RuntimeDyldELF.cpp   [plain text]

//===-- RuntimeDyldELF.cpp - Run-time dynamic linker for MC-JIT -*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Implementation of ELF support for the MC-JIT runtime dynamic linker.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "dyld"
#include "RuntimeDyldELF.h"
#include "JITRegistrar.h"
#include "ObjectImageCommon.h"
#include "llvm/ADT/IntervalMap.h"
#include "llvm/ADT/OwningPtr.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Triple.h"
#include "llvm/ExecutionEngine/ObjectBuffer.h"
#include "llvm/ExecutionEngine/ObjectImage.h"
#include "llvm/Object/ELF.h"
#include "llvm/Object/ObjectFile.h"
#include "llvm/Support/ELF.h"
using namespace llvm;
using namespace llvm::object;

namespace {

static inline
error_code check(error_code Err) {
  if (Err) {
    report_fatal_error(Err.message());
  }
  return Err;
}

template<class ELFT>
class DyldELFObject
  : public ELFObjectFile<ELFT> {
  LLVM_ELF_IMPORT_TYPES(ELFT)

  typedef Elf_Shdr_Impl<ELFT> Elf_Shdr;
  typedef Elf_Sym_Impl<ELFT> Elf_Sym;
  typedef
    Elf_Rel_Impl<ELFT, false> Elf_Rel;
  typedef
    Elf_Rel_Impl<ELFT, true> Elf_Rela;

  typedef Elf_Ehdr_Impl<ELFT> Elf_Ehdr;

  typedef typename ELFDataTypeTypedefHelper<
          ELFT>::value_type addr_type;

public:
  DyldELFObject(MemoryBuffer *Wrapper, error_code &ec);

  void updateSectionAddress(const SectionRef &Sec, uint64_t Addr);
  void updateSymbolAddress(const SymbolRef &Sym, uint64_t Addr);

  // Methods for type inquiry through isa, cast and dyn_cast
  static inline bool classof(const Binary *v) {
    return (isa<ELFObjectFile<ELFT> >(v)
            && classof(cast<ELFObjectFile
                <ELFT> >(v)));
  }
  static inline bool classof(
      const ELFObjectFile<ELFT> *v) {
    return v->isDyldType();
  }
};

template<class ELFT>
class ELFObjectImage : public ObjectImageCommon {
  protected:
    DyldELFObject<ELFT> *DyldObj;
    bool Registered;

  public:
    ELFObjectImage(ObjectBuffer *Input,
                 DyldELFObject<ELFT> *Obj)
    : ObjectImageCommon(Input, Obj),
      DyldObj(Obj),
      Registered(false) {}

    virtual ~ELFObjectImage() {
      if (Registered)
        deregisterWithDebugger();
    }

    // Subclasses can override these methods to update the image with loaded
    // addresses for sections and common symbols
    virtual void updateSectionAddress(const SectionRef &Sec, uint64_t Addr)
    {
      DyldObj->updateSectionAddress(Sec, Addr);
    }

    virtual void updateSymbolAddress(const SymbolRef &Sym, uint64_t Addr)
    {
      DyldObj->updateSymbolAddress(Sym, Addr);
    }

    virtual void registerWithDebugger()
    {
      JITRegistrar::getGDBRegistrar().registerObject(*Buffer);
      Registered = true;
    }
    virtual void deregisterWithDebugger()
    {
      JITRegistrar::getGDBRegistrar().deregisterObject(*Buffer);
    }
};

// The MemoryBuffer passed into this constructor is just a wrapper around the
// actual memory.  Ultimately, the Binary parent class will take ownership of
// this MemoryBuffer object but not the underlying memory.
template<class ELFT>
DyldELFObject<ELFT>::DyldELFObject(MemoryBuffer *Wrapper, error_code &ec)
  : ELFObjectFile<ELFT>(Wrapper, ec) {
  this->isDyldELFObject = true;
}

template<class ELFT>
void DyldELFObject<ELFT>::updateSectionAddress(const SectionRef &Sec,
                                               uint64_t Addr) {
  DataRefImpl ShdrRef = Sec.getRawDataRefImpl();
  Elf_Shdr *shdr = const_cast<Elf_Shdr*>(
                          reinterpret_cast<const Elf_Shdr *>(ShdrRef.p));

  // This assumes the address passed in matches the target address bitness
  // The template-based type cast handles everything else.
  shdr->sh_addr = static_cast<addr_type>(Addr);
}

template<class ELFT>
void DyldELFObject<ELFT>::updateSymbolAddress(const SymbolRef &SymRef,
                                              uint64_t Addr) {

  Elf_Sym *sym = const_cast<Elf_Sym*>(
    ELFObjectFile<ELFT>::getSymbol(SymRef.getRawDataRefImpl()));

  // This assumes the address passed in matches the target address bitness
  // The template-based type cast handles everything else.
  sym->st_value = static_cast<addr_type>(Addr);
}

} // namespace

namespace llvm {

ObjectImage *RuntimeDyldELF::createObjectImage(ObjectBuffer *Buffer) {
  if (Buffer->getBufferSize() < ELF::EI_NIDENT)
    llvm_unreachable("Unexpected ELF object size");
  std::pair<unsigned char, unsigned char> Ident = std::make_pair(
                         (uint8_t)Buffer->getBufferStart()[ELF::EI_CLASS],
                         (uint8_t)Buffer->getBufferStart()[ELF::EI_DATA]);
  error_code ec;

  if (Ident.first == ELF::ELFCLASS32 && Ident.second == ELF::ELFDATA2LSB) {
    DyldELFObject<ELFType<support::little, 4, false> > *Obj =
      new DyldELFObject<ELFType<support::little, 4, false> >(
        Buffer->getMemBuffer(), ec);
    return new ELFObjectImage<ELFType<support::little, 4, false> >(Buffer, Obj);
  }
  else if (Ident.first == ELF::ELFCLASS32 && Ident.second == ELF::ELFDATA2MSB) {
    DyldELFObject<ELFType<support::big, 4, false> > *Obj =
      new DyldELFObject<ELFType<support::big, 4, false> >(
        Buffer->getMemBuffer(), ec);
    return new ELFObjectImage<ELFType<support::big, 4, false> >(Buffer, Obj);
  }
  else if (Ident.first == ELF::ELFCLASS64 && Ident.second == ELF::ELFDATA2MSB) {
    DyldELFObject<ELFType<support::big, 8, true> > *Obj =
      new DyldELFObject<ELFType<support::big, 8, true> >(
        Buffer->getMemBuffer(), ec);
    return new ELFObjectImage<ELFType<support::big, 8, true> >(Buffer, Obj);
  }
  else if (Ident.first == ELF::ELFCLASS64 && Ident.second == ELF::ELFDATA2LSB) {
    DyldELFObject<ELFType<support::little, 8, true> > *Obj =
      new DyldELFObject<ELFType<support::little, 8, true> >(
        Buffer->getMemBuffer(), ec);
    return new ELFObjectImage<ELFType<support::little, 8, true> >(Buffer, Obj);
  }
  else
    llvm_unreachable("Unexpected ELF format");
}

RuntimeDyldELF::~RuntimeDyldELF() {
}

void RuntimeDyldELF::resolveX86_64Relocation(const SectionEntry &Section,
                                             uint64_t Offset,
                                             uint64_t Value,
                                             uint32_t Type,
                                             int64_t Addend) {
  switch (Type) {
  default:
    llvm_unreachable("Relocation type not implemented yet!");
  break;
  case ELF::R_X86_64_64: {
    uint64_t *Target = reinterpret_cast<uint64_t*>(Section.Address + Offset);
    *Target = Value + Addend;
    DEBUG(dbgs() << "Writing " << format("%p", (Value + Addend))
                 << " at " << format("%p\n",Target));
    break;
  }
  case ELF::R_X86_64_32:
  case ELF::R_X86_64_32S: {
    Value += Addend;
    assert((Type == ELF::R_X86_64_32 && (Value <= UINT32_MAX)) ||
           (Type == ELF::R_X86_64_32S &&
             ((int64_t)Value <= INT32_MAX && (int64_t)Value >= INT32_MIN)));
    uint32_t TruncatedAddr = (Value & 0xFFFFFFFF);
    uint32_t *Target = reinterpret_cast<uint32_t*>(Section.Address + Offset);
    *Target = TruncatedAddr;
    DEBUG(dbgs() << "Writing " << format("%p", TruncatedAddr)
                 << " at " << format("%p\n",Target));
    break;
  }
  case ELF::R_X86_64_PC32: {
    // Get the placeholder value from the generated object since
    // a previous relocation attempt may have overwritten the loaded version
    uint32_t *Placeholder = reinterpret_cast<uint32_t*>(Section.ObjAddress
                                                                   + Offset);
    uint32_t *Target = reinterpret_cast<uint32_t*>(Section.Address + Offset);
    uint64_t  FinalAddress = Section.LoadAddress + Offset;
    int64_t RealOffset = *Placeholder + Value + Addend - FinalAddress;
    assert(RealOffset <= INT32_MAX && RealOffset >= INT32_MIN);
    int32_t TruncOffset = (RealOffset & 0xFFFFFFFF);
    *Target = TruncOffset;
    break;
  }
  }
}

void RuntimeDyldELF::resolveX86Relocation(const SectionEntry &Section,
                                          uint64_t Offset,
                                          uint32_t Value,
                                          uint32_t Type,
                                          int32_t Addend) {
  switch (Type) {
  case ELF::R_386_32: {
    // Get the placeholder value from the generated object since
    // a previous relocation attempt may have overwritten the loaded version
    uint32_t *Placeholder = reinterpret_cast<uint32_t*>(Section.ObjAddress
                                                                   + Offset);
    uint32_t *Target = reinterpret_cast<uint32_t*>(Section.Address + Offset);
    *Target = *Placeholder + Value + Addend;
    break;
  }
  case ELF::R_386_PC32: {
    // Get the placeholder value from the generated object since
    // a previous relocation attempt may have overwritten the loaded version
    uint32_t *Placeholder = reinterpret_cast<uint32_t*>(Section.ObjAddress
                                                                   + Offset);
    uint32_t *Target = reinterpret_cast<uint32_t*>(Section.Address + Offset);
    uint32_t  FinalAddress = ((Section.LoadAddress + Offset) & 0xFFFFFFFF);
    uint32_t RealOffset = *Placeholder + Value + Addend - FinalAddress;
    *Target = RealOffset;
    break;
    }
    default:
      // There are other relocation types, but it appears these are the
      // only ones currently used by the LLVM ELF object writer
      llvm_unreachable("Relocation type not implemented yet!");
      break;
  }
}

void RuntimeDyldELF::resolveARMRelocation(const SectionEntry &Section,
                                          uint64_t Offset,
                                          uint32_t Value,
                                          uint32_t Type,
                                          int32_t Addend) {
  // TODO: Add Thumb relocations.
  uint32_t* TargetPtr = (uint32_t*)(Section.Address + Offset);
  uint32_t FinalAddress = ((Section.LoadAddress + Offset) & 0xFFFFFFFF);
  Value += Addend;

  DEBUG(dbgs() << "resolveARMRelocation, LocalAddress: "
               << Section.Address + Offset
               << " FinalAddress: " << format("%p",FinalAddress)
               << " Value: " << format("%x",Value)
               << " Type: " << format("%x",Type)
               << " Addend: " << format("%x",Addend)
               << "\n");

  switch(Type) {
  default:
    llvm_unreachable("Not implemented relocation type!");

  // Write a 32bit value to relocation address, taking into account the
  // implicit addend encoded in the target.
  case ELF::R_ARM_TARGET1 :
  case ELF::R_ARM_ABS32 :
    *TargetPtr += Value;
    break;

  // Write first 16 bit of 32 bit value to the mov instruction.
  // Last 4 bit should be shifted.
  case ELF::R_ARM_MOVW_ABS_NC :
    // We are not expecting any other addend in the relocation address.
    // Using 0x000F0FFF because MOVW has its 16 bit immediate split into 2
    // non-contiguous fields.
    assert((*TargetPtr & 0x000F0FFF) == 0);
    Value = Value & 0xFFFF;
    *TargetPtr |= Value & 0xFFF;
    *TargetPtr |= ((Value >> 12) & 0xF) << 16;
    break;

  // Write last 16 bit of 32 bit value to the mov instruction.
  // Last 4 bit should be shifted.
  case ELF::R_ARM_MOVT_ABS :
    // We are not expecting any other addend in the relocation address.
    // Use 0x000F0FFF for the same reason as R_ARM_MOVW_ABS_NC.
    assert((*TargetPtr & 0x000F0FFF) == 0);
    Value = (Value >> 16) & 0xFFFF;
    *TargetPtr |= Value & 0xFFF;
    *TargetPtr |= ((Value >> 12) & 0xF) << 16;
    break;

  // Write 24 bit relative value to the branch instruction.
  case ELF::R_ARM_PC24 :    // Fall through.
  case ELF::R_ARM_CALL :    // Fall through.
  case ELF::R_ARM_JUMP24 :
    int32_t RelValue = static_cast<int32_t>(Value - FinalAddress - 8);
    RelValue = (RelValue & 0x03FFFFFC) >> 2;
    *TargetPtr &= 0xFF000000;
    *TargetPtr |= RelValue;
    break;
  }
}

void RuntimeDyldELF::resolveMIPSRelocation(const SectionEntry &Section,
                                           uint64_t Offset,
                                           uint32_t Value,
                                           uint32_t Type,
                                           int32_t Addend) {
  uint32_t* TargetPtr = (uint32_t*)(Section.Address + Offset);
  Value += Addend;

  DEBUG(dbgs() << "resolveMipselocation, LocalAddress: "
               << Section.Address + Offset
               << " FinalAddress: "
               << format("%p",Section.LoadAddress + Offset)
               << " Value: " << format("%x",Value)
               << " Type: " << format("%x",Type)
               << " Addend: " << format("%x",Addend)
               << "\n");

  switch(Type) {
  default:
    llvm_unreachable("Not implemented relocation type!");
    break;
  case ELF::R_MIPS_32:
    *TargetPtr = Value + (*TargetPtr);
    break;
  case ELF::R_MIPS_26:
    *TargetPtr = ((*TargetPtr) & 0xfc000000) | (( Value & 0x0fffffff) >> 2);
    break;
  case ELF::R_MIPS_HI16:
    // Get the higher 16-bits. Also add 1 if bit 15 is 1.
    Value += ((*TargetPtr) & 0x0000ffff) << 16;
    *TargetPtr = ((*TargetPtr) & 0xffff0000) |
                 (((Value + 0x8000) >> 16) & 0xffff);
    break;
   case ELF::R_MIPS_LO16:
    Value += ((*TargetPtr) & 0x0000ffff);
    *TargetPtr = ((*TargetPtr) & 0xffff0000) | (Value & 0xffff);
    break;
   }
}

// Return the .TOC. section address to R_PPC64_TOC relocations.
uint64_t RuntimeDyldELF::findPPC64TOC() const {
  // The TOC consists of sections .got, .toc, .tocbss, .plt in that
  // order. The TOC starts where the first of these sections starts.
  SectionList::const_iterator it = Sections.begin();
  SectionList::const_iterator ite = Sections.end();
  for (; it != ite; ++it) {
    if (it->Name == ".got" ||
        it->Name == ".toc" ||
        it->Name == ".tocbss" ||
        it->Name == ".plt")
      break;
  }
  if (it == ite) {
    // This may happen for
    // * references to TOC base base (sym@toc, .odp relocation) without
    // a .toc directive.
    // In this case just use the first section (which is usually
    // the .odp) since the code won't reference the .toc base
    // directly.
    it = Sections.begin();
  }
  assert (it != ite);
  // Per the ppc64-elf-linux ABI, The TOC base is TOC value plus 0x8000
  // thus permitting a full 64 Kbytes segment.
  return it->LoadAddress + 0x8000;
}

// Returns the sections and offset associated with the ODP entry referenced
// by Symbol.
void RuntimeDyldELF::findOPDEntrySection(ObjectImage &Obj,
                                         ObjSectionToIDMap &LocalSections,
                                         RelocationValueRef &Rel) {
  // Get the ELF symbol value (st_value) to compare with Relocation offset in
  // .opd entries

  error_code err;
  for (section_iterator si = Obj.begin_sections(),
     se = Obj.end_sections(); si != se; si.increment(err)) {
    StringRef SectionName;
    check(si->getName(SectionName));
    if (SectionName != ".opd")
      continue;

    for (relocation_iterator i = si->begin_relocations(),
         e = si->end_relocations(); i != e;) {
      check(err);

      // The R_PPC64_ADDR64 relocation indicates the first field
      // of a .opd entry
      uint64_t TypeFunc;
      check(i->getType(TypeFunc));
      if (TypeFunc != ELF::R_PPC64_ADDR64) {
        i.increment(err);
        continue;
      }

      SymbolRef TargetSymbol;
      uint64_t TargetSymbolOffset;
      int64_t TargetAdditionalInfo;
      check(i->getSymbol(TargetSymbol));
      check(i->getOffset(TargetSymbolOffset));
      check(i->getAdditionalInfo(TargetAdditionalInfo));

      i = i.increment(err);
      if (i == e)
        break;
      check(err);

      // Just check if following relocation is a R_PPC64_TOC
      uint64_t TypeTOC;
      check(i->getType(TypeTOC));
      if (TypeTOC != ELF::R_PPC64_TOC)
        continue;

      // Finally compares the Symbol value and the target symbol offset
      // to check if this .opd entry refers to the symbol the relocation
      // points to.
      if (Rel.Addend != (intptr_t)TargetSymbolOffset)
        continue;

      section_iterator tsi(Obj.end_sections());
      check(TargetSymbol.getSection(tsi));
      Rel.SectionID = findOrEmitSection(Obj, (*tsi), true, LocalSections);
      Rel.Addend = (intptr_t)TargetAdditionalInfo;
      return;
    }
  }
  llvm_unreachable("Attempting to get address of ODP entry!");
}

// Relocation masks following the #lo(value), #hi(value), #higher(value),
// and #highest(value) macros defined in section 4.5.1. Relocation Types
// in PPC-elf64abi document.
//
static inline
uint16_t applyPPClo (uint64_t value)
{
  return value & 0xffff;
}

static inline
uint16_t applyPPChi (uint64_t value)
{
  return (value >> 16) & 0xffff;
}

static inline
uint16_t applyPPChigher (uint64_t value)
{
  return (value >> 32) & 0xffff;
}

static inline
uint16_t applyPPChighest (uint64_t value)
{
  return (value >> 48) & 0xffff;
}

void RuntimeDyldELF::resolvePPC64Relocation(const SectionEntry &Section,
                                            uint64_t Offset,
                                            uint64_t Value,
                                            uint32_t Type,
                                            int64_t Addend) {
  uint8_t* LocalAddress = Section.Address + Offset;
  switch (Type) {
  default:
    llvm_unreachable("Relocation type not implemented yet!");
  break;
  case ELF::R_PPC64_ADDR16_LO :
    writeInt16BE(LocalAddress, applyPPClo (Value + Addend));
    break;
  case ELF::R_PPC64_ADDR16_HI :
    writeInt16BE(LocalAddress, applyPPChi (Value + Addend));
    break;
  case ELF::R_PPC64_ADDR16_HIGHER :
    writeInt16BE(LocalAddress, applyPPChigher (Value + Addend));
    break;
  case ELF::R_PPC64_ADDR16_HIGHEST :
    writeInt16BE(LocalAddress, applyPPChighest (Value + Addend));
    break;
  case ELF::R_PPC64_ADDR14 : {
    assert(((Value + Addend) & 3) == 0);
    // Preserve the AA/LK bits in the branch instruction
    uint8_t aalk = *(LocalAddress+3);
    writeInt16BE(LocalAddress + 2, (aalk & 3) | ((Value + Addend) & 0xfffc));
  } break;
  case ELF::R_PPC64_ADDR32 : {
    int32_t Result = static_cast<int32_t>(Value + Addend);
    if (SignExtend32<32>(Result) != Result)
      llvm_unreachable("Relocation R_PPC64_ADDR32 overflow");
    writeInt32BE(LocalAddress, Result);
  } break;
  case ELF::R_PPC64_REL24 : {
    uint64_t FinalAddress = (Section.LoadAddress + Offset);
    int32_t delta = static_cast<int32_t>(Value - FinalAddress + Addend);
    if (SignExtend32<24>(delta) != delta)
      llvm_unreachable("Relocation R_PPC64_REL24 overflow");
    // Generates a 'bl <address>' instruction
    writeInt32BE(LocalAddress, 0x48000001 | (delta & 0x03FFFFFC));
  } break;
  case ELF::R_PPC64_REL32 : {
    uint64_t FinalAddress = (Section.LoadAddress + Offset);
    int32_t delta = static_cast<int32_t>(Value - FinalAddress + Addend);
    if (SignExtend32<32>(delta) != delta)
      llvm_unreachable("Relocation R_PPC64_REL32 overflow");
    writeInt32BE(LocalAddress, delta);
  } break;
  case ELF::R_PPC64_ADDR64 :
    writeInt64BE(LocalAddress, Value + Addend);
    break;
  case ELF::R_PPC64_TOC :
    writeInt64BE(LocalAddress, findPPC64TOC());
    break;
  case ELF::R_PPC64_TOC16 : {
    uint64_t TOCStart = findPPC64TOC();
    Value = applyPPClo((Value + Addend) - TOCStart);
    writeInt16BE(LocalAddress, applyPPClo(Value));
  } break;
  case ELF::R_PPC64_TOC16_DS : {
    uint64_t TOCStart = findPPC64TOC();
    Value = ((Value + Addend) - TOCStart);
    writeInt16BE(LocalAddress, applyPPClo(Value));
  } break;
  }
}

void RuntimeDyldELF::resolveRelocation(const SectionEntry &Section,
                                       uint64_t Offset,
                                       uint64_t Value,
                                       uint32_t Type,
                                       int64_t Addend) {
  switch (Arch) {
  case Triple::x86_64:
    resolveX86_64Relocation(Section, Offset, Value, Type, Addend);
    break;
  case Triple::x86:
    resolveX86Relocation(Section, Offset,
                         (uint32_t)(Value & 0xffffffffL), Type,
                         (uint32_t)(Addend & 0xffffffffL));
    break;
  case Triple::arm:    // Fall through.
  case Triple::thumb:
    resolveARMRelocation(Section, Offset,
                         (uint32_t)(Value & 0xffffffffL), Type,
                         (uint32_t)(Addend & 0xffffffffL));
    break;
  case Triple::mips:    // Fall through.
  case Triple::mipsel:
    resolveMIPSRelocation(Section, Offset,
                          (uint32_t)(Value & 0xffffffffL), Type,
                          (uint32_t)(Addend & 0xffffffffL));
    break;
  case Triple::ppc64:
    resolvePPC64Relocation(Section, Offset, Value, Type, Addend);
    break;
  default: llvm_unreachable("Unsupported CPU type!");
  }
}

void RuntimeDyldELF::processRelocationRef(const ObjRelocationInfo &Rel,
                                          ObjectImage &Obj,
                                          ObjSectionToIDMap &ObjSectionToID,
                                          const SymbolTableMap &Symbols,
                                          StubMap &Stubs) {

  uint32_t RelType = (uint32_t)(Rel.Type & 0xffffffffL);
  intptr_t Addend = (intptr_t)Rel.AdditionalInfo;
  const SymbolRef &Symbol = Rel.Symbol;

  // Obtain the symbol name which is referenced in the relocation
  StringRef TargetName;
  Symbol.getName(TargetName);
  DEBUG(dbgs() << "\t\tRelType: " << RelType
               << " Addend: " << Addend
               << " TargetName: " << TargetName
               << "\n");
  RelocationValueRef Value;
  // First search for the symbol in the local symbol table
  SymbolTableMap::const_iterator lsi = Symbols.find(TargetName.data());
  SymbolRef::Type SymType;
  Symbol.getType(SymType);
  if (lsi != Symbols.end()) {
    Value.SectionID = lsi->second.first;
    Value.Addend = lsi->second.second;
  } else {
    // Search for the symbol in the global symbol table
    SymbolTableMap::const_iterator gsi =
        GlobalSymbolTable.find(TargetName.data());
    if (gsi != GlobalSymbolTable.end()) {
      Value.SectionID = gsi->second.first;
      Value.Addend = gsi->second.second;
    } else {
      switch (SymType) {
        case SymbolRef::ST_Debug: {
          // TODO: Now ELF SymbolRef::ST_Debug = STT_SECTION, it's not obviously
          // and can be changed by another developers. Maybe best way is add
          // a new symbol type ST_Section to SymbolRef and use it.
          section_iterator si(Obj.end_sections());
          Symbol.getSection(si);
          if (si == Obj.end_sections())
            llvm_unreachable("Symbol section not found, bad object file format!");
          DEBUG(dbgs() << "\t\tThis is section symbol\n");
          // Default to 'true' in case isText fails (though it never does).
          bool isCode = true;
          si->isText(isCode);
          Value.SectionID = findOrEmitSection(Obj,
                                              (*si),
                                              isCode,
                                              ObjSectionToID);
          Value.Addend = Addend;
          break;
        }
        case SymbolRef::ST_Unknown: {
          Value.SymbolName = TargetName.data();
          Value.Addend = Addend;
          break;
        }
        default:
          llvm_unreachable("Unresolved symbol type!");
          break;
      }
    }
  }
  DEBUG(dbgs() << "\t\tRel.SectionID: " << Rel.SectionID
               << " Rel.Offset: " << Rel.Offset
               << "\n");
  if (Arch == Triple::arm &&
      (RelType == ELF::R_ARM_PC24 ||
       RelType == ELF::R_ARM_CALL ||
       RelType == ELF::R_ARM_JUMP24)) {
    // This is an ARM branch relocation, need to use a stub function.
    DEBUG(dbgs() << "\t\tThis is an ARM branch relocation.");
    SectionEntry &Section = Sections[Rel.SectionID];

    // Look for an existing stub.
    StubMap::const_iterator i = Stubs.find(Value);
    if (i != Stubs.end()) {
        resolveRelocation(Section, Rel.Offset,
                          (uint64_t)Section.Address + i->second, RelType, 0);
      DEBUG(dbgs() << " Stub function found\n");
    } else {
      // Create a new stub function.
      DEBUG(dbgs() << " Create a new stub function\n");
      Stubs[Value] = Section.StubOffset;
      uint8_t *StubTargetAddr = createStubFunction(Section.Address +
                                                   Section.StubOffset);
      RelocationEntry RE(Rel.SectionID, StubTargetAddr - Section.Address,
                         ELF::R_ARM_ABS32, Value.Addend);
      if (Value.SymbolName)
        addRelocationForSymbol(RE, Value.SymbolName);
      else
        addRelocationForSection(RE, Value.SectionID);

      resolveRelocation(Section, Rel.Offset,
                        (uint64_t)Section.Address + Section.StubOffset,
                        RelType, 0);
      Section.StubOffset += getMaxStubSize();
    }
  } else if ((Arch == Triple::mipsel || Arch == Triple::mips) &&
             RelType == ELF::R_MIPS_26) {
    // This is an Mips branch relocation, need to use a stub function.
    DEBUG(dbgs() << "\t\tThis is a Mips branch relocation.");
    SectionEntry &Section = Sections[Rel.SectionID];
    uint8_t *Target = Section.Address + Rel.Offset;
    uint32_t *TargetAddress = (uint32_t *)Target;

    // Extract the addend from the instruction.
    uint32_t Addend = ((*TargetAddress) & 0x03ffffff) << 2;

    Value.Addend += Addend;

    //  Look up for existing stub.
    StubMap::const_iterator i = Stubs.find(Value);
    if (i != Stubs.end()) {
      resolveRelocation(Section, Rel.Offset,
                        (uint64_t)Section.Address + i->second, RelType, 0);
      DEBUG(dbgs() << " Stub function found\n");
    } else {
      // Create a new stub function.
      DEBUG(dbgs() << " Create a new stub function\n");
      Stubs[Value] = Section.StubOffset;
      uint8_t *StubTargetAddr = createStubFunction(Section.Address +
                                                   Section.StubOffset);

      // Creating Hi and Lo relocations for the filled stub instructions.
      RelocationEntry REHi(Rel.SectionID,
                           StubTargetAddr - Section.Address,
                           ELF::R_MIPS_HI16, Value.Addend);
      RelocationEntry RELo(Rel.SectionID,
                           StubTargetAddr - Section.Address + 4,
                           ELF::R_MIPS_LO16, Value.Addend);

      if (Value.SymbolName) {
        addRelocationForSymbol(REHi, Value.SymbolName);
        addRelocationForSymbol(RELo, Value.SymbolName);
      } else {
        addRelocationForSection(REHi, Value.SectionID);
        addRelocationForSection(RELo, Value.SectionID);
      }

      resolveRelocation(Section, Rel.Offset,
                        (uint64_t)Section.Address + Section.StubOffset,
                        RelType, 0);
      Section.StubOffset += getMaxStubSize();
    }
  } else if (Arch == Triple::ppc64) {
    if (RelType == ELF::R_PPC64_REL24) {
      // A PPC branch relocation will need a stub function if the target is
      // an external symbol (Symbol::ST_Unknown) or if the target address
      // is not within the signed 24-bits branch address.
      SectionEntry &Section = Sections[Rel.SectionID];
      uint8_t *Target = Section.Address + Rel.Offset;
      bool RangeOverflow = false;
      if (SymType != SymbolRef::ST_Unknown) {
        // A function call may points to the .opd entry, so the final symbol value
        // in calculated based in the relocation values in .opd section.
        findOPDEntrySection(Obj, ObjSectionToID, Value);
        uint8_t *RelocTarget = Sections[Value.SectionID].Address + Value.Addend;
        int32_t delta = static_cast<int32_t>(Target - RelocTarget);
        // If it is within 24-bits branch range, just set the branch target
        if (SignExtend32<24>(delta) == delta) {
          RelocationEntry RE(Rel.SectionID, Rel.Offset, RelType, Value.Addend);
          if (Value.SymbolName)
            addRelocationForSymbol(RE, Value.SymbolName);
          else
            addRelocationForSection(RE, Value.SectionID);
        } else {
          RangeOverflow = true;
        }
      }
      if (SymType == SymbolRef::ST_Unknown || RangeOverflow == true) {
        // It is an external symbol (SymbolRef::ST_Unknown) or within a range
        // larger than 24-bits.
        StubMap::const_iterator i = Stubs.find(Value);
        if (i != Stubs.end()) {
          // Symbol function stub already created, just relocate to it
          resolveRelocation(Section, Rel.Offset,
                            (uint64_t)Section.Address + i->second, RelType, 0);
          DEBUG(dbgs() << " Stub function found\n");
        } else {
          // Create a new stub function.
          DEBUG(dbgs() << " Create a new stub function\n");
          Stubs[Value] = Section.StubOffset;
          uint8_t *StubTargetAddr = createStubFunction(Section.Address +
                                                       Section.StubOffset);
          RelocationEntry RE(Rel.SectionID, StubTargetAddr - Section.Address,
                             ELF::R_PPC64_ADDR64, Value.Addend);

          // Generates the 64-bits address loads as exemplified in section
          // 4.5.1 in PPC64 ELF ABI.
          RelocationEntry REhst(Rel.SectionID,
                                StubTargetAddr - Section.Address + 2,
                                ELF::R_PPC64_ADDR16_HIGHEST, Value.Addend);
          RelocationEntry REhr(Rel.SectionID,
                               StubTargetAddr - Section.Address + 6,
                               ELF::R_PPC64_ADDR16_HIGHER, Value.Addend);
          RelocationEntry REh(Rel.SectionID,
                              StubTargetAddr - Section.Address + 14,
                              ELF::R_PPC64_ADDR16_HI, Value.Addend);
          RelocationEntry REl(Rel.SectionID,
                              StubTargetAddr - Section.Address + 18,
                              ELF::R_PPC64_ADDR16_LO, Value.Addend);

          if (Value.SymbolName) {
            addRelocationForSymbol(REhst, Value.SymbolName);
            addRelocationForSymbol(REhr,  Value.SymbolName);
            addRelocationForSymbol(REh,   Value.SymbolName);
            addRelocationForSymbol(REl,   Value.SymbolName);
          } else {
            addRelocationForSection(REhst, Value.SectionID);
            addRelocationForSection(REhr,  Value.SectionID);
            addRelocationForSection(REh,   Value.SectionID);
            addRelocationForSection(REl,   Value.SectionID);
          }

          resolveRelocation(Section, Rel.Offset,
                            (uint64_t)Section.Address + Section.StubOffset,
                            RelType, 0);
          if (SymType == SymbolRef::ST_Unknown)
            // Restore the TOC for external calls
            writeInt32BE(Target+4, 0xE8410028); // ld r2,40(r1)
          Section.StubOffset += getMaxStubSize();
        }
      }
    } else {
      RelocationEntry RE(Rel.SectionID, Rel.Offset, RelType, Value.Addend);
      // Extra check to avoid relocation againt empty symbols (usually
      // the R_PPC64_TOC).
      if (Value.SymbolName && !TargetName.empty())
        addRelocationForSymbol(RE, Value.SymbolName);
      else
        addRelocationForSection(RE, Value.SectionID);
    }
  } else {
    RelocationEntry RE(Rel.SectionID, Rel.Offset, RelType, Value.Addend);
    if (Value.SymbolName)
      addRelocationForSymbol(RE, Value.SymbolName);
    else
      addRelocationForSection(RE, Value.SectionID);
  }
}

unsigned RuntimeDyldELF::getCommonSymbolAlignment(const SymbolRef &Sym) {
  // In ELF, the value of an SHN_COMMON symbol is its alignment requirement.
  uint64_t Align;
  Check(Sym.getValue(Align));
  return Align;
}

bool RuntimeDyldELF::isCompatibleFormat(const ObjectBuffer *Buffer) const {
  if (Buffer->getBufferSize() < strlen(ELF::ElfMagic))
    return false;
  return (memcmp(Buffer->getBufferStart(), ELF::ElfMagic, strlen(ELF::ElfMagic))) == 0;
}
} // namespace llvm