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76e71bd66e
llvm-6502/lib/MC/ELFObjectWriter.cpp
Rafael Espindola 76e71bd66e Write sections mostly in one pass. 
During ELF writing, there is no need to further relax the sections, so we
should not be creating fragments. This patch avoids doing so in all cases
but debug section compression (that is next).

Also, the ELF format is fairly simple to write. We can do a single pass over
the sections to write them out and compute the section header table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236235 91177308-0d34-0410-b5e6-96231b3b80d8
2015-04-30 14:21:49 +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/MCELF.h"
#include "llvm/MC/MCELFSymbolFlags.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSectionELF.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 {
protected:
static bool isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind);
static bool RelocNeedsGOT(MCSymbolRefExpr::VariantKind Variant);
static uint64_t SymbolValue(MCSymbolData &Data, const MCAsmLayout &Layout);
static bool isInSymtab(const MCAsmLayout &Layout, const MCSymbolData &Data,
bool Used, bool Renamed);
static bool isLocal(const MCSymbolData &Data, bool isUsedInReloc);
/// Helper struct for containing some precomputed information on symbols.
struct ELFSymbolData {
MCSymbolData *SymbolData;
uint64_t StringIndex;
uint32_t SectionIndex;
StringRef Name;
// Support lexicographic sorting.
bool operator<(const ELFSymbolData &RHS) const {
unsigned LHSType = MCELF::GetType(*SymbolData);
unsigned RHSType = MCELF::GetType(*RHS.SymbolData);
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;
SmallPtrSet<const MCSymbol *, 16> UsedInReloc;
SmallPtrSet<const MCSymbol *, 16> WeakrefUsedInReloc;
DenseMap<const MCSymbol *, const MCSymbol *> Renames;
llvm::DenseMap<const MCSectionELF *, std::vector<ELFRelocationEntry>>
Relocations;
StringTableBuilder ShStrTabBuilder;
/// @}
/// @name Symbol Table Data
/// @{
StringTableBuilder StrTabBuilder;
std::vector<uint64_t> FileSymbolData;
std::vector<ELFSymbolData> LocalSymbolData;
std::vector<ELFSymbolData> ExternalSymbolData;
std::vector<ELFSymbolData> UndefinedSymbolData;
/// @}
bool NeedsGOT;
// 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;
unsigned ShstrtabIndex;
// 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);
}
public:
ELFObjectWriter(MCELFObjectTargetWriter *MOTW, raw_pwrite_stream &OS,
bool IsLittleEndian)
: MCObjectWriter(OS, IsLittleEndian), TargetObjectWriter(MOTW),
NeedsGOT(false) {}
void reset() override {
UsedInReloc.clear();
WeakrefUsedInReloc.clear();
Renames.clear();
Relocations.clear();
ShStrTabBuilder.clear();
StrTabBuilder.clear();
FileSymbolData.clear();
LocalSymbolData.clear();
ExternalSymbolData.clear();
UndefinedSymbolData.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);
}
template <typename T> void write(MCDataFragment &F, T Value);
void writeHeader(const MCAssembler &Asm);
void WriteSymbol(SymbolTableWriter &Writer, ELFSymbolData &MSD,
const MCAsmLayout &Layout);
// Start and end offset of each section
typedef std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>
SectionOffsetsTy;
void WriteSymbolTable(MCAssembler &Asm, const MCAsmLayout &Layout,
std::vector<const MCSectionELF *> &Sections,
SectionOffsetsTy &SectionOffsets);
bool shouldRelocateWithSymbol(const MCAssembler &Asm,
const MCSymbolRefExpr *RefA,
const MCSymbolData *SD, 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;
uint64_t getSymbolIndexInSymbolTable(const MCAssembler &Asm,
const MCSymbol *S);
// 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);
const MCSectionELF *createRelocationSection(MCAssembler &Asm,
const MCSectionELF &Sec);
void CompressDebugSections(MCAssembler &Asm, MCAsmLayout &Layout);
const MCSectionELF *
createSectionHeaderStringTable(MCAssembler &Asm,
std::vector<const MCSectionELF *> &Sections);
const MCSectionELF *
createStringTable(MCAssembler &Asm,
std::vector<const MCSectionELF *> &Sections);
void ExecutePostLayoutBinding(MCAssembler &Asm,
const MCAsmLayout &Layout) override;
void writeSectionHeader(ArrayRef<const MCSectionELF *> Sections,
MCAssembler &Asm, const MCAsmLayout &Layout,
const SectionIndexMapTy &SectionIndexMap,
const SectionOffsetsTy &SectionOffsets);
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 MCSymbolData &DataA,
const MCFragment &FB,
bool InSet,
bool IsPCRel) const override;
bool isWeak(const MCSymbolData &SD) const override;
void WriteObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
void writeSection(MCAssembler &Asm,
const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex,
uint64_t Offset, uint64_t Size, uint64_t Alignment,
const MCSectionELF &Section);
};
}
template <typename T> void ELFObjectWriter::write(MCDataFragment &F, T Val) {
if (IsLittleEndian)
Val = support::endian::byte_swap<T, support::little>(Val);
else
Val = support::endian::byte_swap<T, support::big>(Val);
const char *Start = (const char *)&Val;
F.getContents().append(Start, Start + sizeof(T));
}
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;
}
bool ELFObjectWriter::RelocNeedsGOT(MCSymbolRefExpr::VariantKind Variant) {
switch (Variant) {
default:
return false;
case MCSymbolRefExpr::VK_GOT:
case MCSymbolRefExpr::VK_PLT:
case MCSymbolRefExpr::VK_GOTPCREL:
case MCSymbolRefExpr::VK_GOTOFF:
case MCSymbolRefExpr::VK_TPOFF:
case MCSymbolRefExpr::VK_TLSGD:
case MCSymbolRefExpr::VK_GOTTPOFF:
case MCSymbolRefExpr::VK_INDNTPOFF:
case MCSymbolRefExpr::VK_NTPOFF:
case MCSymbolRefExpr::VK_GOTNTPOFF:
case MCSymbolRefExpr::VK_TLSLDM:
case MCSymbolRefExpr::VK_DTPOFF:
case MCSymbolRefExpr::VK_TLSLD:
return true;
}
}
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(ShstrtabIndex < ELF::SHN_LORESERVE);
Write16(ShstrtabIndex);
}
uint64_t ELFObjectWriter::SymbolValue(MCSymbolData &Data,
const MCAsmLayout &Layout) {
if (Data.isCommon() && Data.isExternal())
return Data.getCommonAlignment();
uint64_t Res;
if (!Layout.getSymbolOffset(&Data, Res))
return 0;
if (Layout.getAssembler().isThumbFunc(&Data.getSymbol()))
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 (MCSymbolData &OriginalData : Asm.symbols()) {
const MCSymbol &Alias = OriginalData.getSymbol();
// Not an alias.
if (!Alias.isVariable())
continue;
auto *Ref = dyn_cast<MCSymbolRefExpr>(Alias.getVariableValue());
if (!Ref)
continue;
const MCSymbol &Symbol = Ref->getSymbol();
MCSymbolData &SD = Asm.getSymbolData(Symbol);
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.
OriginalData.setExternal(SD.isExternal());
MCELF::SetBinding(OriginalData, MCELF::GetBinding(SD));
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, ELFSymbolData &MSD,
const MCAsmLayout &Layout) {
MCSymbolData &OrigData = *MSD.SymbolData;
assert((!OrigData.getFragment() ||
(&OrigData.getFragment()->getParent()->getSection() ==
&OrigData.getSymbol().getSection())) &&
"The symbol's section doesn't match the fragment's symbol");
const MCSymbol *Base = Layout.getBaseSymbol(OrigData.getSymbol());
// This has to be in sync with when computeSymbolTable uses SHN_ABS or
// SHN_COMMON.
bool IsReserved = !Base || OrigData.isCommon();
// Binding and Type share the same byte as upper and lower nibbles
uint8_t Binding = MCELF::GetBinding(OrigData);
uint8_t Type = MCELF::GetType(OrigData);
MCSymbolData *BaseSD = nullptr;
if (Base) {
BaseSD = &Layout.getAssembler().getSymbolData(*Base);
Type = mergeTypeForSet(Type, MCELF::GetType(*BaseSD));
}
uint8_t Info = (Binding << ELF_STB_Shift) | (Type << ELF_STT_Shift);
// Other and Visibility share the same byte with Visibility using the lower
// 2 bits
uint8_t Visibility = MCELF::GetVisibility(OrigData);
uint8_t Other = MCELF::getOther(OrigData) << (ELF_STO_Shift - ELF_STV_Shift);
Other |= Visibility;
uint64_t Value = SymbolValue(OrigData, Layout);
uint64_t Size = 0;
const MCExpr *ESize = OrigData.getSize();
if (!ESize && Base)
ESize = BaseSD->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(MSD.StringIndex, Info, Value, Size, Other,
MSD.SectionIndex, IsReserved);
}
void ELFObjectWriter::WriteSymbolTable(
MCAssembler &Asm, const MCAsmLayout &Layout,
std::vector<const MCSectionELF *> &Sections,
SectionOffsetsTy &SectionOffsets) {
MCContext &Ctx = Asm.getContext();
unsigned EntrySize = is64Bit() ? ELF::SYMENTRY_SIZE64 : ELF::SYMENTRY_SIZE32;
// Symbol table
const MCSectionELF *SymtabSection =
Ctx.getELFSection(".symtab", ELF::SHT_SYMTAB, 0, EntrySize, "");
MCSectionData &SymtabSD = Asm.getOrCreateSectionData(*SymtabSection);
SymtabSD.setAlignment(is64Bit() ? 8 : 4);
SymbolTableIndex = Sections.size() + 1;
Sections.push_back(SymtabSection);
// The string table must be emitted first because we need the index
// into the string table for all the symbol names.
SymbolTableWriter Writer(*this, is64Bit());
uint64_t Padding = OffsetToAlignment(OS.tell(), SymtabSD.getAlignment());
WriteZeros(Padding);
uint64_t SecStart = OS.tell();
// The first entry is the undefined symbol entry.
Writer.writeSymbol(0, 0, 0, 0, 0, 0, false);
for (unsigned i = 0, e = FileSymbolData.size(); i != e; ++i) {
Writer.writeSymbol(FileSymbolData[i], ELF::STT_FILE | ELF::STB_LOCAL, 0, 0,
ELF::STV_DEFAULT, ELF::SHN_ABS, true);
}
// Write the symbol table entries.
LastLocalSymbolIndex = FileSymbolData.size() + LocalSymbolData.size() + 1;
for (unsigned i = 0, e = LocalSymbolData.size(); i != e; ++i) {
ELFSymbolData &MSD = LocalSymbolData[i];
WriteSymbol(Writer, MSD, Layout);
}
for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i) {
ELFSymbolData &MSD = ExternalSymbolData[i];
MCSymbolData &Data = *MSD.SymbolData;
assert(((Data.getFlags() & ELF_STB_Global) ||
(Data.getFlags() & ELF_STB_Weak)) &&
"External symbol requires STB_GLOBAL or STB_WEAK flag");
WriteSymbol(Writer, MSD, Layout);
if (MCELF::GetBinding(Data) == ELF::STB_LOCAL)
LastLocalSymbolIndex++;
}
for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i) {
ELFSymbolData &MSD = UndefinedSymbolData[i];
MCSymbolData &Data = *MSD.SymbolData;
WriteSymbol(Writer, MSD, Layout);
if (MCELF::GetBinding(Data) == ELF::STB_LOCAL)
LastLocalSymbolIndex++;
}
uint64_t SecEnd = OS.tell();
SectionOffsets[SymtabSection] = std::make_pair(SecStart, SecEnd);
ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
if (ShndxIndexes.empty())
return;
SecStart = OS.tell();
const MCSectionELF *SymtabShndxSection =
Ctx.getELFSection(".symtab_shndxr", ELF::SHT_SYMTAB_SHNDX, 0, 4, "");
Sections.push_back(SymtabShndxSection);
MCSectionData *SymtabShndxSD =
&Asm.getOrCreateSectionData(*SymtabShndxSection);
SymtabShndxSD->setAlignment(4);
for (uint32_t Index : ShndxIndexes)
write(Index);
SecEnd = OS.tell();
SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
}
// 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 MCSymbolData *SD,
uint64_t C,
unsigned Type) const {
// 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.
const MCSymbol &Sym = SD->getSymbol();
if (Sym.isUndefined())
return true;
unsigned Binding = MCELF::GetBinding(*SD);
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(*SD, Type))
return true;
return false;
}
static const MCSymbol *getWeakRef(const MCSymbolRefExpr &Ref) {
const MCSymbol &Sym = Ref.getSymbol();
if (Ref.getKind() == MCSymbolRefExpr::VK_WEAKREF)
return &Sym;
if (!Sym.isVariable())
return nullptr;
const MCExpr *Expr = Sym.getVariableValue();
const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr);
if (!Inner)
return nullptr;
if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
return &Inner->getSymbol();
return nullptr;
}
// 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 MCSymbolData &D) {
if (MCELF::GetType(D) == ELF::STT_GNU_IFUNC)
return true;
switch (MCELF::GetBinding(D)) {
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 MCSectionData *FixupSectionD = Fragment->getParent();
const MCSectionELF &FixupSection =
cast<MCSectionELF>(FixupSectionD->getSection());
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().FatalError(
Fixup.getLoc(),
"No relocation available to represent this relative expression");
const MCSymbol &SymB = RefB->getSymbol();
if (SymB.isUndefined())
Asm.getContext().FatalError(
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().FatalError(
Fixup.getLoc(), "Cannot represent a difference across sections");
const MCSymbolData &SymBD = Asm.getSymbolData(SymB);
if (::isWeak(SymBD))
Asm.getContext().FatalError(
Fixup.getLoc(), "Cannot represent a subtraction with a weak symbol");
uint64_t SymBOffset = Layout.getSymbolOffset(&SymBD);
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 MCSymbol *SymA = RefA ? &RefA->getSymbol() : nullptr;
const MCSymbolData *SymAD = SymA ? &Asm.getSymbolData(*SymA) : nullptr;
unsigned Type = GetRelocType(Target, Fixup, IsPCRel);
bool RelocateWithSymbol = shouldRelocateWithSymbol(Asm, RefA, SymAD, C, Type);
if (!RelocateWithSymbol && SymA && !SymA->isUndefined())
C += Layout.getSymbolOffset(SymAD);
uint64_t Addend = 0;
if (hasRelocationAddend()) {
Addend = C;
C = 0;
}
FixedValue = C;
// FIXME: What is this!?!?
MCSymbolRefExpr::VariantKind Modifier =
RefA ? RefA->getKind() : MCSymbolRefExpr::VK_None;
if (RelocNeedsGOT(Modifier))
NeedsGOT = true;
if (!RelocateWithSymbol) {
const MCSection *SecA =
(SymA && !SymA->isUndefined()) ? &SymA->getSection() : nullptr;
auto *ELFSec = cast_or_null<MCSectionELF>(SecA);
MCSymbol *SectionSymbol =
ELFSec ? Asm.getContext().getOrCreateSectionSymbol(*ELFSec)
: nullptr;
ELFRelocationEntry Rec(FixupOffset, SectionSymbol, Type, Addend);
Relocations[&FixupSection].push_back(Rec);
return;
}
if (SymA) {
if (const MCSymbol *R = Renames.lookup(SymA))
SymA = R;
if (const MCSymbol *WeakRef = getWeakRef(*RefA))
WeakrefUsedInReloc.insert(WeakRef);
else
UsedInReloc.insert(SymA);
}
ELFRelocationEntry Rec(FixupOffset, SymA, Type, Addend);
Relocations[&FixupSection].push_back(Rec);
return;
}
uint64_t
ELFObjectWriter::getSymbolIndexInSymbolTable(const MCAssembler &Asm,
const MCSymbol *S) {
const MCSymbolData &SD = Asm.getSymbolData(*S);
return SD.getIndex();
}
bool ELFObjectWriter::isInSymtab(const MCAsmLayout &Layout,
const MCSymbolData &Data, bool Used,
bool Renamed) {
const MCSymbol &Symbol = Data.getSymbol();
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.getName() == "_GLOBAL_OFFSET_TABLE_")
return true;
if (Symbol.isVariable()) {
const MCSymbol *Base = Layout.getBaseSymbol(Symbol);
if (Base && Base->isUndefined())
return false;
}
bool IsGlobal = MCELF::GetBinding(Data) == ELF::STB_GLOBAL;
if (!Symbol.isVariable() && Symbol.isUndefined() && !IsGlobal)
return false;
if (Symbol.isTemporary())
return false;
return true;
}
bool ELFObjectWriter::isLocal(const MCSymbolData &Data, bool isUsedInReloc) {
if (Data.isExternal())
return false;
const MCSymbol &Symbol = Data.getSymbol();
if (Symbol.isDefined())
return true;
if (isUsedInReloc)
return false;
return true;
}
void ELFObjectWriter::computeSymbolTable(
MCAssembler &Asm, const MCAsmLayout &Layout,
const SectionIndexMapTy &SectionIndexMap,
const RevGroupMapTy &RevGroupMap) {
// FIXME: Is this the correct place to do this?
// FIXME: Why is an undefined reference to _GLOBAL_OFFSET_TABLE_ needed?
if (NeedsGOT) {
StringRef Name = "_GLOBAL_OFFSET_TABLE_";
MCSymbol *Sym = Asm.getContext().GetOrCreateSymbol(Name);
MCSymbolData &Data = Asm.getOrCreateSymbolData(*Sym);
Data.setExternal(true);
MCELF::SetBinding(Data, ELF::STB_GLOBAL);
}
// Add the data for the symbols.
for (MCSymbolData &SD : Asm.symbols()) {
const MCSymbol &Symbol = SD.getSymbol();
bool Used = UsedInReloc.count(&Symbol);
bool WeakrefUsed = WeakrefUsedInReloc.count(&Symbol);
bool isSignature = RevGroupMap.count(&Symbol);
if (!isInSymtab(Layout, SD,
Used || WeakrefUsed || isSignature,
Renames.count(&Symbol)))
continue;
ELFSymbolData MSD;
MSD.SymbolData = &SD;
const MCSymbol *BaseSymbol = Layout.getBaseSymbol(Symbol);
// Undefined symbols are global, but this is the first place we
// are able to set it.
bool Local = isLocal(SD, Used);
if (!Local && MCELF::GetBinding(SD) == ELF::STB_LOCAL) {
assert(BaseSymbol);
MCSymbolData &BaseData = Asm.getSymbolData(*BaseSymbol);
MCELF::SetBinding(SD, ELF::STB_GLOBAL);
MCELF::SetBinding(BaseData, ELF::STB_GLOBAL);
}
if (!BaseSymbol) {
MSD.SectionIndex = ELF::SHN_ABS;
} else if (SD.isCommon()) {
assert(!Local);
MSD.SectionIndex = ELF::SHN_COMMON;
} else if (BaseSymbol->isUndefined()) {
if (isSignature && !Used)
MSD.SectionIndex = RevGroupMap.lookup(&Symbol);
else
MSD.SectionIndex = ELF::SHN_UNDEF;
if (!Used && WeakrefUsed)
MCELF::SetBinding(SD, ELF::STB_WEAK);
} else {
const MCSectionELF &Section =
static_cast<const MCSectionELF&>(BaseSymbol->getSection());
MSD.SectionIndex = SectionIndexMap.lookup(&Section);
assert(MSD.SectionIndex && "Invalid section index!");
}
// 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 (MCELF::GetType(SD) != ELF::STT_SECTION)
MSD.Name = StrTabBuilder.add(Name);
if (MSD.SectionIndex == ELF::SHN_UNDEF)
UndefinedSymbolData.push_back(MSD);
else if (Local)
LocalSymbolData.push_back(MSD);
else
ExternalSymbolData.push_back(MSD);
}
for (auto i = Asm.file_names_begin(), e = Asm.file_names_end(); i != e; ++i)
StrTabBuilder.add(*i);
StrTabBuilder.finalize(StringTableBuilder::ELF);
for (auto i = Asm.file_names_begin(), e = Asm.file_names_end(); i != e; ++i)
FileSymbolData.push_back(StrTabBuilder.getOffset(*i));
for (ELFSymbolData &MSD : LocalSymbolData)
MSD.StringIndex = MCELF::GetType(*MSD.SymbolData) == ELF::STT_SECTION
? 0
: StrTabBuilder.getOffset(MSD.Name);
for (ELFSymbolData &MSD : ExternalSymbolData)
MSD.StringIndex = StrTabBuilder.getOffset(MSD.Name);
for (ELFSymbolData& MSD : UndefinedSymbolData)
MSD.StringIndex = StrTabBuilder.getOffset(MSD.Name);
// Symbols are required to be in lexicographic order.
array_pod_sort(LocalSymbolData.begin(), LocalSymbolData.end());
array_pod_sort(ExternalSymbolData.begin(), ExternalSymbolData.end());
array_pod_sort(UndefinedSymbolData.begin(), UndefinedSymbolData.end());
// Set the symbol indices. Local symbols must come before all other
// symbols with non-local bindings.
unsigned Index = FileSymbolData.size() + 1;
for (unsigned i = 0, e = LocalSymbolData.size(); i != e; ++i)
LocalSymbolData[i].SymbolData->setIndex(Index++);
for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i)
ExternalSymbolData[i].SymbolData->setIndex(Index++);
for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i)
UndefinedSymbolData[i].SymbolData->setIndex(Index++);
}
const MCSectionELF *
ELFObjectWriter::createRelocationSection(MCAssembler &Asm,
const MCSectionELF &Sec) {
if (Relocations[&Sec].empty())
return nullptr;
MCContext &Ctx = Asm.getContext();
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;
const MCSectionELF *RelaSection = Ctx.createELFRelSection(
RelaSectionName, hasRelocationAddend() ? ELF::SHT_RELA : ELF::SHT_REL,
Flags, EntrySize, Sec.getGroup(), &Sec);
MCSectionData &RelSD = Asm.getOrCreateSectionData(*RelaSection);
RelSD.setAlignment(is64Bit() ? 8 : 4);
return RelaSection;
}
static SmallVector<char, 128>
getUncompressedData(const MCAsmLayout &Layout,
MCSectionData::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;
}
// Return a single fragment containing the compressed contents of the whole
// section. Null if the section was not compressed for any reason.
static std::unique_ptr<MCDataFragment>
getCompressedFragment(const MCAsmLayout &Layout,
MCSectionData::FragmentListType &Fragments) {
std::unique_ptr<MCDataFragment> CompressedFragment(new MCDataFragment());
// Gather the uncompressed data from all the fragments, recording the
// alignment fragment, if seen, and any fixups.
SmallVector<char, 128> UncompressedData =
getUncompressedData(Layout, Fragments);
SmallVectorImpl<char> &CompressedContents = CompressedFragment->getContents();
zlib::Status Success = zlib::compress(
StringRef(UncompressedData.data(), UncompressedData.size()),
CompressedContents);
if (Success != zlib::StatusOK)
return nullptr;
if (!prependCompressionHeader(UncompressedData.size(), CompressedContents))
return nullptr;
return CompressedFragment;
}
typedef DenseMap<const MCSectionData *, std::vector<MCSymbolData *>>
DefiningSymbolMap;
static void UpdateSymbols(const MCAsmLayout &Layout,
const std::vector<MCSymbolData *> &Symbols,
MCFragment &NewFragment) {
for (MCSymbolData *Sym : Symbols) {
Sym->setOffset(Sym->getOffset() +
Layout.getFragmentOffset(Sym->getFragment()));
Sym->setFragment(&NewFragment);
}
}
static void CompressDebugSection(MCAssembler &Asm, MCAsmLayout &Layout,
const DefiningSymbolMap &DefiningSymbols,
const MCSectionELF &Section,
MCSectionData &SD) {
StringRef SectionName = Section.getSectionName();
MCSectionData::FragmentListType &Fragments = SD.getFragmentList();
std::unique_ptr<MCDataFragment> CompressedFragment =
getCompressedFragment(Layout, Fragments);
// Leave the section as-is if the fragments could not be compressed.
if (!CompressedFragment)
return;
// Update the fragment+offsets of any symbols referring to fragments in this
// section to refer to the new fragment.
auto I = DefiningSymbols.find(&SD);
if (I != DefiningSymbols.end())
UpdateSymbols(Layout, I->second, *CompressedFragment);
// Invalidate the layout for the whole section since it will have new and
// different fragments now.
Layout.invalidateFragmentsFrom(&Fragments.front());
Fragments.clear();
// Complete the initialization of the new fragment
CompressedFragment->setParent(&SD);
CompressedFragment->setLayoutOrder(0);
Fragments.push_back(CompressedFragment.release());
// Rename from .debug_* to .zdebug_*
Asm.getContext().renameELFSection(&Section,
(".z" + SectionName.drop_front(1)).str());
}
void ELFObjectWriter::CompressDebugSections(MCAssembler &Asm,
MCAsmLayout &Layout) {
if (!Asm.getContext().getAsmInfo()->compressDebugSections())
return;
DefiningSymbolMap DefiningSymbols;
for (MCSymbolData &SD : Asm.symbols())
if (MCFragment *F = SD.getFragment())
DefiningSymbols[F->getParent()].push_back(&SD);
for (MCSectionData &SD : Asm) {
const MCSectionELF &Section =
static_cast<const MCSectionELF &>(SD.getSection());
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 (!SectionName.startswith(".debug_") || SectionName == ".debug_frame")
continue;
CompressDebugSection(Asm, Layout, DefiningSymbols, Section, SD);
}
}
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 ? getSymbolIndexInSymbolTable(Asm, Entry.Symbol) : 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::createSectionHeaderStringTable(
MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections) {
const MCSectionELF *ShstrtabSection = Sections[ShstrtabIndex - 1];
Asm.getOrCreateSectionData(*ShstrtabSection);
for (MCSectionData &SD : Asm) {
const MCSectionELF &Section =
static_cast<const MCSectionELF &>(SD.getSection());
ShStrTabBuilder.add(Section.getSectionName());
}
ShStrTabBuilder.finalize(StringTableBuilder::ELF);
OS << ShStrTabBuilder.data();
return ShstrtabSection;
}
const MCSectionELF *ELFObjectWriter::createStringTable(
MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections) {
MCContext &Ctx = Asm.getContext();
const MCSectionELF *StrtabSection =
Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
Asm.getOrCreateSectionData(*StrtabSection);
Sections.push_back(StrtabSection);
StringTableIndex = Sections.size();
OS << StrTabBuilder.data();
return StrtabSection;
}
void ELFObjectWriter::writeSection(MCAssembler &Asm,
const SectionIndexMapTy &SectionIndexMap,
uint32_t GroupSymbolIndex,
uint64_t Offset, uint64_t Size,
uint64_t Alignment,
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:
sh_link = ShStrTabBuilder.getOffset(Section.getSectionName());
break;
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(ShStrTabBuilder.getOffset(Section.getSectionName()),
Section.getType(),
Section.getFlags(), 0, Offset, Size, sh_link, sh_info,
Alignment, Section.getEntrySize());
}
void ELFObjectWriter::writeSectionHeader(
ArrayRef<const MCSectionELF *> Sections, MCAssembler &Asm,
const MCAsmLayout &Layout, const SectionIndexMapTy &SectionIndexMap,
const SectionOffsetsTy &SectionOffsets) {
const unsigned NumSections = Asm.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 (unsigned i = 0; i < NumSections; ++i) {
const MCSectionELF &Section = *Sections[i];
const MCSectionData &SD = Asm.getOrCreateSectionData(Section);
uint32_t GroupSymbolIndex;
if (Section.getType() != ELF::SHT_GROUP)
GroupSymbolIndex = 0;
else
GroupSymbolIndex = getSymbolIndexInSymbolTable(Asm, Section.getGroup());
const std::pair<uint64_t, uint64_t> &Offsets =
SectionOffsets.find(&Section)->second;
uint64_t Size = Section.getType() == ELF::SHT_NOBITS
? Layout.getSectionAddressSize(&SD)
: Offsets.second - Offsets.first;
writeSection(Asm, SectionIndexMap, GroupSymbolIndex, Offsets.first, Size,
SD.getAlignment(), Section);
}
}
void ELFObjectWriter::WriteObject(MCAssembler &Asm,
const MCAsmLayout &Layout) {
CompressDebugSections(Asm, const_cast<MCAsmLayout &>(Layout));
std::vector<const MCSectionELF *> Sections;
MCContext &Ctx = Asm.getContext();
const MCSectionELF *ShstrtabSection =
Ctx.getELFSection(".shstrtab", ELF::SHT_STRTAB, 0);
Sections.push_back(ShstrtabSection);
ShstrtabIndex = Sections.size();
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;
bool ComputedSymtab = false;
for (const MCSectionData &SD : Asm) {
const MCSectionELF &Section =
static_cast<const MCSectionELF &>(SD.getSection());
uint64_t Padding = OffsetToAlignment(OS.tell(), SD.getAlignment());
WriteZeros(Padding);
// Remember the offset into the file for this section.
uint64_t SecStart = OS.tell();
const MCSymbol *SignatureSymbol = Section.getGroup();
unsigned Type = Section.getType();
if (Type == ELF::SHT_GROUP) {
assert(SignatureSymbol);
write(uint32_t(ELF::GRP_COMDAT));
for (const MCSectionELF *Member : GroupMembers[SignatureSymbol]) {
uint32_t SecIndex = SectionIndexMap.lookup(Member);
write(SecIndex);
}
} else if (Type == ELF::SHT_REL || Type == ELF::SHT_RELA) {
if (!ComputedSymtab) {
// Compute symbol table information.
computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap);
ComputedSymtab = true;
}
writeRelocations(Asm, *Section.getAssociatedSection());
} else {
Asm.writeSectionData(&SD, Layout);
}
uint64_t SecEnd = OS.tell();
SectionOffsets[&Section] = std::make_pair(SecStart, SecEnd);
if (Type == ELF::SHT_GROUP || Type == ELF::SHT_REL || Type == ELF::SHT_RELA)
continue;
const MCSectionELF *RelSection = createRelocationSection(Asm, Section);
if (SignatureSymbol) {
Asm.getOrCreateSymbolData(*SignatureSymbol);
unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
if (!GroupIdx) {
const MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
Sections.push_back(Group);
GroupIdx = Sections.size();
MCSectionData *GroupD = &Asm.getOrCreateSectionData(*Group);
GroupD->setAlignment(4);
}
GroupMembers[SignatureSymbol].push_back(&Section);
if (RelSection)
GroupMembers[SignatureSymbol].push_back(RelSection);
}
Sections.push_back(&Section);
SectionIndexMap[&Section] = Sections.size();
if (RelSection) {
Sections.push_back(RelSection);
SectionIndexMap[RelSection] = Sections.size();
}
}
if (!ComputedSymtab) {
// Compute symbol table information.
computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap);
ComputedSymtab = true;
}
WriteSymbolTable(Asm, Layout, Sections, SectionOffsets);
{
uint64_t SecStart = OS.tell();
const MCSectionELF *Sec = createStringTable(Asm, Sections);
uint64_t SecEnd = OS.tell();
SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
}
{
uint64_t SecStart = OS.tell();
const MCSectionELF *Sec = createSectionHeaderStringTable(Asm, Sections);
uint64_t SecEnd = OS.tell();
SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
}
uint64_t NaturalAlignment = is64Bit() ? 8 : 4;
uint64_t Padding = OffsetToAlignment(OS.tell(), NaturalAlignment);
WriteZeros(Padding);
const unsigned SectionHeaderOffset = OS.tell();
// ... then the section header table ...
writeSectionHeader(Sections, Asm, Layout, SectionIndexMap, SectionOffsets);
uint16_t NumSections = (Sections.size() + 1 >= ELF::SHN_LORESERVE)
? (uint16_t)ELF::SHN_UNDEF
: Sections.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 MCSymbolData &DataA, const MCFragment &FB,
bool InSet, bool IsPCRel) const {
if (IsPCRel) {
assert(!InSet);
if (::isWeak(DataA))
return false;
}
return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(Asm, DataA, FB,
InSet, IsPCRel);
}
bool ELFObjectWriter::isWeak(const MCSymbolData &SD) const {
if (::isWeak(SD))
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 (MCELF::GetBinding(SD) != ELF::STB_GLOBAL)
return false;
const MCSymbol &Sym = SD.getSymbol();
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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