llvm-objcopy/MachO/Object.h

//===- Object.h - Mach-O object file model ----------------------*- 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
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_OBJCOPY_MACHO_OBJECT_H
#define LLVM_OBJCOPY_MACHO_OBJECT_H

#include "llvm/ADT/Optional.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/ObjectYAML/DWARFYAML.h"
#include "llvm/Support/YAMLTraits.h"
#include <cstdint>
#include <string>
#include <vector>

namespace llvm {
namespace objcopy {
namespace macho {

struct MachHeader {
  uint32_t Magic;
  uint32_t CPUType;
  uint32_t CPUSubType;
  uint32_t FileType;
  uint32_t NCmds;
  uint32_t SizeOfCmds;
  uint32_t Flags;
  uint32_t Reserved = 0;
};

struct RelocationInfo;
struct Section {
  std::string Sectname;
  std::string Segname;
  uint64_t Addr;
  uint64_t Size;
  uint32_t Offset;
  uint32_t Align;
  uint32_t RelOff;
  uint32_t NReloc;
  uint32_t Flags;
  uint32_t Reserved1;
  uint32_t Reserved2;
  uint32_t Reserved3;

  StringRef Content;
  std::vector<RelocationInfo> Relocations;

  MachO::SectionType getType() const {
    return static_cast<MachO::SectionType>(Flags & MachO::SECTION_TYPE);
  }

  bool isVirtualSection() const {
    return (getType() == MachO::S_ZEROFILL ||
            getType() == MachO::S_GB_ZEROFILL ||
            getType() == MachO::S_THREAD_LOCAL_ZEROFILL);
  }
};

struct LoadCommand {
  // The type MachO::macho_load_command is defined in llvm/BinaryFormat/MachO.h
  // and it is a union of all the structs corresponding to various load
  // commands.
  MachO::macho_load_command MachOLoadCommand;

  // The raw content of the payload of the load command (located right after the
  // corresponding struct). In some cases it is either empty or can be
  // copied-over without digging into its structure.
  ArrayRef<uint8_t> Payload;

  // Some load commands can contain (inside the payload) an array of sections,
  // though the contents of the sections are stored separately. The struct
  // Section describes only sections' metadata and where to find the
  // corresponding content inside the binary.
  std::vector<Section> Sections;
};

// A symbol information. Fields which starts with "n_" are same as them in the
// nlist.
struct SymbolEntry {
  std::string Name;
  uint32_t Index;
  uint8_t n_type;
  uint8_t n_sect;
  uint16_t n_desc;
  uint64_t n_value;
};

/// The location of the symbol table inside the binary is described by LC_SYMTAB
/// load command.
struct SymbolTable {
  std::vector<std::unique_ptr<SymbolEntry>> Symbols;

  const SymbolEntry *getSymbolByIndex(uint32_t Index) const;
};

/// The location of the string table inside the binary is described by LC_SYMTAB
/// load command.
struct StringTable {
  std::vector<std::string> Strings;
};

struct RelocationInfo {
  const SymbolEntry *Symbol;
  // True if Info is a scattered_relocation_info.
  bool Scattered;
  MachO::any_relocation_info Info;
};

/// The location of the rebase info inside the binary is described by
/// LC_DYLD_INFO load command. Dyld rebases an image whenever dyld loads it at
/// an address different from its preferred address.  The rebase information is
/// a stream of byte sized opcodes whose symbolic names start with
/// REBASE_OPCODE_. Conceptually the rebase information is a table of tuples:
///   <seg-index, seg-offset, type>
/// The opcodes are a compressed way to encode the table by only
/// encoding when a column changes.  In addition simple patterns
/// like "every n'th offset for m times" can be encoded in a few
/// bytes.
struct RebaseInfo {
  // At the moment we do not parse this info (and it is simply copied over),
  // but the proper support will be added later.
  ArrayRef<uint8_t> Opcodes;
};

/// The location of the bind info inside the binary is described by
/// LC_DYLD_INFO load command. Dyld binds an image during the loading process,
/// if the image requires any pointers to be initialized to symbols in other
/// images. The bind information is a stream of byte sized opcodes whose
/// symbolic names start with BIND_OPCODE_. Conceptually the bind information is
/// a table of tuples: <seg-index, seg-offset, type, symbol-library-ordinal,
/// symbol-name, addend> The opcodes are a compressed way to encode the table by
/// only encoding when a column changes.  In addition simple patterns like for
/// runs of pointers initialized to the same value can be encoded in a few
/// bytes.
struct BindInfo {
  // At the moment we do not parse this info (and it is simply copied over),
  // but the proper support will be added later.
  ArrayRef<uint8_t> Opcodes;
};

/// The location of the weak bind info inside the binary is described by
/// LC_DYLD_INFO load command. Some C++ programs require dyld to unique symbols
/// so that all images in the process use the same copy of some code/data. This
/// step is done after binding. The content of the weak_bind info is an opcode
/// stream like the bind_info.  But it is sorted alphabetically by symbol name.
/// This enable dyld to walk all images with weak binding information in order
/// and look for collisions.  If there are no collisions, dyld does no updating.
/// That means that some fixups are also encoded in the bind_info.  For
/// instance, all calls to "operator new" are first bound to libstdc++.dylib
/// using the information in bind_info.  Then if some image overrides operator
/// new that is detected when the weak_bind information is processed and the
/// call to operator new is then rebound.
struct WeakBindInfo {
  // At the moment we do not parse this info (and it is simply copied over),
  // but the proper support will be added later.
  ArrayRef<uint8_t> Opcodes;
};

/// The location of the lazy bind info inside the binary is described by
/// LC_DYLD_INFO load command. Some uses of external symbols do not need to be
/// bound immediately. Instead they can be lazily bound on first use.  The
/// lazy_bind contains a stream of BIND opcodes to bind all lazy symbols. Normal
/// use is that dyld ignores the lazy_bind section when loading an image.
/// Instead the static linker arranged for the lazy pointer to initially point
/// to a helper function which pushes the offset into the lazy_bind area for the
/// symbol needing to be bound, then jumps to dyld which simply adds the offset
/// to lazy_bind_off to get the information on what to bind.
struct LazyBindInfo {
  ArrayRef<uint8_t> Opcodes;
};

/// The location of the export info inside the binary is described by
/// LC_DYLD_INFO load command. The symbols exported by a dylib are encoded in a
/// trie.  This is a compact representation that factors out common prefixes. It
/// also reduces LINKEDIT pages in RAM because it encodes all information (name,
/// address, flags) in one small, contiguous range. The export area is a stream
/// of nodes.  The first node sequentially is the start node for the trie. Nodes
/// for a symbol start with a uleb128 that is the length of the exported symbol
/// information for the string so far. If there is no exported symbol, the node
/// starts with a zero byte. If there is exported info, it follows the length.
/// First is a uleb128 containing flags. Normally, it is followed by
/// a uleb128 encoded offset which is location of the content named
/// by the symbol from the mach_header for the image.  If the flags
/// is EXPORT_SYMBOL_FLAGS_REEXPORT, then following the flags is
/// a uleb128 encoded library ordinal, then a zero terminated
/// UTF8 string.  If the string is zero length, then the symbol
/// is re-export from the specified dylib with the same name.
/// If the flags is EXPORT_SYMBOL_FLAGS_STUB_AND_RESOLVER, then following
/// the flags is two uleb128s: the stub offset and the resolver offset.
/// The stub is used by non-lazy pointers.  The resolver is used
/// by lazy pointers and must be called to get the actual address to use.
/// After the optional exported symbol information is a byte of
/// how many edges (0-255) that this node has leaving it,
/// followed by each edge.
/// Each edge is a zero terminated UTF8 of the addition chars
/// in the symbol, followed by a uleb128 offset for the node that
/// edge points to.
struct ExportInfo {
  ArrayRef<uint8_t> Trie;
};

struct Object {
  MachHeader Header;
  std::vector<LoadCommand> LoadCommands;

  SymbolTable SymTable;
  StringTable StrTable;

  RebaseInfo Rebases;
  BindInfo Binds;
  WeakBindInfo WeakBinds;
  LazyBindInfo LazyBinds;
  ExportInfo Exports;

  /// The index of LC_SYMTAB load command if present.
  Optional<size_t> SymTabCommandIndex;
  /// The index of LC_DYLD_INFO or LC_DYLD_INFO_ONLY load command if present.
  Optional<size_t> DyLdInfoCommandIndex;
};

} // end namespace macho
} // end namespace objcopy
} // end namespace llvm

#endif // LLVM_OBJCOPY_MACHO_OBJECT_H