      : stream_(ASSERT_NOTNULL(stream)) {}

  intptr_t position() const { return stream_->position(); }
  void Align(const intptr_t alignment) {
    ASSERT(Utils::IsPowerOfTwo(alignment));
    stream_->Align(alignment);
  }
  void WriteBytes(const uint8_t* b, intptr_t size) {
    stream_->WriteBytes(b, size);
  }
  void WriteByte(uint8_t value) {
    stream_->WriteBytes(reinterpret_cast<uint8_t*>(&value), sizeof(value));
  }
  void WriteHalf(uint16_t value) {
    stream_->WriteBytes(reinterpret_cast<uint8_t*>(&value), sizeof(value));
  }
  void WriteWord(uint32_t value) {
    stream_->WriteBytes(reinterpret_cast<uint8_t*>(&value), sizeof(value));
  }
  void WriteAddr(compiler::target::uword value) {
    stream_->WriteBytes(reinterpret_cast<uint8_t*>(&value), sizeof(value));
  }
  void WriteOff(compiler::target::uword value) {
    stream_->WriteBytes(reinterpret_cast<uint8_t*>(&value), sizeof(value));
  }
#if defined(TARGET_ARCH_IS_64_BIT)
  void WriteXWord(uint64_t value) {
    stream_->WriteBytes(reinterpret_cast<uint8_t*>(&value), sizeof(value));
  }
#endif

 private:
  StreamingWriteStream* const stream_;
};

static constexpr intptr_t kLinearInitValue = -1;

#define DEFINE_LINEAR_FIELD_METHODS(name)                                      \
  intptr_t name() const {                                                      \
    ASSERT(name##_ != kLinearInitValue);                                       \
    return name##_;                                                            \
  }                                                                            \
  bool name##_is_set() const { return name##_ != kLinearInitValue; }           \
  void set_##name(intptr_t value) {                                            \
    ASSERT(value != kLinearInitValue);                                         \
    ASSERT_EQUAL(name##_, kLinearInitValue);                                   \
    name##_ = value;                                                           \
  }

#define DEFINE_LINEAR_FIELD(name) intptr_t name##_ = kLinearInitValue;

class BitsContainer;
class Segment;

static constexpr intptr_t kDefaultAlignment = -1;
// Align note sections and segments to 4 byte boundries.
static constexpr intptr_t kNoteAlignment = 4;

class Section : public ZoneAllocated {
 public:
  Section(elf::SectionHeaderType t,
          bool allocate,
          bool executable,
          bool writable,
          intptr_t align = kDefaultAlignment)
      : type(t),
        flags(EncodeFlags(allocate, executable, writable)),
        alignment(align == kDefaultAlignment ? DefaultAlignment(t) : align),
        // Non-segments will never have a memory offset, here represented by 0.
        memory_offset_(allocate ? kLinearInitValue : 0) {
    // Only sections with type SHT_NULL are allowed to have an alignment of 0.
    ASSERT(type == elf::SectionHeaderType::SHT_NULL || alignment > 0);
    // Non-zero alignments must be a power of 2.
    ASSERT(alignment == 0 || Utils::IsPowerOfTwo(alignment));
  }

  virtual ~Section() {}

  // Linker view.
  const elf::SectionHeaderType type;
  const intptr_t flags;
  const intptr_t alignment;

  // These are fields that only are not set for most kinds of sections and so we
  // set them to a reasonable default.
  intptr_t link = elf::SHN_UNDEF;
  intptr_t info = 0;
  intptr_t entry_size = 0;

  // Stores the name for the symbol that should be created in the dynamic (and
  // static, if unstripped) tables for this section.
  const char* symbol_name = nullptr;

#define FOR_EACH_SECTION_LINEAR_FIELD(M)                                       \
  M(name)                                                                      \
  M(index)                                                                     \
  M(file_offset)

  FOR_EACH_SECTION_LINEAR_FIELD(DEFINE_LINEAR_FIELD_METHODS);

  virtual intptr_t FileSize() const = 0;

  // Loader view.
#define FOR_EACH_SEGMENT_LINEAR_FIELD(M) M(memory_offset)

  FOR_EACH_SEGMENT_LINEAR_FIELD(DEFINE_LINEAR_FIELD_METHODS);

  // Each section belongs to at most one PT_LOAD segment.
  const Segment* load_segment = nullptr;

  virtual intptr_t MemorySize() const = 0;

  // Other methods.

  bool IsAllocated() const {
    return (flags & elf::SHF_ALLOC) == elf::SHF_ALLOC;
  }
  bool IsExecutable() const {
    return (flags & elf::SHF_EXECINSTR) == elf::SHF_EXECINSTR;
  }
  bool IsWritable() const { return (flags & elf::SHF_WRITE) == elf::SHF_WRITE; }

  // Returns whether new content can be added to a section.
  bool HasBeenFinalized() const {
    if (IsAllocated()) {
      // The contents of a section that is allocated (part of a segment) must
      // not change after the section is added.
      return memory_offset_is_set();
    } else {
      // Unallocated sections can have new content added until we calculate
      // file offsets.
      return file_offset_is_set();
    }
  }

  virtual const BitsContainer* AsBitsContainer() const { return nullptr; }

  // Writes the file contents of the section.
  virtual void Write(ElfWriteStream* stream) = 0;

  virtual void WriteSectionHeader(ElfWriteStream* stream) {
#if defined(TARGET_ARCH_IS_32_BIT)
    stream->WriteWord(name());
    stream->WriteWord(static_cast<uint32_t>(type));
    stream->WriteWord(flags);
    stream->WriteAddr(memory_offset());
    stream->WriteOff(file_offset());
    stream->WriteWord(FileSize());  // Has different meaning for BSS.
    stream->WriteWord(link);
    stream->WriteWord(info);
    stream->WriteWord(alignment);
    stream->WriteWord(entry_size);
#else
    stream->WriteWord(name());
    stream->WriteWord(static_cast<uint32_t>(type));
    stream->WriteXWord(flags);
    stream->WriteAddr(memory_offset());
    stream->WriteOff(file_offset());
    stream->WriteXWord(FileSize());  // Has different meaning for BSS.
    stream->WriteWord(link);
    stream->WriteWord(info);
    stream->WriteXWord(alignment);
    stream->WriteXWord(entry_size);
#endif
  }

 private:
  static intptr_t EncodeFlags(bool allocate, bool executable, bool writable) {
    if (!allocate) return 0;
    intptr_t flags = elf::SHF_ALLOC;
    if (executable) flags |= elf::SHF_EXECINSTR;
    if (writable) flags |= elf::SHF_WRITE;
    return flags;
  }

  static intptr_t DefaultAlignment(elf::SectionHeaderType type) {
    switch (type) {
      case elf::SectionHeaderType::SHT_SYMTAB:
      case elf::SectionHeaderType::SHT_DYNSYM:
      case elf::SectionHeaderType::SHT_HASH:
      case elf::SectionHeaderType::SHT_DYNAMIC:
        return compiler::target::kWordSize;
      default:
        return 1;
    }
  }

  FOR_EACH_SECTION_LINEAR_FIELD(DEFINE_LINEAR_FIELD);
  FOR_EACH_SEGMENT_LINEAR_FIELD(DEFINE_LINEAR_FIELD);

#undef FOR_EACH_SECTION_LINEAR_FIELD
#undef FOR_EACH_SEGMENT_LINEAR_FIELD
};

#undef DEFINE_LINEAR_FIELD
#undef DEFINE_LINEAR_FIELD_METHODS

class Segment : public ZoneAllocated {
 public:
  Segment(Zone* zone,
          Section* initial_section,
          elf::ProgramHeaderType segment_type)
      : type(segment_type),
        // Flags for the segment are the same as the initial section.
        flags(EncodeFlags(ASSERT_NOTNULL(initial_section)->IsExecutable(),
                          ASSERT_NOTNULL(initial_section)->IsWritable())),
        sections_(zone, 0) {
    // Unlike sections, we don't have a reserved segment with the null type,
    // so we never should pass this value.
    ASSERT(segment_type != elf::ProgramHeaderType::PT_NULL);
    // All segments should have at least one section. The first one is added
    // during initialization. Unlike others added later, it should already have
    // a memory offset since we use it to determine the segment memory offset.
    ASSERT(initial_section->IsAllocated());
    ASSERT(initial_section->memory_offset_is_set());
    sections_.Add(initial_section);
    if (type == elf::ProgramHeaderType::PT_LOAD) {
      ASSERT(initial_section->load_segment == nullptr);
      initial_section->load_segment = this;
    }
  }

  virtual ~Segment() {}

  static intptr_t Alignment(elf::ProgramHeaderType segment_type) {
    switch (segment_type) {
      case elf::ProgramHeaderType::PT_DYNAMIC:
        return compiler::target::kWordSize;
      case elf::ProgramHeaderType::PT_NOTE:
        return kNoteAlignment;
      default:
        return Elf::kPageSize;
    }
  }

  bool IsExecutable() const { return (flags & elf::PF_X) == elf::PF_X; }
  bool IsWritable() const { return (flags & elf::PF_W) == elf::PF_W; }

  void WriteProgramHeader(ElfWriteStream* stream) {
#if defined(TARGET_ARCH_IS_32_BIT)
    stream->WriteWord(static_cast<uint32_t>(type));
    stream->WriteOff(FileOffset());
    stream->WriteAddr(MemoryOffset());  // Virtual address.
    stream->WriteAddr(MemoryOffset());  // Physical address, not used.
    stream->WriteWord(FileSize());
    stream->WriteWord(MemorySize());
    stream->WriteWord(flags);
    stream->WriteWord(Alignment(type));
#else
    stream->WriteWord(static_cast<uint32_t>(type));
    stream->WriteWord(flags);
    stream->WriteOff(FileOffset());
    stream->WriteAddr(MemoryOffset());  // Virtual address.
    stream->WriteAddr(MemoryOffset());  // Physical address, not used.
    stream->WriteXWord(FileSize());
    stream->WriteXWord(MemorySize());
    stream->WriteXWord(Alignment(type));
#endif
  }

  // Adds the given section to this segment.
  //
  // Returns whether the Section could be added to the segment. If not, a
  // new segment will need to be created for this section.
  //
  // Sets the memory offset of the section if added.
  bool Add(Section* section) {
    // We only add additional sections to load segments.
    ASSERT(type == elf::ProgramHeaderType::PT_LOAD);
    ASSERT(section != nullptr);
    // Only sections with the allocate flag set should be added to segments,
    // and sections with already-set memory offsets cannot be added.
    ASSERT(section->IsAllocated());
    ASSERT(!section->memory_offset_is_set());
    ASSERT(section->load_segment == nullptr);
    switch (sections_.Last()->type) {
      // We only use SHT_NULL sections as pseudo sections that will not appear
      // in the final ELF file. Don't pack sections into these segments, as we
      // may remove/replace the segments during finalization.
      case elf::SectionHeaderType::SHT_NULL:
      // If the last section in the segments is NOBITS, then we don't add it,
      // as otherwise we'll be guaranteed the file offset and memory offset
      // won't be page aligned without padding.
      case elf::SectionHeaderType::SHT_NOBITS:
        return false;
      default:
        break;
    }
    // We don't add if the W or X bits don't match.
    if (IsExecutable() != section->IsExecutable() ||
        IsWritable() != section->IsWritable()) {
      return false;
    }
    auto const start_address = Utils::RoundUp(MemoryEnd(), section->alignment);
    section->set_memory_offset(start_address);
    sections_.Add(section);
    section->load_segment = this;
    return true;
  }

  intptr_t FileOffset() const { return sections_[0]->file_offset(); }

  intptr_t FileSize() const {
    auto const last = sections_.Last();
    const intptr_t end = last->file_offset() + last->FileSize();
    return end - FileOffset();
  }

  intptr_t MemoryOffset() const { return sections_[0]->memory_offset(); }

  intptr_t MemorySize() const {
    auto const last = sections_.Last();
    const intptr_t end = last->memory_offset() + last->MemorySize();
    return end - MemoryOffset();
  }

  intptr_t MemoryEnd() const { return MemoryOffset() + MemorySize(); }

 private:
  static constexpr intptr_t kInitValue = -1;
  static_assert(kInitValue < 0, "init value must be negative");

  static intptr_t EncodeFlags(bool executable, bool writable) {
    intptr_t flags = elf::PF_R;
    if (executable) flags |= elf::PF_X;
    if (writable) flags |= elf::PF_W;
    return flags;
  }

 public:
  const elf::ProgramHeaderType type;
  const intptr_t flags;

 private:
  GrowableArray<const Section*> sections_;
};

// Represents the first entry in the section table, which should only contain
// zero values and does not correspond to a memory segment.
class ReservedSection : public Section {
 public:
  ReservedSection()
      : Section(elf::SectionHeaderType::SHT_NULL,
                /*allocate=*/false,
                /*executable=*/false,
                /*writable=*/false,
                /*alignment=*/0) {
    set_name(0);
    set_index(0);
    set_file_offset(0);
  }

  intptr_t FileSize() const { return 0; }
  intptr_t MemorySize() const { return 0; }
  void Write(ElfWriteStream* stream) {}
};

// Represents portions of the file/memory space which do not correspond to
// actual sections. Should never be added to sections_.
class PseudoSection : public Section {
 public:
  PseudoSection(bool executable,
                bool writable,
                intptr_t file_offset,
                intptr_t file_size,
                intptr_t memory_offset,
                intptr_t memory_size)
      : Section(elf::SectionHeaderType::SHT_NULL,
                /*allocate=*/true,
                executable,
                writable,
                /*alignment=*/0),
        file_size_(file_size),
        memory_size_(memory_size) {
    set_file_offset(file_offset);
    set_memory_offset(memory_offset);
  }

  intptr_t FileSize() const { return file_size_; }
  intptr_t MemorySize() const { return memory_size_; }
  void WriteSectionHeader(ElfWriteStream* stream) { UNREACHABLE(); }
  void Write(ElfWriteStream* stream) { UNREACHABLE(); }

 private:
  const intptr_t file_size_;
  const intptr_t memory_size_;
};

// A segment for representing the program header table self-reference in the
// program header table.
class ProgramTableSelfSegment : public Segment {
 public:
  ProgramTableSelfSegment(Zone* zone, intptr_t offset, intptr_t size)
      : Segment(zone,
                new (zone) PseudoSection(/*executable=*/false,
                                         /*writable=*/false,
                                         offset,
                                         size,
                                         offset,
                                         size),
                elf::ProgramHeaderType::PT_PHDR) {}
};

// A segment for representing the program header table load segment in the
// program header table.
class ProgramTableLoadSegment : public Segment {
 public:
  // The Android dynamic linker in Jelly Bean incorrectly assumes that all
  // non-writable segments are continguous. Since the BSS segment comes directly
  // after the program header segment, we must make this segment writable so
  // later non-writable segments does not cause the BSS to be also marked as
  // read-only.
  //
  // The bug is here:
  //   https://github.com/aosp-mirror/platform_bionic/blob/94963af28e445384e19775a838a29e6a71708179/linker/linker.c#L1991-L2001
  explicit ProgramTableLoadSegment(Zone* zone, intptr_t size)
      : Segment(zone,
                // This segment should always start at address 0.
                new (zone) PseudoSection(/*executable=*/false,
                                         /*writable=*/true,
                                         0,
                                         size,
                                         0,
                                         size),
                elf::ProgramHeaderType::PT_LOAD) {}
};

class BitsContainer : public Section {
 public:
  // Fully specified BitsContainer information.
  BitsContainer(elf::SectionHeaderType type,
                bool allocate,
                bool executable,
                bool writable,
                intptr_t size,
                const uint8_t* bytes,
                int alignment = kDefaultAlignment)
      : Section(type, allocate, executable, writable, alignment),
        file_size_(type == elf::SectionHeaderType::SHT_NOBITS ? 0 : size),
        memory_size_(allocate ? size : 0),
        bytes_(bytes) {
    ASSERT(type == elf::SectionHeaderType::SHT_NOBITS || bytes != nullptr);
  }

  // For BitsContainers used only as sections.
  BitsContainer(elf::SectionHeaderType type,
                intptr_t size,
                const uint8_t* bytes,
                intptr_t alignment = kDefaultAlignment)
      : BitsContainer(type,
                      /*allocate=*/false,
                      /*executable=*/false,
                      /*writable=*/false,
                      size,
                      bytes,
                      alignment) {}

  // For BitsContainers used as segments whose type differ on the type of the
  // ELF file. Creates an elf::SHT_NOBITS section if type is DebugInfo,
  // otherwise creates an elf::SHT_PROGBITS section.
  BitsContainer(Elf::Type t,
                bool executable,
                bool writable,
                intptr_t size,
                const uint8_t* bytes,
                intptr_t alignment = kDefaultAlignment)
      : BitsContainer(t == Elf::Type::DebugInfo
                          ? elf::SectionHeaderType::SHT_NOBITS
                          : elf::SectionHeaderType::SHT_PROGBITS,
                      /*allocate=*/true,
                      executable,
                      writable,
                      size,
                      bytes,
                      alignment) {}

  const BitsContainer* AsBitsContainer() const { return this; }

  void Write(ElfWriteStream* stream) {
    if (type != elf::SectionHeaderType::SHT_NOBITS) {
      stream->WriteBytes(bytes_, FileSize());
    }
  }

  intptr_t FileSize() const { return file_size_; }
  intptr_t MemorySize() const { return memory_size_; }
  const uint8_t* bytes() const { return bytes_; }

 private:
  const intptr_t file_size_;
  const intptr_t memory_size_;
  const uint8_t* const bytes_;
};

class StringTable : public Section {
 public:
  explicit StringTable(Zone* zone, bool allocate)
      : Section(elf::SectionHeaderType::SHT_STRTAB,
                allocate,
                /*executable=*/false,
                /*writable=*/false),
        dynamic_(allocate),
        text_(zone, 128),
        text_indices_(zone) {
    text_.AddChar('\0');
    text_indices_.Insert({"", 1});
  }

  intptr_t FileSize() const { return text_.length(); }
  intptr_t MemorySize() const { return dynamic_ ? FileSize() : 0; }

  void Write(ElfWriteStream* stream) {
    stream->WriteBytes(reinterpret_cast<const uint8_t*>(text_.buffer()),
                       text_.length());
  }

  intptr_t AddString(const char* str) {
    if (auto const kv = text_indices_.Lookup(str)) return kv->value - 1;
    intptr_t offset = text_.length();
    text_.AddString(str);
    text_.AddChar('\0');
    text_indices_.Insert({str, offset + 1});
    return offset;
  }

  const char* At(intptr_t index) {
    ASSERT(index < text_.length());
    return text_.buffer() + index;
  }
  intptr_t Lookup(const char* str) const {
    return text_indices_.LookupValue(str) - 1;
  }

  const bool dynamic_;
  ZoneTextBuffer text_;
  // To avoid kNoValue for intptr_t (0), we store an index n as n + 1.
  CStringMap<intptr_t> text_indices_;
};

class Symbol : public ZoneAllocated {
 public:
  Symbol(const char* cstr,
         intptr_t name,
         intptr_t info,
         intptr_t section,
         intptr_t offset,
         intptr_t size)
      : name_index(name),
        info(info),
        section_index(section),
        offset(offset),
        size(size),
        cstr_(cstr) {}

  void Write(ElfWriteStream* stream) const {
    const intptr_t start = stream->position();
    stream->WriteWord(name_index);
#if defined(TARGET_ARCH_IS_32_BIT)
    stream->WriteAddr(offset);
    stream->WriteWord(size);
    stream->WriteByte(info);
    stream->WriteByte(0);
    stream->WriteHalf(section_index);
#else
    stream->WriteByte(info);
    stream->WriteByte(0);
    stream->WriteHalf(section_index);
    stream->WriteAddr(offset);
    stream->WriteXWord(size);
#endif
    ASSERT_EQUAL(stream->position() - start, sizeof(elf::Symbol));
  }

  const intptr_t name_index;
  const intptr_t info;
  const intptr_t section_index;
  const intptr_t offset;
  const intptr_t size;

 private:
  friend class SymbolHashTable;  // For cstr_ access.

  const char* const cstr_;
};

class SymbolTable : public Section {
 public:
  SymbolTable(Zone* zone, bool dynamic)
      : Section(dynamic ? elf::SectionHeaderType::SHT_DYNSYM
                        : elf::SectionHeaderType::SHT_SYMTAB,
                dynamic,
                /*executable=*/false,
                /*writable=*/false),
        dynamic_(dynamic),
        reserved_("", 0, 0, 0, 0, 0),
        symbols_(zone, 1) {
    entry_size = sizeof(elf::Symbol);
    // The first symbol table entry is reserved and must be all zeros.
    symbols_.Add(&reserved_);
    info = 1;  // One "local" symbol, the reserved first entry.
  }

  intptr_t FileSize() const { return Length() * entry_size; }
  intptr_t MemorySize() const { return dynamic_ ? FileSize() : 0; }

  void Write(ElfWriteStream* stream) {
    for (intptr_t i = 0; i < Length(); i++) {
      auto const symbol = At(i);
      const intptr_t start = stream->position();
      symbol->Write(stream);
      ASSERT_EQUAL(stream->position() - start, entry_size);
    }
  }

  void AddSymbol(const Symbol* symbol) { symbols_.Add(symbol); }
  intptr_t Length() const { return symbols_.length(); }
  const Symbol* At(intptr_t i) const { return symbols_[i]; }

  const Symbol* FindSymbolWithNameIndex(intptr_t name_index) const {
    for (intptr_t i = 0; i < Length(); i++) {
      auto const symbol = At(i);
      if (symbol->name_index == name_index) return symbol;
    }
    return nullptr;
  }

 private:
  const bool dynamic_;
  const Symbol reserved_;
  GrowableArray<const Symbol*> symbols_;
};

static uint32_t ElfHash(const unsigned char* name) {
  uint32_t h = 0;
  while (*name != '\0') {
    h = (h << 4) + *name++;
    uint32_t g = h & 0xf0000000;
    h ^= g;
    h ^= g >> 24;
  }
  return h;
}

class SymbolHashTable : public Section {
 public:
  SymbolHashTable(Zone* zone, StringTable* strtab, SymbolTable* symtab)
      : Section(elf::SectionHeaderType::SHT_HASH,
                /*allocate=*/true,
                /*executable=*/false,
                /*writable=*/false) {
    link = symtab->index();
    entry_size = sizeof(int32_t);

    nchain_ = symtab->Length();
    nbucket_ = symtab->Length();

    bucket_ = zone->Alloc<int32_t>(nbucket_);
    for (intptr_t i = 0; i < nbucket_; i++) {
      bucket_[i] = elf::STN_UNDEF;
    }

    chain_ = zone->Alloc<int32_t>(nchain_);
    for (intptr_t i = 0; i < nchain_; i++) {
      chain_[i] = elf::STN_UNDEF;
    }

    for (intptr_t i = 1; i < symtab->Length(); i++) {
      auto const symbol = symtab->At(i);
      uint32_t hash = ElfHash((const unsigned char*)symbol->cstr_);
      uint32_t probe = hash % nbucket_;
      chain_[i] = bucket_[probe];  // next = head
      bucket_[probe] = i;          // head = symbol
    }
  }

  intptr_t FileSize() const { return entry_size * (nbucket_ + nchain_ + 2); }
  intptr_t MemorySize() const { return FileSize(); }

  void Write(ElfWriteStream* stream) {
    stream->WriteWord(nbucket_);
    stream->WriteWord(nchain_);
    for (intptr_t i = 0; i < nbucket_; i++) {
      stream->WriteWord(bucket_[i]);
    }
    for (intptr_t i = 0; i < nchain_; i++) {
      stream->WriteWord(chain_[i]);
    }
  }

 private:
  int32_t nbucket_;
  int32_t nchain_;
  int32_t* bucket_;  // "Head"
  int32_t* chain_;   // "Next"
};

class DynamicTable : public Section {
 public:
  DynamicTable(Zone* zone,
               StringTable* strtab,
               SymbolTable* symtab,
               SymbolHashTable* hash)
      : Section(elf::SectionHeaderType::SHT_DYNAMIC,
                /*allocate=*/true,
                /*executable=*/false,
                /*writable=*/true) {
    link = strtab->index();
    entry_size = sizeof(elf::DynamicEntry);

    AddEntry(zone, elf::DynamicEntryType::DT_HASH, hash->memory_offset());
    AddEntry(zone, elf::DynamicEntryType::DT_STRTAB, strtab->memory_offset());
    AddEntry(zone, elf::DynamicEntryType::DT_STRSZ, strtab->MemorySize());
    AddEntry(zone, elf::DynamicEntryType::DT_SYMTAB, symtab->memory_offset());
    AddEntry(zone, elf::DynamicEntryType::DT_SYMENT, sizeof(elf::Symbol));
    AddEntry(zone, elf::DynamicEntryType::DT_NULL, 0);
  }

  intptr_t FileSize() const { return entries_.length() * entry_size; }
  intptr_t MemorySize() const { return FileSize(); }

  void Write(ElfWriteStream* stream) {
    for (intptr_t i = 0; i < entries_.length(); i++) {
      entries_[i]->Write(stream);
    }
  }

  struct Entry : public ZoneAllocated {
    Entry(elf::DynamicEntryType tag, intptr_t value) : tag(tag), value(value) {}

    void Write(ElfWriteStream* stream) {
      const intptr_t start = stream->position();
#if defined(TARGET_ARCH_IS_32_BIT)
      stream->WriteWord(static_cast<uint32_t>(tag));
      stream->WriteAddr(value);
#else
      stream->WriteXWord(static_cast<uint64_t>(tag));
      stream->WriteAddr(value);
#endif
      ASSERT_EQUAL(stream->position() - start, sizeof(elf::DynamicEntry));
    }

    elf::DynamicEntryType tag;
    intptr_t value;
  };

  void AddEntry(Zone* zone, elf::DynamicEntryType tag, intptr_t value) {
    auto const entry = new (zone) Entry(tag, value);
    entries_.Add(entry);
  }

 private:
  GrowableArray<Entry*> entries_;
};

// A segment for representing the dynamic table segment in the program header
// table. There is no corresponding section for this segment.
class DynamicSegment : public Segment {
 public:
  explicit DynamicSegment(Zone* zone, DynamicTable* dynamic)
      : Segment(zone, dynamic, elf::ProgramHeaderType::PT_DYNAMIC) {}
};

// A segment for representing the dynamic table segment in the program header
// table. There is no corresponding section for this segment.
class NoteSegment : public Segment {
 public:
  NoteSegment(Zone* zone, Section* note)
      : Segment(zone, note, elf::ProgramHeaderType::PT_NOTE) {
    ASSERT_EQUAL(static_cast<uint32_t>(note->type),
                 static_cast<uint32_t>(elf::SectionHeaderType::SHT_NOTE));
  }
};

static const intptr_t kProgramTableSegmentSize = Elf::kPageSize;

// Here, both VM and isolate will be compiled into a single snapshot.
// In assembly generation, each serialized text section gets a separate
// pointer into the BSS segment and BSS slots are created for each, since
// we may not serialize both VM and isolate. Here, we always serialize both,
// so make a BSS segment large enough for both, with the VM entries coming
// first.
static constexpr const char* kSnapshotBssAsmSymbol = "_kDartBSSData";
static const intptr_t kBssIsolateOffset =
    BSS::kVmEntryCount * compiler::target::kWordSize;
static const intptr_t kBssSize =
    kBssIsolateOffset + BSS::kIsolateEntryCount * compiler::target::kWordSize;

Elf::Elf(Zone* zone, StreamingWriteStream* stream, Type type, Dwarf* dwarf)
    : zone_(zone),
      unwrapped_stream_(stream),
      type_(type),
      dwarf_(dwarf),
      bss_(CreateBSS(zone, type, kBssSize)),
      shstrtab_(new (zone) StringTable(zone, /*allocate=*/false)),
      dynstrtab_(new (zone) StringTable(zone, /*allocate=*/true)),
      dynsym_(new (zone) SymbolTable(zone, /*dynamic=*/true)) {
  // Separate debugging information should always have a Dwarf object.
  ASSERT(type_ == Type::Snapshot || dwarf_ != nullptr);
  // Assumed by various offset logic in this file.
  ASSERT_EQUAL(unwrapped_stream_->position(), 0);
  // The first section in the section header table is always a reserved
  // entry containing only 0 values.
  sections_.Add(new (zone_) ReservedSection());
  if (!IsStripped()) {
    // Not a stripped ELF file, so allocate static string and symbol tables.
    strtab_ = new (zone_) StringTable(zone_, /* allocate= */ false);
    symtab_ = new (zone_) SymbolTable(zone, /*dynamic=*/false);
  }
  // We add an initial segment to represent reserved space for the program
  // header, and so we can always assume there's at least one segment in the
  // segments_ array. We later remove this and replace it with appropriately
  // calculated segments in Elf::FinalizeProgramTable().
  auto const start_segment =
      new (zone_) ProgramTableLoadSegment(zone_, kProgramTableSegmentSize);
  segments_.Add(start_segment);
  // Note that the BSS segment must be the first user-defined segment because
  // it cannot be placed in between any two non-writable segments, due to a bug
  // in Jelly Bean's ELF loader. See also Elf::WriteProgramTable().
  //
  // We add it in all cases, even to the separate debugging information ELF,
  // to ensure that relocated addresses are consistent between ELF snapshots
  // and ELF separate debugging information.
  AddSection(bss_, ".bss", kSnapshotBssAsmSymbol);
}

intptr_t Elf::NextMemoryOffset() const {
  return Utils::RoundUp(LastLoadSegment()->MemoryEnd(), Elf::kPageSize);
}

uword Elf::BssStart(bool vm) const {
  return bss_->memory_offset() + (vm ? 0 : kBssIsolateOffset);
}

intptr_t Elf::AddSection(Section* section,
                         const char* name,
                         const char* symbol_name) {
  ASSERT(section_table_file_size_ < 0);
  ASSERT(!shstrtab_->HasBeenFinalized());
  section->set_name(shstrtab_->AddString(name));
  section->set_index(sections_.length());
  sections_.Add(section);

  // No memory offset, so just return -1.
  if (!section->IsAllocated()) return -1;

  ASSERT(program_table_file_size_ < 0);
  auto const last_load = LastLoadSegment();
  if (!last_load->Add(section)) {
    // We can't add this section to the last load segment, so create a new one.
    // The new segment starts at the next aligned address.
    auto const type = elf::ProgramHeaderType::PT_LOAD;
    auto const start_address =
        Utils::RoundUp(last_load->MemoryEnd(), Segment::Alignment(type));
    section->set_memory_offset(start_address);
    auto const segment = new (zone_) Segment(zone_, section, type);
    segments_.Add(segment);
  }
  if (symbol_name != nullptr) {
    section->symbol_name = symbol_name;
  }
  return section->memory_offset();
}

intptr_t Elf::AddText(const char* name, const uint8_t* bytes, intptr_t size) {
  // When making a separate debugging info file for assembly, we don't have
  // the binary text segment contents.
  ASSERT(type_ == Type::DebugInfo || bytes != nullptr);
  auto const image = new (zone_)
      BitsContainer(type_, /*executable=*/true,
                    /*writable=*/false, size, bytes, Elf::kPageSize);
  return AddSection(image, ".text", name);
}

Section* Elf::CreateBSS(Zone* zone, Type type, intptr_t size) {
  uint8_t* bytes = nullptr;
  if (type != Type::DebugInfo) {
    // Ideally the BSS segment would take no space in the object, but Android's
    // "strip" utility truncates the memory-size of our segments to their
    // file-size.
    //
    // Therefore we must insert zero-filled pages for the BSS.
    bytes = zone->Alloc<uint8_t>(size);
    memset(bytes, 0, size);
  }
  return new (zone) BitsContainer(type, /*executable=*/false, /*writable=*/true,
                                  kBssSize, bytes, Image::kBssAlignment);
}

intptr_t Elf::AddROData(const char* name, const uint8_t* bytes, intptr_t size) {
  ASSERT(bytes != nullptr);
  auto const image = new (zone_)
      BitsContainer(type_, /*executable=*/false,
                    /*writable=*/false, size, bytes, kMaxObjectAlignment);
  return AddSection(image, ".rodata", name);
}

void Elf::AddDebug(const char* name, const uint8_t* bytes, intptr_t size) {
  ASSERT(!IsStripped());
  ASSERT(bytes != nullptr);
  auto const image = new (zone_)
      BitsContainer(elf::SectionHeaderType::SHT_PROGBITS, size, bytes);
  AddSection(image, name);
}

void Elf::AddDynamicSymbol(const char* name,
                           intptr_t info,
                           intptr_t section_index,
                           intptr_t address,
                           intptr_t size) {
  ASSERT(!dynstrtab_->HasBeenFinalized() && !dynsym_->HasBeenFinalized());
  auto const name_index = dynstrtab_->AddString(name);
  auto const symbol =
      new (zone_) Symbol(name, name_index, info, section_index, address, size);
  dynsym_->AddSymbol(symbol);

  // Some tools assume the static symbol table is a superset of the dynamic
  // symbol table when it exists (see dartbug.com/41783).
  AddStaticSymbol(name, info, section_index, address, size);
}

void Elf::AddStaticSymbol(const char* name,
                          intptr_t info,
                          intptr_t section_index,
                          intptr_t address,
                          intptr_t size) {
  if (IsStripped()) return;  // No static info kept in stripped ELF files.
  ASSERT(!symtab_->HasBeenFinalized() && !strtab_->HasBeenFinalized());
  auto const name_index = strtab_->AddString(name);
  auto const symbol =
      new (zone_) Symbol(name, name_index, info, section_index, address, size);
  symtab_->AddSymbol(symbol);
}

#if defined(DART_PRECOMPILER)
class DwarfElfStream : public DwarfWriteStream {
 public:
  explicit DwarfElfStream(Zone* zone,
                          WriteStream* stream,
                          const CStringMap<intptr_t>& address_map)
      : zone_(zone),
        stream_(ASSERT_NOTNULL(stream)),
        address_map_(address_map) {}

  void sleb128(intptr_t value) {
    bool is_last_part = false;
    while (!is_last_part) {
      uint8_t part = value & 0x7F;
      value >>= 7;
      if ((value == 0 && (part & 0x40) == 0) ||
          (value == static_cast<intptr_t>(-1) && (part & 0x40) != 0)) {
        is_last_part = true;
      } else {
        part |= 0x80;
      }
      stream_->WriteFixed(part);
    }
  }

  void uleb128(uintptr_t value) {
    bool is_last_part = false;
    while (!is_last_part) {
      uint8_t part = value & 0x7F;
      value >>= 7;
      if (value == 0) {
        is_last_part = true;
      } else {
        part |= 0x80;
      }
      stream_->WriteFixed(part);
    }
  }

  void u1(uint8_t value) { stream_->WriteFixed(value); }
  // Can't use WriteFixed for these, as we may not be at aligned positions.
  void u2(uint16_t value) { stream_->WriteBytes(&value, sizeof(value)); }
  void u4(uint32_t value) { stream_->WriteBytes(&value, sizeof(value)); }
  void u8(uint64_t value) { stream_->WriteBytes(&value, sizeof(value)); }
  void string(const char* cstr) {  // NOLINT
    stream_->WriteBytes(reinterpret_cast<const uint8_t*>(cstr),
                        strlen(cstr) + 1);
  }
  intptr_t position() { return stream_->Position(); }
  intptr_t ReserveSize(const char* prefix, intptr_t* start) {
    ASSERT(start != nullptr);
    intptr_t fixup = position();
    // We assume DWARF v2, so all sizes are 32-bit.
    u4(0);
    // All sizes for DWARF sections measure the size of the section data _after_
    // the size value.
    *start = position();
    return fixup;
  }
  void SetSize(intptr_t fixup, const char* prefix, intptr_t start) {
    const uint32_t value = position() - start;
    memmove(stream_->buffer() + fixup, &value, sizeof(value));
  }
  void OffsetFromSymbol(const char* symbol, intptr_t offset) {
    auto const address = address_map_.LookupValue(symbol);
    ASSERT(address != 0);
    addr(address + offset);
  }
  void DistanceBetweenSymbolOffsets(const char* symbol1,
                                    intptr_t offset1,
                                    const char* symbol2,
                                    intptr_t offset2) {
    auto const address1 = address_map_.LookupValue(symbol1);
    ASSERT(address1 != 0);
    auto const address2 = address_map_.LookupValue(symbol2);
    ASSERT(address2 != 0);
    auto const delta = (address1 + offset1) - (address2 + offset2);
    RELEASE_ASSERT(delta >= 0);
    uleb128(delta);
  }
  void InitializeAbstractOrigins(intptr_t size) {
    abstract_origins_size_ = size;
    abstract_origins_ = zone_->Alloc<uint32_t>(abstract_origins_size_);
  }
  void RegisterAbstractOrigin(intptr_t index) {
    ASSERT(abstract_origins_ != nullptr);
    ASSERT(index < abstract_origins_size_);
    abstract_origins_[index] = position();
  }
  void AbstractOrigin(intptr_t index) { u4(abstract_origins_[index]); }

 private:
  void addr(uword value) {
#if defined(TARGET_ARCH_IS_32_BIT)
    u4(value);
#else
    u8(value);
#endif
  }

  Zone* const zone_;
  WriteStream* const stream_;
  const CStringMap<intptr_t>& address_map_;
  uint32_t* abstract_origins_ = nullptr;
  intptr_t abstract_origins_size_ = -1;

  DISALLOW_COPY_AND_ASSIGN(DwarfElfStream);
};

static constexpr intptr_t kInitialDwarfBufferSize = 64 * KB;
#endif

static uint8_t* ZoneReallocate(uint8_t* ptr, intptr_t len, intptr_t new_len) {
  return Thread::Current()->zone()->Realloc<uint8_t>(ptr, len, new_len);
}

Segment* Elf::LastLoadSegment() const {
  for (intptr_t i = segments_.length() - 1; i >= 0; i--) {
    auto const segment = segments_.At(i);
    if (segment->type == elf::ProgramHeaderType::PT_LOAD) {
      return segment;
    }
  }
  // There should always be a load segment, since one is added in construction.
  UNREACHABLE();
}

const Section* Elf::FindSectionForAddress(intptr_t address) const {
  for (auto const section : sections_) {
    if (!section->IsAllocated()) continue;
    auto const start = section->memory_offset();
    auto const end = start + section->MemorySize();
    if (address >= start && address < end) {
      return section;
    }
  }
  return nullptr;
}

void Elf::AddSectionSymbols() {
  for (auto const section : sections_) {
    if (section->symbol_name == nullptr) continue;
    ASSERT(section->memory_offset_is_set());
    // While elf::STT_SECTION might seem more appropriate, those symbols are
    // usually local and dlsym won't return them.
    auto const info = (elf::STB_GLOBAL << 4) | elf::STT_FUNC;
    AddDynamicSymbol(section->symbol_name, info, section->index(),
                     section->memory_offset(), section->MemorySize());
  }
}

void Elf::FinalizeDwarfSections() {
  if (dwarf_ == nullptr) return;
#if defined(DART_PRECOMPILER)
  // Add all the static symbols for Code objects. We'll keep a table of
  // symbol names to relocated addresses for use in the DwarfElfStream.
  // The default kNoValue of 0 is okay here, as no symbols are defined for
  // relocated address 0.
  CStringMap<intptr_t> symbol_to_address_map;
  // Prime the map with any existing static symbols.
  if (symtab_ != nullptr) {
    ASSERT(strtab_ != nullptr);
    // Skip the initial reserved entry in the symbol table.
    for (intptr_t i = 1; i < symtab_->Length(); i++) {
      auto const symbol = symtab_->At(i);
      auto const name = strtab_->At(symbol->name_index);
      symbol_to_address_map.Insert({name, symbol->offset});
    }
  }

  // Need these to turn offsets into relocated addresses.
  auto const vm_start =
      symbol_to_address_map.LookupValue(kVmSnapshotInstructionsAsmSymbol);
  // vm_start is absent in deferred loading peices.
  auto const isolate_start =
      symbol_to_address_map.LookupValue(kIsolateSnapshotInstructionsAsmSymbol);
  ASSERT(isolate_start > 0);
  auto const vm_text = FindSectionForAddress(vm_start);
  // vm_text is absent in deferred loading peices.
  auto const isolate_text = FindSectionForAddress(isolate_start);
  ASSERT(isolate_text != nullptr);

  SnapshotTextObjectNamer namer(zone_);
  const auto& codes = dwarf_->codes();
  if (codes.length() == 0) {
    return;
  }
  for (intptr_t i = 0; i < codes.length(); i++) {
    const auto& code = *codes[i];
    auto const name = namer.SnapshotNameFor(i, code);
    const auto& pair = dwarf_->CodeAddress(code);
    ASSERT(pair.offset > 0);
    auto const section = pair.vm ? vm_text : isolate_text;
    const intptr_t address = section->memory_offset() + pair.offset;
    auto const info = (elf::STB_GLOBAL << 4) | elf::STT_FUNC;
    AddStaticSymbol(name, info, section->index(), address, code.Size());
    symbol_to_address_map.Insert({name, address});
  }

  // TODO(rmacnak): Generate .debug_frame / .eh_frame / .arm.exidx to
  // provide unwinding information.

  {
    uint8_t* buffer = nullptr;
    WriteStream stream(&buffer, ZoneReallocate, kInitialDwarfBufferSize);
    DwarfElfStream dwarf_stream(zone_, &stream, symbol_to_address_map);
    dwarf_->WriteAbbreviations(&dwarf_stream);
    AddDebug(".debug_abbrev", buffer, stream.bytes_written());
  }

  {
    uint8_t* buffer = nullptr;
    WriteStream stream(&buffer, ZoneReallocate, kInitialDwarfBufferSize);
    DwarfElfStream dwarf_stream(zone_, &stream, symbol_to_address_map);
    dwarf_->WriteDebugInfo(&dwarf_stream);
    AddDebug(".debug_info", buffer, stream.bytes_written());
  }

  {
    uint8_t* buffer = nullptr;
    WriteStream stream(&buffer, ZoneReallocate, kInitialDwarfBufferSize);
    DwarfElfStream dwarf_stream(zone_, &stream, symbol_to_address_map);
    dwarf_->WriteLineNumberProgram(&dwarf_stream);
    AddDebug(".debug_line", buffer, stream.bytes_written());
  }
#endif
}

void Elf::Finalize() {
  AddSectionSymbols();

  // The Build ID depends on the symbols being in place, so must be run after
  // AddSectionSymbols(). Unfortunately, it currently depends on the contents
  // of the .text and .rodata sections, so it can't come earlier in the file
  // without changing how we add the .text and .rodata sections (since we
  // determine memory offsets for those sections when we add them, and the
  // text sections must have the memory offsets to do BSS relocations).
  if (auto const build_id = GenerateBuildId()) {
    AddSection(build_id, ".note.gnu.build-id", kSnapshotBuildIdAsmSymbol);

    // Add a PT_NOTE segment for the build ID.
    segments_.Add(new (zone_) NoteSegment(zone_, build_id));
  }

  // Adding the dynamic symbol table and associated sections.
  AddSection(dynstrtab_, ".dynstr");
  AddSection(dynsym_, ".dynsym");
  dynsym_->link = dynstrtab_->index();

  auto const hash = new (zone_) SymbolHashTable(zone_, dynstrtab_, dynsym_);
  AddSection(hash, ".hash");

  auto const dynamic =
      new (zone_) DynamicTable(zone_, dynstrtab_, dynsym_, hash);
  AddSection(dynamic, ".dynamic");

  // Add a PT_DYNAMIC segment for the dynamic symbol table.
  segments_.Add(new (zone_) DynamicSegment(zone_, dynamic));

  // Currently, we add all (non-reserved) unallocated sections after all
  // allocated sections. If we put unallocated sections between allocated
  // sections, they would affect the file offset but not the memory offset
  // of the later allocated sections.
  //
  // However, memory offsets must be page-aligned to the file offset for the
  // ELF file to be successfully loaded. This means we'd either have to add
  // extra padding _or_ determine file offsets before memory offsets. The
  // latter would require us to handle BSS relocations during ELF finalization,
  // instead of while writing the .text section content.
  FinalizeDwarfSections();
  if (!IsStripped()) {
    AddSection(strtab_, ".strtab");
    AddSection(symtab_, ".symtab");
    symtab_->link = strtab_->index();
  }
  AddSection(shstrtab_, ".shstrtab");

  // At this point, all non-programmatically calculated sections and segments
  // have been added. Add any programatically calculated sections and segments
  // and then calculate file offsets.
  FinalizeProgramTable();
  ComputeFileOffsets();

  // Finally, write the ELF file contents.
  ElfWriteStream wrapped(unwrapped_stream_);
  WriteHeader(&wrapped);
  WriteProgramTable(&wrapped);
  WriteSections(&wrapped);
  WriteSectionTable(&wrapped);
}

// Need to include the final \0 terminator in both byte count and byte output.
static const uint32_t kBuildIdNameLength = strlen(elf::ELF_NOTE_GNU) + 1;
// We generate a 128-bit hash, where each 32 bits is a hash of the contents of
// the following segments in order:
//
// .text(VM) | .text(Isolate) | .rodata(VM) | .rodata(Isolate)
static constexpr intptr_t kBuildIdSegmentNamesLength = 4;
static constexpr const char* kBuildIdSegmentNames[kBuildIdSegmentNamesLength]{
    kVmSnapshotInstructionsAsmSymbol,
    kIsolateSnapshotInstructionsAsmSymbol,
    kVmSnapshotDataAsmSymbol,
    kIsolateSnapshotDataAsmSymbol,
};
static constexpr uint32_t kBuildIdDescriptionLength =
    kBuildIdSegmentNamesLength * sizeof(uint32_t);
static const intptr_t kBuildIdDescriptionOffset =
    sizeof(elf::Note) + kBuildIdNameLength;
static const intptr_t kBuildIdSize =
    kBuildIdDescriptionOffset + kBuildIdDescriptionLength;

static const Symbol* LookupSymbol(StringTable* strings,
                                  SymbolTable* symbols,
                                  const char* name) {
  ASSERT(strings != nullptr);
  ASSERT(symbols != nullptr);
  auto const name_index = strings->Lookup(name);
  if (name_index < 0) return nullptr;
  return symbols->FindSymbolWithNameIndex(name_index);
}

static uint32_t HashBitsContainer(const BitsContainer* bits) {
  uint32_t hash = 0;
  auto const size = bits->MemorySize();
  auto const end = bits->bytes() + size;
  auto const non_word_size = size % kWordSize;
  auto const end_of_words =
      reinterpret_cast<const uword*>(bits->bytes() + (size - non_word_size));
  for (auto cursor = reinterpret_cast<const uword*>(bits->bytes());
       cursor < end_of_words; cursor++) {
    hash = CombineHashes(hash, *cursor);
  }
  for (auto cursor = reinterpret_cast<const uint8_t*>(end_of_words);
       cursor < end; cursor++) {
    hash = CombineHashes(hash, *cursor);
  }
  return FinalizeHash(hash, 32);
}

Section* Elf::GenerateBuildId() {
  uint8_t* notes_buffer = nullptr;
  WriteStream stream(&notes_buffer, ZoneReallocate, kBuildIdSize);
  stream.WriteFixed(kBuildIdNameLength);
  stream.WriteFixed(kBuildIdDescriptionLength);
  stream.WriteFixed(static_cast<uint32_t>(elf::NoteType::NT_GNU_BUILD_ID));
  stream.WriteBytes(elf::ELF_NOTE_GNU, kBuildIdNameLength);
  const intptr_t description_start = stream.bytes_written();
  for (intptr_t i = 0; i < kBuildIdSegmentNamesLength; i++) {
    auto const name = kBuildIdSegmentNames[i];
    auto const symbol = LookupSymbol(dynstrtab_, dynsym_, name);
    if (symbol == nullptr) {
      stream.WriteFixed(static_cast<uint32_t>(0));
      continue;
    }
    auto const bits = sections_[symbol->section_index]->AsBitsContainer();
    if (bits == nullptr) {
      FATAL1("Section for symbol %s is not a BitsContainer", name);
    }
    ASSERT_EQUAL(bits->MemorySize(), symbol->size);
    // We don't actually have the bytes (i.e., this is a separate debugging
    // info file for an assembly snapshot), so we can't calculate the build ID.
    if (bits->bytes() == nullptr) return nullptr;

    stream.WriteFixed(HashBitsContainer(bits));
  }
  ASSERT_EQUAL(stream.bytes_written() - description_start,
               kBuildIdDescriptionLength);
  return new (zone_) BitsContainer(
      elf::SectionHeaderType::SHT_NOTE, /*allocate=*/true, /*executable=*/false,
      /*writable=*/false, stream.bytes_written(), notes_buffer, kNoteAlignment);
}

void Elf::FinalizeProgramTable() {
  ASSERT(program_table_file_size_ < 0);

  program_table_file_offset_ = sizeof(elf::ElfHeader);

  // There are two segments we need the size of the program table to create, so
  // calculate it as if those two segments were already in place.
  program_table_file_size_ =
      (2 + segments_.length()) * sizeof(elf::ProgramHeader);

  // We pre-allocated the virtual memory space for the program table itself.
  // Check that we didn't generate too many segments. Currently we generate a
  // fixed num of segments based on the four pieces of a snapshot, but if we
  // use more in the future we'll likely need to do something more compilated
  // to generate DWARF without knowing a piece's virtual address in advance.
  auto const program_table_segment_size =
      program_table_file_offset_ + program_table_file_size_;
  RELEASE_ASSERT(program_table_segment_size < kProgramTableSegmentSize);

  // Remove the original stand-in segment we added in the constructor.
  segments_.EraseAt(0);

  // Self-reference to program header table. Required by Android but not by
  // Linux. Must appear before any PT_LOAD entries.
  segments_.InsertAt(
      0, new (zone_) ProgramTableSelfSegment(zone_, program_table_file_offset_,
                                             program_table_file_size_));

  // Segment for loading the initial part of the ELF file, including the
  // program header table. Required by Android but not by Linux.
  segments_.InsertAt(1, new (zone_) ProgramTableLoadSegment(
                            zone_, program_table_segment_size));
}

static const intptr_t kElfSectionTableAlignment = compiler::target::kWordSize;

void Elf::ComputeFileOffsets() {
  // We calculate the size and offset of the program header table during
  // finalization.
  ASSERT(program_table_file_offset_ > 0 && program_table_file_size_ > 0);
  intptr_t file_offset = program_table_file_offset_ + program_table_file_size_;
  // When calculating file offsets for sections, we'll need to know if we've
  // changed segments. Start with the one for the program table.
  const auto* current_segment = segments_[1];

  // The non-reserved sections are output to the file in order after the program
  // header table. If we're entering a new segment, then we need to align
  // according to the PT_LOAD segment alignment as well to keep the file offsets
  // aligned with the memory addresses.
  auto const load_align = Segment::Alignment(elf::ProgramHeaderType::PT_LOAD);
  for (intptr_t i = 1; i < sections_.length(); i++) {
    auto const section = sections_[i];
    file_offset = Utils::RoundUp(file_offset, section->alignment);
    if (section->IsAllocated() && section->load_segment != current_segment) {
      file_offset = Utils::RoundUp(file_offset, load_align);
      current_segment = section->load_segment;
    }
    section->set_file_offset(file_offset);
#if defined(DEBUG)
    if (section->IsAllocated()) {
      // For files that will be dynamically loaded, make sure the file offsets
      // of allocated sections are page aligned to the memory offsets.
      ASSERT_EQUAL(section->file_offset() % load_align,
                   section->memory_offset() % load_align);
    }
#endif
    file_offset += section->FileSize();
  }

  file_offset = Utils::RoundUp(file_offset, kElfSectionTableAlignment);
  section_table_file_offset_ = file_offset;
  section_table_file_size_ = sections_.length() * sizeof(elf::SectionHeader);
  file_offset += section_table_file_size_;
}

void Elf::WriteHeader(ElfWriteStream* stream) {
#if defined(TARGET_ARCH_IS_32_BIT)
  uint8_t size = elf::ELFCLASS32;
#else
  uint8_t size = elf::ELFCLASS64;
#endif
  uint8_t e_ident[16] = {0x7f,
                         'E',
                         'L',
                         'F',
                         size,
                         elf::ELFDATA2LSB,
                         elf::EV_CURRENT,
                         elf::ELFOSABI_SYSV,
                         0,
                         0,
                         0,
                         0,
                         0,
                         0,
                         0,
                         0};
  stream->WriteBytes(e_ident, 16);

  stream->WriteHalf(elf::ET_DYN);  // Shared library.

#if defined(TARGET_ARCH_IA32)
  stream->WriteHalf(elf::EM_386);
#elif defined(TARGET_ARCH_X64)
  stream->WriteHalf(elf::EM_X86_64);
#elif defined(TARGET_ARCH_ARM)
  stream->WriteHalf(elf::EM_ARM);
#elif defined(TARGET_ARCH_ARM64)
  stream->WriteHalf(elf::EM_AARCH64);
#else
  FATAL("Unknown ELF architecture");
#endif

  stream->WriteWord(elf::EV_CURRENT);  // Version
  stream->WriteAddr(0);                // "Entry point"
  stream->WriteOff(program_table_file_offset_);
  stream->WriteOff(section_table_file_offset_);

#if defined(TARGET_ARCH_ARM)
  uword flags = elf::EF_ARM_ABI | (TargetCPUFeatures::hardfp_supported()
                                       ? elf::EF_ARM_ABI_FLOAT_HARD
                                       : elf::EF_ARM_ABI_FLOAT_SOFT);
#else
  uword flags = 0;
#endif
  stream->WriteWord(flags);

  stream->WriteHalf(sizeof(elf::ElfHeader));
  stream->WriteHalf(sizeof(elf::ProgramHeader));
  stream->WriteHalf(segments_.length());
  stream->WriteHalf(sizeof(elf::SectionHeader));
  stream->WriteHalf(sections_.length());
  stream->WriteHalf(shstrtab_->index());

  ASSERT_EQUAL(stream->position(), sizeof(elf::ElfHeader));
}

void Elf::WriteProgramTable(ElfWriteStream* stream) {
  ASSERT(program_table_file_size_ >= 0);  // Check for finalization.
  ASSERT(stream->position() == program_table_file_offset_);
#if defined(DEBUG)
  // Here, we count the number of times that a PT_LOAD writable segment is
  // followed by a non-writable segment. We initialize last_writable to true so
  // that we catch the case where the first segment is non-writable.
  bool last_writable = true;
  int non_writable_groups = 0;
#endif
  for (auto const segment : segments_) {
#if defined(DEBUG)
    if (segment->type == elf::ProgramHeaderType::PT_LOAD) {
      if (last_writable && !segment->IsWritable()) {
        non_writable_groups++;
      }
      last_writable = segment->IsWritable();
    }
#endif
    const intptr_t start = stream->position();
    segment->WriteProgramHeader(stream);
    const intptr_t end = stream->position();
    ASSERT_EQUAL(end - start, sizeof(elf::ProgramHeader));
  }
#if defined(DEBUG)
  // All PT_LOAD non-writable segments must be contiguous. If not, some older
  // Android dynamic linkers fail to handle writable segments between
  // non-writable ones. See https://github.com/flutter/flutter/issues/43259.
  ASSERT(non_writable_groups <= 1);
#endif
}

void Elf::WriteSectionTable(ElfWriteStream* stream) {
  ASSERT(section_table_file_size_ >= 0);  // Check for finalization.
  stream->Align(kElfSectionTableAlignment);
  ASSERT_EQUAL(stream->position(), section_table_file_offset_);

  for (auto const section : sections_) {
    const intptr_t start = stream->position();
    section->WriteSectionHeader(stream);
    const intptr_t end = stream->position();
    ASSERT_EQUAL(end - start, sizeof(elf::SectionHeader));
  }
}

void Elf::WriteSections(ElfWriteStream* stream) {
  ASSERT(section_table_file_size_ >= 0);  // Check for finalization.

  // Skip the reserved first section, as its alignment is 0 (which will cause
  // stream->Align() to fail) and it never contains file contents anyway.
  ASSERT_EQUAL(static_cast<uint32_t>(sections_[0]->type),
               static_cast<uint32_t>(elf::SectionHeaderType::SHT_NULL));
  ASSERT_EQUAL(sections_[0]->alignment, 0);
  auto const load_align = Segment::Alignment(elf::ProgramHeaderType::PT_LOAD);
  const Segment* current_segment = segments_[1];
  for (intptr_t i = 1; i < sections_.length(); i++) {
    Section* section = sections_[i];
    stream->Align(section->alignment);
    if (section->IsAllocated() && section->load_segment != current_segment) {
      // Changing segments, so align accordingly.
      stream->Align(load_align);
      current_segment = section->load_segment;
    }
    ASSERT_EQUAL(stream->position(), section->file_offset());
    section->Write(stream);
    ASSERT_EQUAL(stream->position(),
                 section->file_offset() + section->FileSize());
  }
}

}  // namespace dart