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ELFObjectWriter.cpp
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//===- lib/MC/ELFObjectWriter.cpp - ELF File Writer -----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements ELF object file writer information.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCAsmLayout.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/MC/MCObjectFileInfo.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCSymbolELF.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/EndianStream.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/Host.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/SMLoc.h"
#include "llvm/Support/StringSaver.h"
#include "llvm/Support/SwapByteOrder.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <map>
#include <memory>
#include <string>
#include <utility>
#include <vector>
using namespace llvm;
#undef DEBUG_TYPE
#define DEBUG_TYPE "reloc-info"
namespace {
using SectionIndexMapTy = DenseMap<const MCSectionELF *, uint32_t>;
class ELFObjectWriter;
struct ELFWriter;
bool isDwoSection(const MCSectionELF &Sec) {
return Sec.getName().endswith(".dwo");
}
class SymbolTableWriter {
ELFWriter &EWriter;
bool Is64Bit;
// indexes we are going to write to .symtab_shndx.
std::vector<uint32_t> ShndxIndexes;
// The numbel of symbols written so far.
unsigned NumWritten;
void createSymtabShndx();
template <typename T> void write(T Value);
public:
SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit);
void writeSymbol(uint32_t name, uint8_t info, uint64_t value, uint64_t size,
uint8_t other, uint32_t shndx, bool Reserved);
ArrayRef<uint32_t> getShndxIndexes() const { return ShndxIndexes; }
};
struct ELFWriter {
ELFObjectWriter &OWriter;
support::endian::Writer W;
enum DwoMode {
AllSections,
NonDwoOnly,
DwoOnly,
} Mode;
static uint64_t SymbolValue(const MCSymbol &Sym, const MCAsmLayout &Layout);
static bool isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
bool Used, bool Renamed);
/// Helper struct for containing some precomputed information on symbols.
struct ELFSymbolData {
const MCSymbolELF *Symbol;
StringRef Name;
uint32_t SectionIndex;
uint32_t Order;
};
/// @}
/// @name Symbol Table Data
/// @{
StringTableBuilder StrTabBuilder{StringTableBuilder::ELF};
/// @}
// This holds the symbol table index of the last local symbol.
unsigned LastLocalSymbolIndex;
// This holds the .strtab section index.
unsigned StringTableIndex;
// This holds the .symtab section index.
unsigned SymbolTableIndex;
// Sections in the order they are to be output in the section table.
std::vector<const MCSectionELF *> SectionTable;
unsigned addToSectionTable(const MCSectionELF *Sec);
// TargetObjectWriter wrappers.
bool is64Bit() const;
bool hasRelocationAddend() const;
uint64_t align(unsigned Alignment);
bool maybeWriteCompression(uint64_t Size,
SmallVectorImpl<char> &CompressedContents,
bool ZLibStyle, unsigned Alignment);
public:
ELFWriter(ELFObjectWriter &OWriter, raw_pwrite_stream &OS,
bool IsLittleEndian, DwoMode Mode)
: OWriter(OWriter),
W(OS, IsLittleEndian ? support::little : support::big), Mode(Mode) {}
void WriteWord(uint64_t Word) {
if (is64Bit())
W.write<uint64_t>(Word);
else
W.write<uint32_t>(Word);
}
template <typename T> void write(T Val) {
W.write(Val);
}
void writeHeader(const MCAssembler &Asm);
void writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
ELFSymbolData &MSD, const MCAsmLayout &Layout);
// Start and end offset of each section
using SectionOffsetsTy =
std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>;
// Map from a signature symbol to the group section index
using RevGroupMapTy = DenseMap<const MCSymbol *, unsigned>;
/// Compute the symbol table data
///
/// \param Asm - The assembler.
/// \param SectionIndexMap - Maps a section to its index.
/// \param RevGroupMap - Maps a signature symbol to the group section.
void computeSymbolTable(MCAssembler &Asm, const MCAsmLayout &Layout,
const SectionIndexMapTy &SectionIndexMap,
const RevGroupMapTy &RevGroupMap,
SectionOffsetsTy &SectionOffsets);
void writeAddrsigSection();
MCSectionELF *createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec);
void writeSectionHeader(const MCAsmLayout &Layout,
const SectionIndexMapTy &SectionIndexMap,
const SectionOffsetsTy &SectionOffsets);
void writeSectionData(const MCAssembler &Asm, MCSection &Sec,
const MCAsmLayout &Layout);
void WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
uint64_t Address, uint64_t Offset, uint64_t Size,
uint32_t Link, uint32_t Info, uint64_t Alignment,
uint64_t EntrySize);
void writeRelocations(const MCAssembler &Asm, const MCSectionELF &Sec);
uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout);
void writeSection(const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex, uint64_t Offset, uint64_t Size,
const MCSectionELF &Section);
};
class ELFObjectWriter : public MCObjectWriter {
/// The target specific ELF writer instance.
std::unique_ptr<MCELFObjectTargetWriter> TargetObjectWriter;
DenseMap<const MCSectionELF *, std::vector<ELFRelocationEntry>> Relocations;
DenseMap<const MCSymbolELF *, const MCSymbolELF *> Renames;
bool SeenGnuAbi = false;
bool EmitAddrsigSection = false;
std::vector<const MCSymbol *> AddrsigSyms;
bool hasRelocationAddend() const;
bool shouldRelocateWithSymbol(const MCAssembler &Asm,
const MCSymbolRefExpr *RefA,
const MCSymbolELF *Sym, uint64_t C,
unsigned Type) const;
public:
ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW)
: TargetObjectWriter(std::move(MOTW)) {}
void reset() override {
SeenGnuAbi = false;
Relocations.clear();
Renames.clear();
MCObjectWriter::reset();
}
bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
const MCSymbol &SymA,
const MCFragment &FB, bool InSet,
bool IsPCRel) const override;
virtual bool checkRelocation(MCContext &Ctx, SMLoc Loc,
const MCSectionELF *From,
const MCSectionELF *To) {
return true;
}
void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup,
MCValue Target, uint64_t &FixedValue) override;
void executePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) override;
void markGnuAbi() override { SeenGnuAbi = true; }
bool seenGnuAbi() const { return SeenGnuAbi; }
void emitAddrsigSection() override { EmitAddrsigSection = true; }
void addAddrsigSymbol(const MCSymbol *Sym) override {
AddrsigSyms.push_back(Sym);
}
friend struct ELFWriter;
};
class ELFSingleObjectWriter : public ELFObjectWriter {
raw_pwrite_stream &OS;
bool IsLittleEndian;
public:
ELFSingleObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
raw_pwrite_stream &OS, bool IsLittleEndian)
: ELFObjectWriter(std::move(MOTW)), OS(OS),
IsLittleEndian(IsLittleEndian) {}
uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override {
return ELFWriter(*this, OS, IsLittleEndian, ELFWriter::AllSections)
.writeObject(Asm, Layout);
}
friend struct ELFWriter;
};
class ELFDwoObjectWriter : public ELFObjectWriter {
raw_pwrite_stream &OS, &DwoOS;
bool IsLittleEndian;
public:
ELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS,
bool IsLittleEndian)
: ELFObjectWriter(std::move(MOTW)), OS(OS), DwoOS(DwoOS),
IsLittleEndian(IsLittleEndian) {}
virtual bool checkRelocation(MCContext &Ctx, SMLoc Loc,
const MCSectionELF *From,
const MCSectionELF *To) override {
if (isDwoSection(*From)) {
Ctx.reportError(Loc, "A dwo section may not contain relocations");
return false;
}
if (To && isDwoSection(*To)) {
Ctx.reportError(Loc, "A relocation may not refer to a dwo section");
return false;
}
return true;
}
uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override {
uint64_t Size = ELFWriter(*this, OS, IsLittleEndian, ELFWriter::NonDwoOnly)
.writeObject(Asm, Layout);
Size += ELFWriter(*this, DwoOS, IsLittleEndian, ELFWriter::DwoOnly)
.writeObject(Asm, Layout);
return Size;
}
};
} // end anonymous namespace
uint64_t ELFWriter::align(unsigned Alignment) {
uint64_t Offset = W.OS.tell(), NewOffset = alignTo(Offset, Alignment);
W.OS.write_zeros(NewOffset - Offset);
return NewOffset;
}
unsigned ELFWriter::addToSectionTable(const MCSectionELF *Sec) {
SectionTable.push_back(Sec);
StrTabBuilder.add(Sec->getName());
return SectionTable.size();
}
void SymbolTableWriter::createSymtabShndx() {
if (!ShndxIndexes.empty())
return;
ShndxIndexes.resize(NumWritten);
}
template <typename T> void SymbolTableWriter::write(T Value) {
EWriter.write(Value);
}
SymbolTableWriter::SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit)
: EWriter(EWriter), Is64Bit(Is64Bit), NumWritten(0) {}
void SymbolTableWriter::writeSymbol(uint32_t name, uint8_t info, uint64_t value,
uint64_t size, uint8_t other,
uint32_t shndx, bool Reserved) {
bool LargeIndex = shndx >= ELF::SHN_LORESERVE && !Reserved;
if (LargeIndex)
createSymtabShndx();
if (!ShndxIndexes.empty()) {
if (LargeIndex)
ShndxIndexes.push_back(shndx);
else
ShndxIndexes.push_back(0);
}
uint16_t Index = LargeIndex ? uint16_t(ELF::SHN_XINDEX) : shndx;
if (Is64Bit) {
write(name); // st_name
write(info); // st_info
write(other); // st_other
write(Index); // st_shndx
write(value); // st_value
write(size); // st_size
} else {
write(name); // st_name
write(uint32_t(value)); // st_value
write(uint32_t(size)); // st_size
write(info); // st_info
write(other); // st_other
write(Index); // st_shndx
}
++NumWritten;
}
bool ELFWriter::is64Bit() const {
return OWriter.TargetObjectWriter->is64Bit();
}
bool ELFWriter::hasRelocationAddend() const {
return OWriter.hasRelocationAddend();
}
// Emit the ELF header.
void ELFWriter::writeHeader(const MCAssembler &Asm) {
// ELF Header
// ----------
//
// Note
// ----
// emitWord method behaves differently for ELF32 and ELF64, writing
// 4 bytes in the former and 8 in the latter.
W.OS << ELF::ElfMagic; // e_ident[EI_MAG0] to e_ident[EI_MAG3]
W.OS << char(is64Bit() ? ELF::ELFCLASS64 : ELF::ELFCLASS32); // e_ident[EI_CLASS]
// e_ident[EI_DATA]
W.OS << char(W.Endian == support::little ? ELF::ELFDATA2LSB
: ELF::ELFDATA2MSB);
W.OS << char(ELF::EV_CURRENT); // e_ident[EI_VERSION]
// e_ident[EI_OSABI]
uint8_t OSABI = OWriter.TargetObjectWriter->getOSABI();
W.OS << char(OSABI == ELF::ELFOSABI_NONE && OWriter.seenGnuAbi()
? int(ELF::ELFOSABI_GNU)
: OSABI);
// e_ident[EI_ABIVERSION]
W.OS << char(OWriter.TargetObjectWriter->getABIVersion());
W.OS.write_zeros(ELF::EI_NIDENT - ELF::EI_PAD);
W.write<uint16_t>(ELF::ET_REL); // e_type
W.write<uint16_t>(OWriter.TargetObjectWriter->getEMachine()); // e_machine = target
W.write<uint32_t>(ELF::EV_CURRENT); // e_version
WriteWord(0); // e_entry, no entry point in .o file
WriteWord(0); // e_phoff, no program header for .o
WriteWord(0); // e_shoff = sec hdr table off in bytes
// e_flags = whatever the target wants
W.write<uint32_t>(Asm.getELFHeaderEFlags());
// e_ehsize = ELF header size
W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Ehdr)
: sizeof(ELF::Elf32_Ehdr));
W.write<uint16_t>(0); // e_phentsize = prog header entry size
W.write<uint16_t>(0); // e_phnum = # prog header entries = 0
// e_shentsize = Section header entry size
W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Shdr)
: sizeof(ELF::Elf32_Shdr));
// e_shnum = # of section header ents
W.write<uint16_t>(0);
// e_shstrndx = Section # of '.strtab'
assert(StringTableIndex < ELF::SHN_LORESERVE);
W.write<uint16_t>(StringTableIndex);
}
uint64_t ELFWriter::SymbolValue(const MCSymbol &Sym,
const MCAsmLayout &Layout) {
if (Sym.isCommon())
return Sym.getCommonAlignment();
uint64_t Res;
if (!Layout.getSymbolOffset(Sym, Res))
return 0;
if (Layout.getAssembler().isThumbFunc(&Sym))
Res |= 1;
return Res;
}
static uint8_t mergeTypeForSet(uint8_t origType, uint8_t newType) {
uint8_t Type = newType;
// Propagation rules:
// IFUNC > FUNC > OBJECT > NOTYPE
// TLS_OBJECT > OBJECT > NOTYPE
//
// dont let the new type degrade the old type
switch (origType) {
default:
break;
case ELF::STT_GNU_IFUNC:
if (Type == ELF::STT_FUNC || Type == ELF::STT_OBJECT ||
Type == ELF::STT_NOTYPE || Type == ELF::STT_TLS)
Type = ELF::STT_GNU_IFUNC;
break;
case ELF::STT_FUNC:
if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
Type == ELF::STT_TLS)
Type = ELF::STT_FUNC;
break;
case ELF::STT_OBJECT:
if (Type == ELF::STT_NOTYPE)
Type = ELF::STT_OBJECT;
break;
case ELF::STT_TLS:
if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
Type == ELF::STT_GNU_IFUNC || Type == ELF::STT_FUNC)
Type = ELF::STT_TLS;
break;
}
return Type;
}
static bool isIFunc(const MCSymbolELF *Symbol) {
while (Symbol->getType() != ELF::STT_GNU_IFUNC) {
const MCSymbolRefExpr *Value;
if (!Symbol->isVariable() ||
!(Value = dyn_cast<MCSymbolRefExpr>(Symbol->getVariableValue())) ||
Value->getKind() != MCSymbolRefExpr::VK_None ||
mergeTypeForSet(Symbol->getType(), ELF::STT_GNU_IFUNC) != ELF::STT_GNU_IFUNC)
return false;
Symbol = &cast<MCSymbolELF>(Value->getSymbol());
}
return true;
}
void ELFWriter::writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
ELFSymbolData &MSD, const MCAsmLayout &Layout) {
const auto &Symbol = cast<MCSymbolELF>(*MSD.Symbol);
const MCSymbolELF *Base =
cast_or_null<MCSymbolELF>(Layout.getBaseSymbol(Symbol));
// This has to be in sync with when computeSymbolTable uses SHN_ABS or
// SHN_COMMON.
bool IsReserved = !Base || Symbol.isCommon();
// Binding and Type share the same byte as upper and lower nibbles
uint8_t Binding = Symbol.getBinding();
uint8_t Type = Symbol.getType();
if (isIFunc(&Symbol))
Type = ELF::STT_GNU_IFUNC;
if (Base) {
Type = mergeTypeForSet(Type, Base->getType());
}
uint8_t Info = (Binding << 4) | Type;
// Other and Visibility share the same byte with Visibility using the lower
// 2 bits
uint8_t Visibility = Symbol.getVisibility();
uint8_t Other = Symbol.getOther() | Visibility;
uint64_t Value = SymbolValue(*MSD.Symbol, Layout);
uint64_t Size = 0;
const MCExpr *ESize = MSD.Symbol->getSize();
if (!ESize && Base)
ESize = Base->getSize();
if (ESize) {
int64_t Res;
if (!ESize->evaluateKnownAbsolute(Res, Layout))
report_fatal_error("Size expression must be absolute.");
Size = Res;
}
// Write out the symbol table entry
Writer.writeSymbol(StringIndex, Info, Value, Size, Other, MSD.SectionIndex,
IsReserved);
}
bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
bool Used, bool Renamed) {
if (Symbol.isVariable()) {
const MCExpr *Expr = Symbol.getVariableValue();
// Target Expressions that are always inlined do not appear in the symtab
if (const auto *T = dyn_cast<MCTargetExpr>(Expr))
if (T->inlineAssignedExpr())
return false;
if (const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr)) {
if (Ref->getKind() == MCSymbolRefExpr::VK_WEAKREF)
return false;
}
}
if (Used)
return true;
if (Renamed)
return false;
if (Symbol.isVariable() && Symbol.isUndefined()) {
// FIXME: this is here just to diagnose the case of a var = commmon_sym.
Layout.getBaseSymbol(Symbol);
return false;
}
if (Symbol.isTemporary())
return false;
if (Symbol.getType() == ELF::STT_SECTION)
return false;
return true;
}
void ELFWriter::computeSymbolTable(
MCAssembler &Asm, const MCAsmLayout &Layout,
const SectionIndexMapTy &SectionIndexMap, const RevGroupMapTy &RevGroupMap,
SectionOffsetsTy &SectionOffsets) {
MCContext &Ctx = Asm.getContext();
SymbolTableWriter Writer(*this, is64Bit());
// Symbol table
unsigned EntrySize = is64Bit() ? ELF::SYMENTRY_SIZE64 : ELF::SYMENTRY_SIZE32;
MCSectionELF *SymtabSection =
Ctx.getELFSection(".symtab", ELF::SHT_SYMTAB, 0, EntrySize);
SymtabSection->setAlignment(is64Bit() ? Align(8) : Align(4));
SymbolTableIndex = addToSectionTable(SymtabSection);
uint64_t SecStart = align(SymtabSection->getAlignment());
// The first entry is the undefined symbol entry.
Writer.writeSymbol(0, 0, 0, 0, 0, 0, false);
std::vector<ELFSymbolData> LocalSymbolData;
std::vector<ELFSymbolData> ExternalSymbolData;
MutableArrayRef<std::pair<std::string, size_t>> FileNames =
Asm.getFileNames();
for (const std::pair<std::string, size_t> &F : FileNames)
StrTabBuilder.add(F.first);
// Add the data for the symbols.
bool HasLargeSectionIndex = false;
for (auto It : llvm::enumerate(Asm.symbols())) {
const auto &Symbol = cast<MCSymbolELF>(It.value());
bool Used = Symbol.isUsedInReloc();
bool WeakrefUsed = Symbol.isWeakrefUsedInReloc();
bool isSignature = Symbol.isSignature();
if (!isInSymtab(Layout, Symbol, Used || WeakrefUsed || isSignature,
OWriter.Renames.count(&Symbol)))
continue;
if (Symbol.isTemporary() && Symbol.isUndefined()) {
Ctx.reportError(SMLoc(), "Undefined temporary symbol " + Symbol.getName());
continue;
}
ELFSymbolData MSD;
MSD.Symbol = cast<MCSymbolELF>(&Symbol);
MSD.Order = It.index();
bool Local = Symbol.getBinding() == ELF::STB_LOCAL;
assert(Local || !Symbol.isTemporary());
if (Symbol.isAbsolute()) {
MSD.SectionIndex = ELF::SHN_ABS;
} else if (Symbol.isCommon()) {
if (Symbol.isTargetCommon()) {
MSD.SectionIndex = Symbol.getIndex();
} else {
assert(!Local);
MSD.SectionIndex = ELF::SHN_COMMON;
}
} else if (Symbol.isUndefined()) {
if (isSignature && !Used) {
MSD.SectionIndex = RevGroupMap.lookup(&Symbol);
if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
HasLargeSectionIndex = true;
} else {
MSD.SectionIndex = ELF::SHN_UNDEF;
}
} else {
const MCSectionELF &Section =
static_cast<const MCSectionELF &>(Symbol.getSection());
// We may end up with a situation when section symbol is technically
// defined, but should not be. That happens because we explicitly
// pre-create few .debug_* sections to have accessors.
// And if these sections were not really defined in the code, but were
// referenced, we simply error out.
if (!Section.isRegistered()) {
assert(static_cast<const MCSymbolELF &>(Symbol).getType() ==
ELF::STT_SECTION);
Ctx.reportError(SMLoc(),
"Undefined section reference: " + Symbol.getName());
continue;
}
if (Mode == NonDwoOnly && isDwoSection(Section))
continue;
MSD.SectionIndex = SectionIndexMap.lookup(&Section);
assert(MSD.SectionIndex && "Invalid section index!");
if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
HasLargeSectionIndex = true;
}
StringRef Name = Symbol.getName();
// Sections have their own string table
if (Symbol.getType() != ELF::STT_SECTION) {
MSD.Name = Name;
StrTabBuilder.add(Name);
}
if (Local)
LocalSymbolData.push_back(MSD);
else
ExternalSymbolData.push_back(MSD);
}
// This holds the .symtab_shndx section index.
unsigned SymtabShndxSectionIndex = 0;
if (HasLargeSectionIndex) {
MCSectionELF *SymtabShndxSection =
Ctx.getELFSection(".symtab_shndx", ELF::SHT_SYMTAB_SHNDX, 0, 4);
SymtabShndxSectionIndex = addToSectionTable(SymtabShndxSection);
SymtabShndxSection->setAlignment(Align(4));
}
StrTabBuilder.finalize();
// Make the first STT_FILE precede previous local symbols.
unsigned Index = 1;
auto FileNameIt = FileNames.begin();
if (!FileNames.empty())
FileNames[0].second = 0;
for (ELFSymbolData &MSD : LocalSymbolData) {
// Emit STT_FILE symbols before their associated local symbols.
for (; FileNameIt != FileNames.end() && FileNameIt->second <= MSD.Order;
++FileNameIt) {
Writer.writeSymbol(StrTabBuilder.getOffset(FileNameIt->first),
ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
ELF::SHN_ABS, true);
++Index;
}
unsigned StringIndex = MSD.Symbol->getType() == ELF::STT_SECTION
? 0
: StrTabBuilder.getOffset(MSD.Name);
MSD.Symbol->setIndex(Index++);
writeSymbol(Writer, StringIndex, MSD, Layout);
}
for (; FileNameIt != FileNames.end(); ++FileNameIt) {
Writer.writeSymbol(StrTabBuilder.getOffset(FileNameIt->first),
ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
ELF::SHN_ABS, true);
++Index;
}
// Write the symbol table entries.
LastLocalSymbolIndex = Index;
for (ELFSymbolData &MSD : ExternalSymbolData) {
unsigned StringIndex = StrTabBuilder.getOffset(MSD.Name);
MSD.Symbol->setIndex(Index++);
writeSymbol(Writer, StringIndex, MSD, Layout);
assert(MSD.Symbol->getBinding() != ELF::STB_LOCAL);
}
uint64_t SecEnd = W.OS.tell();
SectionOffsets[SymtabSection] = std::make_pair(SecStart, SecEnd);
ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
if (ShndxIndexes.empty()) {
assert(SymtabShndxSectionIndex == 0);
return;
}
assert(SymtabShndxSectionIndex != 0);
SecStart = W.OS.tell();
const MCSectionELF *SymtabShndxSection =
SectionTable[SymtabShndxSectionIndex - 1];
for (uint32_t Index : ShndxIndexes)
write(Index);
SecEnd = W.OS.tell();
SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
}
void ELFWriter::writeAddrsigSection() {
for (const MCSymbol *Sym : OWriter.AddrsigSyms)
encodeULEB128(Sym->getIndex(), W.OS);
}
MCSectionELF *ELFWriter::createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec) {
if (OWriter.Relocations[&Sec].empty())
return nullptr;
const StringRef SectionName = Sec.getName();
std::string RelaSectionName = hasRelocationAddend() ? ".rela" : ".rel";
RelaSectionName += SectionName;
unsigned EntrySize;
if (hasRelocationAddend())
EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rela) : sizeof(ELF::Elf32_Rela);
else
EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rel) : sizeof(ELF::Elf32_Rel);
unsigned Flags = 0;
if (Sec.getFlags() & ELF::SHF_GROUP)
Flags = ELF::SHF_GROUP;
MCSectionELF *RelaSection = Ctx.createELFRelSection(
RelaSectionName, hasRelocationAddend() ? ELF::SHT_RELA : ELF::SHT_REL,
Flags, EntrySize, Sec.getGroup(), &Sec);
RelaSection->setAlignment(is64Bit() ? Align(8) : Align(4));
return RelaSection;
}
// Include the debug info compression header.
bool ELFWriter::maybeWriteCompression(
uint64_t Size, SmallVectorImpl<char> &CompressedContents, bool ZLibStyle,
unsigned Alignment) {
if (ZLibStyle) {
uint64_t HdrSize =
is64Bit() ? sizeof(ELF::Elf32_Chdr) : sizeof(ELF::Elf64_Chdr);
if (Size <= HdrSize + CompressedContents.size())
return false;
// Platform specific header is followed by compressed data.
if (is64Bit()) {
// Write Elf64_Chdr header.
write(static_cast<ELF::Elf64_Word>(ELF::ELFCOMPRESS_ZLIB));
write(static_cast<ELF::Elf64_Word>(0)); // ch_reserved field.
write(static_cast<ELF::Elf64_Xword>(Size));
write(static_cast<ELF::Elf64_Xword>(Alignment));
} else {
// Write Elf32_Chdr header otherwise.
write(static_cast<ELF::Elf32_Word>(ELF::ELFCOMPRESS_ZLIB));
write(static_cast<ELF::Elf32_Word>(Size));
write(static_cast<ELF::Elf32_Word>(Alignment));
}
return true;
}
// "ZLIB" followed by 8 bytes representing the uncompressed size of the section,
// useful for consumers to preallocate a buffer to decompress into.
const StringRef Magic = "ZLIB";
if (Size <= Magic.size() + sizeof(Size) + CompressedContents.size())
return false;
W.OS << Magic;
support::endian::write(W.OS, Size, support::big);
return true;
}
void ELFWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
const MCAsmLayout &Layout) {
MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
StringRef SectionName = Section.getName();
auto &MC = Asm.getContext();
const auto &MAI = MC.getAsmInfo();
// Compressing debug_frame requires handling alignment fragments which is
// more work (possibly generalizing MCAssembler.cpp:writeFragment to allow
// for writing to arbitrary buffers) for little benefit.
bool CompressionEnabled =
MAI->compressDebugSections() != DebugCompressionType::None;
if (!CompressionEnabled || !SectionName.startswith(".debug_") ||
SectionName == ".debug_frame") {
Asm.writeSectionData(W.OS, &Section, Layout);
return;
}
assert((MAI->compressDebugSections() == DebugCompressionType::Z ||
MAI->compressDebugSections() == DebugCompressionType::GNU) &&
"expected zlib or zlib-gnu style compression");
SmallVector<char, 128> UncompressedData;
raw_svector_ostream VecOS(UncompressedData);
Asm.writeSectionData(VecOS, &Section, Layout);
SmallVector<char, 128> CompressedContents;
if (Error E = zlib::compress(
StringRef(UncompressedData.data(), UncompressedData.size()),
CompressedContents)) {
consumeError(std::move(E));
W.OS << UncompressedData;
return;
}
bool ZlibStyle = MAI->compressDebugSections() == DebugCompressionType::Z;
if (!maybeWriteCompression(UncompressedData.size(), CompressedContents,
ZlibStyle, Sec.getAlignment())) {
W.OS << UncompressedData;
return;
}
if (ZlibStyle) {
// Set the compressed flag. That is zlib style.
Section.setFlags(Section.getFlags() | ELF::SHF_COMPRESSED);
// Alignment field should reflect the requirements of
// the compressed section header.
Section.setAlignment(is64Bit() ? Align(8) : Align(4));
} else {
// Add "z" prefix to section name. This is zlib-gnu style.
MC.renameELFSection(&Section, (".z" + SectionName.drop_front(1)).str());
}
W.OS << CompressedContents;
}
void ELFWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
uint64_t Address, uint64_t Offset,
uint64_t Size, uint32_t Link, uint32_t Info,
uint64_t Alignment, uint64_t EntrySize) {
W.write<uint32_t>(Name); // sh_name: index into string table
W.write<uint32_t>(Type); // sh_type
WriteWord(Flags); // sh_flags
WriteWord(Address); // sh_addr
WriteWord(Offset); // sh_offset
WriteWord(Size); // sh_size
W.write<uint32_t>(Link); // sh_link
W.write<uint32_t>(Info); // sh_info
WriteWord(Alignment); // sh_addralign
WriteWord(EntrySize); // sh_entsize
}
void ELFWriter::writeRelocations(const MCAssembler &Asm,
const MCSectionELF &Sec) {
std::vector<ELFRelocationEntry> &Relocs = OWriter.Relocations[&Sec];
// We record relocations by pushing to the end of a vector. Reverse the vector
// to get the relocations in the order they were created.
// In most cases that is not important, but it can be for special sections
// (.eh_frame) or specific relocations (TLS optimizations on SystemZ).
std::reverse(Relocs.begin(), Relocs.end());
// Sort the relocation entries. MIPS needs this.
OWriter.TargetObjectWriter->sortRelocs(Asm, Relocs);
for (unsigned i = 0, e = Relocs.size(); i != e; ++i) {
const ELFRelocationEntry &Entry = Relocs[e - i - 1];
unsigned Index = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
if (is64Bit()) {
write(Entry.Offset);
if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
write(uint32_t(Index));
write(OWriter.TargetObjectWriter->getRSsym(Entry.Type));
write(OWriter.TargetObjectWriter->getRType3(Entry.Type));
write(OWriter.TargetObjectWriter->getRType2(Entry.Type));
write(OWriter.TargetObjectWriter->getRType(Entry.Type));
} else {
struct ELF::Elf64_Rela ERE64;
ERE64.setSymbolAndType(Index, Entry.Type);
write(ERE64.r_info);
}
if (hasRelocationAddend())
write(Entry.Addend);
} else {
write(uint32_t(Entry.Offset));
struct ELF::Elf32_Rela ERE32;
ERE32.setSymbolAndType(Index, Entry.Type);
write(ERE32.r_info);
if (hasRelocationAddend())
write(uint32_t(Entry.Addend));
if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
if (uint32_t RType =
OWriter.TargetObjectWriter->getRType2(Entry.Type)) {
write(uint32_t(Entry.Offset));
ERE32.setSymbolAndType(0, RType);
write(ERE32.r_info);
write(uint32_t(0));
}
if (uint32_t RType =
OWriter.TargetObjectWriter->getRType3(Entry.Type)) {
write(uint32_t(Entry.Offset));
ERE32.setSymbolAndType(0, RType);
write(ERE32.r_info);
write(uint32_t(0));
}
}
}
}
}
void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex, uint64_t Offset,
uint64_t Size, const MCSectionELF &Section) {
uint64_t sh_link = 0;
uint64_t sh_info = 0;
switch(Section.getType()) {
default:
// Nothing to do.
break;
case ELF::SHT_DYNAMIC:
llvm_unreachable("SHT_DYNAMIC in a relocatable object");
case ELF::SHT_REL:
case ELF::SHT_RELA: {
sh_link = SymbolTableIndex;
assert(sh_link && ".symtab not found");
const MCSection *InfoSection = Section.getLinkedToSection();
sh_info = SectionIndexMap.lookup(cast<MCSectionELF>(InfoSection));
break;