1 files changed, 590 insertions, 0 deletions
diff --git a/tools/llvm-objcopy/MachO/MachOWriter.cpp b/tools/llvm-objcopy/MachO/MachOWriter.cpp
new file mode 100644
index 000000000000..74200c5aa62a
--- /dev/null
+++ b/tools/llvm-objcopy/MachO/MachOWriter.cpp
@@ -0,0 +1,590 @@
+//===- MachOWriter.cpp ------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "MachOWriter.h"
+#include "Object.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/BinaryFormat/MachO.h"
+#include "llvm/Object/MachO.h"
+#include "llvm/Support/Errc.h"
+#include "llvm/Support/ErrorHandling.h"
+#include <memory>
+
+namespace llvm {
+namespace objcopy {
+namespace macho {
+
+size_t MachOWriter::headerSize() const {
+  return Is64Bit ? sizeof(MachO::mach_header_64) : sizeof(MachO::mach_header);
+}
+
+size_t MachOWriter::loadCommandsSize() const { return O.Header.SizeOfCmds; }
+
+size_t MachOWriter::symTableSize() const {
+  return O.SymTable.Symbols.size() *
+         (Is64Bit ? sizeof(MachO::nlist_64) : sizeof(MachO::nlist));
+}
+
+size_t MachOWriter::totalSize() const {
+  // Going from tail to head and looking for an appropriate "anchor" to
+  // calculate the total size assuming that all the offsets are either valid
+  // ("true") or 0 (0 indicates that the corresponding part is missing).
+
+  SmallVector<size_t, 7> Ends;
+  if (O.SymTabCommandIndex) {
+    const MachO::symtab_command &SymTabCommand =
+        O.LoadCommands[*O.SymTabCommandIndex]
+            .MachOLoadCommand.symtab_command_data;
+    if (SymTabCommand.symoff) {
+      assert((SymTabCommand.nsyms == O.SymTable.Symbols.size()) &&
+             "Incorrect number of symbols");
+      Ends.push_back(SymTabCommand.symoff + symTableSize());
+    }
+    if (SymTabCommand.stroff) {
+      assert((SymTabCommand.strsize == StrTableBuilder.getSize()) &&
+             "Incorrect string table size");
+      Ends.push_back(SymTabCommand.stroff + SymTabCommand.strsize);
+    }
+  }
+  if (O.DyLdInfoCommandIndex) {
+    const MachO::dyld_info_command &DyLdInfoCommand =
+        O.LoadCommands[*O.DyLdInfoCommandIndex]
+            .MachOLoadCommand.dyld_info_command_data;
+    if (DyLdInfoCommand.rebase_off) {
+      assert((DyLdInfoCommand.rebase_size == O.Rebases.Opcodes.size()) &&
+             "Incorrect rebase opcodes size");
+      Ends.push_back(DyLdInfoCommand.rebase_off + DyLdInfoCommand.rebase_size);
+    }
+    if (DyLdInfoCommand.bind_off) {
+      assert((DyLdInfoCommand.bind_size == O.Binds.Opcodes.size()) &&
+             "Incorrect bind opcodes size");
+      Ends.push_back(DyLdInfoCommand.bind_off + DyLdInfoCommand.bind_size);
+    }
+    if (DyLdInfoCommand.weak_bind_off) {
+      assert((DyLdInfoCommand.weak_bind_size == O.WeakBinds.Opcodes.size()) &&
+             "Incorrect weak bind opcodes size");
+      Ends.push_back(DyLdInfoCommand.weak_bind_off +
+                     DyLdInfoCommand.weak_bind_size);
+    }
+    if (DyLdInfoCommand.lazy_bind_off) {
+      assert((DyLdInfoCommand.lazy_bind_size == O.LazyBinds.Opcodes.size()) &&
+             "Incorrect lazy bind opcodes size");
+      Ends.push_back(DyLdInfoCommand.lazy_bind_off +
+                     DyLdInfoCommand.lazy_bind_size);
+    }
+    if (DyLdInfoCommand.export_off) {
+      assert((DyLdInfoCommand.export_size == O.Exports.Trie.size()) &&
+             "Incorrect trie size");
+      Ends.push_back(DyLdInfoCommand.export_off + DyLdInfoCommand.export_size);
+    }
+  }
+
+  // Otherwise, use the last section / reloction.
+  for (const auto &LC : O.LoadCommands)
+    for (const auto &S : LC.Sections) {
+      Ends.push_back(S.Offset + S.Size);
+      if (S.RelOff)
+        Ends.push_back(S.RelOff +
+                       S.NReloc * sizeof(MachO::any_relocation_info));
+    }
+
+  if (!Ends.empty())
+    return *std::max_element(Ends.begin(), Ends.end());
+
+  // Otherwise, we have only Mach header and load commands.
+  return headerSize() + loadCommandsSize();
+}
+
+void MachOWriter::writeHeader() {
+  MachO::mach_header_64 Header;
+
+  Header.magic = O.Header.Magic;
+  Header.cputype = O.Header.CPUType;
+  Header.cpusubtype = O.Header.CPUSubType;
+  Header.filetype = O.Header.FileType;
+  Header.ncmds = O.Header.NCmds;
+  Header.sizeofcmds = O.Header.SizeOfCmds;
+  Header.flags = O.Header.Flags;
+  Header.reserved = O.Header.Reserved;
+
+  if (IsLittleEndian != sys::IsLittleEndianHost)
+    MachO::swapStruct(Header);
+
+  auto HeaderSize =
+      Is64Bit ? sizeof(MachO::mach_header_64) : sizeof(MachO::mach_header);
+  memcpy(B.getBufferStart(), &Header, HeaderSize);
+}
+
+void MachOWriter::updateSymbolIndexes() {
+  uint32_t Index = 0;
+  for (auto &Symbol : O.SymTable.Symbols) {
+    Symbol->Index = Index;
+    Index++;
+  }
+}
+
+void MachOWriter::writeLoadCommands() {
+  uint8_t *Begin = B.getBufferStart() + headerSize();
+  for (const auto &LC : O.LoadCommands) {
+    // Construct a load command.
+    MachO::macho_load_command MLC = LC.MachOLoadCommand;
+    switch (MLC.load_command_data.cmd) {
+    case MachO::LC_SEGMENT:
+      if (IsLittleEndian != sys::IsLittleEndianHost)
+        MachO::swapStruct(MLC.segment_command_data);
+      memcpy(Begin, &MLC.segment_command_data, sizeof(MachO::segment_command));
+      Begin += sizeof(MachO::segment_command);
+
+      for (const auto &Sec : LC.Sections)
+        writeSectionInLoadCommand<MachO::section>(Sec, Begin);
+      continue;
+    case MachO::LC_SEGMENT_64:
+      if (IsLittleEndian != sys::IsLittleEndianHost)
+        MachO::swapStruct(MLC.segment_command_64_data);
+      memcpy(Begin, &MLC.segment_command_64_data,
+             sizeof(MachO::segment_command_64));
+      Begin += sizeof(MachO::segment_command_64);
+
+      for (const auto &Sec : LC.Sections)
+        writeSectionInLoadCommand<MachO::section_64>(Sec, Begin);
+      continue;
+    }
+
+#define HANDLE_LOAD_COMMAND(LCName, LCValue, LCStruct)                         \
+  case MachO::LCName:                                                          \
+    assert(sizeof(MachO::LCStruct) + LC.Payload.size() ==                      \
+           MLC.load_command_data.cmdsize);                                     \
+    if (IsLittleEndian != sys::IsLittleEndianHost)                             \
+      MachO::swapStruct(MLC.LCStruct##_data);                                  \
+    memcpy(Begin, &MLC.LCStruct##_data, sizeof(MachO::LCStruct));              \
+    Begin += sizeof(MachO::LCStruct);                                          \
+    memcpy(Begin, LC.Payload.data(), LC.Payload.size());                       \
+    Begin += LC.Payload.size();                                                \
+    break;
+
+    // Copy the load command as it is.
+    switch (MLC.load_command_data.cmd) {
+    default:
+      assert(sizeof(MachO::load_command) + LC.Payload.size() ==
+             MLC.load_command_data.cmdsize);
+      if (IsLittleEndian != sys::IsLittleEndianHost)
+        MachO::swapStruct(MLC.load_command_data);
+      memcpy(Begin, &MLC.load_command_data, sizeof(MachO::load_command));
+      Begin += sizeof(MachO::load_command);
+      memcpy(Begin, LC.Payload.data(), LC.Payload.size());
+      Begin += LC.Payload.size();
+      break;
+#include "llvm/BinaryFormat/MachO.def"
+    }
+  }
+}
+
+template <typename StructType>
+void MachOWriter::writeSectionInLoadCommand(const Section &Sec, uint8_t *&Out) {
+  StructType Temp;
+  assert(Sec.Segname.size() <= sizeof(Temp.segname) && "too long segment name");
+  assert(Sec.Sectname.size() <= sizeof(Temp.sectname) &&
+         "too long section name");
+  memset(&Temp, 0, sizeof(StructType));
+  memcpy(Temp.segname, Sec.Segname.data(), Sec.Segname.size());
+  memcpy(Temp.sectname, Sec.Sectname.data(), Sec.Sectname.size());
+  Temp.addr = Sec.Addr;
+  Temp.size = Sec.Size;
+  Temp.offset = Sec.Offset;
+  Temp.align = Sec.Align;
+  Temp.reloff = Sec.RelOff;
+  Temp.nreloc = Sec.NReloc;
+  Temp.flags = Sec.Flags;
+  Temp.reserved1 = Sec.Reserved1;
+  Temp.reserved2 = Sec.Reserved2;
+
+  if (IsLittleEndian != sys::IsLittleEndianHost)
+    MachO::swapStruct(Temp);
+  memcpy(Out, &Temp, sizeof(StructType));
+  Out += sizeof(StructType);
+}
+
+void MachOWriter::writeSections() {
+  for (const auto &LC : O.LoadCommands)
+    for (const auto &Sec : LC.Sections) {
+      if (Sec.isVirtualSection())
+        continue;
+
+      assert(Sec.Offset && "Section offset can not be zero");
+      assert((Sec.Size == Sec.Content.size()) && "Incorrect section size");
+      memcpy(B.getBufferStart() + Sec.Offset, Sec.Content.data(),
+             Sec.Content.size());
+      for (size_t Index = 0; Index < Sec.Relocations.size(); ++Index) {
+        auto RelocInfo = Sec.Relocations[Index];
+        if (!RelocInfo.Scattered) {
+          auto *Info =
+              reinterpret_cast<MachO::relocation_info *>(&RelocInfo.Info);
+          Info->r_symbolnum = RelocInfo.Symbol->Index;
+        }
+
+        if (IsLittleEndian != sys::IsLittleEndianHost)
+          MachO::swapStruct(
+              reinterpret_cast<MachO::any_relocation_info &>(RelocInfo.Info));
+        memcpy(B.getBufferStart() + Sec.RelOff +
+                   Index * sizeof(MachO::any_relocation_info),
+               &RelocInfo.Info, sizeof(RelocInfo.Info));
+      }
+    }
+}
+
+template <typename NListType>
+void writeNListEntry(const SymbolEntry &SE, bool IsLittleEndian, char *&Out,
+                     uint32_t Nstrx) {
+  NListType ListEntry;
+  ListEntry.n_strx = Nstrx;
+  ListEntry.n_type = SE.n_type;
+  ListEntry.n_sect = SE.n_sect;
+  ListEntry.n_desc = SE.n_desc;
+  ListEntry.n_value = SE.n_value;
+
+  if (IsLittleEndian != sys::IsLittleEndianHost)
+    MachO::swapStruct(ListEntry);
+  memcpy(Out, reinterpret_cast<const char *>(&ListEntry), sizeof(NListType));
+  Out += sizeof(NListType);
+}
+
+void MachOWriter::writeSymbolTable() {
+  if (!O.SymTabCommandIndex)
+    return;
+  const MachO::symtab_command &SymTabCommand =
+      O.LoadCommands[*O.SymTabCommandIndex]
+          .MachOLoadCommand.symtab_command_data;
+
+  uint8_t *StrTable = (uint8_t *)B.getBufferStart() + SymTabCommand.stroff;
+  StrTableBuilder.write(StrTable);
+}
+
+void MachOWriter::writeStringTable() {
+  if (!O.SymTabCommandIndex)
+    return;
+  const MachO::symtab_command &SymTabCommand =
+      O.LoadCommands[*O.SymTabCommandIndex]
+          .MachOLoadCommand.symtab_command_data;
+
+  char *SymTable = (char *)B.getBufferStart() + SymTabCommand.symoff;
+  for (auto Iter = O.SymTable.Symbols.begin(), End = O.SymTable.Symbols.end();
+       Iter != End; Iter++) {
+    SymbolEntry *Sym = Iter->get();
+    auto Nstrx = StrTableBuilder.getOffset(Sym->Name);
+
+    if (Is64Bit)
+      writeNListEntry<MachO::nlist_64>(*Sym, IsLittleEndian, SymTable, Nstrx);
+    else
+      writeNListEntry<MachO::nlist>(*Sym, IsLittleEndian, SymTable, Nstrx);
+  }
+}
+
+void MachOWriter::writeRebaseInfo() {
+  if (!O.DyLdInfoCommandIndex)
+    return;
+  const MachO::dyld_info_command &DyLdInfoCommand =
+      O.LoadCommands[*O.DyLdInfoCommandIndex]
+          .MachOLoadCommand.dyld_info_command_data;
+  char *Out = (char *)B.getBufferStart() + DyLdInfoCommand.rebase_off;
+  assert((DyLdInfoCommand.rebase_size == O.Rebases.Opcodes.size()) &&
+         "Incorrect rebase opcodes size");
+  memcpy(Out, O.Rebases.Opcodes.data(), O.Rebases.Opcodes.size());
+}
+
+void MachOWriter::writeBindInfo() {
+  if (!O.DyLdInfoCommandIndex)
+    return;
+  const MachO::dyld_info_command &DyLdInfoCommand =
+      O.LoadCommands[*O.DyLdInfoCommandIndex]
+          .MachOLoadCommand.dyld_info_command_data;
+  char *Out = (char *)B.getBufferStart() + DyLdInfoCommand.bind_off;
+  assert((DyLdInfoCommand.bind_size == O.Binds.Opcodes.size()) &&
+         "Incorrect bind opcodes size");
+  memcpy(Out, O.Binds.Opcodes.data(), O.Binds.Opcodes.size());
+}
+
+void MachOWriter::writeWeakBindInfo() {
+  if (!O.DyLdInfoCommandIndex)
+    return;
+  const MachO::dyld_info_command &DyLdInfoCommand =
+      O.LoadCommands[*O.DyLdInfoCommandIndex]
+          .MachOLoadCommand.dyld_info_command_data;
+  char *Out = (char *)B.getBufferStart() + DyLdInfoCommand.weak_bind_off;
+  assert((DyLdInfoCommand.weak_bind_size == O.WeakBinds.Opcodes.size()) &&
+         "Incorrect weak bind opcodes size");
+  memcpy(Out, O.WeakBinds.Opcodes.data(), O.WeakBinds.Opcodes.size());
+}
+
+void MachOWriter::writeLazyBindInfo() {
+  if (!O.DyLdInfoCommandIndex)
+    return;
+  const MachO::dyld_info_command &DyLdInfoCommand =
+      O.LoadCommands[*O.DyLdInfoCommandIndex]
+          .MachOLoadCommand.dyld_info_command_data;
+  char *Out = (char *)B.getBufferStart() + DyLdInfoCommand.lazy_bind_off;
+  assert((DyLdInfoCommand.lazy_bind_size == O.LazyBinds.Opcodes.size()) &&
+         "Incorrect lazy bind opcodes size");
+  memcpy(Out, O.LazyBinds.Opcodes.data(), O.LazyBinds.Opcodes.size());
+}
+
+void MachOWriter::writeExportInfo() {
+  if (!O.DyLdInfoCommandIndex)
+    return;
+  const MachO::dyld_info_command &DyLdInfoCommand =
+      O.LoadCommands[*O.DyLdInfoCommandIndex]
+          .MachOLoadCommand.dyld_info_command_data;
+  char *Out = (char *)B.getBufferStart() + DyLdInfoCommand.export_off;
+  assert((DyLdInfoCommand.export_size == O.Exports.Trie.size()) &&
+         "Incorrect export trie size");
+  memcpy(Out, O.Exports.Trie.data(), O.Exports.Trie.size());
+}
+
+void MachOWriter::writeTail() {
+  typedef void (MachOWriter::*WriteHandlerType)(void);
+  typedef std::pair<uint64_t, WriteHandlerType> WriteOperation;
+  SmallVector<WriteOperation, 7> Queue;
+
+  if (O.SymTabCommandIndex) {
+    const MachO::symtab_command &SymTabCommand =
+        O.LoadCommands[*O.SymTabCommandIndex]
+            .MachOLoadCommand.symtab_command_data;
+    if (SymTabCommand.symoff)
+      Queue.push_back({SymTabCommand.symoff, &MachOWriter::writeSymbolTable});
+    if (SymTabCommand.stroff)
+      Queue.push_back({SymTabCommand.stroff, &MachOWriter::writeStringTable});
+  }
+
+  if (O.DyLdInfoCommandIndex) {
+    const MachO::dyld_info_command &DyLdInfoCommand =
+        O.LoadCommands[*O.DyLdInfoCommandIndex]
+            .MachOLoadCommand.dyld_info_command_data;
+    if (DyLdInfoCommand.rebase_off)
+      Queue.push_back(
+          {DyLdInfoCommand.rebase_off, &MachOWriter::writeRebaseInfo});
+    if (DyLdInfoCommand.bind_off)
+      Queue.push_back({DyLdInfoCommand.bind_off, &MachOWriter::writeBindInfo});
+    if (DyLdInfoCommand.weak_bind_off)
+      Queue.push_back(
+          {DyLdInfoCommand.weak_bind_off, &MachOWriter::writeWeakBindInfo});
+    if (DyLdInfoCommand.lazy_bind_off)
+      Queue.push_back(
+          {DyLdInfoCommand.lazy_bind_off, &MachOWriter::writeLazyBindInfo});
+    if (DyLdInfoCommand.export_off)
+      Queue.push_back(
+          {DyLdInfoCommand.export_off, &MachOWriter::writeExportInfo});
+  }
+
+  llvm::sort(Queue, [](const WriteOperation &LHS, const WriteOperation &RHS) {
+    return LHS.first < RHS.first;
+  });
+
+  for (auto WriteOp : Queue)
+    (this->*WriteOp.second)();
+}
+
+void MachOWriter::updateSizeOfCmds() {
+  auto Size = 0;
+  for (const auto &LC : O.LoadCommands) {
+    auto &MLC = LC.MachOLoadCommand;
+    auto cmd = MLC.load_command_data.cmd;
+
+    switch (cmd) {
+    case MachO::LC_SEGMENT:
+      Size += sizeof(MachO::segment_command) +
+              sizeof(MachO::section) * LC.Sections.size();
+      continue;
+    case MachO::LC_SEGMENT_64:
+      Size += sizeof(MachO::segment_command_64) +
+              sizeof(MachO::section_64) * LC.Sections.size();
+      continue;
+    }
+
+    switch (cmd) {
+#define HANDLE_LOAD_COMMAND(LCName, LCValue, LCStruct)                         \
+  case MachO::LCName:                                                          \
+    Size += sizeof(MachO::LCStruct);                                           \
+    break;
+#include "llvm/BinaryFormat/MachO.def"
+#undef HANDLE_LOAD_COMMAND
+    }
+  }
+
+  O.Header.SizeOfCmds = Size;
+}
+
+// Updates the index and the number of local/external/undefined symbols. Here we
+// assume that MLC is a LC_DYSYMTAB and the nlist entries in the symbol table
+// are already sorted by the those types.
+void MachOWriter::updateDySymTab(MachO::macho_load_command &MLC) {
+  uint32_t NumLocalSymbols = 0;
+  auto Iter = O.SymTable.Symbols.begin();
+  auto End = O.SymTable.Symbols.end();
+  for (; Iter != End; Iter++) {
+    if ((*Iter)->n_type & (MachO::N_EXT | MachO::N_PEXT))
+      break;
+
+    NumLocalSymbols++;
+  }
+
+  uint32_t NumExtDefSymbols = 0;
+  for (; Iter != End; Iter++) {
+    if (((*Iter)->n_type & MachO::N_TYPE) == MachO::N_UNDF)
+      break;
+
+    NumExtDefSymbols++;
+  }
+
+  MLC.dysymtab_command_data.ilocalsym = 0;
+  MLC.dysymtab_command_data.nlocalsym = NumLocalSymbols;
+  MLC.dysymtab_command_data.iextdefsym = NumLocalSymbols;
+  MLC.dysymtab_command_data.nextdefsym = NumExtDefSymbols;
+  MLC.dysymtab_command_data.iundefsym = NumLocalSymbols + NumExtDefSymbols;
+  MLC.dysymtab_command_data.nundefsym =
+      O.SymTable.Symbols.size() - (NumLocalSymbols + NumExtDefSymbols);
+}
+
+// Recomputes and updates offset and size fields in load commands and sections
+// since they could be modified.
+Error MachOWriter::layout() {
+  auto SizeOfCmds = loadCommandsSize();
+  auto Offset = headerSize() + SizeOfCmds;
+  O.Header.NCmds = O.LoadCommands.size();
+  O.Header.SizeOfCmds = SizeOfCmds;
+
+  // Lay out sections.
+  for (auto &LC : O.LoadCommands) {
+    uint64_t FileOff = Offset;
+    uint64_t VMSize = 0;
+    uint64_t FileOffsetInSegment = 0;
+    for (auto &Sec : LC.Sections) {
+      if (!Sec.isVirtualSection()) {
+        auto FilePaddingSize =
+            OffsetToAlignment(FileOffsetInSegment, 1ull << Sec.Align);
+        Sec.Offset = Offset + FileOffsetInSegment + FilePaddingSize;
+        Sec.Size = Sec.Content.size();
+        FileOffsetInSegment += FilePaddingSize + Sec.Size;
+      }
+
+      VMSize = std::max(VMSize, Sec.Addr + Sec.Size);
+    }
+
+    // TODO: Handle the __PAGEZERO segment.
+    auto &MLC = LC.MachOLoadCommand;
+    switch (MLC.load_command_data.cmd) {
+    case MachO::LC_SEGMENT:
+      MLC.segment_command_data.cmdsize =
+          sizeof(MachO::segment_command) +
+          sizeof(MachO::section) * LC.Sections.size();
+      MLC.segment_command_data.nsects = LC.Sections.size();
+      MLC.segment_command_data.fileoff = FileOff;
+      MLC.segment_command_data.vmsize = VMSize;
+      MLC.segment_command_data.filesize = FileOffsetInSegment;
+      break;
+    case MachO::LC_SEGMENT_64:
+      MLC.segment_command_64_data.cmdsize =
+          sizeof(MachO::segment_command_64) +
+          sizeof(MachO::section_64) * LC.Sections.size();
+      MLC.segment_command_64_data.nsects = LC.Sections.size();
+      MLC.segment_command_64_data.fileoff = FileOff;
+      MLC.segment_command_64_data.vmsize = VMSize;
+      MLC.segment_command_64_data.filesize = FileOffsetInSegment;
+      break;
+    }
+
+    Offset += FileOffsetInSegment;
+  }
+
+  // Lay out relocations.
+  for (auto &LC : O.LoadCommands)
+    for (auto &Sec : LC.Sections) {
+      Sec.RelOff = Sec.Relocations.empty() ? 0 : Offset;
+      Sec.NReloc = Sec.Relocations.size();
+      Offset += sizeof(MachO::any_relocation_info) * Sec.NReloc;
+    }
+
+  // Lay out tail stuff.
+  auto NListSize = Is64Bit ? sizeof(MachO::nlist_64) : sizeof(MachO::nlist);
+  for (auto &LC : O.LoadCommands) {
+    auto &MLC = LC.MachOLoadCommand;
+    auto cmd = MLC.load_command_data.cmd;
+    switch (cmd) {
+    case MachO::LC_SYMTAB:
+      MLC.symtab_command_data.nsyms = O.SymTable.Symbols.size();
+      MLC.symtab_command_data.strsize = StrTableBuilder.getSize();
+      MLC.symtab_command_data.symoff = Offset;
+      Offset += NListSize * MLC.symtab_command_data.nsyms;
+      MLC.symtab_command_data.stroff = Offset;
+      Offset += MLC.symtab_command_data.strsize;
+      break;
+    case MachO::LC_DYSYMTAB: {
+      if (MLC.dysymtab_command_data.ntoc != 0 ||
+          MLC.dysymtab_command_data.nmodtab != 0 ||
+          MLC.dysymtab_command_data.nextrefsyms != 0 ||
+          MLC.dysymtab_command_data.nlocrel != 0 ||
+          MLC.dysymtab_command_data.nextrel != 0)
+        return createStringError(llvm::errc::not_supported,
+                                 "shared library is not yet supported");
+
+      if (MLC.dysymtab_command_data.nindirectsyms != 0)
+        return createStringError(llvm::errc::not_supported,
+                                 "indirect symbol table is not yet supported");
+
+      updateDySymTab(MLC);
+      break;
+    }
+    case MachO::LC_SEGMENT:
+    case MachO::LC_SEGMENT_64:
+    case MachO::LC_VERSION_MIN_MACOSX:
+    case MachO::LC_BUILD_VERSION:
+    case MachO::LC_ID_DYLIB:
+    case MachO::LC_LOAD_DYLIB:
+    case MachO::LC_UUID:
+    case MachO::LC_SOURCE_VERSION:
+      // Nothing to update.
+      break;
+    default:
+      // Abort if it's unsupported in order to prevent corrupting the object.
+      return createStringError(llvm::errc::not_supported,
+                               "unsupported load command (cmd=0x%x)", cmd);
+    }
+  }
+
+  return Error::success();
+}
+
+void MachOWriter::constructStringTable() {
+  for (std::unique_ptr<SymbolEntry> &Sym : O.SymTable.Symbols)
+    StrTableBuilder.add(Sym->Name);
+  StrTableBuilder.finalize();
+}
+
+Error MachOWriter::finalize() {
+  updateSizeOfCmds();
+  constructStringTable();
+
+  if (auto E = layout())
+    return E;
+
+  return Error::success();
+}
+
+Error MachOWriter::write() {
+  if (Error E = B.allocate(totalSize()))
+    return E;
+  memset(B.getBufferStart(), 0, totalSize());
+  writeHeader();
+  updateSymbolIndexes();
+  writeLoadCommands();
+  writeSections();
+  writeTail();
+  return B.commit();
+}
+
+} // end namespace macho
+} // end namespace objcopy
+} // end namespace llvm