llvm-project/lld/ELF/OutputSections.cpp

//===- OutputSections.cpp -------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "OutputSections.h"
#include "Config.h"
#include "LinkerScript.h"
#include "Memory.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Threads.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/MD5.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/SHA1.h"

using namespace llvm;
using namespace llvm::dwarf;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace llvm::ELF;

using namespace lld;
using namespace lld::elf;

uint8_t Out::First;
OutputSection *Out::Opd;
uint8_t *Out::OpdBuf;
PhdrEntry *Out::TlsPhdr;
OutputSection *Out::DebugInfo;
OutputSection *Out::ElfHeader;
OutputSection *Out::ProgramHeaders;
OutputSection *Out::PreinitArray;
OutputSection *Out::InitArray;
OutputSection *Out::FiniArray;

std::vector<OutputSection *> elf::OutputSections;

uint32_t OutputSection::getPhdrFlags() const {
  uint32_t Ret = PF_R;
  if (Flags & SHF_WRITE)
    Ret |= PF_W;
  if (Flags & SHF_EXECINSTR)
    Ret |= PF_X;
  return Ret;
}

template <class ELFT>
void OutputSection::writeHeaderTo(typename ELFT::Shdr *Shdr) {
  Shdr->sh_entsize = Entsize;
  Shdr->sh_addralign = Alignment;
  Shdr->sh_type = Type;
  Shdr->sh_offset = Offset;
  Shdr->sh_flags = Flags;
  Shdr->sh_info = Info;
  Shdr->sh_link = Link;
  Shdr->sh_addr = Addr;
  Shdr->sh_size = Size;
  Shdr->sh_name = ShName;
}

OutputSection::OutputSection(StringRef Name, uint32_t Type, uint64_t Flags)
    : BaseCommand(OutputSectionKind),
      SectionBase(Output, Name, Flags, /*Entsize*/ 0, /*Alignment*/ 1, Type,
                  /*Info*/ 0,
                  /*Link*/ 0),
      SectionIndex(INT_MAX) {
  Live = false;
}

static uint64_t updateOffset(uint64_t Off, InputSection *S) {
  Off = alignTo(Off, S->Alignment);
  S->OutSecOff = Off;
  return Off + S->getSize();
}

void OutputSection::addSection(InputSection *S) {
  assert(S->Live);
  Live = true;
  S->Parent = this;
  this->updateAlignment(S->Alignment);

  // The actual offsets will be computed by assignAddresses. For now, use
  // crude approximation so that it is at least easy for other code to know the
  // section order. It is also used to calculate the output section size early
  // for compressed debug sections.
  this->Size = updateOffset(Size, S);

  // If this section contains a table of fixed-size entries, sh_entsize
  // holds the element size. Consequently, if this contains two or more
  // input sections, all of them must have the same sh_entsize. However,
  // you can put different types of input sections into one output
  // sectin by using linker scripts. I don't know what to do here.
  // Probably we sholuld handle that as an error. But for now we just
  // pick the largest sh_entsize.
  this->Entsize = std::max(this->Entsize, S->Entsize);

  if (!S->Assigned) {
    S->Assigned = true;
    if (Commands.empty() || !isa<InputSectionDescription>(Commands.back()))
      Commands.push_back(make<InputSectionDescription>(""));
    auto *ISD = cast<InputSectionDescription>(Commands.back());
    ISD->Sections.push_back(S);
  }
}

static SectionKey createKey(InputSectionBase *C, StringRef OutsecName) {
  //  The ELF spec just says
  // ----------------------------------------------------------------
  // In the first phase, input sections that match in name, type and
  // attribute flags should be concatenated into single sections.
  // ----------------------------------------------------------------
  //
  // However, it is clear that at least some flags have to be ignored for
  // section merging. At the very least SHF_GROUP and SHF_COMPRESSED have to be
  // ignored. We should not have two output .text sections just because one was
  // in a group and another was not for example.
  //
  // It also seems that that wording was a late addition and didn't get the
  // necessary scrutiny.
  //
  // Merging sections with different flags is expected by some users. One
  // reason is that if one file has
  //
  // int *const bar __attribute__((section(".foo"))) = (int *)0;
  //
  // gcc with -fPIC will produce a read only .foo section. But if another
  // file has
  //
  // int zed;
  // int *const bar __attribute__((section(".foo"))) = (int *)&zed;
  //
  // gcc with -fPIC will produce a read write section.
  //
  // Last but not least, when using linker script the merge rules are forced by
  // the script. Unfortunately, linker scripts are name based. This means that
  // expressions like *(.foo*) can refer to multiple input sections with
  // different flags. We cannot put them in different output sections or we
  // would produce wrong results for
  //
  // start = .; *(.foo.*) end = .; *(.bar)
  //
  // and a mapping of .foo1 and .bar1 to one section and .foo2 and .bar2 to
  // another. The problem is that there is no way to layout those output
  // sections such that the .foo sections are the only thing between the start
  // and end symbols.
  //
  // Given the above issues, we instead merge sections by name and error on
  // incompatible types and flags.

  uint32_t Alignment = 0;
  uint64_t Flags = 0;
  if (Config->Relocatable && (C->Flags & SHF_MERGE)) {
    Alignment = std::max<uint64_t>(C->Alignment, C->Entsize);
    Flags = C->Flags & (SHF_MERGE | SHF_STRINGS);
  }

  return SectionKey{OutsecName, Flags, Alignment};
}

OutputSectionFactory::OutputSectionFactory() {}

static uint64_t getIncompatibleFlags(uint64_t Flags) {
  return Flags & (SHF_ALLOC | SHF_TLS);
}

// We allow sections of types listed below to merged into a
// single progbits section. This is typically done by linker
// scripts. Merging nobits and progbits will force disk space
// to be allocated for nobits sections. Other ones don't require
// any special treatment on top of progbits, so there doesn't
// seem to be a harm in merging them.
static bool canMergeToProgbits(unsigned Type) {
  return Type == SHT_NOBITS || Type == SHT_PROGBITS || Type == SHT_INIT_ARRAY ||
         Type == SHT_PREINIT_ARRAY || Type == SHT_FINI_ARRAY ||
         Type == SHT_NOTE;
}

void elf::sortByOrder(MutableArrayRef<InputSection *> In,
                      std::function<int(InputSectionBase *S)> Order) {
  typedef std::pair<int, InputSection *> Pair;
  auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };

  std::vector<Pair> V;
  for (InputSection *S : In)
    V.push_back({Order(S), S});
  std::stable_sort(V.begin(), V.end(), Comp);

  for (size_t I = 0; I < V.size(); ++I)
    In[I] = V[I].second;
}

void elf::reportDiscarded(InputSectionBase *IS) {
  if (!Config->PrintGcSections)
    return;
  message("removing unused section from '" + IS->Name + "' in file '" +
          IS->File->getName() + "'");
}

static OutputSection *addSection(InputSectionBase *IS, StringRef OutsecName,
                                 OutputSection *Sec) {
  if (Sec && Sec->Live) {
    if (getIncompatibleFlags(Sec->Flags) != getIncompatibleFlags(IS->Flags))
      error("incompatible section flags for " + Sec->Name + "\n>>> " +
            toString(IS) + ": 0x" + utohexstr(IS->Flags) +
            "\n>>> output section " + Sec->Name + ": 0x" +
            utohexstr(Sec->Flags));
    if (Sec->Type != IS->Type) {
      if (canMergeToProgbits(Sec->Type) && canMergeToProgbits(IS->Type))
        Sec->Type = SHT_PROGBITS;
      else
        error("section type mismatch for " + IS->Name + "\n>>> " +
              toString(IS) + ": " +
              getELFSectionTypeName(Config->EMachine, IS->Type) +
              "\n>>> output section " + Sec->Name + ": " +
              getELFSectionTypeName(Config->EMachine, Sec->Type));
    }
    Sec->Flags |= IS->Flags;
  } else {
    if (!Sec) {
      Sec = Script->createOutputSection(OutsecName, "<internal>");
      Script->Opt.Commands.push_back(Sec);
    }
    Sec->Type = IS->Type;
    Sec->Flags = IS->Flags;
  }

  Sec->addSection(cast<InputSection>(IS));
  return Sec;
}

void OutputSectionFactory::addInputSec(InputSectionBase *IS,
                                       StringRef OutsecName,
                                       OutputSection *OS) {
  if (!IS->Live) {
    reportDiscarded(IS);
    return;
  }

  // If we have destination output section - use it directly.
  if (OS) {
    addSection(IS, OutsecName, OS);
    return;
  }

  // Sections with SHT_GROUP or SHF_GROUP attributes reach here only when the -r
  // option is given. A section with SHT_GROUP defines a "section group", and
  // its members have SHF_GROUP attribute. Usually these flags have already been
  // stripped by InputFiles.cpp as section groups are processed and uniquified.
  // However, for the -r option, we want to pass through all section groups
  // as-is because adding/removing members or merging them with other groups
  // change their semantics.
  if (IS->Type == SHT_GROUP || (IS->Flags & SHF_GROUP)) {
    addSection(IS, OutsecName, nullptr);
    return;
  }

  // Imagine .zed : { *(.foo) *(.bar) } script. Both foo and bar may have
  // relocation sections .rela.foo and .rela.bar for example. Most tools do
  // not allow multiple REL[A] sections for output section. Hence we
  // should combine these relocation sections into single output.
  // We skip synthetic sections because it can be .rela.dyn/.rela.plt or any
  // other REL[A] sections created by linker itself.
  if (!isa<SyntheticSection>(IS) &&
      (IS->Type == SHT_REL || IS->Type == SHT_RELA)) {
    auto *Sec = cast<InputSection>(IS);
    OutputSection *Out = Sec->getRelocatedSection()->getOutputSection();
    Out->RelocationSection = addSection(IS, OutsecName, Out->RelocationSection);
    return;
  }

  SectionKey Key = createKey(IS, OutsecName);
  OutputSection *&Sec = Map[Key];
  Sec = addSection(IS, OutsecName, Sec);
}

OutputSectionFactory::~OutputSectionFactory() {}

SectionKey DenseMapInfo<SectionKey>::getEmptyKey() {
  return SectionKey{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
}

SectionKey DenseMapInfo<SectionKey>::getTombstoneKey() {
  return SectionKey{DenseMapInfo<StringRef>::getTombstoneKey(), 0, 0};
}

unsigned DenseMapInfo<SectionKey>::getHashValue(const SectionKey &Val) {
  return hash_combine(Val.Name, Val.Flags, Val.Alignment);
}

bool DenseMapInfo<SectionKey>::isEqual(const SectionKey &LHS,
                                       const SectionKey &RHS) {
  return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
         LHS.Flags == RHS.Flags && LHS.Alignment == RHS.Alignment;
}

uint64_t elf::getHeaderSize() {
  if (Config->OFormatBinary)
    return 0;
  return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
}

bool OutputSection::classof(const BaseCommand *C) {
  return C->Kind == OutputSectionKind;
}

void OutputSection::sort(std::function<int(InputSectionBase *S)> Order) {
  assert(Commands.size() == 1);
  sortByOrder(cast<InputSectionDescription>(Commands[0])->Sections, Order);
}

// Fill [Buf, Buf + Size) with Filler.
// This is used for linker script "=fillexp" command.
static void fill(uint8_t *Buf, size_t Size, uint32_t Filler) {
  size_t I = 0;
  for (; I + 4 < Size; I += 4)
    memcpy(Buf + I, &Filler, 4);
  memcpy(Buf + I, &Filler, Size - I);
}

// Compress section contents if this section contains debug info.
template <class ELFT> void OutputSection::maybeCompress() {
  typedef typename ELFT::Chdr Elf_Chdr;

  // Compress only DWARF debug sections.
  if (!Config->CompressDebugSections || (Flags & SHF_ALLOC) ||
      !Name.startswith(".debug_"))
    return;

  // Create a section header.
  ZDebugHeader.resize(sizeof(Elf_Chdr));
  auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
  Hdr->ch_type = ELFCOMPRESS_ZLIB;
  Hdr->ch_size = Size;
  Hdr->ch_addralign = Alignment;

  // Write section contents to a temporary buffer and compress it.
  std::vector<uint8_t> Buf(Size);
  writeTo<ELFT>(Buf.data());
  if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
    fatal("compress failed: " + llvm::toString(std::move(E)));

  // Update section headers.
  Size = sizeof(Elf_Chdr) + CompressedData.size();
  Flags |= SHF_COMPRESSED;
}

static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
  if (Size == 1)
    *Buf = Data;
  else if (Size == 2)
    write16(Buf, Data, Config->Endianness);
  else if (Size == 4)
    write32(Buf, Data, Config->Endianness);
  else if (Size == 8)
    write64(Buf, Data, Config->Endianness);
  else
    llvm_unreachable("unsupported Size argument");
}

template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
  if (Type == SHT_NOBITS)
    return;

  Loc = Buf;

  // If -compress-debug-section is specified and if this is a debug seciton,
  // we've already compressed section contents. If that's the case,
  // just write it down.
  if (!CompressedData.empty()) {
    memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
    memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
           CompressedData.size());
    return;
  }

  // Write leading padding.
  std::vector<InputSection *> Sections;
  for (BaseCommand *Cmd : Commands)
    if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
      for (InputSection *IS : ISD->Sections)
        if (IS->Live)
          Sections.push_back(IS);
  uint32_t Filler = getFiller();
  if (Filler)
    fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);

  parallelForEachN(0, Sections.size(), [=](size_t I) {
    InputSection *IS = Sections[I];
    IS->writeTo<ELFT>(Buf);

    // Fill gaps between sections.
    if (Filler) {
      uint8_t *Start = Buf + IS->OutSecOff + IS->getSize();
      uint8_t *End;
      if (I + 1 == Sections.size())
        End = Buf + Size;
      else
        End = Buf + Sections[I + 1]->OutSecOff;
      fill(Start, End - Start, Filler);
    }
  });

  // Linker scripts may have BYTE()-family commands with which you
  // can write arbitrary bytes to the output. Process them if any.
  for (BaseCommand *Base : Commands)
    if (auto *Data = dyn_cast<BytesDataCommand>(Base))
      writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
}

static bool compareByFilePosition(InputSection *A, InputSection *B) {
  // Synthetic doesn't have link order dependecy, stable_sort will keep it last
  if (A->kind() == InputSectionBase::Synthetic ||
      B->kind() == InputSectionBase::Synthetic)
    return false;
  InputSection *LA = A->getLinkOrderDep();
  InputSection *LB = B->getLinkOrderDep();
  OutputSection *AOut = LA->getParent();
  OutputSection *BOut = LB->getParent();
  if (AOut != BOut)
    return AOut->SectionIndex < BOut->SectionIndex;
  return LA->OutSecOff < LB->OutSecOff;
}

template <class ELFT>
static void finalizeShtGroup(OutputSection *OS,
                             ArrayRef<InputSection *> Sections) {
  assert(Config->Relocatable && Sections.size() == 1);

  // sh_link field for SHT_GROUP sections should contain the section index of
  // the symbol table.
  OS->Link = InX::SymTab->getParent()->SectionIndex;

  // sh_info then contain index of an entry in symbol table section which
  // provides signature of the section group.
  ObjFile<ELFT> *Obj = Sections[0]->getFile<ELFT>();
  ArrayRef<SymbolBody *> Symbols = Obj->getSymbols();
  OS->Info = InX::SymTab->getSymbolIndex(Symbols[Sections[0]->Info]);
}

template <class ELFT> void OutputSection::finalize() {
  // Link order may be distributed across several InputSectionDescriptions
  // but sort must consider them all at once.
  std::vector<InputSection **> ScriptSections;
  std::vector<InputSection *> Sections;
  for (BaseCommand *Base : Commands) {
    if (auto *ISD = dyn_cast<InputSectionDescription>(Base)) {
      for (InputSection *&IS : ISD->Sections) {
        ScriptSections.push_back(&IS);
        Sections.push_back(IS);
      }
    }
  }

  if (Flags & SHF_LINK_ORDER) {
    std::stable_sort(Sections.begin(), Sections.end(), compareByFilePosition);
    for (int I = 0, N = Sections.size(); I < N; ++I)
      *ScriptSections[I] = Sections[I];

    // We must preserve the link order dependency of sections with the
    // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
    // need to translate the InputSection sh_link to the OutputSection sh_link,
    // all InputSections in the OutputSection have the same dependency.
    if (auto *D = Sections.front()->getLinkOrderDep())
      Link = D->getParent()->SectionIndex;
  }

  if (Type == SHT_GROUP) {
    finalizeShtGroup<ELFT>(this, Sections);
    return;
  }

  if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
    return;

  InputSection *First = Sections[0];
  if (isa<SyntheticSection>(First))
    return;

  Link = InX::SymTab->getParent()->SectionIndex;
  // sh_info for SHT_REL[A] sections should contain the section header index of
  // the section to which the relocation applies.
  InputSectionBase *S = First->getRelocatedSection();
  Info = S->getOutputSection()->SectionIndex;
  Flags |= SHF_INFO_LINK;
}

// Returns true if S matches /Filename.?\.o$/.
static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
  if (!S.endswith(".o"))
    return false;
  S = S.drop_back(2);
  if (S.endswith(Filename))
    return true;
  return !S.empty() && S.drop_back().endswith(Filename);
}

static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }

// .ctors and .dtors are sorted by this priority from highest to lowest.
//
//  1. The section was contained in crtbegin (crtbegin contains
//     some sentinel value in its .ctors and .dtors so that the runtime
//     can find the beginning of the sections.)
//
//  2. The section has an optional priority value in the form of ".ctors.N"
//     or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
//     they are compared as string rather than number.
//
//  3. The section is just ".ctors" or ".dtors".
//
//  4. The section was contained in crtend, which contains an end marker.
//
// In an ideal world, we don't need this function because .init_array and
// .ctors are duplicate features (and .init_array is newer.) However, there
// are too many real-world use cases of .ctors, so we had no choice to
// support that with this rather ad-hoc semantics.
static bool compCtors(const InputSection *A, const InputSection *B) {
  bool BeginA = isCrtbegin(A->File->getName());
  bool BeginB = isCrtbegin(B->File->getName());
  if (BeginA != BeginB)
    return BeginA;
  bool EndA = isCrtend(A->File->getName());
  bool EndB = isCrtend(B->File->getName());
  if (EndA != EndB)
    return EndB;
  StringRef X = A->Name;
  StringRef Y = B->Name;
  assert(X.startswith(".ctors") || X.startswith(".dtors"));
  assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
  X = X.substr(6);
  Y = Y.substr(6);
  if (X.empty() && Y.empty())
    return false;
  return X < Y;
}

// Sorts input sections by the special rules for .ctors and .dtors.
// Unfortunately, the rules are different from the one for .{init,fini}_array.
// Read the comment above.
void OutputSection::sortCtorsDtors() {
  assert(Commands.size() == 1);
  auto *ISD = cast<InputSectionDescription>(Commands[0]);
  std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
}

// If an input string is in the form of "foo.N" where N is a number,
// return N. Otherwise, returns 65536, which is one greater than the
// lowest priority.
int elf::getPriority(StringRef S) {
  size_t Pos = S.rfind('.');
  if (Pos == StringRef::npos)
    return 65536;
  int V;
  if (!to_integer(S.substr(Pos + 1), V, 10))
    return 65536;
  return V;
}

// Sorts input sections by section name suffixes, so that .foo.N comes
// before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
// We want to keep the original order if the priorities are the same
// because the compiler keeps the original initialization order in a
// translation unit and we need to respect that.
// For more detail, read the section of the GCC's manual about init_priority.
void OutputSection::sortInitFini() {
  // Sort sections by priority.
  sort([](InputSectionBase *S) { return getPriority(S->Name); });
}

uint32_t OutputSection::getFiller() {
  if (Filler)
    return *Filler;
  if (Flags & SHF_EXECINSTR)
    return Target->TrapInstr;
  return 0;
}

template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);

template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);

template void OutputSection::maybeCompress<ELF32LE>();
template void OutputSection::maybeCompress<ELF32BE>();
template void OutputSection::maybeCompress<ELF64LE>();
template void OutputSection::maybeCompress<ELF64BE>();

template void OutputSection::finalize<ELF32LE>();
template void OutputSection::finalize<ELF32BE>();
template void OutputSection::finalize<ELF64LE>();
template void OutputSection::finalize<ELF64BE>();