Reland Implement .eh_frame writer and disassembler.

src/eh-frame.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/eh-frame.h"
-#include "src/objects-inl.h"
-#include "src/objects.h"
+
+#include <iomanip>
+#include <ostream>
+
+#if !defined(V8_TARGET_ARCH_X64) && !defined(V8_TARGET_ARCH_ARM) && \
+    !defined(V8_TARGET_ARCH_ARM64)
+
+// Placeholders for unsupported architectures.
 
 namespace v8 {
 namespace internal {
 
-static const int DW_EH_PE_pcrel = 0x10;
-static const int DW_EH_PE_datarel = 0x30;
-static const int DW_EH_PE_udata4 = 0x03;
-static const int DW_EH_PE_sdata4 = 0x0b;
-
-const int EhFrameHdr::kCIESize = 0;
-
-static const int kVersionSize = 1;
-static const int kEncodingSpecifiersSize = 3;
+STATIC_CONST_MEMBER_DEFINITION const int EhFrameWriter::kDataAlignmentFactor =
+    1;
+
+void EhFrameWriter::WriteReturnAddressRegisterCode() { UNIMPLEMENTED(); }
+
+void EhFrameWriter::WriteInitialStateInCIE() { UNIMPLEMENTED(); }
+
+int EhFrameWriter::RegisterToDwarfCode(Register) {
+  UNIMPLEMENTED();
+  return -1;
+}
+
+#ifdef ENABLE_DISASSEMBLER
+
+const char* EhFrameDisassembler::DwarfRegisterCodeToString(int) {
+  UNIMPLEMENTED();
+  return nullptr;
+}
+
+#endif
+
+}  // namespace internal
+}  // namespace v8
+
+#endif
+
+namespace v8 {
+namespace internal {
+
+STATIC_CONST_MEMBER_DEFINITION const int EhFrameWriter::kEhFrameTerminatorSize;
+STATIC_CONST_MEMBER_DEFINITION const uint32_t EhFrameWriter::kInt32Placeholder;
+
+EhFrameWriter::EhFrameWriter()
+    : cie_size_(0),
+      last_pc_offset_(0),
+      eh_frame_finalised_(false),
+      base_register_(no_reg),
+      base_offset_(0) {
+  eh_frame_buffer_.reserve(100);
+  WriteCIE();
+  WriteFDEHeader();
+}
+
+void EhFrameWriter::WriteCIE() {
+  static const int kCIEIdentifier = 0;
+  static const int kCIEVersion = 3;
+  static const int kCodeAlignmentFactor = 1;
+  static const int kAugmentationDataSize = 2;
+  static const byte kAugmentationString[] = {'z', 'L', 'R', 0};
+
+  int size_offset = eh_frame_offset();
+  WriteInt32(kInt32Placeholder);
+
+  int record_start_offset = eh_frame_offset();
+  WriteInt32(kCIEIdentifier);
+  WriteByte(kCIEVersion);
+
+  WriteBytes(&kAugmentationString[0], sizeof(kAugmentationString));
+
+  WriteSLEB128(kCodeAlignmentFactor);
+  WriteSLEB128(kDataAlignmentFactor);
+
+  WriteReturnAddressRegisterCode();
+
+  WriteByte(kAugmentationDataSize);
+  WriteByte(kOmit);
+  WriteByte(kSData4 | kPcRel);
+
+  DCHECK_EQ(eh_frame_offset() - size_offset, kInitialStateOffsetInCIE);
+  WriteInitialStateInCIE();
+
+  WritePaddingTo8ByteAlignment();
+
+  int record_end_offset = eh_frame_offset();
+  int encoded_cie_size = record_end_offset - record_start_offset;
+  cie_size_ = record_end_offset - size_offset;
+
+  PatchInt32(size_offset, encoded_cie_size);
+}
+
+void EhFrameWriter::WriteFDEHeader() {
+  DCHECK_NE(cie_size_, 0);
+
+  // Placeholder for size of the FDE.
+  DCHECK_EQ(eh_frame_offset(), fde_offset());
+  WriteInt32(kInt32Placeholder);
+
+  // Backwards offset to the CIE.
+  WriteInt32(cie_size_ + kInt32Size);
+
+  // Placeholder for pointer to procedure.
+  DCHECK_EQ(eh_frame_offset(), procedure_address_offset());
+  WriteInt32(kInt32Placeholder);
+
+  // Placeholder for size of the procedure.
+  DCHECK_EQ(eh_frame_offset(), procedure_size_offset());
+  WriteInt32(kInt32Placeholder);
+
+  // No augmentation data.
+  WriteByte(0);
+}
+
+void EhFrameWriter::WritePaddingTo8ByteAlignment() {
+  DCHECK(!eh_frame_finalised_);
+
+  int unpadded_size = eh_frame_offset();
+  int padded_size = RoundUp(unpadded_size, 8);
+  int padding_size = padded_size - unpadded_size;
+
+  byte nop = static_cast<byte>(DwarfOpcodes::kNop);
+  static const byte kPadding[] = {nop, nop, nop, nop, nop, nop, nop, nop};
+  DCHECK_LE(padding_size, static_cast<int>(sizeof(kPadding)));
+  WriteBytes(&kPadding[0], padding_size);
+}
+
+void EhFrameWriter::AdvanceLocation(int pc_offset) {
+  DCHECK(!eh_frame_finalised_);
+  DCHECK_GE(pc_offset, last_pc_offset_);
+  uint32_t delta = pc_offset - last_pc_offset_;
+
+  if (delta <= kLocationMask) {
+    WriteByte((kLocationTag << kLocationMaskSize) | (delta & kLocationMask));
+  } else if (delta <= kMaxUInt8) {
+    WriteOpcode(DwarfOpcodes::kAdvanceLoc1);
+    WriteByte(delta);
+  } else if (delta <= kMaxUInt16) {
+    WriteOpcode(DwarfOpcodes::kAdvanceLoc2);
+    WriteInt16(delta);
+  } else {
+    WriteOpcode(DwarfOpcodes::kAdvanceLoc4);
+    WriteInt32(delta);
+  }
+
+  last_pc_offset_ = pc_offset;
+}
+
+void EhFrameWriter::SetBaseAddressOffset(int base_offset) {
+  DCHECK(!eh_frame_finalised_);
+  DCHECK_GE(base_offset, 0);
+  WriteOpcode(DwarfOpcodes::kDefCfaOffset);
+  WriteULEB128(base_offset);
+  base_offset_ = base_offset;
+}
+
+void EhFrameWriter::SetBaseAddressRegister(Register base_register) {
+  DCHECK(!eh_frame_finalised_);
+  int code = RegisterToDwarfCode(base_register);
+  WriteOpcode(DwarfOpcodes::kDefCfaRegister);
+  WriteULEB128(code);
+  base_register_ = base_register;
+}
+
+void EhFrameWriter::SetBaseAddressRegisterAndOffset(Register base_register,
+                                                    int base_offset) {
+  DCHECK(!eh_frame_finalised_);
+  int code = RegisterToDwarfCode(base_register);
+  WriteOpcode(DwarfOpcodes::kDefCfa);
+  WriteULEB128(code);
+  WriteULEB128(base_offset);
+  base_offset_ = base_offset;
+  base_register_ = base_register;
+}
+
+void EhFrameWriter::RecordRegisterSavedToStack(int register_code, int offset) {
+  DCHECK(!eh_frame_finalised_);
+  DCHECK_EQ(offset % kDataAlignmentFactor, 0);
+  int factored_offset = offset / std::abs(kDataAlignmentFactor);
+  if (factored_offset >= 0) {
+    DCHECK_LE(register_code, kSavedRegisterMask);
+    WriteByte((kSavedRegisterTag << kSavedRegisterMaskSize) |
+              (register_code & kSavedRegisterMask));
+    WriteULEB128(factored_offset);
+  } else {
+    WriteOpcode(DwarfOpcodes::kOffsetExtendedSf);
+    WriteULEB128(register_code);
+    WriteSLEB128(factored_offset);
+  }
+}
+
+void EhFrameWriter::RecordRegisterIsValid(Register name) {
+  DCHECK(!eh_frame_finalised_);
+  WriteOpcode(DwarfOpcodes::kSameValue);
+  WriteULEB128(RegisterToDwarfCode(name));
+}
+
+void EhFrameWriter::RecordRegisterFollowsInitialRule(Register name) {
+  DCHECK(!eh_frame_finalised_);
+  int code = RegisterToDwarfCode(name);
+  DCHECK_LE(code, kFollowInitialRuleMask);
+  WriteByte((kFollowInitialRuleTag << kFollowInitialRuleMaskSize) |
+            (code & kFollowInitialRuleMask));
+}
+
+void EhFrameWriter::Finish(int code_size) {
+  DCHECK(!eh_frame_finalised_);
+  DCHECK_GE(eh_frame_offset(), cie_size_);
+
+  WritePaddingTo8ByteAlignment();
+
+  // Write the size of the FDE now that we know it.
+  // The encoded size does not include the size field itself.
+  int fde_size = eh_frame_offset() - fde_offset() - kInt32Size;
+  PatchInt32(fde_offset(), fde_size);
+
+  // Write the size and offset to procedure.
+  PatchInt32(procedure_address_offset(),
+             -(RoundUp(code_size, 8) + procedure_address_offset()));
+  PatchInt32(procedure_size_offset(), code_size);
+
+  // Terminate the .eh_frame.
+  static const byte kTerminator[kEhFrameTerminatorSize] = {0};
+  WriteBytes(&kTerminator[0], kEhFrameTerminatorSize);
+
+  // Write .eh_frame_hdr
+  EhFrameHdr eh_frame_hdr(code_size, eh_frame_offset(), cie_size_);
+  WriteBytes(reinterpret_cast<const byte*>(&eh_frame_hdr),
+             EhFrameHdr::kRecordSize);
+
+  eh_frame_finalised_ = true;
+}
+
+void EhFrameWriter::GetEhFrame(CodeDesc* desc) {
+  DCHECK(eh_frame_finalised_);
+  desc->unwinding_info_size = static_cast<int>(eh_frame_buffer_.size());
+  desc->unwinding_info = eh_frame_buffer_.data();
+}
+
+void EhFrameWriter::WriteULEB128(uint32_t value) {
+  do {
+    byte chunk = value & 0x7f;
+    value >>= 7;
+    if (value != 0) chunk |= 0x80;
+    eh_frame_buffer_.push_back(chunk);
+  } while (value != 0);
+}
+
+void EhFrameWriter::WriteSLEB128(int32_t value) {
+  static const int kSignBitMask = 0x40;
+  bool done;
+  do {
+    byte chunk = value & 0x7f;
+    value >>= 7;
+    done = ((value == 0) && ((chunk & kSignBitMask) == 0)) ||
+           ((value == -1) && ((chunk & kSignBitMask) != 0));
+    if (!done) chunk |= 0x80;
+    eh_frame_buffer_.push_back(chunk);
+  } while (!done);
+}
+
+EhFrameIterator::EhFrameIterator(EhFrameWriter* writer) {
+  CodeDesc desc;
+  writer->GetEhFrame(&desc);
+  start_ = desc.unwinding_info;
+  end_ = start_ + desc.unwinding_info_size;
+  next_ = start_;
+}
+
+uint32_t EhFrameIterator::GetNextULEB128() {
+  int size = 0;
+  uint32_t result = DecodeULEB128(next_, &size);
+  DCHECK_LE(next_ + size, end_);
+  next_ += size;
+  return result;
+}
+
+int32_t EhFrameIterator::GetNextSLEB128() {
+  int size = 0;
+  int32_t result = DecodeSLEB128(next_, &size);
+  DCHECK_LE(next_ + size, end_);
+  next_ += size;
+  return result;
+}
+
+// static
+uint32_t EhFrameIterator::DecodeULEB128(const byte* encoded,
+                                        int* encoded_size) {
+  const byte* current = encoded;
+  uint32_t result = 0;
+  int shift = 0;
+
+  do {
+    DCHECK_LT(shift, 8 * static_cast<int>(sizeof(result)));
+    result |= (*current & 0x7f) << shift;
+    shift += 7;
+  } while (*current++ >= 128);
+
+  DCHECK_NOT_NULL(encoded_size);
+  *encoded_size = static_cast<int>(current - encoded);
+
+  return result;
+}
+
+// static
+int32_t EhFrameIterator::DecodeSLEB128(const byte* encoded, int* encoded_size) {
+  static const byte kSignBitMask = 0x40;
+
+  const byte* current = encoded;
+  int32_t result = 0;
+  int shift = 0;
+  byte chunk;
+
+  do {
+    chunk = *current++;
+    DCHECK_LT(shift, 8 * static_cast<int>(sizeof(result)));
+    result |= (chunk & 0x7f) << shift;
+    shift += 7;
+  } while (chunk >= 128);
+
+  // Sign extend the result if the last chunk has the sign bit set.
+  if (chunk & kSignBitMask) result |= (~0ull) << shift;
+
+  DCHECK_NOT_NULL(encoded_size);
+  *encoded_size = static_cast<int>(current - encoded);
+
+  return result;
+}
+
+#ifdef ENABLE_DISASSEMBLER
+
+namespace {
+
+class StreamModifiersScope final {
+ public:
+  explicit StreamModifiersScope(std::ostream* stream)
+      : stream_(stream), flags_(stream->flags()) {}
+  ~StreamModifiersScope() { stream_->flags(flags_); }
+
+ private:
+  std::ostream* stream_;
+  std::ios::fmtflags flags_;
+};
+
+}  // namespace
+
+// static
+void EhFrameDisassembler::DumpDWARFDirectives(std::ostream& stream,  // NOLINT
+                                              const byte* start,
+                                              const byte* end) {
+  StreamModifiersScope modifiers_scope(&stream);
+
+  EhFrameIterator eh_frame_iterator(start, end);
+  uint32_t offset_in_procedure = 0;
+
+  while (!eh_frame_iterator.ReachedEnd()) {
+    stream << eh_frame_iterator.current_address() << "  ";
+
+    byte bytecode = eh_frame_iterator.GetNextByte();
+
+    if (((bytecode >> EhFrameWriter::kLocationMaskSize) & 0xff) ==
+        EhFrameWriter::kLocationTag) {
+      int value = bytecode & EhFrameWriter::kLocationMask;
+      offset_in_procedure += value;
+      stream << "| pc_offset=" << std::dec << offset_in_procedure
+             << " (delta=0x" << std::hex << value << ")\n";
+      continue;
+    }
+
+    if (((bytecode >> EhFrameWriter::kSavedRegisterMaskSize) & 0xff) ==
+        EhFrameWriter::kSavedRegisterTag) {
+      int decoded_offset = static_cast<int>(eh_frame_iterator.GetNextULEB128());
+      stream << "| " << DwarfRegisterCodeToString(bytecode &
+                                                  EhFrameWriter::kLocationMask)
+             << " saved at base" << std::showpos << std::dec
+             << decoded_offset * EhFrameWriter::kDataAlignmentFactor << '\n';
+      continue;
+    }
+
+    if (((bytecode >> EhFrameWriter::kFollowInitialRuleMaskSize) & 0xff) ==
+        EhFrameWriter::kFollowInitialRuleTag) {
+      stream << "| " << DwarfRegisterCodeToString(bytecode &
+                                                  EhFrameWriter::kLocationMask)
+             << " follows initial rule\n";
+      continue;
+    }
+
+    switch (static_cast<EhFrameWriter::DwarfOpcodes>(bytecode)) {
+      case EhFrameWriter::DwarfOpcodes::kOffsetExtendedSf: {
+        stream << "| "
+               << DwarfRegisterCodeToString(eh_frame_iterator.GetNextULEB128());
+        int32_t decoded_offset = eh_frame_iterator.GetNextSLEB128();
+        stream << " saved at base" << std::showpos << std::dec
+               << decoded_offset * EhFrameWriter::kDataAlignmentFactor << '\n';
+      }
+      case EhFrameWriter::DwarfOpcodes::kAdvanceLoc1: {
+        unsigned value = eh_frame_iterator.GetNextByte();
+        offset_in_procedure += value;
+        stream << "| pc_offset=" << std::dec << offset_in_procedure
+               << " (delta=0x" << std::hex << value << ")\n";
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kAdvanceLoc2: {
+        uint16_t value = eh_frame_iterator.GetNextUInt16();
+        offset_in_procedure += value;
+        stream << "| pc_offset=" << std::dec << offset_in_procedure
+               << " (delta=0x" << std::hex << value << ")\n";
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kAdvanceLoc4: {
+        uint32_t value = eh_frame_iterator.GetNextUInt32();
+        offset_in_procedure += value;
+        stream << "| pc_offset=" << std::dec << offset_in_procedure
+               << " (delta=0x" << std::hex << value << ")\n";
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kDefCfa: {
+        int base_register = eh_frame_iterator.GetNextULEB128();
+        int base_offset = eh_frame_iterator.GetNextULEB128();
+        stream << "| base_register=" << DwarfRegisterCodeToString(base_register)
+               << ", base_offset=0x" << std::hex << base_offset << '\n';
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kDefCfaOffset: {
+        stream << "| base_offset=0x" << std::hex
+               << eh_frame_iterator.GetNextULEB128() << '\n';
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kDefCfaRegister: {
+        stream << "| base_register="
+               << DwarfRegisterCodeToString(eh_frame_iterator.GetNextULEB128())
+               << '\n';
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kSameValue: {
+        stream << "| "
+               << DwarfRegisterCodeToString(eh_frame_iterator.GetNextULEB128())
+               << " to initial value\n";
+        break;
+      }
+      case EhFrameWriter::DwarfOpcodes::kNop:
+        stream << "| nop\n";
+        break;
+      default:
+        UNREACHABLE();
+        return;
+    }
+  }
+}
+
+// static
+void EhFrameDisassembler::DisassembleToStream(std::ostream& stream) {  // NOLINT
+  // The encoded CIE size does not include the size field itself.
+  const int cie_size = ReadUnalignedUInt32(start_) + kInt32Size;
+  const int fde_offset = cie_size;
+
+  const byte* cie_directives_start =
+      start_ + EhFrameWriter::kInitialStateOffsetInCIE;
+  const byte* cie_directives_end = start_ + cie_size;
+  DCHECK_LE(cie_directives_start, cie_directives_end);
+
+  stream << reinterpret_cast<const void*>(start_) << "  .eh_frame: CIE\n";
+  DumpDWARFDirectives(stream, cie_directives_start, cie_directives_end);
+
+  const byte* procedure_offset_address =
+      start_ + fde_offset + EhFrameWriter::kProcedureAddressOffsetInFde;
+  int32_t procedure_offset =
+      ReadUnalignedValue<int32_t>(procedure_offset_address);
+
+  const byte* procedure_size_address =
+      start_ + fde_offset + EhFrameWriter::kProcedureSizeOffsetInFde;
+  uint32_t procedure_size = ReadUnalignedUInt32(procedure_size_address);
+
+  const byte* fde_start = start_ + fde_offset;
+  stream << reinterpret_cast<const void*>(fde_start) << "  .eh_frame: FDE\n"
+         << reinterpret_cast<const void*>(procedure_offset_address)
+         << "  | procedure offset=" << procedure_offset << '\n'
+         << reinterpret_cast<const void*>(procedure_size_address)
+         << "  | procedure size=" << procedure_size << '\n';
+
+  const int fde_directives_offset = fde_offset + 4 * kInt32Size + 1;
+
+  const byte* fde_directives_start = start_ + fde_directives_offset;
+  const byte* fde_directives_end =
+      end_ - EhFrameHdr::kRecordSize - EhFrameWriter::kEhFrameTerminatorSize;
+  DCHECK_LE(fde_directives_start, fde_directives_end);
+
+  DumpDWARFDirectives(stream, fde_directives_start, fde_directives_end);
+
+  const byte* fde_terminator_start = fde_directives_end;
+  stream << reinterpret_cast<const void*>(fde_terminator_start)
+         << "  .eh_frame: terminator\n";
+
+  const byte* eh_frame_hdr_start =
+      fde_terminator_start + EhFrameWriter::kEhFrameTerminatorSize;
+  stream << reinterpret_cast<const void*>(eh_frame_hdr_start)
+         << "  .eh_frame_hdr: placeholder\n";
+}
+
+#endif
 
 //
 // In order to calculate offsets in the .eh_frame_hdr, we must know the layout
 // of the DSO generated by perf inject, which is assumed to be the following:
 //
 //  |      ...      |                        |
 //  +---------------+ <-- (F) ---            |  Larger offsets in file
 //  |               |           ^            |
 //  |  Instructions |           | .text      v
 //  |               |           v
@@ skipping 13 matching lines @@
 //  |    version    |           ^
 //  +---------------+           |
 //  |   encoding    |           |
 //  |  specifiers   |           |
 //  +---------------+ <---(A)   | .eh_frame_hdr
 //  |   offset to   |           |
 //  |   .eh_frame   |           |
 //  +---------------+           |
 //  |      ...      |          ...
 //
-// (F) is aligned at a 16-byte boundary.
-// (D) is aligned at a  8-byte boundary.
-// (B) is aligned at a  4-byte boundary.
-// (E), (C) and (A) have no alignment requirements.
+// (F) is aligned to a 16-byte boundary.
+// (D) is aligned to a  8-byte boundary.
+// (B) is aligned to a  4-byte boundary.
+// (C) is aligned to an addressing unit size boundary.
+// (E) and (A) have no alignment requirements.
 //
 // The distance between (A) and (B) is 4 bytes.
 //
-// The size of the .eh_frame is required to be a multiple of the pointer size,
-// which means that (B) will be naturally aligned to a 4-byte boundary on all
-// the architectures we support.
+// The size of the FDE is required to be a multiple of the pointer size, which
+// means that (B) will be naturally aligned to a 4-byte boundary on all the
+// architectures we support.
 //
 // Because (E) has no alignment requirements, there is padding between (E) and
 // (D). (F) is aligned at a 16-byte boundary, thus to a 8-byte one as well.
 //
-EhFrameHdr::EhFrameHdr(Code* code) {
-  int code_size = code->is_crankshafted() ? code->safepoint_table_offset()
-                                          : code->instruction_size();
-  version_ = 1;
-  eh_frame_ptr_encoding_ = DW_EH_PE_sdata4 | DW_EH_PE_pcrel;
-  lut_size_encoding_ = DW_EH_PE_udata4;
-  lut_entries_encoding_ = DW_EH_PE_sdata4 | DW_EH_PE_datarel;
+EhFrameHdr::EhFrameHdr(int code_size, int eh_frame_size, int cie_size) {
+  static const int kFdeVersionSize = 1;
+  static const int kFdeEncodingSpecifiersSize = 3;
 
-  // .eh_frame pointer and LUT
-  if (code->has_unwinding_info()) {
-    DCHECK_GE(code->unwinding_info_size(), EhFrameHdr::kRecordSize);
-    int eh_frame_size = code->unwinding_info_size() - EhFrameHdr::kRecordSize;
+  version_ = kEhFrameHdrVersion;
 
-    offset_to_eh_frame_ =
-        -(eh_frame_size + kVersionSize + kEncodingSpecifiersSize);  // A -> D
-    lut_entries_number_ = 1;
-    offset_to_procedure_ = -(RoundUp(code_size, 8) + eh_frame_size);  // B -> F
-    offset_to_fde_ = -(eh_frame_size - kCIESize);                     // B -> C
-  } else {
-    // Create a dummy table
-    offset_to_eh_frame_ = 0;
-    lut_entries_number_ = 0;
-    offset_to_procedure_ = 0;
-    offset_to_fde_ = 0;
-  }
+  eh_frame_ptr_encoding_ = EhFrameWriter::kSData4 | EhFrameWriter::kPcRel;
+  lut_size_encoding_ = EhFrameWriter::kUData4;
+  lut_entries_encoding_ = EhFrameWriter::kSData4 | EhFrameWriter::kDataRel;
+  offset_to_eh_frame_ = -(eh_frame_size + kFdeVersionSize +
+                          kFdeEncodingSpecifiersSize);  // A -> D
+  lut_entries_number_ = 1;
+  offset_to_procedure_ = -(RoundUp(code_size, 8) + eh_frame_size);  // B -> F
+  offset_to_fde_ = -(eh_frame_size - cie_size);                     // B -> C
+}
+
+// static
+EhFrameHdr EhFrameHdr::CreateEmptyHeader() {
+  EhFrameHdr dummy_frame;
+  dummy_frame.version_ = kEhFrameHdrVersion;
+  dummy_frame.eh_frame_ptr_encoding_ =
+      EhFrameWriter::kSData4 | EhFrameWriter::kPcRel;
+  dummy_frame.lut_size_encoding_ = EhFrameWriter::kUData4;
+  dummy_frame.lut_entries_encoding_ =
+      EhFrameWriter::kSData4 | EhFrameWriter::kDataRel;
+  dummy_frame.offset_to_eh_frame_ = 0;
+  dummy_frame.lut_entries_number_ = 0;
+  dummy_frame.offset_to_procedure_ = 0;
+  dummy_frame.offset_to_fde_ = 0;
+  return dummy_frame;
 }
 
 }  // namespace internal
 }  // namespace v8