llvm-project/lld/COFF/Chunks.h

//===- Chunks.h -------------------------------------------------*- C++ -*-===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#ifndef LLD_COFF_CHUNKS_H
#define LLD_COFF_CHUNKS_H

#include "Config.h"
#include "InputFiles.h"
#include "lld/Core/LLVM.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/iterator.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Object/COFF.h"
#include <utility>
#include <vector>

namespace lld {
namespace coff {

using llvm::COFF::ImportDirectoryTableEntry;
using llvm::object::COFFSymbolRef;
using llvm::object::SectionRef;
using llvm::object::coff_relocation;
using llvm::object::coff_section;

class Baserel;
class Defined;
class DefinedImportData;
class DefinedRegular;
class ObjectFile;
class OutputSection;
class SymbolBody;

// Mask for section types (code, data, bss, disacardable, etc.)
// and permissions (writable, readable or executable).
const uint32_t PermMask = 0xFF0000F0;

// A Chunk represents a chunk of data that will occupy space in the
// output (if the resolver chose that). It may or may not be backed by
// a section of an input file. It could be linker-created data, or
// doesn't even have actual data (if common or bss).
class Chunk {
public:
  enum Kind { SectionKind, OtherKind };
  Kind kind() const { return ChunkKind; }
  virtual ~Chunk() = default;

  // Returns the size of this chunk (even if this is a common or BSS.)
  virtual size_t getSize() const = 0;

  // Write this chunk to a mmap'ed file, assuming Buf is pointing to
  // beginning of the file. Because this function may use RVA values
  // of other chunks for relocations, you need to set them properly
  // before calling this function.
  virtual void writeTo(uint8_t *Buf) const {}

  // The writer sets and uses the addresses.
  uint64_t getRVA() const { return RVA; }
  uint32_t getAlign() const { return Align; }
  void setRVA(uint64_t V) { RVA = V; }
  void setOutputSectionOff(uint64_t V) { OutputSectionOff = V; }

  // Returns true if this has non-zero data. BSS chunks return
  // false. If false is returned, the space occupied by this chunk
  // will be filled with zeros.
  virtual bool hasData() const { return true; }

  // Returns readable/writable/executable bits.
  virtual uint32_t getPermissions() const { return 0; }

  // Returns the section name if this is a section chunk.
  // It is illegal to call this function on non-section chunks.
  virtual StringRef getSectionName() const {
    llvm_unreachable("unimplemented getSectionName");
  }

  // An output section has pointers to chunks in the section, and each
  // chunk has a back pointer to an output section.
  void setOutputSection(OutputSection *O) { Out = O; }
  OutputSection *getOutputSection() { return Out; }

  // Windows-specific.
  // Collect all locations that contain absolute addresses for base relocations.
  virtual void getBaserels(std::vector<Baserel> *Res) {}

  // Returns a human-readable name of this chunk. Chunks are unnamed chunks of
  // bytes, so this is used only for logging or debugging.
  virtual StringRef getDebugName() { return ""; }

protected:
  Chunk(Kind K = OtherKind) : ChunkKind(K) {}
  const Kind ChunkKind;

  // The RVA of this chunk in the output. The writer sets a value.
  uint64_t RVA = 0;

  // The offset from beginning of the output section. The writer sets a value.
  uint64_t OutputSectionOff = 0;

  // The output section for this chunk.
  OutputSection *Out = nullptr;

  // The alignment of this chunk. The writer uses the value.
  uint32_t Align = 1;
};

// A chunk corresponding a section of an input file.
class SectionChunk : public Chunk {
  // Identical COMDAT Folding feature accesses section internal data.
  friend class ICF;

public:
  class symbol_iterator : public llvm::iterator_adaptor_base<
                              symbol_iterator, const coff_relocation *,
                              std::random_access_iterator_tag, SymbolBody *> {
    friend SectionChunk;

    ObjectFile *File;

    symbol_iterator(ObjectFile *File, const coff_relocation *I)
        : symbol_iterator::iterator_adaptor_base(I), File(File) {}

  public:
    symbol_iterator() = default;

    SymbolBody *operator*() const {
      return File->getSymbolBody(I->SymbolTableIndex);
    }
  };

  SectionChunk(ObjectFile *File, const coff_section *Header);
  static bool classof(const Chunk *C) { return C->kind() == SectionKind; }
  size_t getSize() const override { return Header->SizeOfRawData; }
  ArrayRef<uint8_t> getContents() const;
  void writeTo(uint8_t *Buf) const override;
  bool hasData() const override;
  uint32_t getPermissions() const override;
  StringRef getSectionName() const override { return SectionName; }
  void getBaserels(std::vector<Baserel> *Res) override;
  bool isCOMDAT() const;
  void applyRelX64(uint8_t *Off, uint16_t Type, Defined *Sym, uint64_t P) const;
  void applyRelX86(uint8_t *Off, uint16_t Type, Defined *Sym, uint64_t P) const;
  void applyRelARM(uint8_t *Off, uint16_t Type, Defined *Sym, uint64_t P) const;

  // Called if the garbage collector decides to not include this chunk
  // in a final output. It's supposed to print out a log message to stdout.
  void printDiscardedMessage() const;

  // Adds COMDAT associative sections to this COMDAT section. A chunk
  // and its children are treated as a group by the garbage collector.
  void addAssociative(SectionChunk *Child);

  StringRef getDebugName() override;
  void setSymbol(DefinedRegular *S) { if (!Sym) Sym = S; }

  // Returns true if the chunk was not dropped by GC or COMDAT deduplication.
  bool isLive() { return Live && !Discarded; }

  // Used by the garbage collector.
  void markLive() {
    assert(Config->DoGC && "should only mark things live from GC");
    assert(!isLive() && "Cannot mark an already live section!");
    Live = true;
  }

  // Returns true if this chunk was dropped by COMDAT deduplication.
  bool isDiscarded() const { return Discarded; }

  // Used by the SymbolTable when discarding unused comdat sections. This is
  // redundant when GC is enabled, as all comdat sections will start out dead.
  void markDiscarded() { Discarded = true; }

  // Allow iteration over the bodies of this chunk's relocated symbols.
  llvm::iterator_range<symbol_iterator> symbols() const {
    return llvm::make_range(symbol_iterator(File, Relocs.begin()),
                            symbol_iterator(File, Relocs.end()));
  }

  // Allow iteration over the associated child chunks for this section.
  ArrayRef<SectionChunk *> children() const { return AssocChildren; }

  // A pointer pointing to a replacement for this chunk.
  // Initially it points to "this" object. If this chunk is merged
  // with other chunk by ICF, it points to another chunk,
  // and this chunk is considrered as dead.
  SectionChunk *Repl;

  // The CRC of the contents as described in the COFF spec 4.5.5.
  // Auxiliary Format 5: Section Definitions. Used for ICF.
  uint32_t Checksum = 0;

  const coff_section *Header;

  // The file that this chunk was created from.
  ObjectFile *File;

private:
  StringRef SectionName;
  std::vector<SectionChunk *> AssocChildren;
  llvm::iterator_range<const coff_relocation *> Relocs;
  size_t NumRelocs;

  // True if this chunk was discarded because it was a duplicate comdat section.
  bool Discarded;

  // Used by the garbage collector.
  bool Live;

  // Used for ICF (Identical COMDAT Folding)
  void replace(SectionChunk *Other);
  uint32_t Class[2] = {0, 0};

  // Sym points to a section symbol if this is a COMDAT chunk.
  DefinedRegular *Sym = nullptr;
};

// A chunk for common symbols. Common chunks don't have actual data.
class CommonChunk : public Chunk {
public:
  CommonChunk(const COFFSymbolRef Sym);
  size_t getSize() const override { return Sym.getValue(); }
  bool hasData() const override { return false; }
  uint32_t getPermissions() const override;
  StringRef getSectionName() const override { return ".bss"; }

private:
  const COFFSymbolRef Sym;
};

// A chunk for linker-created strings.
class StringChunk : public Chunk {
public:
  explicit StringChunk(StringRef S) : Str(S) {}
  size_t getSize() const override { return Str.size() + 1; }
  void writeTo(uint8_t *Buf) const override;

private:
  StringRef Str;
};

static const uint8_t ImportThunkX86[] = {
    0xff, 0x25, 0x00, 0x00, 0x00, 0x00, // JMP *0x0
};

static const uint8_t ImportThunkARM[] = {
    0x40, 0xf2, 0x00, 0x0c, // mov.w ip, #0
    0xc0, 0xf2, 0x00, 0x0c, // mov.t ip, #0
    0xdc, 0xf8, 0x00, 0xf0, // ldr.w pc, [ip]
};

// Windows-specific.
// A chunk for DLL import jump table entry. In a final output, it's
// contents will be a JMP instruction to some __imp_ symbol.
class ImportThunkChunkX64 : public Chunk {
public:
  explicit ImportThunkChunkX64(Defined *S);
  size_t getSize() const override { return sizeof(ImportThunkX86); }
  void writeTo(uint8_t *Buf) const override;

private:
  Defined *ImpSymbol;
};

class ImportThunkChunkX86 : public Chunk {
public:
  explicit ImportThunkChunkX86(Defined *S) : ImpSymbol(S) {}
  size_t getSize() const override { return sizeof(ImportThunkX86); }
  void getBaserels(std::vector<Baserel> *Res) override;
  void writeTo(uint8_t *Buf) const override;

private:
  Defined *ImpSymbol;
};

class ImportThunkChunkARM : public Chunk {
public:
  explicit ImportThunkChunkARM(Defined *S) : ImpSymbol(S) {}
  size_t getSize() const override { return sizeof(ImportThunkARM); }
  void getBaserels(std::vector<Baserel> *Res) override;
  void writeTo(uint8_t *Buf) const override;

private:
  Defined *ImpSymbol;
};

// Windows-specific.
// See comments for DefinedLocalImport class.
class LocalImportChunk : public Chunk {
public:
  explicit LocalImportChunk(Defined *S) : Sym(S) {}
  size_t getSize() const override;
  void getBaserels(std::vector<Baserel> *Res) override;
  void writeTo(uint8_t *Buf) const override;

private:
  Defined *Sym;
};

// Windows-specific.
// A chunk for SEH table which contains RVAs of safe exception handler
// functions. x86-only.
class SEHTableChunk : public Chunk {
public:
  explicit SEHTableChunk(std::set<Defined *> S) : Syms(std::move(S)) {}
  size_t getSize() const override { return Syms.size() * 4; }
  void writeTo(uint8_t *Buf) const override;

private:
  std::set<Defined *> Syms;
};

// Windows-specific.
// This class represents a block in .reloc section.
// See the PE/COFF spec 5.6 for details.
class BaserelChunk : public Chunk {
public:
  BaserelChunk(uint32_t Page, Baserel *Begin, Baserel *End);
  size_t getSize() const override { return Data.size(); }
  void writeTo(uint8_t *Buf) const override;

private:
  std::vector<uint8_t> Data;
};

class Baserel {
public:
  Baserel(uint32_t V, uint8_t Ty) : RVA(V), Type(Ty) {}
  explicit Baserel(uint32_t V) : Baserel(V, getDefaultType()) {}
  uint8_t getDefaultType();

  uint32_t RVA;
  uint8_t Type;
};

} // namespace coff
} // namespace lld

#endif