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llvm/lib/MC/ELFObjectWriter.cpp
Rafael Espindola aa4466db93 Refactor padding writing into a helper function. 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239174 91177308-0d34-0410-b5e6-96231b3b80d8
2015-06-05 18:21:00 +00:00

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//===- lib/MC/ELFObjectWriter.cpp - ELF File Writer -----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements ELF object file writer information.
//
//===----------------------------------------------------------------------===//
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringMap.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/MCExpr.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCSymbolELF.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ELF.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/ErrorHandling.h"
#include <vector>
using namespace llvm;
#undef DEBUG_TYPE
#define DEBUG_TYPE "reloc-info"
namespace {
typedef DenseMap<const MCSectionELF *, uint32_t> SectionIndexMapTy;
class ELFObjectWriter;
class SymbolTableWriter {
ELFObjectWriter &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(ELFObjectWriter &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; }
};
class ELFObjectWriter : public MCObjectWriter {
static bool isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind);
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;
uint32_t SectionIndex;
StringRef Name;
// Support lexicographic sorting.
bool operator<(const ELFSymbolData &RHS) const {
unsigned LHSType = Symbol->getType();
unsigned RHSType = RHS.Symbol->getType();
if (LHSType == ELF::STT_SECTION && RHSType != ELF::STT_SECTION)
return false;
if (LHSType != ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
return true;
if (LHSType == ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
return SectionIndex < RHS.SectionIndex;
return Name < RHS.Name;
}
};
/// The target specific ELF writer instance.
std::unique_ptr<MCELFObjectTargetWriter> TargetObjectWriter;
DenseMap<const MCSymbolELF *, const MCSymbolELF *> Renames;
llvm::DenseMap<const MCSectionELF *, std::vector<ELFRelocationEntry>>
Relocations;
/// @}
/// @name Symbol Table Data
/// @{
StringTableBuilder StrTabBuilder;
/// @}
// 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;
// This holds the .symtab_shndx section index.
unsigned SymtabShndxSectionIndex = 0;
// 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 { return TargetObjectWriter->is64Bit(); }
bool hasRelocationAddend() const {
return TargetObjectWriter->hasRelocationAddend();
}
unsigned GetRelocType(const MCValue &Target, const MCFixup &Fixup,
bool IsPCRel) const {
return TargetObjectWriter->GetRelocType(Target, Fixup, IsPCRel);
}
void align(unsigned Alignment);
public:
ELFObjectWriter(MCELFObjectTargetWriter *MOTW, raw_pwrite_stream &OS,
bool IsLittleEndian)
: MCObjectWriter(OS, IsLittleEndian), TargetObjectWriter(MOTW) {}
void reset() override {
Renames.clear();
Relocations.clear();
StrTabBuilder.clear();
SectionTable.clear();
MCObjectWriter::reset();
}
~ELFObjectWriter() override;
void WriteWord(uint64_t W) {
if (is64Bit())
write64(W);
else
write32(W);
}
template <typename T> void write(T Val) {
if (IsLittleEndian)
support::endian::Writer<support::little>(OS).write(Val);
else
support::endian::Writer<support::big>(OS).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
typedef std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>
SectionOffsetsTy;
bool shouldRelocateWithSymbol(const MCAssembler &Asm,
const MCSymbolRefExpr *RefA,
const MCSymbol *Sym, uint64_t C,
unsigned Type) const;
void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup,
MCValue Target, bool &IsPCRel,
uint64_t &FixedValue) override;
// Map from a signature symbol to the group section index
typedef DenseMap<const MCSymbol *, unsigned> RevGroupMapTy;
/// 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);
MCSectionELF *createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec);
const MCSectionELF *createStringTable(MCContext &Ctx);
void executePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) override;
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);
bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
const MCSymbol &SymA,
const MCFragment &FB,
bool InSet,
bool IsPCRel) const override;
bool isWeak(const MCSymbol &Sym) const override;
void writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
void writeSection(const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex, uint64_t Offset, uint64_t Size,
const MCSectionELF &Section);
};
}
void ELFObjectWriter::align(unsigned Alignment) {
uint64_t Padding = OffsetToAlignment(OS.tell(), Alignment);
WriteZeros(Padding);
}
unsigned ELFObjectWriter::addToSectionTable(const MCSectionELF *Sec) {
SectionTable.push_back(Sec);
StrTabBuilder.add(Sec->getSectionName());
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(ELFObjectWriter &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 ELFObjectWriter::isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind) {
const MCFixupKindInfo &FKI =
Asm.getBackend().getFixupKindInfo((MCFixupKind) Kind);
return FKI.Flags & MCFixupKindInfo::FKF_IsPCRel;
}
ELFObjectWriter::~ELFObjectWriter()
{}
// Emit the ELF header.
void ELFObjectWriter::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.
writeBytes(ELF::ElfMagic); // e_ident[EI_MAG0] to e_ident[EI_MAG3]
write8(is64Bit() ? ELF::ELFCLASS64 : ELF::ELFCLASS32); // e_ident[EI_CLASS]
// e_ident[EI_DATA]
write8(isLittleEndian() ? ELF::ELFDATA2LSB : ELF::ELFDATA2MSB);
write8(ELF::EV_CURRENT); // e_ident[EI_VERSION]
// e_ident[EI_OSABI]
write8(TargetObjectWriter->getOSABI());
write8(0); // e_ident[EI_ABIVERSION]
WriteZeros(ELF::EI_NIDENT - ELF::EI_PAD);
write16(ELF::ET_REL); // e_type
write16(TargetObjectWriter->getEMachine()); // e_machine = target
write32(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
write32(Asm.getELFHeaderEFlags());
// e_ehsize = ELF header size
write16(is64Bit() ? sizeof(ELF::Elf64_Ehdr) : sizeof(ELF::Elf32_Ehdr));
write16(0); // e_phentsize = prog header entry size
write16(0); // e_phnum = # prog header entries = 0
// e_shentsize = Section header entry size
write16(is64Bit() ? sizeof(ELF::Elf64_Shdr) : sizeof(ELF::Elf32_Shdr));
// e_shnum = # of section header ents
write16(0);
// e_shstrndx = Section # of '.shstrtab'
assert(StringTableIndex < ELF::SHN_LORESERVE);
write16(StringTableIndex);
}
uint64_t ELFObjectWriter::SymbolValue(const MCSymbol &Sym,
const MCAsmLayout &Layout) {
if (Sym.isCommon() && Sym.isExternal())
return Sym.getCommonAlignment();
uint64_t Res;
if (!Layout.getSymbolOffset(Sym, Res))
return 0;
if (Layout.getAssembler().isThumbFunc(&Sym))
Res |= 1;
return Res;
}
void ELFObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) {
// The presence of symbol versions causes undefined symbols and
// versions declared with @@@ to be renamed.
for (const MCSymbol &A : Asm.symbols()) {
const auto &Alias = cast<MCSymbolELF>(A);
// Not an alias.
if (!Alias.isVariable())
continue;
auto *Ref = dyn_cast<MCSymbolRefExpr>(Alias.getVariableValue());
if (!Ref)
continue;
const auto &Symbol = cast<MCSymbolELF>(Ref->getSymbol());
StringRef AliasName = Alias.getName();
size_t Pos = AliasName.find('@');
if (Pos == StringRef::npos)
continue;
// Aliases defined with .symvar copy the binding from the symbol they alias.
// This is the first place we are able to copy this information.
Alias.setExternal(Symbol.isExternal());
Alias.setBinding(Symbol.getBinding());
StringRef Rest = AliasName.substr(Pos);
if (!Symbol.isUndefined() && !Rest.startswith("@@@"))
continue;
// FIXME: produce a better error message.
if (Symbol.isUndefined() && Rest.startswith("@@") &&
!Rest.startswith("@@@"))
report_fatal_error("A @@ version cannot be undefined");
Renames.insert(std::make_pair(&Symbol, &Alias));
}
}
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;
}
void ELFObjectWriter::writeSymbol(SymbolTableWriter &Writer,
uint32_t StringIndex, ELFSymbolData &MSD,
const MCAsmLayout &Layout) {
const auto &Symbol = cast<MCSymbolELF>(*MSD.Symbol);
assert((!Symbol.getFragment() ||
(Symbol.getFragment()->getParent() == &Symbol.getSection())) &&
"The symbol's section doesn't match the fragment's 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 (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);
}
// It is always valid to create a relocation with a symbol. It is preferable
// to use a relocation with a section if that is possible. Using the section
// allows us to omit some local symbols from the symbol table.
bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
const MCSymbolRefExpr *RefA,
const MCSymbol *S, uint64_t C,
unsigned Type) const {
const auto *Sym = cast_or_null<MCSymbolELF>(S);
// A PCRel relocation to an absolute value has no symbol (or section). We
// represent that with a relocation to a null section.
if (!RefA)
return false;
MCSymbolRefExpr::VariantKind Kind = RefA->getKind();
switch (Kind) {
default:
break;
// The .odp creation emits a relocation against the symbol ".TOC." which
// create a R_PPC64_TOC relocation. However the relocation symbol name
// in final object creation should be NULL, since the symbol does not
// really exist, it is just the reference to TOC base for the current
// object file. Since the symbol is undefined, returning false results
// in a relocation with a null section which is the desired result.
case MCSymbolRefExpr::VK_PPC_TOCBASE:
return false;
// These VariantKind cause the relocation to refer to something other than
// the symbol itself, like a linker generated table. Since the address of
// symbol is not relevant, we cannot replace the symbol with the
// section and patch the difference in the addend.
case MCSymbolRefExpr::VK_GOT:
case MCSymbolRefExpr::VK_PLT:
case MCSymbolRefExpr::VK_GOTPCREL:
case MCSymbolRefExpr::VK_Mips_GOT:
case MCSymbolRefExpr::VK_PPC_GOT_LO:
case MCSymbolRefExpr::VK_PPC_GOT_HI:
case MCSymbolRefExpr::VK_PPC_GOT_HA:
return true;
}
// An undefined symbol is not in any section, so the relocation has to point
// to the symbol itself.
assert(Sym && "Expected a symbol");
if (Sym->isUndefined())
return true;
unsigned Binding = Sym->getBinding();
switch(Binding) {
default:
llvm_unreachable("Invalid Binding");
case ELF::STB_LOCAL:
break;
case ELF::STB_WEAK:
// If the symbol is weak, it might be overridden by a symbol in another
// file. The relocation has to point to the symbol so that the linker
// can update it.
return true;
case ELF::STB_GLOBAL:
// Global ELF symbols can be preempted by the dynamic linker. The relocation
// has to point to the symbol for a reason analogous to the STB_WEAK case.
return true;
}
// If a relocation points to a mergeable section, we have to be careful.
// If the offset is zero, a relocation with the section will encode the
// same information. With a non-zero offset, the situation is different.
// For example, a relocation can point 42 bytes past the end of a string.
// If we change such a relocation to use the section, the linker would think
// that it pointed to another string and subtracting 42 at runtime will
// produce the wrong value.
auto &Sec = cast<MCSectionELF>(Sym->getSection());
unsigned Flags = Sec.getFlags();
if (Flags & ELF::SHF_MERGE) {
if (C != 0)
return true;
// It looks like gold has a bug (http://sourceware.org/PR16794) and can
// only handle section relocations to mergeable sections if using RELA.
if (!hasRelocationAddend())
return true;
}
// Most TLS relocations use a got, so they need the symbol. Even those that
// are just an offset (@tpoff), require a symbol in gold versions before
// 5efeedf61e4fe720fd3e9a08e6c91c10abb66d42 (2014-09-26) which fixed
// http://sourceware.org/PR16773.
if (Flags & ELF::SHF_TLS)
return true;
// If the symbol is a thumb function the final relocation must set the lowest
// bit. With a symbol that is done by just having the symbol have that bit
// set, so we would lose the bit if we relocated with the section.
// FIXME: We could use the section but add the bit to the relocation value.
if (Asm.isThumbFunc(Sym))
return true;
if (TargetObjectWriter->needsRelocateWithSymbol(*Sym, Type))
return true;
return false;
}
// True if the assembler knows nothing about the final value of the symbol.
// This doesn't cover the comdat issues, since in those cases the assembler
// can at least know that all symbols in the section will move together.
static bool isWeak(const MCSymbolELF &Sym) {
if (Sym.getType() == ELF::STT_GNU_IFUNC)
return true;
switch (Sym.getBinding()) {
default:
llvm_unreachable("Unknown binding");
case ELF::STB_LOCAL:
return false;
case ELF::STB_GLOBAL:
return false;
case ELF::STB_WEAK:
case ELF::STB_GNU_UNIQUE:
return true;
}
}
void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup, MCValue Target,
bool &IsPCRel, uint64_t &FixedValue) {
const MCSectionELF &FixupSection = cast<MCSectionELF>(*Fragment->getParent());
uint64_t C = Target.getConstant();
uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
"Should not have constructed this");
// Let A, B and C being the components of Target and R be the location of
// the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
// If it is pcrel, we want to compute (A - B + C - R).
// In general, ELF has no relocations for -B. It can only represent (A + C)
// or (A + C - R). If B = R + K and the relocation is not pcrel, we can
// replace B to implement it: (A - R - K + C)
if (IsPCRel)
Asm.getContext().reportFatalError(
Fixup.getLoc(),
"No relocation available to represent this relative expression");
const auto &SymB = cast<MCSymbolELF>(RefB->getSymbol());
if (SymB.isUndefined())
Asm.getContext().reportFatalError(
Fixup.getLoc(),
Twine("symbol '") + SymB.getName() +
"' can not be undefined in a subtraction expression");
assert(!SymB.isAbsolute() && "Should have been folded");
const MCSection &SecB = SymB.getSection();
if (&SecB != &FixupSection)
Asm.getContext().reportFatalError(
Fixup.getLoc(), "Cannot represent a difference across sections");
if (::isWeak(SymB))
Asm.getContext().reportFatalError(
Fixup.getLoc(), "Cannot represent a subtraction with a weak symbol");
uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
uint64_t K = SymBOffset - FixupOffset;
IsPCRel = true;
C -= K;
}
// We either rejected the fixup or folded B into C at this point.
const MCSymbolRefExpr *RefA = Target.getSymA();
const auto *SymA = RefA ? cast<MCSymbolELF>(&RefA->getSymbol()) : nullptr;
bool ViaWeakRef = false;
if (SymA && SymA->isVariable()) {
const MCExpr *Expr = SymA->getVariableValue();
if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF) {
SymA = cast<MCSymbolELF>(&Inner->getSymbol());
ViaWeakRef = true;
}
}
}
unsigned Type = GetRelocType(Target, Fixup, IsPCRel);
bool RelocateWithSymbol = shouldRelocateWithSymbol(Asm, RefA, SymA, C, Type);
if (!RelocateWithSymbol && SymA && !SymA->isUndefined())
C += Layout.getSymbolOffset(*SymA);
uint64_t Addend = 0;
if (hasRelocationAddend()) {
Addend = C;
C = 0;
}
FixedValue = C;
if (!RelocateWithSymbol) {
const MCSection *SecA =
(SymA && !SymA->isUndefined()) ? &SymA->getSection() : nullptr;
auto *ELFSec = cast_or_null<MCSectionELF>(SecA);
const auto *SectionSymbol =
ELFSec ? cast<MCSymbolELF>(ELFSec->getBeginSymbol()) : nullptr;
if (SectionSymbol)
SectionSymbol->setUsedInReloc();
ELFRelocationEntry Rec(FixupOffset, SectionSymbol, Type, Addend);
Relocations[&FixupSection].push_back(Rec);
return;
}
if (SymA) {
if (const MCSymbolELF *R = Renames.lookup(SymA))
SymA = R;
if (ViaWeakRef)
SymA->setIsWeakrefUsedInReloc();
else
SymA->setUsedInReloc();
}
ELFRelocationEntry Rec(FixupOffset, SymA, Type, Addend);
Relocations[&FixupSection].push_back(Rec);
return;
}
bool ELFObjectWriter::isInSymtab(const MCAsmLayout &Layout,
const MCSymbolELF &Symbol, bool Used,
bool Renamed) {
if (Symbol.isVariable()) {
const MCExpr *Expr = Symbol.getVariableValue();
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.isUndefined() && !Symbol.isBindingSet())
return false;
if (Symbol.isTemporary())
return false;
if (Symbol.getType() == ELF::STT_SECTION)
return false;
return true;
}
void ELFObjectWriter::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() ? 8 : 4);
SymbolTableIndex = addToSectionTable(SymtabSection);
align(SymtabSection->getAlignment());
uint64_t SecStart = OS.tell();
// 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;
// Add the data for the symbols.
bool HasLargeSectionIndex = false;
for (const MCSymbol &S : Asm.symbols()) {
const auto &Symbol = cast<MCSymbolELF>(S);
bool Used = Symbol.isUsedInReloc();
bool WeakrefUsed = Symbol.isWeakrefUsedInReloc();
bool isSignature = Symbol.isSignature();
if (!isInSymtab(Layout, Symbol, Used || WeakrefUsed || isSignature,
Renames.count(&Symbol)))
continue;
ELFSymbolData MSD;
MSD.Symbol = cast<MCSymbolELF>(&Symbol);
bool Local = Symbol.getBinding() == ELF::STB_LOCAL;
if (Symbol.isAbsolute()) {
MSD.SectionIndex = ELF::SHN_ABS;
} else if (Symbol.isCommon()) {
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());
MSD.SectionIndex = SectionIndexMap.lookup(&Section);
assert(MSD.SectionIndex && "Invalid section index!");
if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
HasLargeSectionIndex = true;
}
// The @@@ in symbol version is replaced with @ in undefined symbols and @@
// in defined ones.
//
// FIXME: All name handling should be done before we get to the writer,
// including dealing with GNU-style version suffixes. Fixing this isn't
// trivial.
//
// We thus have to be careful to not perform the symbol version replacement
// blindly:
//
// The ELF format is used on Windows by the MCJIT engine. Thus, on
// Windows, the ELFObjectWriter can encounter symbols mangled using the MS
// Visual Studio C++ name mangling scheme. Symbols mangled using the MSVC
// C++ name mangling can legally have "@@@" as a sub-string. In that case,
// the EFLObjectWriter should not interpret the "@@@" sub-string as
// specifying GNU-style symbol versioning. The ELFObjectWriter therefore
// checks for the MSVC C++ name mangling prefix which is either "?", "@?",
// "__imp_?" or "__imp_@?".
//
// It would have been interesting to perform the MS mangling prefix check
// only when the target triple is of the form *-pc-windows-elf. But, it
// seems that this information is not easily accessible from the
// ELFObjectWriter.
StringRef Name = Symbol.getName();
if (!Name.startswith("?") && !Name.startswith("@?") &&
!Name.startswith("__imp_?") && !Name.startswith("__imp_@?")) {
// This symbol isn't following the MSVC C++ name mangling convention. We
// can thus safely interpret the @@@ in symbol names as specifying symbol
// versioning.
SmallString<32> Buf;
size_t Pos = Name.find("@@@");
if (Pos != StringRef::npos) {
Buf += Name.substr(0, Pos);
unsigned Skip = MSD.SectionIndex == ELF::SHN_UNDEF ? 2 : 1;
Buf += Name.substr(Pos + Skip);
Name = Buf;
}
}
// Sections have their own string table
if (Symbol.getType() != ELF::STT_SECTION)
MSD.Name = StrTabBuilder.add(Name);
if (Local)
LocalSymbolData.push_back(MSD);
else
ExternalSymbolData.push_back(MSD);
}
if (HasLargeSectionIndex) {
MCSectionELF *SymtabShndxSection =
Ctx.getELFSection(".symtab_shndxr", ELF::SHT_SYMTAB_SHNDX, 0, 4, "");
SymtabShndxSectionIndex = addToSectionTable(SymtabShndxSection);
SymtabShndxSection->setAlignment(4);
}
ArrayRef<std::string> FileNames = Asm.getFileNames();
for (const std::string &Name : FileNames)
StrTabBuilder.add(Name);
StrTabBuilder.finalize(StringTableBuilder::ELF);
for (const std::string &Name : FileNames)
Writer.writeSymbol(StrTabBuilder.getOffset(Name),
ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
ELF::SHN_ABS, true);
// Symbols are required to be in lexicographic order.
array_pod_sort(LocalSymbolData.begin(), LocalSymbolData.end());
array_pod_sort(ExternalSymbolData.begin(), ExternalSymbolData.end());
// Set the symbol indices. Local symbols must come before all other
// symbols with non-local bindings.
unsigned Index = FileNames.size() + 1;
for (ELFSymbolData &MSD : LocalSymbolData) {
unsigned StringIndex = MSD.Symbol->getType() == ELF::STT_SECTION
? 0
: StrTabBuilder.getOffset(MSD.Name);
MSD.Symbol->setIndex(Index++);
writeSymbol(Writer, StringIndex, MSD, Layout);
}
// 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 = 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 = OS.tell();
const MCSectionELF *SymtabShndxSection =
SectionTable[SymtabShndxSectionIndex - 1];
for (uint32_t Index : ShndxIndexes)
write(Index);
SecEnd = OS.tell();
SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
}
MCSectionELF *
ELFObjectWriter::createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec) {
if (Relocations[&Sec].empty())
return nullptr;
const StringRef SectionName = Sec.getSectionName();
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() ? 8 : 4);
return RelaSection;
}
static SmallVector<char, 128>
getUncompressedData(const MCAsmLayout &Layout,
const MCSection::FragmentListType &Fragments) {
SmallVector<char, 128> UncompressedData;
for (const MCFragment &F : Fragments) {
const SmallVectorImpl<char> *Contents;
switch (F.getKind()) {
case MCFragment::FT_Data:
Contents = &cast<MCDataFragment>(F).getContents();
break;
case MCFragment::FT_Dwarf:
Contents = &cast<MCDwarfLineAddrFragment>(F).getContents();
break;
case MCFragment::FT_DwarfFrame:
Contents = &cast<MCDwarfCallFrameFragment>(F).getContents();
break;
default:
llvm_unreachable(
"Not expecting any other fragment types in a debug_* section");
}
UncompressedData.append(Contents->begin(), Contents->end());
}
return UncompressedData;
}
// Include the debug info compression header:
// "ZLIB" followed by 8 bytes representing the uncompressed size of the section,
// useful for consumers to preallocate a buffer to decompress into.
static bool
prependCompressionHeader(uint64_t Size,
SmallVectorImpl<char> &CompressedContents) {
const StringRef Magic = "ZLIB";
if (Size <= Magic.size() + sizeof(Size) + CompressedContents.size())
return false;
if (sys::IsLittleEndianHost)
sys::swapByteOrder(Size);
CompressedContents.insert(CompressedContents.begin(),
Magic.size() + sizeof(Size), 0);
std::copy(Magic.begin(), Magic.end(), CompressedContents.begin());
std::copy(reinterpret_cast<char *>(&Size),
reinterpret_cast<char *>(&Size + 1),
CompressedContents.begin() + Magic.size());
return true;
}
void ELFObjectWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
const MCAsmLayout &Layout) {
MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
StringRef SectionName = Section.getSectionName();
// 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.
if (!Asm.getContext().getAsmInfo()->compressDebugSections() ||
!SectionName.startswith(".debug_") || SectionName == ".debug_frame") {
Asm.writeSectionData(&Section, Layout);
return;
}
// Gather the uncompressed data from all the fragments.
const MCSection::FragmentListType &Fragments = Section.getFragmentList();
SmallVector<char, 128> UncompressedData =
getUncompressedData(Layout, Fragments);
SmallVector<char, 128> CompressedContents;
zlib::Status Success = zlib::compress(
StringRef(UncompressedData.data(), UncompressedData.size()),
CompressedContents);
if (Success != zlib::StatusOK) {
Asm.writeSectionData(&Section, Layout);
return;
}
if (!prependCompressionHeader(UncompressedData.size(), CompressedContents)) {
Asm.writeSectionData(&Section, Layout);
return;
}
Asm.getContext().renameELFSection(&Section,
(".z" + SectionName.drop_front(1)).str());
OS << CompressedContents;
}
void ELFObjectWriter::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) {
write32(Name); // sh_name: index into string table
write32(Type); // sh_type
WriteWord(Flags); // sh_flags
WriteWord(Address); // sh_addr
WriteWord(Offset); // sh_offset
WriteWord(Size); // sh_size
write32(Link); // sh_link
write32(Info); // sh_info
WriteWord(Alignment); // sh_addralign
WriteWord(EntrySize); // sh_entsize
}
void ELFObjectWriter::writeRelocations(const MCAssembler &Asm,
const MCSectionELF &Sec) {
std::vector<ELFRelocationEntry> &Relocs = Relocations[&Sec];
// Sort the relocation entries. Most targets just sort by Offset, but some
// (e.g., MIPS) have additional constraints.
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 (TargetObjectWriter->isN64()) {
write(uint32_t(Index));
write(TargetObjectWriter->getRSsym(Entry.Type));
write(TargetObjectWriter->getRType3(Entry.Type));
write(TargetObjectWriter->getRType2(Entry.Type));
write(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));
}
}
}
const MCSectionELF *ELFObjectWriter::createStringTable(MCContext &Ctx) {
const MCSectionELF *StrtabSection = SectionTable[StringTableIndex - 1];
OS << StrTabBuilder.data();
return StrtabSection;
}
void ELFObjectWriter::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 MCSectionELF *InfoSection = Section.getAssociatedSection();
sh_info = SectionIndexMap.lookup(InfoSection);
break;
}
case ELF::SHT_SYMTAB:
case ELF::SHT_DYNSYM:
sh_link = StringTableIndex;
sh_info = LastLocalSymbolIndex;
break;
case ELF::SHT_SYMTAB_SHNDX:
sh_link = SymbolTableIndex;
break;
case ELF::SHT_GROUP:
sh_link = SymbolTableIndex;
sh_info = GroupSymbolIndex;
break;
}
if (TargetObjectWriter->getEMachine() == ELF::EM_ARM &&
Section.getType() == ELF::SHT_ARM_EXIDX)
sh_link = SectionIndexMap.lookup(Section.getAssociatedSection());
WriteSecHdrEntry(StrTabBuilder.getOffset(Section.getSectionName()),
Section.getType(), Section.getFlags(), 0, Offset, Size,
sh_link, sh_info, Section.getAlignment(),
Section.getEntrySize());
}
void ELFObjectWriter::writeSectionHeader(
const MCAsmLayout &Layout, const SectionIndexMapTy &SectionIndexMap,
const SectionOffsetsTy &SectionOffsets) {
const unsigned NumSections = SectionTable.size();
// Null section first.
uint64_t FirstSectionSize =
(NumSections + 1) >= ELF::SHN_LORESERVE ? NumSections + 1 : 0;
WriteSecHdrEntry(0, 0, 0, 0, 0, FirstSectionSize, 0, 0, 0, 0);
for (const MCSectionELF *Section : SectionTable) {
uint32_t GroupSymbolIndex;
unsigned Type = Section->getType();
if (Type != ELF::SHT_GROUP)
GroupSymbolIndex = 0;
else
GroupSymbolIndex = Section->getGroup()->getIndex();
const std::pair<uint64_t, uint64_t> &Offsets =
SectionOffsets.find(Section)->second;
uint64_t Size;
if (Type == ELF::SHT_NOBITS)
Size = Layout.getSectionAddressSize(Section);
else
Size = Offsets.second - Offsets.first;
writeSection(SectionIndexMap, GroupSymbolIndex, Offsets.first, Size,
*Section);
}
}
void ELFObjectWriter::writeObject(MCAssembler &Asm,
const MCAsmLayout &Layout) {
MCContext &Ctx = Asm.getContext();
MCSectionELF *StrtabSection =
Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
StringTableIndex = addToSectionTable(StrtabSection);
RevGroupMapTy RevGroupMap;
SectionIndexMapTy SectionIndexMap;
std::map<const MCSymbol *, std::vector<const MCSectionELF *>> GroupMembers;
// Write out the ELF header ...
writeHeader(Asm);
// ... then the sections ...
SectionOffsetsTy SectionOffsets;
std::vector<MCSectionELF *> Groups;
std::vector<MCSectionELF *> Relocations;
for (MCSection &Sec : Asm) {
MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
align(Section.getAlignment());
// Remember the offset into the file for this section.
uint64_t SecStart = OS.tell();
const MCSymbolELF *SignatureSymbol = Section.getGroup();
writeSectionData(Asm, Section, Layout);
uint64_t SecEnd = OS.tell();
SectionOffsets[&Section] = std::make_pair(SecStart, SecEnd);
MCSectionELF *RelSection = createRelocationSection(Ctx, Section);
if (SignatureSymbol) {
Asm.registerSymbol(*SignatureSymbol);
unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
if (!GroupIdx) {
MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
GroupIdx = addToSectionTable(Group);
Group->setAlignment(4);
Groups.push_back(Group);
}
std::vector<const MCSectionELF *> &Members =
GroupMembers[SignatureSymbol];
Members.push_back(&Section);
if (RelSection)
Members.push_back(RelSection);
}
SectionIndexMap[&Section] = addToSectionTable(&Section);
if (RelSection) {
SectionIndexMap[RelSection] = addToSectionTable(RelSection);
Relocations.push_back(RelSection);
}
}
for (MCSectionELF *Group : Groups) {
align(Group->getAlignment());
// Remember the offset into the file for this section.
uint64_t SecStart = OS.tell();
const MCSymbol *SignatureSymbol = Group->getGroup();
assert(SignatureSymbol);
write(uint32_t(ELF::GRP_COMDAT));
for (const MCSectionELF *Member : GroupMembers[SignatureSymbol]) {
uint32_t SecIndex = SectionIndexMap.lookup(Member);
write(SecIndex);
}
uint64_t SecEnd = OS.tell();
SectionOffsets[Group] = std::make_pair(SecStart, SecEnd);
}
// Compute symbol table information.
computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap, SectionOffsets);
for (MCSectionELF *RelSection : Relocations) {
align(RelSection->getAlignment());
// Remember the offset into the file for this section.
uint64_t SecStart = OS.tell();
writeRelocations(Asm, *RelSection->getAssociatedSection());
uint64_t SecEnd = OS.tell();
SectionOffsets[RelSection] = std::make_pair(SecStart, SecEnd);
}
{
uint64_t SecStart = OS.tell();
const MCSectionELF *Sec = createStringTable(Ctx);
uint64_t SecEnd = OS.tell();
SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
}
uint64_t NaturalAlignment = is64Bit() ? 8 : 4;
align(NaturalAlignment);
const unsigned SectionHeaderOffset = OS.tell();
// ... then the section header table ...
writeSectionHeader(Layout, SectionIndexMap, SectionOffsets);
uint16_t NumSections = (SectionTable.size() + 1 >= ELF::SHN_LORESERVE)
? (uint16_t)ELF::SHN_UNDEF
: SectionTable.size() + 1;
if (sys::IsLittleEndianHost != IsLittleEndian)
sys::swapByteOrder(NumSections);
unsigned NumSectionsOffset;
if (is64Bit()) {
uint64_t Val = SectionHeaderOffset;
if (sys::IsLittleEndianHost != IsLittleEndian)
sys::swapByteOrder(Val);
OS.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
offsetof(ELF::Elf64_Ehdr, e_shoff));
NumSectionsOffset = offsetof(ELF::Elf64_Ehdr, e_shnum);
} else {
uint32_t Val = SectionHeaderOffset;
if (sys::IsLittleEndianHost != IsLittleEndian)
sys::swapByteOrder(Val);
OS.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
offsetof(ELF::Elf32_Ehdr, e_shoff));
NumSectionsOffset = offsetof(ELF::Elf32_Ehdr, e_shnum);
}
OS.pwrite(reinterpret_cast<char *>(&NumSections), sizeof(NumSections),
NumSectionsOffset);
}
bool ELFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
const MCAssembler &Asm, const MCSymbol &SA, const MCFragment &FB,
bool InSet, bool IsPCRel) const {
const auto &SymA = cast<MCSymbolELF>(SA);
if (IsPCRel) {
assert(!InSet);
if (::isWeak(SymA))
return false;
}
return MCObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(Asm, SymA, FB,
InSet, IsPCRel);
}
bool ELFObjectWriter::isWeak(const MCSymbol &S) const {
const auto &Sym = cast<MCSymbolELF>(S);
if (::isWeak(Sym))
return true;
// It is invalid to replace a reference to a global in a comdat
// with a reference to a local since out of comdat references
// to a local are forbidden.
// We could try to return false for more cases, like the reference
// being in the same comdat or Sym being an alias to another global,
// but it is not clear if it is worth the effort.
if (Sym.getBinding() != ELF::STB_GLOBAL)
return false;
if (!Sym.isInSection())
return false;
const auto &Sec = cast<MCSectionELF>(Sym.getSection());
return Sec.getGroup();
}
MCObjectWriter *llvm::createELFObjectWriter(MCELFObjectTargetWriter *MOTW,
raw_pwrite_stream &OS,
bool IsLittleEndian) {
return new ELFObjectWriter(MOTW, OS, IsLittleEndian);
}
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