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llvm-project/llvm/lib/MC/ELFObjectWriter.cpp
Fangrui Song 734660b6dd ELFObjectWriter: Simplify STT_SECTION adjustment
2025-04-12 21:34:57 -07:00

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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/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.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/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCELFExtras.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/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/MCTargetOptions.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/EndianStream.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/SMLoc.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/TargetParser/Host.h"
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <memory>
#include <string>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "elf-object-writer"
namespace {
namespace stats {
STATISTIC(ELFHeaderBytes, "Total size of ELF headers");
STATISTIC(SectionHeaderBytes, "Total size of section headers table");
STATISTIC(AllocTextBytes, "Total size of SHF_ALLOC text sections");
STATISTIC(AllocROBytes, "Total size of SHF_ALLOC readonly sections");
STATISTIC(AllocRWBytes, "Total size of SHF_ALLOC read-write sections");
STATISTIC(StrtabBytes, "Total size of SHT_STRTAB sections");
STATISTIC(SymtabBytes, "Total size of SHT_SYMTAB sections");
STATISTIC(RelocationBytes, "Total size of relocation sections");
STATISTIC(DynsymBytes, "Total size of SHT_DYNSYM sections");
STATISTIC(
DebugBytes,
"Total size of debug info sections (not including those written to .dwo)");
STATISTIC(UnwindBytes, "Total size of unwind sections");
STATISTIC(OtherBytes, "Total size of uncategorized sections");
STATISTIC(DwoBytes, "Total size of sections written to .dwo file");
} // namespace stats
struct ELFWriter;
bool isDwoSection(const MCSectionELF &Sec) {
return Sec.getName().ends_with(".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 MCAssembler &Asm, const MCSymbol &Sym);
static bool isInSymtab(const MCAssembler &Asm, 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 = ~0u;
// This holds the .strtab section index.
unsigned StringTableIndex = ~0u;
// This holds the .symtab section index.
unsigned SymbolTableIndex = ~0u;
// Sections in the order they are to be output in the section table.
std::vector<MCSectionELF *> SectionTable;
unsigned addToSectionTable(MCSectionELF *Sec);
// TargetObjectWriter wrappers.
bool is64Bit() const;
uint64_t align(Align Alignment);
bool maybeWriteCompression(uint32_t ChType, uint64_t Size,
SmallVectorImpl<uint8_t> &CompressedContents,
Align Alignment);
public:
ELFWriter(ELFObjectWriter &OWriter, raw_pwrite_stream &OS,
bool IsLittleEndian, DwoMode Mode)
: OWriter(OWriter), W(OS, IsLittleEndian ? llvm::endianness::little
: llvm::endianness::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(const MCAssembler &Asm, SymbolTableWriter &Writer,
uint32_t StringIndex, ELFSymbolData &MSD);
// 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 RevGroupMap - Maps a signature symbol to the group section.
void computeSymbolTable(MCAssembler &Asm, const RevGroupMapTy &RevGroupMap);
void writeAddrsigSection();
MCSectionELF *createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec);
void writeSectionHeaders(const MCAssembler &Asm);
void writeSectionData(const MCAssembler &Asm, MCSection &Sec);
void writeSectionHeaderEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
uint64_t Address, uint64_t Offset, uint64_t Size,
uint32_t Link, uint32_t Info,
MaybeAlign Alignment, uint64_t EntrySize);
void writeRelocations(const MCAssembler &Asm, const MCSectionELF &Sec);
uint64_t writeObject(MCAssembler &Asm);
void writeSectionHeader(uint32_t GroupSymbolIndex, uint64_t Offset,
uint64_t Size, const MCSectionELF &Section);
};
} // end anonymous namespace
uint64_t ELFWriter::align(Align Alignment) {
uint64_t Offset = W.OS.tell();
uint64_t NewOffset = alignTo(Offset, Alignment);
W.OS.write_zeros(NewOffset - Offset);
return NewOffset;
}
unsigned ELFWriter::addToSectionTable(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();
}
// 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 == llvm::endianness::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.OverrideABIVersion
? *OWriter.OverrideABIVersion
: 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>(OWriter.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 MCAssembler &Asm, const MCSymbol &Sym) {
if (Sym.isCommon())
return Sym.getCommonAlignment()->value();
uint64_t Res;
if (!Asm.getSymbolOffset(Sym, Res))
return 0;
if (Asm.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(const MCAssembler &Asm, SymbolTableWriter &Writer,
uint32_t StringIndex, ELFSymbolData &MSD) {
const auto &Symbol = cast<MCSymbolELF>(*MSD.Symbol);
const MCSymbolELF *Base =
cast_or_null<MCSymbolELF>(Asm.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(Asm, *MSD.Symbol);
uint64_t Size = 0;
const MCExpr *ESize = MSD.Symbol->getSize();
if (!ESize && Base) {
// For expressions like .set y, x+1, if y's size is unset, inherit from x.
ESize = Base->getSize();
// For `.size x, 2; y = x; .size y, 1; z = y; z1 = z; .symver y, y@v1`, z,
// z1, and y@v1's st_size equals y's. However, `Base` is `x` which will give
// us 2. Follow the MCSymbolRefExpr assignment chain, which covers most
// needs. MCBinaryExpr is not handled.
const MCSymbolELF *Sym = &Symbol;
while (Sym->isVariable()) {
if (auto *Expr =
dyn_cast<MCSymbolRefExpr>(Sym->getVariableValue(false))) {
Sym = cast<MCSymbolELF>(&Expr->getSymbol());
if (!Sym->getSize())
continue;
ESize = Sym->getSize();
}
break;
}
}
if (ESize) {
int64_t Res;
if (!ESize->evaluateKnownAbsolute(Res, Asm))
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 MCAssembler &Asm, 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.
Asm.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 RevGroupMapTy &RevGroupMap) {
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->getAlign());
// 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 =
OWriter.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(Asm, 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 = Section.getOrdinal();
assert(MSD.SectionIndex && "Invalid section index!");
if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
HasLargeSectionIndex = true;
}
// Temporary symbols generated for certain assembler features (.eh_frame,
// .debug_line) of an empty name may be referenced by relocations due to
// linker relaxation. Rename them to ".L0 " to match the gas fake label name
// and allow ld/objcopy --discard-locals to discard such symbols.
StringRef Name = Symbol.getName();
if (Name.empty())
Name = ".L0 ";
// 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(Asm, Writer, StringIndex, MSD);
}
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(Asm, Writer, StringIndex, MSD);
assert(MSD.Symbol->getBinding() != ELF::STB_LOCAL);
}
uint64_t SecEnd = W.OS.tell();
SymtabSection->setOffsets(SecStart, SecEnd);
ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
if (ShndxIndexes.empty()) {
assert(SymtabShndxSectionIndex == 0);
return;
}
assert(SymtabShndxSectionIndex != 0);
SecStart = W.OS.tell();
MCSectionELF *SymtabShndxSection = SectionTable[SymtabShndxSectionIndex - 1];
for (uint32_t Index : ShndxIndexes)
write(Index);
SecEnd = W.OS.tell();
SymtabShndxSection->setOffsets(SecStart, SecEnd);
}
void ELFWriter::writeAddrsigSection() {
for (const MCSymbol *Sym : OWriter.getAddrsigSyms())
if (Sym->getIndex() != 0)
encodeULEB128(Sym->getIndex(), W.OS);
}
MCSectionELF *ELFWriter::createRelocationSection(MCContext &Ctx,
const MCSectionELF &Sec) {
if (OWriter.Relocations[&Sec].empty())
return nullptr;
unsigned Flags = ELF::SHF_INFO_LINK;
if (Sec.getFlags() & ELF::SHF_GROUP)
Flags = ELF::SHF_GROUP;
const StringRef SectionName = Sec.getName();
const MCTargetOptions *TO = Ctx.getTargetOptions();
if (TO && TO->Crel) {
MCSectionELF *RelaSection =
Ctx.createELFRelSection(".crel" + SectionName, ELF::SHT_CREL, Flags,
/*EntrySize=*/1, Sec.getGroup(), &Sec);
return RelaSection;
}
const bool Rela = OWriter.usesRela(TO, Sec);
unsigned EntrySize;
if (Rela)
EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rela) : sizeof(ELF::Elf32_Rela);
else
EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rel) : sizeof(ELF::Elf32_Rel);
MCSectionELF *RelaSection =
Ctx.createELFRelSection(((Rela ? ".rela" : ".rel") + SectionName),
Rela ? 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(
uint32_t ChType, uint64_t Size,
SmallVectorImpl<uint8_t> &CompressedContents, Align Alignment) {
uint64_t HdrSize =
is64Bit() ? sizeof(ELF::Elf64_Chdr) : sizeof(ELF::Elf32_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>(ChType));
write(static_cast<ELF::Elf64_Word>(0)); // ch_reserved field.
write(static_cast<ELF::Elf64_Xword>(Size));
write(static_cast<ELF::Elf64_Xword>(Alignment.value()));
} else {
// Write Elf32_Chdr header otherwise.
write(static_cast<ELF::Elf32_Word>(ChType));
write(static_cast<ELF::Elf32_Word>(Size));
write(static_cast<ELF::Elf32_Word>(Alignment.value()));
}
return true;
}
void ELFWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec) {
MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
StringRef SectionName = Section.getName();
auto &Ctx = Asm.getContext();
const DebugCompressionType CompressionType =
Ctx.getTargetOptions() ? Ctx.getTargetOptions()->CompressDebugSections
: DebugCompressionType::None;
if (CompressionType == DebugCompressionType::None ||
!SectionName.starts_with(".debug_")) {
Asm.writeSectionData(W.OS, &Section);
return;
}
SmallVector<char, 128> UncompressedData;
raw_svector_ostream VecOS(UncompressedData);
Asm.writeSectionData(VecOS, &Section);
ArrayRef<uint8_t> Uncompressed =
ArrayRef(reinterpret_cast<uint8_t *>(UncompressedData.data()),
UncompressedData.size());
SmallVector<uint8_t, 128> Compressed;
uint32_t ChType;
switch (CompressionType) {
case DebugCompressionType::None:
llvm_unreachable("has been handled");
case DebugCompressionType::Zlib:
ChType = ELF::ELFCOMPRESS_ZLIB;
break;
case DebugCompressionType::Zstd:
ChType = ELF::ELFCOMPRESS_ZSTD;
break;
}
compression::compress(compression::Params(CompressionType), Uncompressed,
Compressed);
if (!maybeWriteCompression(ChType, UncompressedData.size(), Compressed,
Sec.getAlign())) {
W.OS << UncompressedData;
return;
}
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));
W.OS << toStringRef(Compressed);
}
void ELFWriter::writeSectionHeaderEntry(uint32_t Name, uint32_t Type,
uint64_t Flags, uint64_t Address,
uint64_t Offset, uint64_t Size,
uint32_t Link, uint32_t Info,
MaybeAlign 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 ? Alignment->value() : 0); // sh_addralign
writeWord(EntrySize); // sh_entsize
}
template <bool Is64>
static void encodeCrel(ArrayRef<ELFRelocationEntry> Relocs, raw_ostream &OS) {
using uint = std::conditional_t<Is64, uint64_t, uint32_t>;
ELF::encodeCrel<Is64>(OS, Relocs, [&](const ELFRelocationEntry &R) {
uint32_t SymIdx = R.Symbol ? R.Symbol->getIndex() : 0;
return ELF::Elf_Crel<Is64>{static_cast<uint>(R.Offset), SymIdx, R.Type,
std::make_signed_t<uint>(R.Addend)};
});
}
void ELFWriter::writeRelocations(const MCAssembler &Asm,
const MCSectionELF &Sec) {
std::vector<ELFRelocationEntry> &Relocs = OWriter.Relocations[&Sec];
const MCTargetOptions *TO = Asm.getContext().getTargetOptions();
const bool Rela = OWriter.usesRela(TO, Sec);
// Sort the relocation entries. MIPS needs this.
OWriter.TargetObjectWriter->sortRelocs(Asm, Relocs);
if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
for (const ELFRelocationEntry &Entry : Relocs) {
uint32_t SymIdx = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
if (is64Bit()) {
write(Entry.Offset);
write(uint32_t(SymIdx));
write(OWriter.TargetObjectWriter->getRSsym(Entry.Type));
write(OWriter.TargetObjectWriter->getRType3(Entry.Type));
write(OWriter.TargetObjectWriter->getRType2(Entry.Type));
write(OWriter.TargetObjectWriter->getRType(Entry.Type));
if (Rela)
write(Entry.Addend);
} else {
write(uint32_t(Entry.Offset));
ELF::Elf32_Rela ERE32;
ERE32.setSymbolAndType(SymIdx, Entry.Type);
write(ERE32.r_info);
if (Rela)
write(uint32_t(Entry.Addend));
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));
}
}
}
} else if (TO && TO->Crel) {
if (is64Bit())
encodeCrel<true>(Relocs, W.OS);
else
encodeCrel<false>(Relocs, W.OS);
} else {
for (const ELFRelocationEntry &Entry : Relocs) {
uint32_t Symidx = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
if (is64Bit()) {
write(Entry.Offset);
ELF::Elf64_Rela ERE;
ERE.setSymbolAndType(Symidx, Entry.Type);
write(ERE.r_info);
if (Rela)
write(Entry.Addend);
} else {
write(uint32_t(Entry.Offset));
ELF::Elf32_Rela ERE;
ERE.setSymbolAndType(Symidx, Entry.Type);
write(ERE.r_info);
if (Rela)
write(uint32_t(Entry.Addend));
}
}
}
}
void ELFWriter::writeSectionHeader(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:
case ELF::SHT_CREL: {
sh_link = SymbolTableIndex;
assert(sh_link && ".symtab not found");
const MCSection *InfoSection = Section.getLinkedToSection();
sh_info = InfoSection->getOrdinal();
break;
}
case ELF::SHT_SYMTAB:
sh_link = StringTableIndex;
sh_info = LastLocalSymbolIndex;
break;
case ELF::SHT_SYMTAB_SHNDX:
case ELF::SHT_LLVM_CALL_GRAPH_PROFILE:
case ELF::SHT_LLVM_ADDRSIG:
sh_link = SymbolTableIndex;
break;
case ELF::SHT_GROUP:
sh_link = SymbolTableIndex;
sh_info = GroupSymbolIndex;
break;
}
if (Section.getFlags() & ELF::SHF_LINK_ORDER) {
// If the value in the associated metadata is not a definition, Sym will be
// undefined. Represent this with sh_link=0.
const MCSymbol *Sym = Section.getLinkedToSymbol();
if (Sym && Sym->isInSection())
sh_link = Sym->getSection().getOrdinal();
}
writeSectionHeaderEntry(StrTabBuilder.getOffset(Section.getName()),
Section.getType(), Section.getFlags(), 0, Offset,
Size, sh_link, sh_info, Section.getAlign(),
Section.getEntrySize());
}
void ELFWriter::writeSectionHeaders(const MCAssembler &Asm) {
uint64_t Start = W.OS.tell();
const unsigned NumSections = SectionTable.size();
// Null section first.
uint64_t FirstSectionSize =
(NumSections + 1) >= ELF::SHN_LORESERVE ? NumSections + 1 : 0;
writeSectionHeaderEntry(0, 0, 0, 0, 0, FirstSectionSize, 0, 0, std::nullopt,
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();
std::pair<uint64_t, uint64_t> Offsets = Section->getOffsets();
uint64_t Size;
if (Type == ELF::SHT_NOBITS)
Size = Asm.getSectionAddressSize(*Section);
else
Size = Offsets.second - Offsets.first;
auto SectionHasFlag = [&](uint64_t Flag) -> bool {
return Section->getFlags() & Flag;
};
if (Mode == DwoOnly) {
stats::DwoBytes += Size;
} else if (Section->getName().starts_with(".debug")) {
stats::DebugBytes += Size;
} else if (Section->getName().starts_with(".eh_frame")) {
stats::UnwindBytes += Size;
} else if (SectionHasFlag(ELF::SHF_ALLOC)) {
if (SectionHasFlag(ELF::SHF_EXECINSTR)) {
stats::AllocTextBytes += Size;
} else if (SectionHasFlag(ELF::SHF_WRITE)) {
stats::AllocRWBytes += Size;
} else {
stats::AllocROBytes += Size;
}
} else {
switch (Section->getType()) {
case ELF::SHT_STRTAB:
stats::StrtabBytes += Size;
break;
case ELF::SHT_SYMTAB:
stats::SymtabBytes += Size;
break;
case ELF::SHT_DYNSYM:
stats::DynsymBytes += Size;
break;
case ELF::SHT_REL:
case ELF::SHT_RELA:
case ELF::SHT_CREL:
stats::RelocationBytes += Size;
break;
default:
stats::OtherBytes += Size;
break;
}
}
writeSectionHeader(GroupSymbolIndex, Offsets.first, Size, *Section);
}
stats::SectionHeaderBytes += W.OS.tell() - Start;
}
uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
uint64_t StartOffset = W.OS.tell();
MCContext &Ctx = Asm.getContext();
MCSectionELF *StrtabSection =
Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
StringTableIndex = addToSectionTable(StrtabSection);
RevGroupMapTy RevGroupMap;
// Write out the ELF header ...
writeHeader(Asm);
stats::ELFHeaderBytes += W.OS.tell() - StartOffset;
// ... then the sections ...
SmallVector<std::pair<MCSectionELF *, SmallVector<unsigned>>, 0> Groups;
// Map from group section index to group
SmallVector<unsigned, 0> GroupMap;
SmallVector<MCSectionELF *> Relocations;
for (MCSection &Sec : Asm) {
MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
if (Mode == NonDwoOnly && isDwoSection(Section))
continue;
if (Mode == DwoOnly && !isDwoSection(Section))
continue;
// Remember the offset into the file for this section.
const uint64_t SecStart = align(Section.getAlign());
const MCSymbolELF *SignatureSymbol = Section.getGroup();
writeSectionData(Asm, Section);
uint64_t SecEnd = W.OS.tell();
Section.setOffsets(SecStart, SecEnd);
MCSectionELF *RelSection = createRelocationSection(Ctx, Section);
unsigned *GroupIdxEntry = nullptr;
if (SignatureSymbol) {
GroupIdxEntry = &RevGroupMap[SignatureSymbol];
if (!*GroupIdxEntry) {
MCSectionELF *Group =
Ctx.createELFGroupSection(SignatureSymbol, Section.isComdat());
*GroupIdxEntry = addToSectionTable(Group);
Group->setAlignment(Align(4));
GroupMap.resize(*GroupIdxEntry + 1);
GroupMap[*GroupIdxEntry] = Groups.size();
Groups.emplace_back(Group, SmallVector<unsigned>{});
}
}
Section.setOrdinal(addToSectionTable(&Section));
if (RelSection) {
RelSection->setOrdinal(addToSectionTable(RelSection));
Relocations.push_back(RelSection);
}
if (GroupIdxEntry) {
auto &Members = Groups[GroupMap[*GroupIdxEntry]];
Members.second.push_back(Section.getOrdinal());
if (RelSection)
Members.second.push_back(RelSection->getOrdinal());
}
}
for (auto &[Group, Members] : Groups) {
// Remember the offset into the file for this section.
const uint64_t SecStart = align(Group->getAlign());
write(uint32_t(Group->isComdat() ? unsigned(ELF::GRP_COMDAT) : 0));
W.write<unsigned>(Members);
uint64_t SecEnd = W.OS.tell();
Group->setOffsets(SecStart, SecEnd);
}
if (Mode == DwoOnly) {
// dwo files don't have symbol tables or relocations, but they do have
// string tables.
StrTabBuilder.finalize();
} else {
MCSectionELF *AddrsigSection;
if (OWriter.getEmitAddrsigSection()) {
AddrsigSection = Ctx.getELFSection(".llvm_addrsig", ELF::SHT_LLVM_ADDRSIG,
ELF::SHF_EXCLUDE);
addToSectionTable(AddrsigSection);
}
// Compute symbol table information.
computeSymbolTable(Asm, RevGroupMap);
for (MCSectionELF *RelSection : Relocations) {
// Remember the offset into the file for this section.
const uint64_t SecStart = align(RelSection->getAlign());
writeRelocations(Asm,
cast<MCSectionELF>(*RelSection->getLinkedToSection()));
uint64_t SecEnd = W.OS.tell();
RelSection->setOffsets(SecStart, SecEnd);
}
if (OWriter.getEmitAddrsigSection()) {
uint64_t SecStart = W.OS.tell();
writeAddrsigSection();
uint64_t SecEnd = W.OS.tell();
AddrsigSection->setOffsets(SecStart, SecEnd);
}
}
{
uint64_t SecStart = W.OS.tell();
StrTabBuilder.write(W.OS);
StrtabSection->setOffsets(SecStart, W.OS.tell());
}
const uint64_t SectionHeaderOffset = align(is64Bit() ? Align(8) : Align(4));
// ... then the section header table ...
writeSectionHeaders(Asm);
uint16_t NumSections = support::endian::byte_swap<uint16_t>(
(SectionTable.size() + 1 >= ELF::SHN_LORESERVE) ? (uint16_t)ELF::SHN_UNDEF
: SectionTable.size() + 1,
W.Endian);
unsigned NumSectionsOffset;
auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
if (is64Bit()) {
uint64_t Val =
support::endian::byte_swap<uint64_t>(SectionHeaderOffset, W.Endian);
Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
offsetof(ELF::Elf64_Ehdr, e_shoff));
NumSectionsOffset = offsetof(ELF::Elf64_Ehdr, e_shnum);
} else {
uint32_t Val =
support::endian::byte_swap<uint32_t>(SectionHeaderOffset, W.Endian);
Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
offsetof(ELF::Elf32_Ehdr, e_shoff));
NumSectionsOffset = offsetof(ELF::Elf32_Ehdr, e_shnum);
}
Stream.pwrite(reinterpret_cast<char *>(&NumSections), sizeof(NumSections),
NumSectionsOffset);
return W.OS.tell() - StartOffset;
}
ELFObjectWriter::ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
raw_pwrite_stream &OS, bool IsLittleEndian)
: TargetObjectWriter(std::move(MOTW)), OS(OS),
IsLittleEndian(IsLittleEndian) {}
ELFObjectWriter::ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
raw_pwrite_stream &OS,
raw_pwrite_stream &DwoOS, bool IsLittleEndian)
: TargetObjectWriter(std::move(MOTW)), OS(OS), DwoOS(&DwoOS),
IsLittleEndian(IsLittleEndian) {}
void ELFObjectWriter::reset() {
ELFHeaderEFlags = 0;
SeenGnuAbi = false;
OverrideABIVersion.reset();
Relocations.clear();
Renames.clear();
Symvers.clear();
MCObjectWriter::reset();
}
bool ELFObjectWriter::hasRelocationAddend() const {
return TargetObjectWriter->hasRelocationAddend();
}
void ELFObjectWriter::executePostLayoutBinding(MCAssembler &Asm) {
// The presence of symbol versions causes undefined symbols and
// versions declared with @@@ to be renamed.
for (const Symver &S : Symvers) {
StringRef AliasName = S.Name;
const auto &Symbol = cast<MCSymbolELF>(*S.Sym);
size_t Pos = AliasName.find('@');
assert(Pos != StringRef::npos);
StringRef Prefix = AliasName.substr(0, Pos);
StringRef Rest = AliasName.substr(Pos);
StringRef Tail = Rest;
if (Rest.starts_with("@@@"))
Tail = Rest.substr(Symbol.isUndefined() ? 2 : 1);
auto *Alias =
cast<MCSymbolELF>(Asm.getContext().getOrCreateSymbol(Prefix + Tail));
Asm.registerSymbol(*Alias);
const MCExpr *Value = MCSymbolRefExpr::create(&Symbol, Asm.getContext());
Alias->setVariableValue(Value);
// 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->setBinding(Symbol.getBinding());
Alias->setVisibility(Symbol.getVisibility());
Alias->setOther(Symbol.getOther());
if (!Symbol.isUndefined() && S.KeepOriginalSym)
continue;
if (Symbol.isUndefined() && Rest.starts_with("@@") &&
!Rest.starts_with("@@@")) {
Asm.getContext().reportError(S.Loc, "default version symbol " +
AliasName + " must be defined");
continue;
}
if (auto It = Renames.find(&Symbol);
It != Renames.end() && It->second != Alias) {
Asm.getContext().reportError(S.Loc, Twine("multiple versions for ") +
Symbol.getName());
continue;
}
Renames.insert(std::make_pair(&Symbol, Alias));
}
for (const MCSymbol *&Sym : AddrsigSyms) {
if (const MCSymbol *R = Renames.lookup(cast<MCSymbolELF>(Sym)))
Sym = R;
if (Sym->isInSection() && Sym->getName().starts_with(".L"))
Sym = Sym->getSection().getBeginSymbol();
Sym->setUsedInReloc();
}
}
// 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::useSectionSymbol(const MCAssembler &Asm,
const MCValue &Val,
const MCSymbolELF *Sym, uint64_t C,
unsigned Type) const {
// Keep symbol type for a local ifunc because it may result in an IRELATIVE
// reloc that the dynamic loader will use to resolve the address at startup
// time.
if (Sym->getType() == ELF::STT_GNU_IFUNC)
return false;
// 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.
if (Sym->isInSection()) {
auto &Sec = cast<MCSectionELF>(Sym->getSection());
unsigned Flags = Sec.getFlags();
if (Flags & ELF::SHF_MERGE) {
if (C != 0)
return false;
// gold<2.34 incorrectly ignored the addend for R_386_GOTOFF (9)
// (http://sourceware.org/PR16794).
if (TargetObjectWriter->getEMachine() == ELF::EM_386 &&
Type == ELF::R_386_GOTOFF)
return false;
// ld.lld handles R_MIPS_HI16/R_MIPS_LO16 separately, not as a whole, so
// it doesn't know that an R_MIPS_HI16 with implicit addend 1 and an
// R_MIPS_LO16 with implicit addend -32768 represents 32768, which is in
// range of a MergeInputSection. We could introduce a new RelExpr member
// (like R_RISCV_PC_INDIRECT for R_RISCV_PCREL_HI20 / R_RISCV_PCREL_LO12)
// but the complexity is unnecessary given that GNU as keeps the original
// symbol for this case as well.
if (TargetObjectWriter->getEMachine() == ELF::EM_MIPS &&
!hasRelocationAddend())
return false;
}
// 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 false;
}
// 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 false;
return !TargetObjectWriter->needsRelocateWithSymbol(Val, *Sym, Type);
}
bool ELFObjectWriter::checkRelocation(MCContext &Ctx, SMLoc Loc,
const MCSectionELF *From,
const MCSectionELF *To) {
if (DwoOS) {
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;
}
void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
const MCFragment *Fragment,
const MCFixup &Fixup, MCValue Target,
uint64_t &FixedValue) {
MCAsmBackend &Backend = Asm.getBackend();
bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
MCFixupKindInfo::FKF_IsPCRel;
const MCSectionELF &FixupSection = cast<MCSectionELF>(*Fragment->getParent());
uint64_t C = Target.getConstant();
uint64_t FixupOffset = Asm.getFragmentOffset(*Fragment) + Fixup.getOffset();
MCContext &Ctx = Asm.getContext();
const MCTargetOptions *TO = Ctx.getTargetOptions();
if (auto *RefB = Target.getSubSym()) {
// When there is no relocation specifier, a linker relaxation target may
// emit ADD/SUB relocations for A-B+C.
if (Target.getAddSym() && Backend.handleAddSubRelocations(
Asm, *Fragment, Fixup, Target, FixedValue))
return;
const auto &SymB = cast<MCSymbolELF>(*RefB);
if (SymB.isUndefined()) {
Ctx.reportError(Fixup.getLoc(),
Twine("symbol '") + SymB.getName() +
"' can not be undefined in a subtraction expression");
return;
}
assert(!SymB.isAbsolute() && "Should have been folded");
const MCSection &SecB = SymB.getSection();
if (&SecB != &FixupSection) {
Ctx.reportError(Fixup.getLoc(),
"Cannot represent a difference across sections");
return;
}
assert(!IsPCRel && "should have been folded");
IsPCRel = true;
C += FixupOffset - Asm.getSymbolOffset(SymB);
}
// We either rejected the fixup or folded B into C at this point.
const auto *SymA = cast_or_null<MCSymbolELF>(Target.getAddSym());
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;
}
}
}
const MCSectionELF *SecA = (SymA && SymA->isInSection())
? cast<MCSectionELF>(&SymA->getSection())
: nullptr;
if (!checkRelocation(Ctx, Fixup.getLoc(), &FixupSection, SecA))
return;
unsigned Type = TargetObjectWriter->getRelocType(Ctx, Target, Fixup, IsPCRel);
bool UseSectionSym =
SymA && SymA->getBinding() == ELF::STB_LOCAL && !SymA->isUndefined();
if (UseSectionSym) {
UseSectionSym = useSectionSymbol(Asm, Target, SymA, C, Type);
// Disable STT_SECTION adjustment for CG Profile to help with --cg-profile.
const auto *Parent = cast<MCSectionELF>(Fragment->getParent());
UseSectionSym &= Parent->getType() != ELF::SHT_LLVM_CALL_GRAPH_PROFILE;
}
uint64_t Addend = UseSectionSym ? C + Asm.getSymbolOffset(*SymA) : C;
FixedValue = usesRela(TO, FixupSection) ? 0 : Addend;
if (UseSectionSym) {
SymA = cast<MCSymbolELF>(SecA->getBeginSymbol());
SymA->setUsedInReloc();
} else {
// In PPC64 ELFv1, .quad .TOC.@tocbase in the .opd section is expected to
// reference the null symbol.
if (Type == ELF::R_PPC64_TOC &&
TargetObjectWriter->getEMachine() == ELF::EM_PPC64)
SymA = nullptr;
if (SymA) {
if (const MCSymbolELF *R = Renames.lookup(SymA))
SymA = R;
if (ViaWeakRef)
SymA->setIsWeakrefUsedInReloc();
else
SymA->setUsedInReloc();
}
}
Relocations[&FixupSection].emplace_back(FixupOffset, SymA, Type, Addend);
}
bool ELFObjectWriter::usesRela(const MCTargetOptions *TO,
const MCSectionELF &Sec) const {
return (hasRelocationAddend() &&
Sec.getType() != ELF::SHT_LLVM_CALL_GRAPH_PROFILE) ||
(TO && TO->Crel);
}
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 (SymA.getBinding() != ELF::STB_LOCAL ||
SymA.getType() == ELF::STT_GNU_IFUNC)
return false;
}
return &SymA.getSection() == FB.getParent();
}
uint64_t ELFObjectWriter::writeObject(MCAssembler &Asm) {
uint64_t Size =
ELFWriter(*this, OS, IsLittleEndian,
DwoOS ? ELFWriter::NonDwoOnly : ELFWriter::AllSections)
.writeObject(Asm);
if (DwoOS)
Size += ELFWriter(*this, *DwoOS, IsLittleEndian, ELFWriter::DwoOnly)
.writeObject(Asm);
return Size;
}
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