llvm-project/lld/ELF/LinkerScript.cpp

//===- LinkerScript.cpp ---------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the parser/evaluator of the linker script.
//
//===----------------------------------------------------------------------===//

#include "LinkerScript.h"
#include "Config.h"
#include "InputSection.h"
#include "Memory.h"
#include "OutputSections.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Threads.h"
#include "Writer.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/Path.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <limits>
#include <string>
#include <vector>

using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;

LinkerScript *elf::Script;

uint64_t ExprValue::getValue() const {
  if (Sec) {
    if (OutputSection *OS = Sec->getOutputSection())
      return alignTo(Sec->getOffset(Val) + OS->Addr, Alignment);
    error(Loc + ": unable to evaluate expression: input section " + Sec->Name +
          " has no output section assigned");
  }
  return alignTo(Val, Alignment);
}

uint64_t ExprValue::getSecAddr() const {
  if (Sec)
    return Sec->getOffset(0) + Sec->getOutputSection()->Addr;
  return 0;
}

template <class ELFT> static SymbolBody *addRegular(SymbolAssignment *Cmd) {
  Symbol *Sym;
  uint8_t Visibility = Cmd->Hidden ? STV_HIDDEN : STV_DEFAULT;
  std::tie(Sym, std::ignore) = Symtab<ELFT>::X->insert(
      Cmd->Name, /*Type*/ 0, Visibility, /*CanOmitFromDynSym*/ false,
      /*File*/ nullptr);
  Sym->Binding = STB_GLOBAL;
  ExprValue Value = Cmd->Expression();
  SectionBase *Sec = Value.isAbsolute() ? nullptr : Value.Sec;

  // We want to set symbol values early if we can. This allows us to use symbols
  // as variables in linker scripts. Doing so allows us to write expressions
  // like this: `alignment = 16; . = ALIGN(., alignment)`
  uint64_t SymValue = Value.isAbsolute() ? Value.getValue() : 0;
  replaceBody<DefinedRegular>(Sym, Cmd->Name, /*IsLocal=*/false, Visibility,
                              STT_NOTYPE, SymValue, 0, Sec, nullptr);
  return Sym->body();
}

OutputSectionCommand *
LinkerScript::createOutputSectionCommand(StringRef Name, StringRef Location) {
  OutputSectionCommand *&CmdRef = NameToOutputSectionCommand[Name];
  OutputSectionCommand *Cmd;
  if (CmdRef && CmdRef->Location.empty()) {
    // There was a forward reference.
    Cmd = CmdRef;
  } else {
    Cmd = make<OutputSectionCommand>(Name);
    if (!CmdRef)
      CmdRef = Cmd;
  }
  Cmd->Location = Location;
  return Cmd;
}

OutputSectionCommand *
LinkerScript::getOrCreateOutputSectionCommand(StringRef Name) {
  OutputSectionCommand *&CmdRef = NameToOutputSectionCommand[Name];
  if (!CmdRef)
    CmdRef = make<OutputSectionCommand>(Name);
  return CmdRef;
}

void LinkerScript::setDot(Expr E, const Twine &Loc, bool InSec) {
  uint64_t Val = E().getValue();
  if (Val < Dot && InSec)
    error(Loc + ": unable to move location counter backward for: " +
          CurAddressState->OutSec->Name);
  Dot = Val;
  // Update to location counter means update to section size.
  if (InSec)
    CurAddressState->OutSec->Size = Dot - CurAddressState->OutSec->Addr;
}

// Sets value of a symbol. Two kinds of symbols are processed: synthetic
// symbols, whose value is an offset from beginning of section and regular
// symbols whose value is absolute.
void LinkerScript::assignSymbol(SymbolAssignment *Cmd, bool InSec) {
  if (Cmd->Name == ".") {
    setDot(Cmd->Expression, Cmd->Location, InSec);
    return;
  }

  if (!Cmd->Sym)
    return;

  auto *Sym = cast<DefinedRegular>(Cmd->Sym);
  ExprValue V = Cmd->Expression();
  if (V.isAbsolute()) {
    Sym->Value = V.getValue();
  } else {
    Sym->Section = V.Sec;
    Sym->Value = alignTo(V.Val, V.Alignment);
  }
}

static SymbolBody *findSymbol(StringRef S) {
  switch (Config->EKind) {
  case ELF32LEKind:
    return Symtab<ELF32LE>::X->find(S);
  case ELF32BEKind:
    return Symtab<ELF32BE>::X->find(S);
  case ELF64LEKind:
    return Symtab<ELF64LE>::X->find(S);
  case ELF64BEKind:
    return Symtab<ELF64BE>::X->find(S);
  default:
    llvm_unreachable("unknown Config->EKind");
  }
}

static SymbolBody *addRegularSymbol(SymbolAssignment *Cmd) {
  switch (Config->EKind) {
  case ELF32LEKind:
    return addRegular<ELF32LE>(Cmd);
  case ELF32BEKind:
    return addRegular<ELF32BE>(Cmd);
  case ELF64LEKind:
    return addRegular<ELF64LE>(Cmd);
  case ELF64BEKind:
    return addRegular<ELF64BE>(Cmd);
  default:
    llvm_unreachable("unknown Config->EKind");
  }
}

void LinkerScript::addSymbol(SymbolAssignment *Cmd) {
  if (Cmd->Name == ".")
    return;

  // If a symbol was in PROVIDE(), we need to define it only when
  // it is a referenced undefined symbol.
  SymbolBody *B = findSymbol(Cmd->Name);
  if (Cmd->Provide && (!B || B->isDefined()))
    return;

  Cmd->Sym = addRegularSymbol(Cmd);
}

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

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

// 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);
}

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

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

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

static StringRef basename(InputSectionBase *S) {
  if (S->File)
    return sys::path::filename(S->File->getName());
  return "";
}

bool LinkerScript::shouldKeep(InputSectionBase *S) {
  for (InputSectionDescription *ID : Opt.KeptSections)
    if (ID->FilePat.match(basename(S)))
      for (SectionPattern &P : ID->SectionPatterns)
        if (P.SectionPat.match(S->Name))
          return true;
  return false;
}

// 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.
static int 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;
}

// A helper function for the SORT() command.
static std::function<bool(InputSectionBase *, InputSectionBase *)>
getComparator(SortSectionPolicy K) {
  switch (K) {
  case SortSectionPolicy::Alignment:
    return [](InputSectionBase *A, InputSectionBase *B) {
      // ">" is not a mistake. Sections with larger alignments are placed
      // before sections with smaller alignments in order to reduce the
      // amount of padding necessary. This is compatible with GNU.
      return A->Alignment > B->Alignment;
    };
  case SortSectionPolicy::Name:
    return [](InputSectionBase *A, InputSectionBase *B) {
      return A->Name < B->Name;
    };
  case SortSectionPolicy::Priority:
    return [](InputSectionBase *A, InputSectionBase *B) {
      return getPriority(A->Name) < getPriority(B->Name);
    };
  default:
    llvm_unreachable("unknown sort policy");
  }
}

// A helper function for the SORT() command.
static bool matchConstraints(ArrayRef<InputSectionBase *> Sections,
                             ConstraintKind Kind) {
  if (Kind == ConstraintKind::NoConstraint)
    return true;

  bool IsRW = llvm::any_of(Sections, [](InputSectionBase *Sec) {
    return static_cast<InputSectionBase *>(Sec)->Flags & SHF_WRITE;
  });

  return (IsRW && Kind == ConstraintKind::ReadWrite) ||
         (!IsRW && Kind == ConstraintKind::ReadOnly);
}

static void sortSections(InputSection **Begin, InputSection **End,
                         SortSectionPolicy K) {
  if (K != SortSectionPolicy::Default && K != SortSectionPolicy::None)
    std::stable_sort(Begin, End, getComparator(K));
}

// Compute and remember which sections the InputSectionDescription matches.
std::vector<InputSection *>
LinkerScript::computeInputSections(const InputSectionDescription *Cmd) {
  std::vector<InputSection *> Ret;

  // Collects all sections that satisfy constraints of Cmd.
  for (const SectionPattern &Pat : Cmd->SectionPatterns) {
    size_t SizeBefore = Ret.size();

    for (InputSectionBase *Sec : InputSections) {
      if (Sec->Assigned)
        continue;

      if (!Sec->Live) {
        reportDiscarded(Sec);
        continue;
      }

      // For -emit-relocs we have to ignore entries like
      //   .rela.dyn : { *(.rela.data) }
      // which are common because they are in the default bfd script.
      if (Sec->Type == SHT_REL || Sec->Type == SHT_RELA)
        continue;

      StringRef Filename = basename(Sec);
      if (!Cmd->FilePat.match(Filename) ||
          Pat.ExcludedFilePat.match(Filename) ||
          !Pat.SectionPat.match(Sec->Name))
        continue;

      Ret.push_back(cast<InputSection>(Sec));
      Sec->Assigned = true;
    }

    // Sort sections as instructed by SORT-family commands and --sort-section
    // option. Because SORT-family commands can be nested at most two depth
    // (e.g. SORT_BY_NAME(SORT_BY_ALIGNMENT(.text.*))) and because the command
    // line option is respected even if a SORT command is given, the exact
    // behavior we have here is a bit complicated. Here are the rules.
    //
    // 1. If two SORT commands are given, --sort-section is ignored.
    // 2. If one SORT command is given, and if it is not SORT_NONE,
    //    --sort-section is handled as an inner SORT command.
    // 3. If one SORT command is given, and if it is SORT_NONE, don't sort.
    // 4. If no SORT command is given, sort according to --sort-section.
    InputSection **Begin = Ret.data() + SizeBefore;
    InputSection **End = Ret.data() + Ret.size();
    if (Pat.SortOuter != SortSectionPolicy::None) {
      if (Pat.SortInner == SortSectionPolicy::Default)
        sortSections(Begin, End, Config->SortSection);
      else
        sortSections(Begin, End, Pat.SortInner);
      sortSections(Begin, End, Pat.SortOuter);
    }
  }
  return Ret;
}

void LinkerScript::discard(ArrayRef<InputSectionBase *> V) {
  for (InputSectionBase *S : V) {
    S->Live = false;
    if (S == InX::ShStrTab || S == InX::Dynamic || S == InX::DynSymTab ||
        S == InX::DynStrTab)
      error("discarding " + S->Name + " section is not allowed");
    discard(S->DependentSections);
  }
}

std::vector<InputSectionBase *>
LinkerScript::createInputSectionList(OutputSectionCommand &OutCmd) {
  std::vector<InputSectionBase *> Ret;

  for (BaseCommand *Base : OutCmd.Commands) {
    auto *Cmd = dyn_cast<InputSectionDescription>(Base);
    if (!Cmd)
      continue;

    Cmd->Sections = computeInputSections(Cmd);
    Ret.insert(Ret.end(), Cmd->Sections.begin(), Cmd->Sections.end());
  }

  return Ret;
}

void LinkerScript::processCommands(OutputSectionFactory &Factory) {
  // A symbol can be assigned before any section is mentioned in the linker
  // script. In an DSO, the symbol values are addresses, so the only important
  // section values are:
  // * SHN_UNDEF
  // * SHN_ABS
  // * Any value meaning a regular section.
  // To handle that, create a dummy aether section that fills the void before
  // the linker scripts switches to another section. It has an index of one
  // which will map to whatever the first actual section is.
  Aether = make<OutputSection>("", 0, SHF_ALLOC);
  Aether->SectionIndex = 1;
  auto State = make_unique<AddressState>(Opt);
  // CurAddressState captures the local AddressState and makes it accessible
  // deliberately. This is needed as there are some cases where we cannot just
  // thread the current state through to a lambda function created by the
  // script parser.
  CurAddressState = State.get();
  CurAddressState->OutSec = Aether;
  Dot = 0;

  for (size_t I = 0; I < Opt.Commands.size(); ++I) {
    // Handle symbol assignments outside of any output section.
    if (auto *Cmd = dyn_cast<SymbolAssignment>(Opt.Commands[I])) {
      addSymbol(Cmd);
      continue;
    }

    if (auto *Cmd = dyn_cast<OutputSectionCommand>(Opt.Commands[I])) {
      std::vector<InputSectionBase *> V = createInputSectionList(*Cmd);

      // The output section name `/DISCARD/' is special.
      // Any input section assigned to it is discarded.
      if (Cmd->Name == "/DISCARD/") {
        discard(V);
        continue;
      }

      // This is for ONLY_IF_RO and ONLY_IF_RW. An output section directive
      // ".foo : ONLY_IF_R[OW] { ... }" is handled only if all member input
      // sections satisfy a given constraint. If not, a directive is handled
      // as if it wasn't present from the beginning.
      //
      // Because we'll iterate over Commands many more times, the easiest
      // way to "make it as if it wasn't present" is to just remove it.
      if (!matchConstraints(V, Cmd->Constraint)) {
        for (InputSectionBase *S : V)
          S->Assigned = false;
        Opt.Commands.erase(Opt.Commands.begin() + I);
        --I;
        continue;
      }

      // A directive may contain symbol definitions like this:
      // ".foo : { ...; bar = .; }". Handle them.
      for (BaseCommand *Base : Cmd->Commands)
        if (auto *OutCmd = dyn_cast<SymbolAssignment>(Base))
          addSymbol(OutCmd);

      // Handle subalign (e.g. ".foo : SUBALIGN(32) { ... }"). If subalign
      // is given, input sections are aligned to that value, whether the
      // given value is larger or smaller than the original section alignment.
      if (Cmd->SubalignExpr) {
        uint32_t Subalign = Cmd->SubalignExpr().getValue();
        for (InputSectionBase *S : V)
          S->Alignment = Subalign;
      }

      // Add input sections to an output section.
      for (InputSectionBase *S : V)
        Factory.addInputSec(S, Cmd->Name, Cmd->Sec);
      if (OutputSection *Sec = Cmd->Sec) {
        assert(Sec->SectionIndex == INT_MAX);
        Sec->SectionIndex = I;
        if (Cmd->Noload)
          Sec->Type = SHT_NOBITS;
        SecToCommand[Sec] = Cmd;
      }
    }
  }
  CurAddressState = nullptr;
}

void LinkerScript::fabricateDefaultCommands() {
  std::vector<BaseCommand *> Commands;

  // Define start address
  uint64_t StartAddr = -1;

  // The Sections with -T<section> have been sorted in order of ascending
  // address. We must lower StartAddr if the lowest -T<section address> as
  // calls to setDot() must be monotonically increasing.
  for (auto &KV : Config->SectionStartMap)
    StartAddr = std::min(StartAddr, KV.second);

  Commands.push_back(make<SymbolAssignment>(
      ".",
      [=] {
        return std::min(StartAddr, Config->ImageBase + elf::getHeaderSize());
      },
      ""));

  // For each OutputSection that needs a VA fabricate an OutputSectionCommand
  // with an InputSectionDescription describing the InputSections
  for (OutputSection *Sec : OutputSections) {
    auto *OSCmd = createOutputSectionCommand(Sec->Name, "<internal>");
    OSCmd->Sec = Sec;
    SecToCommand[Sec] = OSCmd;

    Commands.push_back(OSCmd);
    if (Sec->Sections.size()) {
      auto *ISD = make<InputSectionDescription>("");
      OSCmd->Commands.push_back(ISD);
      for (InputSection *ISec : Sec->Sections) {
        ISD->Sections.push_back(ISec);
        ISec->Assigned = true;
      }
    }
  }
  // SECTIONS commands run before other non SECTIONS commands
  Commands.insert(Commands.end(), Opt.Commands.begin(), Opt.Commands.end());
  Opt.Commands = std::move(Commands);
}

// Add sections that didn't match any sections command.
void LinkerScript::addOrphanSections(OutputSectionFactory &Factory) {
  unsigned NumCommands = Opt.Commands.size();
  for (InputSectionBase *S : InputSections) {
    if (!S->Live || S->Parent)
      continue;
    StringRef Name = getOutputSectionName(S->Name);
    auto End = Opt.Commands.begin() + NumCommands;
    auto I = std::find_if(Opt.Commands.begin(), End, [&](BaseCommand *Base) {
      if (auto *Cmd = dyn_cast<OutputSectionCommand>(Base))
        return Cmd->Name == Name;
      return false;
    });
    OutputSectionCommand *Cmd;
    if (I == End) {
      Factory.addInputSec(S, Name);
      OutputSection *Sec = S->getOutputSection();
      assert(Sec->SectionIndex == INT_MAX);
      OutputSectionCommand *&CmdRef = SecToCommand[Sec];
      if (!CmdRef) {
        CmdRef = createOutputSectionCommand(Sec->Name, "<internal>");
        CmdRef->Sec = Sec;
        Opt.Commands.push_back(CmdRef);
      }
      Cmd = CmdRef;
    } else {
      Cmd = cast<OutputSectionCommand>(*I);
      Factory.addInputSec(S, Name, Cmd->Sec);
      if (OutputSection *Sec = Cmd->Sec) {
        SecToCommand[Sec] = Cmd;
        unsigned Index = std::distance(Opt.Commands.begin(), I);
        assert(Sec->SectionIndex == INT_MAX || Sec->SectionIndex == Index);
        Sec->SectionIndex = Index;
      }
    }
    auto *ISD = make<InputSectionDescription>("");
    ISD->Sections.push_back(cast<InputSection>(S));
    Cmd->Commands.push_back(ISD);
  }
}

uint64_t LinkerScript::advance(uint64_t Size, unsigned Align) {
  bool IsTbss = (CurAddressState->OutSec->Flags & SHF_TLS) &&
                CurAddressState->OutSec->Type == SHT_NOBITS;
  uint64_t Start = IsTbss ? Dot + CurAddressState->ThreadBssOffset : Dot;
  Start = alignTo(Start, Align);
  uint64_t End = Start + Size;

  if (IsTbss)
    CurAddressState->ThreadBssOffset = End - Dot;
  else
    Dot = End;
  return End;
}

void LinkerScript::output(InputSection *S) {
  uint64_t Before = advance(0, 1);
  uint64_t Pos = advance(S->getSize(), S->Alignment);
  S->OutSecOff = Pos - S->getSize() - CurAddressState->OutSec->Addr;

  // Update output section size after adding each section. This is so that
  // SIZEOF works correctly in the case below:
  // .foo { *(.aaa) a = SIZEOF(.foo); *(.bbb) }
  CurAddressState->OutSec->Size = Pos - CurAddressState->OutSec->Addr;

  // If there is a memory region associated with this input section, then
  // place the section in that region and update the region index.
  if (CurAddressState->MemRegion) {
    uint64_t &CurOffset =
        CurAddressState->MemRegionOffset[CurAddressState->MemRegion];
    CurOffset += Pos - Before;
    uint64_t CurSize = CurOffset - CurAddressState->MemRegion->Origin;
    if (CurSize > CurAddressState->MemRegion->Length) {
      uint64_t OverflowAmt = CurSize - CurAddressState->MemRegion->Length;
      error("section '" + CurAddressState->OutSec->Name +
            "' will not fit in region '" + CurAddressState->MemRegion->Name +
            "': overflowed by " + Twine(OverflowAmt) + " bytes");
    }
  }
}

void LinkerScript::switchTo(OutputSection *Sec) {
  if (CurAddressState->OutSec == Sec)
    return;

  CurAddressState->OutSec = Sec;
  CurAddressState->OutSec->Addr =
      advance(0, CurAddressState->OutSec->Alignment);

  // If neither AT nor AT> is specified for an allocatable section, the linker
  // will set the LMA such that the difference between VMA and LMA for the
  // section is the same as the preceding output section in the same region
  // https://sourceware.org/binutils/docs-2.20/ld/Output-Section-LMA.html
  if (CurAddressState->LMAOffset)
    CurAddressState->OutSec->LMAOffset = CurAddressState->LMAOffset();
}

void LinkerScript::process(BaseCommand &Base) {
  // This handles the assignments to symbol or to the dot.
  if (auto *Cmd = dyn_cast<SymbolAssignment>(&Base)) {
    assignSymbol(Cmd, true);
    return;
  }

  // Handle BYTE(), SHORT(), LONG(), or QUAD().
  if (auto *Cmd = dyn_cast<BytesDataCommand>(&Base)) {
    Cmd->Offset = Dot - CurAddressState->OutSec->Addr;
    Dot += Cmd->Size;
    CurAddressState->OutSec->Size = Dot - CurAddressState->OutSec->Addr;
    return;
  }

  // Handle ASSERT().
  if (auto *Cmd = dyn_cast<AssertCommand>(&Base)) {
    Cmd->Expression();
    return;
  }

  // Handle a single input section description command.
  // It calculates and assigns the offsets for each section and also
  // updates the output section size.
  auto &Cmd = cast<InputSectionDescription>(Base);
  for (InputSection *Sec : Cmd.Sections) {
    // We tentatively added all synthetic sections at the beginning and removed
    // empty ones afterwards (because there is no way to know whether they were
    // going be empty or not other than actually running linker scripts.)
    // We need to ignore remains of empty sections.
    if (auto *S = dyn_cast<SyntheticSection>(Sec))
      if (S->empty())
        continue;

    if (!Sec->Live)
      continue;
    assert(CurAddressState->OutSec == Sec->getParent());
    output(Sec);
  }
}

// This function searches for a memory region to place the given output
// section in. If found, a pointer to the appropriate memory region is
// returned. Otherwise, a nullptr is returned.
MemoryRegion *LinkerScript::findMemoryRegion(OutputSectionCommand *Cmd) {
  // If a memory region name was specified in the output section command,
  // then try to find that region first.
  if (!Cmd->MemoryRegionName.empty()) {
    auto It = Opt.MemoryRegions.find(Cmd->MemoryRegionName);
    if (It != Opt.MemoryRegions.end())
      return &It->second;
    error("memory region '" + Cmd->MemoryRegionName + "' not declared");
    return nullptr;
  }

  // If at least one memory region is defined, all sections must
  // belong to some memory region. Otherwise, we don't need to do
  // anything for memory regions.
  if (Opt.MemoryRegions.empty())
    return nullptr;

  OutputSection *Sec = Cmd->Sec;
  // See if a region can be found by matching section flags.
  for (auto &Pair : Opt.MemoryRegions) {
    MemoryRegion &M = Pair.second;
    if ((M.Flags & Sec->Flags) && (M.NegFlags & Sec->Flags) == 0)
      return &M;
  }

  // Otherwise, no suitable region was found.
  if (Sec->Flags & SHF_ALLOC)
    error("no memory region specified for section '" + Sec->Name + "'");
  return nullptr;
}

// This function assigns offsets to input sections and an output section
// for a single sections command (e.g. ".text { *(.text); }").
void LinkerScript::assignOffsets(OutputSectionCommand *Cmd) {
  OutputSection *Sec = Cmd->Sec;
  if (!Sec)
    return;

  if (!(Sec->Flags & SHF_ALLOC))
    Dot = 0;
  else if (Cmd->AddrExpr)
    setDot(Cmd->AddrExpr, Cmd->Location, false);

  if (Cmd->LMAExpr) {
    uint64_t D = Dot;
    CurAddressState->LMAOffset = [=] { return Cmd->LMAExpr().getValue() - D; };
  }

  CurAddressState->MemRegion = Cmd->MemRegion;
  if (CurAddressState->MemRegion)
    Dot = CurAddressState->MemRegionOffset[CurAddressState->MemRegion];
  switchTo(Sec);

  // We do not support custom layout for compressed debug sectons.
  // At this point we already know their size and have compressed content.
  if (CurAddressState->OutSec->Flags & SHF_COMPRESSED)
    return;

  for (BaseCommand *C : Cmd->Commands)
    process(*C);
}

void LinkerScript::removeEmptyCommands() {
  // It is common practice to use very generic linker scripts. So for any
  // given run some of the output sections in the script will be empty.
  // We could create corresponding empty output sections, but that would
  // clutter the output.
  // We instead remove trivially empty sections. The bfd linker seems even
  // more aggressive at removing them.
  auto Pos = std::remove_if(
      Opt.Commands.begin(), Opt.Commands.end(), [&](BaseCommand *Base) {
        if (auto *Cmd = dyn_cast<OutputSectionCommand>(Base))
          return Cmd->Sec == nullptr;
        return false;
      });
  Opt.Commands.erase(Pos, Opt.Commands.end());
}

static bool isAllSectionDescription(const OutputSectionCommand &Cmd) {
  for (BaseCommand *Base : Cmd.Commands)
    if (!isa<InputSectionDescription>(*Base))
      return false;
  return true;
}

void LinkerScript::adjustSectionsBeforeSorting() {
  // If the output section contains only symbol assignments, create a
  // corresponding output section. The bfd linker seems to only create them if
  // '.' is assigned to, but creating these section should not have any bad
  // consequeces and gives us a section to put the symbol in.
  uint64_t Flags = SHF_ALLOC;

  for (int I = 0, E = Opt.Commands.size(); I != E; ++I) {
    auto *Cmd = dyn_cast<OutputSectionCommand>(Opt.Commands[I]);
    if (!Cmd)
      continue;
    if (OutputSection *Sec = Cmd->Sec) {
      Flags = Sec->Flags;
      continue;
    }

    if (isAllSectionDescription(*Cmd))
      continue;

    auto *OutSec = make<OutputSection>(Cmd->Name, SHT_PROGBITS, Flags);
    OutSec->SectionIndex = I;
    Cmd->Sec = OutSec;
    SecToCommand[OutSec] = Cmd;
  }
}

void LinkerScript::adjustSectionsAfterSorting() {
  // Try and find an appropriate memory region to assign offsets in.
  for (BaseCommand *Base : Opt.Commands) {
    if (auto *Cmd = dyn_cast<OutputSectionCommand>(Base)) {
      Cmd->MemRegion = findMemoryRegion(Cmd);
      // Handle align (e.g. ".foo : ALIGN(16) { ... }").
      if (Cmd->AlignExpr && Cmd->Sec)
        Cmd->Sec->updateAlignment(Cmd->AlignExpr().getValue());
    }
  }

  // If output section command doesn't specify any segments,
  // and we haven't previously assigned any section to segment,
  // then we simply assign section to the very first load segment.
  // Below is an example of such linker script:
  // PHDRS { seg PT_LOAD; }
  // SECTIONS { .aaa : { *(.aaa) } }
  std::vector<StringRef> DefPhdrs;
  auto FirstPtLoad =
      std::find_if(Opt.PhdrsCommands.begin(), Opt.PhdrsCommands.end(),
                   [](const PhdrsCommand &Cmd) { return Cmd.Type == PT_LOAD; });
  if (FirstPtLoad != Opt.PhdrsCommands.end())
    DefPhdrs.push_back(FirstPtLoad->Name);

  // Walk the commands and propagate the program headers to commands that don't
  // explicitly specify them.
  for (BaseCommand *Base : Opt.Commands) {
    auto *Cmd = dyn_cast<OutputSectionCommand>(Base);
    if (!Cmd)
      continue;

    if (Cmd->Phdrs.empty()) {
      OutputSection *Sec = Cmd->Sec;
      // To match the bfd linker script behaviour, only propagate program
      // headers to sections that are allocated.
      if (Sec && (Sec->Flags & SHF_ALLOC))
        Cmd->Phdrs = DefPhdrs;
    } else {
      DefPhdrs = Cmd->Phdrs;
    }
  }

  removeEmptyCommands();
}

void LinkerScript::allocateHeaders(std::vector<PhdrEntry> &Phdrs) {
  uint64_t Min = std::numeric_limits<uint64_t>::max();
  for (OutputSectionCommand *Cmd : OutputSectionCommands) {
    OutputSection *Sec = Cmd->Sec;
    if (Sec->Flags & SHF_ALLOC)
      Min = std::min<uint64_t>(Min, Sec->Addr);
  }

  auto FirstPTLoad = llvm::find_if(
      Phdrs, [](const PhdrEntry &E) { return E.p_type == PT_LOAD; });
  if (FirstPTLoad == Phdrs.end())
    return;

  uint64_t HeaderSize = getHeaderSize();
  if (HeaderSize <= Min || Script->hasPhdrsCommands()) {
    Min = alignDown(Min - HeaderSize, Config->MaxPageSize);
    Out::ElfHeader->Addr = Min;
    Out::ProgramHeaders->Addr = Min + Out::ElfHeader->Size;
    return;
  }

  assert(FirstPTLoad->First == Out::ElfHeader);
  OutputSection *ActualFirst = nullptr;
  for (OutputSectionCommand *Cmd : OutputSectionCommands) {
    OutputSection *Sec = Cmd->Sec;
    if (Sec->FirstInPtLoad == Out::ElfHeader) {
      ActualFirst = Sec;
      break;
    }
  }
  if (ActualFirst) {
    for (OutputSectionCommand *Cmd : OutputSectionCommands) {
      OutputSection *Sec = Cmd->Sec;
      if (Sec->FirstInPtLoad == Out::ElfHeader)
        Sec->FirstInPtLoad = ActualFirst;
    }
    FirstPTLoad->First = ActualFirst;
  } else {
    Phdrs.erase(FirstPTLoad);
  }

  auto PhdrI = llvm::find_if(
      Phdrs, [](const PhdrEntry &E) { return E.p_type == PT_PHDR; });
  if (PhdrI != Phdrs.end())
    Phdrs.erase(PhdrI);
}

LinkerScript::AddressState::AddressState(const ScriptConfiguration &Opt) {
  for (auto &MRI : Opt.MemoryRegions) {
    const MemoryRegion *MR = &MRI.second;
    MemRegionOffset[MR] = MR->Origin;
  }
}

void LinkerScript::assignAddresses() {
  // Assign addresses as instructed by linker script SECTIONS sub-commands.
  Dot = 0;
  auto State = make_unique<AddressState>(Opt);
  // CurAddressState captures the local AddressState and makes it accessible
  // deliberately. This is needed as there are some cases where we cannot just
  // thread the current state through to a lambda function created by the
  // script parser.
  CurAddressState = State.get();
  ErrorOnMissingSection = true;
  switchTo(Aether);

  for (BaseCommand *Base : Opt.Commands) {
    if (auto *Cmd = dyn_cast<SymbolAssignment>(Base)) {
      assignSymbol(Cmd, false);
      continue;
    }

    if (auto *Cmd = dyn_cast<AssertCommand>(Base)) {
      Cmd->Expression();
      continue;
    }

    auto *Cmd = cast<OutputSectionCommand>(Base);
    assignOffsets(Cmd);
  }
  CurAddressState = nullptr;
}

// Creates program headers as instructed by PHDRS linker script command.
std::vector<PhdrEntry> LinkerScript::createPhdrs() {
  std::vector<PhdrEntry> Ret;

  // Process PHDRS and FILEHDR keywords because they are not
  // real output sections and cannot be added in the following loop.
  for (const PhdrsCommand &Cmd : Opt.PhdrsCommands) {
    Ret.emplace_back(Cmd.Type, Cmd.Flags == UINT_MAX ? PF_R : Cmd.Flags);
    PhdrEntry &Phdr = Ret.back();

    if (Cmd.HasFilehdr)
      Phdr.add(Out::ElfHeader);
    if (Cmd.HasPhdrs)
      Phdr.add(Out::ProgramHeaders);

    if (Cmd.LMAExpr) {
      Phdr.p_paddr = Cmd.LMAExpr().getValue();
      Phdr.HasLMA = true;
    }
  }

  // Add output sections to program headers.
  for (OutputSectionCommand *Cmd : OutputSectionCommands) {
    // Assign headers specified by linker script
    for (size_t Id : getPhdrIndices(Cmd)) {
      OutputSection *Sec = Cmd->Sec;
      Ret[Id].add(Sec);
      if (Opt.PhdrsCommands[Id].Flags == UINT_MAX)
        Ret[Id].p_flags |= Sec->getPhdrFlags();
    }
  }
  return Ret;
}

bool LinkerScript::ignoreInterpSection() {
  // Ignore .interp section in case we have PHDRS specification
  // and PT_INTERP isn't listed.
  if (Opt.PhdrsCommands.empty())
    return false;
  for (PhdrsCommand &Cmd : Opt.PhdrsCommands)
    if (Cmd.Type == PT_INTERP)
      return false;
  return true;
}

OutputSectionCommand *LinkerScript::getCmd(OutputSection *Sec) const {
  auto I = SecToCommand.find(Sec);
  if (I == SecToCommand.end())
    return nullptr;
  return I->second;
}

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

  std::vector<Pair> V;
  assert(Commands.size() == 1);
  auto *ISD = cast<InputSectionDescription>(Commands[0]);
  for (InputSection *S : ISD->Sections)
    V.push_back({Order(S), S});
  std::stable_sort(V.begin(), V.end(), Comp);
  ISD->Sections.clear();
  for (Pair &P : V)
    ISD->Sections.push_back(P.second);
}

// 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 OutputSectionCommand::sortCtorsDtors() {
  assert(Commands.size() == 1);
  auto *ISD = cast<InputSectionDescription>(Commands[0]);
  std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
}

// 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 OutputSectionCommand::sortInitFini() {
  // Sort sections by priority.
  sort([](InputSectionBase *S) { return getPriority(S->Name); });
}

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

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");
}

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.
  elf::ObjectFile<ELFT> *Obj = Sections[0]->getFile<ELFT>();
  ArrayRef<SymbolBody *> Symbols = Obj->getSymbols();
  OS->Info = InX::SymTab->getSymbolIndex(Symbols[Sections[0]->Info - 1]);
}

template <class ELFT> void OutputSectionCommand::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 ((Sec->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())
      Sec->Link = D->getParent()->SectionIndex;
  }

  uint32_t Type = Sec->Type;
  if (Type == SHT_GROUP) {
    finalizeShtGroup<ELFT>(Sec, Sections);
    return;
  }

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

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

  Sec->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();
  Sec->Info = S->getOutputSection()->SectionIndex;
  Sec->Flags |= SHF_INFO_LINK;
}

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

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

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

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

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

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

  Sec->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 (!Sec->CompressedData.empty()) {
    memcpy(Buf, Sec->ZDebugHeader.data(), Sec->ZDebugHeader.size());
    memcpy(Buf + Sec->ZDebugHeader.size(), Sec->CompressedData.data(),
           Sec->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() ? Sec->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 + Sec->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);
}

ExprValue LinkerScript::getSymbolValue(const Twine &Loc, StringRef S) {
  if (S == ".")
    return {CurAddressState->OutSec, Dot - CurAddressState->OutSec->Addr, Loc};
  if (SymbolBody *B = findSymbol(S)) {
    if (auto *D = dyn_cast<DefinedRegular>(B))
      return {D->Section, D->Value, Loc};
    if (auto *C = dyn_cast<DefinedCommon>(B))
      return {InX::Common, C->Offset, Loc};
  }
  error(Loc + ": symbol not found: " + S);
  return 0;
}

bool LinkerScript::isDefined(StringRef S) { return findSymbol(S) != nullptr; }

static const size_t NoPhdr = -1;

// Returns indices of ELF headers containing specific section. Each index is a
// zero based number of ELF header listed within PHDRS {} script block.
std::vector<size_t> LinkerScript::getPhdrIndices(OutputSectionCommand *Cmd) {
  std::vector<size_t> Ret;
  for (StringRef PhdrName : Cmd->Phdrs) {
    size_t Index = getPhdrIndex(Cmd->Location, PhdrName);
    if (Index != NoPhdr)
      Ret.push_back(Index);
  }
  return Ret;
}

// Returns the index of the segment named PhdrName if found otherwise
// NoPhdr. When not found, if PhdrName is not the special case value 'NONE'
// (which can be used to explicitly specify that a section isn't assigned to a
// segment) then error.
size_t LinkerScript::getPhdrIndex(const Twine &Loc, StringRef PhdrName) {
  size_t I = 0;
  for (PhdrsCommand &Cmd : Opt.PhdrsCommands) {
    if (Cmd.Name == PhdrName)
      return I;
    ++I;
  }
  if (PhdrName != "NONE")
    error(Loc + ": section header '" + PhdrName + "' is not listed in PHDRS");
  return NoPhdr;
}

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

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

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