1 //===- lib/MC/ELFObjectWriter.cpp - ELF File Writer -----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements ELF object file writer information.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/ADT/ArrayRef.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/ADT/STLExtras.h"
17 #include "llvm/ADT/SmallString.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/StringRef.h"
20 #include "llvm/ADT/Twine.h"
21 #include "llvm/BinaryFormat/ELF.h"
22 #include "llvm/MC/MCAsmBackend.h"
23 #include "llvm/MC/MCAsmInfo.h"
24 #include "llvm/MC/MCAsmLayout.h"
25 #include "llvm/MC/MCAssembler.h"
26 #include "llvm/MC/MCContext.h"
27 #include "llvm/MC/MCELFObjectWriter.h"
28 #include "llvm/MC/MCExpr.h"
29 #include "llvm/MC/MCFixup.h"
30 #include "llvm/MC/MCFixupKindInfo.h"
31 #include "llvm/MC/MCFragment.h"
32 #include "llvm/MC/MCObjectFileInfo.h"
33 #include "llvm/MC/MCObjectWriter.h"
34 #include "llvm/MC/MCSection.h"
35 #include "llvm/MC/MCSectionELF.h"
36 #include "llvm/MC/MCSymbol.h"
37 #include "llvm/MC/MCSymbolELF.h"
38 #include "llvm/MC/MCValue.h"
39 #include "llvm/MC/StringTableBuilder.h"
40 #include "llvm/Support/Allocator.h"
41 #include "llvm/Support/Casting.h"
42 #include "llvm/Support/Compression.h"
43 #include "llvm/Support/Endian.h"
44 #include "llvm/Support/Error.h"
45 #include "llvm/Support/ErrorHandling.h"
46 #include "llvm/Support/Host.h"
47 #include "llvm/Support/LEB128.h"
48 #include "llvm/Support/MathExtras.h"
49 #include "llvm/Support/SMLoc.h"
50 #include "llvm/Support/StringSaver.h"
51 #include "llvm/Support/SwapByteOrder.h"
52 #include "llvm/Support/raw_ostream.h"
66 #define DEBUG_TYPE "reloc-info"
70 using SectionIndexMapTy = DenseMap<const MCSectionELF *, uint32_t>;
72 class ELFObjectWriter;
75 bool isDwoSection(const MCSectionELF &Sec) {
76 return Sec.getSectionName().endswith(".dwo");
79 class SymbolTableWriter {
83 // indexes we are going to write to .symtab_shndx.
84 std::vector<uint32_t> ShndxIndexes;
86 // The numbel of symbols written so far.
89 void createSymtabShndx();
91 template <typename T> void write(T Value);
94 SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit);
96 void writeSymbol(uint32_t name, uint8_t info, uint64_t value, uint64_t size,
97 uint8_t other, uint32_t shndx, bool Reserved);
99 ArrayRef<uint32_t> getShndxIndexes() const { return ShndxIndexes; }
103 ELFObjectWriter &OWriter;
104 support::endian::Writer W;
112 static uint64_t SymbolValue(const MCSymbol &Sym, const MCAsmLayout &Layout);
113 static bool isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
114 bool Used, bool Renamed);
116 /// Helper struct for containing some precomputed information on symbols.
117 struct ELFSymbolData {
118 const MCSymbolELF *Symbol;
119 uint32_t SectionIndex;
122 // Support lexicographic sorting.
123 bool operator<(const ELFSymbolData &RHS) const {
124 unsigned LHSType = Symbol->getType();
125 unsigned RHSType = RHS.Symbol->getType();
126 if (LHSType == ELF::STT_SECTION && RHSType != ELF::STT_SECTION)
128 if (LHSType != ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
130 if (LHSType == ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
131 return SectionIndex < RHS.SectionIndex;
132 return Name < RHS.Name;
137 /// @name Symbol Table Data
140 StringTableBuilder StrTabBuilder{StringTableBuilder::ELF};
144 // This holds the symbol table index of the last local symbol.
145 unsigned LastLocalSymbolIndex;
146 // This holds the .strtab section index.
147 unsigned StringTableIndex;
148 // This holds the .symtab section index.
149 unsigned SymbolTableIndex;
151 // Sections in the order they are to be output in the section table.
152 std::vector<const MCSectionELF *> SectionTable;
153 unsigned addToSectionTable(const MCSectionELF *Sec);
155 // TargetObjectWriter wrappers.
156 bool is64Bit() const;
157 bool hasRelocationAddend() const;
159 void align(unsigned Alignment);
161 bool maybeWriteCompression(uint64_t Size,
162 SmallVectorImpl<char> &CompressedContents,
163 bool ZLibStyle, unsigned Alignment);
166 ELFWriter(ELFObjectWriter &OWriter, raw_pwrite_stream &OS,
167 bool IsLittleEndian, DwoMode Mode)
169 W(OS, IsLittleEndian ? support::little : support::big), Mode(Mode) {}
171 void WriteWord(uint64_t Word) {
173 W.write<uint64_t>(Word);
175 W.write<uint32_t>(Word);
178 template <typename T> void write(T Val) {
182 void writeHeader(const MCAssembler &Asm);
184 void writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
185 ELFSymbolData &MSD, const MCAsmLayout &Layout);
187 // Start and end offset of each section
188 using SectionOffsetsTy =
189 std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>;
191 // Map from a signature symbol to the group section index
192 using RevGroupMapTy = DenseMap<const MCSymbol *, unsigned>;
194 /// Compute the symbol table data
196 /// \param Asm - The assembler.
197 /// \param SectionIndexMap - Maps a section to its index.
198 /// \param RevGroupMap - Maps a signature symbol to the group section.
199 void computeSymbolTable(MCAssembler &Asm, const MCAsmLayout &Layout,
200 const SectionIndexMapTy &SectionIndexMap,
201 const RevGroupMapTy &RevGroupMap,
202 SectionOffsetsTy &SectionOffsets);
204 void writeAddrsigSection();
206 MCSectionELF *createRelocationSection(MCContext &Ctx,
207 const MCSectionELF &Sec);
209 const MCSectionELF *createStringTable(MCContext &Ctx);
211 void writeSectionHeader(const MCAsmLayout &Layout,
212 const SectionIndexMapTy &SectionIndexMap,
213 const SectionOffsetsTy &SectionOffsets);
215 void writeSectionData(const MCAssembler &Asm, MCSection &Sec,
216 const MCAsmLayout &Layout);
218 void WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
219 uint64_t Address, uint64_t Offset, uint64_t Size,
220 uint32_t Link, uint32_t Info, uint64_t Alignment,
223 void writeRelocations(const MCAssembler &Asm, const MCSectionELF &Sec);
225 uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout);
226 void writeSection(const SectionIndexMapTy &SectionIndexMap,
227 uint32_t GroupSymbolIndex, uint64_t Offset, uint64_t Size,
228 const MCSectionELF &Section);
231 class ELFObjectWriter : public MCObjectWriter {
232 /// The target specific ELF writer instance.
233 std::unique_ptr<MCELFObjectTargetWriter> TargetObjectWriter;
235 DenseMap<const MCSectionELF *, std::vector<ELFRelocationEntry>> Relocations;
237 DenseMap<const MCSymbolELF *, const MCSymbolELF *> Renames;
239 bool EmitAddrsigSection = false;
240 std::vector<const MCSymbol *> AddrsigSyms;
242 bool hasRelocationAddend() const;
244 bool shouldRelocateWithSymbol(const MCAssembler &Asm,
245 const MCSymbolRefExpr *RefA,
246 const MCSymbolELF *Sym, uint64_t C,
247 unsigned Type) const;
250 ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW)
251 : TargetObjectWriter(std::move(MOTW)) {}
253 void reset() override {
256 MCObjectWriter::reset();
259 bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
260 const MCSymbol &SymA,
261 const MCFragment &FB, bool InSet,
262 bool IsPCRel) const override;
264 virtual bool checkRelocation(MCContext &Ctx, SMLoc Loc,
265 const MCSectionELF *From,
266 const MCSectionELF *To) {
270 void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
271 const MCFragment *Fragment, const MCFixup &Fixup,
272 MCValue Target, uint64_t &FixedValue) override;
274 void executePostLayoutBinding(MCAssembler &Asm,
275 const MCAsmLayout &Layout) override;
277 void emitAddrsigSection() override { EmitAddrsigSection = true; }
278 void addAddrsigSymbol(const MCSymbol *Sym) override {
279 AddrsigSyms.push_back(Sym);
282 friend struct ELFWriter;
285 class ELFSingleObjectWriter : public ELFObjectWriter {
286 raw_pwrite_stream &OS;
290 ELFSingleObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
291 raw_pwrite_stream &OS, bool IsLittleEndian)
292 : ELFObjectWriter(std::move(MOTW)), OS(OS),
293 IsLittleEndian(IsLittleEndian) {}
295 uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override {
296 return ELFWriter(*this, OS, IsLittleEndian, ELFWriter::AllSections)
297 .writeObject(Asm, Layout);
300 friend struct ELFWriter;
303 class ELFDwoObjectWriter : public ELFObjectWriter {
304 raw_pwrite_stream &OS, &DwoOS;
308 ELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
309 raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS,
311 : ELFObjectWriter(std::move(MOTW)), OS(OS), DwoOS(DwoOS),
312 IsLittleEndian(IsLittleEndian) {}
314 virtual bool checkRelocation(MCContext &Ctx, SMLoc Loc,
315 const MCSectionELF *From,
316 const MCSectionELF *To) override {
317 if (isDwoSection(*From)) {
318 Ctx.reportError(Loc, "A dwo section may not contain relocations");
321 if (To && isDwoSection(*To)) {
322 Ctx.reportError(Loc, "A relocation may not refer to a dwo section");
328 uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override {
329 uint64_t Size = ELFWriter(*this, OS, IsLittleEndian, ELFWriter::NonDwoOnly)
330 .writeObject(Asm, Layout);
331 Size += ELFWriter(*this, DwoOS, IsLittleEndian, ELFWriter::DwoOnly)
332 .writeObject(Asm, Layout);
337 } // end anonymous namespace
339 void ELFWriter::align(unsigned Alignment) {
340 uint64_t Padding = OffsetToAlignment(W.OS.tell(), Alignment);
341 W.OS.write_zeros(Padding);
344 unsigned ELFWriter::addToSectionTable(const MCSectionELF *Sec) {
345 SectionTable.push_back(Sec);
346 StrTabBuilder.add(Sec->getSectionName());
347 return SectionTable.size();
350 void SymbolTableWriter::createSymtabShndx() {
351 if (!ShndxIndexes.empty())
354 ShndxIndexes.resize(NumWritten);
357 template <typename T> void SymbolTableWriter::write(T Value) {
358 EWriter.write(Value);
361 SymbolTableWriter::SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit)
362 : EWriter(EWriter), Is64Bit(Is64Bit), NumWritten(0) {}
364 void SymbolTableWriter::writeSymbol(uint32_t name, uint8_t info, uint64_t value,
365 uint64_t size, uint8_t other,
366 uint32_t shndx, bool Reserved) {
367 bool LargeIndex = shndx >= ELF::SHN_LORESERVE && !Reserved;
372 if (!ShndxIndexes.empty()) {
374 ShndxIndexes.push_back(shndx);
376 ShndxIndexes.push_back(0);
379 uint16_t Index = LargeIndex ? uint16_t(ELF::SHN_XINDEX) : shndx;
382 write(name); // st_name
383 write(info); // st_info
384 write(other); // st_other
385 write(Index); // st_shndx
386 write(value); // st_value
387 write(size); // st_size
389 write(name); // st_name
390 write(uint32_t(value)); // st_value
391 write(uint32_t(size)); // st_size
392 write(info); // st_info
393 write(other); // st_other
394 write(Index); // st_shndx
400 bool ELFWriter::is64Bit() const {
401 return OWriter.TargetObjectWriter->is64Bit();
404 bool ELFWriter::hasRelocationAddend() const {
405 return OWriter.hasRelocationAddend();
408 // Emit the ELF header.
409 void ELFWriter::writeHeader(const MCAssembler &Asm) {
415 // emitWord method behaves differently for ELF32 and ELF64, writing
416 // 4 bytes in the former and 8 in the latter.
418 W.OS << ELF::ElfMagic; // e_ident[EI_MAG0] to e_ident[EI_MAG3]
420 W.OS << char(is64Bit() ? ELF::ELFCLASS64 : ELF::ELFCLASS32); // e_ident[EI_CLASS]
423 W.OS << char(W.Endian == support::little ? ELF::ELFDATA2LSB
426 W.OS << char(ELF::EV_CURRENT); // e_ident[EI_VERSION]
428 W.OS << char(OWriter.TargetObjectWriter->getOSABI());
429 W.OS << char(0); // e_ident[EI_ABIVERSION]
431 W.OS.write_zeros(ELF::EI_NIDENT - ELF::EI_PAD);
433 W.write<uint16_t>(ELF::ET_REL); // e_type
435 W.write<uint16_t>(OWriter.TargetObjectWriter->getEMachine()); // e_machine = target
437 W.write<uint32_t>(ELF::EV_CURRENT); // e_version
438 WriteWord(0); // e_entry, no entry point in .o file
439 WriteWord(0); // e_phoff, no program header for .o
440 WriteWord(0); // e_shoff = sec hdr table off in bytes
442 // e_flags = whatever the target wants
443 W.write<uint32_t>(Asm.getELFHeaderEFlags());
445 // e_ehsize = ELF header size
446 W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Ehdr)
447 : sizeof(ELF::Elf32_Ehdr));
449 W.write<uint16_t>(0); // e_phentsize = prog header entry size
450 W.write<uint16_t>(0); // e_phnum = # prog header entries = 0
452 // e_shentsize = Section header entry size
453 W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Shdr)
454 : sizeof(ELF::Elf32_Shdr));
456 // e_shnum = # of section header ents
457 W.write<uint16_t>(0);
459 // e_shstrndx = Section # of '.shstrtab'
460 assert(StringTableIndex < ELF::SHN_LORESERVE);
461 W.write<uint16_t>(StringTableIndex);
464 uint64_t ELFWriter::SymbolValue(const MCSymbol &Sym,
465 const MCAsmLayout &Layout) {
466 if (Sym.isCommon() && Sym.isExternal())
467 return Sym.getCommonAlignment();
470 if (!Layout.getSymbolOffset(Sym, Res))
473 if (Layout.getAssembler().isThumbFunc(&Sym))
479 static uint8_t mergeTypeForSet(uint8_t origType, uint8_t newType) {
480 uint8_t Type = newType;
482 // Propagation rules:
483 // IFUNC > FUNC > OBJECT > NOTYPE
484 // TLS_OBJECT > OBJECT > NOTYPE
486 // dont let the new type degrade the old type
490 case ELF::STT_GNU_IFUNC:
491 if (Type == ELF::STT_FUNC || Type == ELF::STT_OBJECT ||
492 Type == ELF::STT_NOTYPE || Type == ELF::STT_TLS)
493 Type = ELF::STT_GNU_IFUNC;
496 if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
497 Type == ELF::STT_TLS)
498 Type = ELF::STT_FUNC;
500 case ELF::STT_OBJECT:
501 if (Type == ELF::STT_NOTYPE)
502 Type = ELF::STT_OBJECT;
505 if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
506 Type == ELF::STT_GNU_IFUNC || Type == ELF::STT_FUNC)
514 void ELFWriter::writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
515 ELFSymbolData &MSD, const MCAsmLayout &Layout) {
516 const auto &Symbol = cast<MCSymbolELF>(*MSD.Symbol);
517 const MCSymbolELF *Base =
518 cast_or_null<MCSymbolELF>(Layout.getBaseSymbol(Symbol));
520 // This has to be in sync with when computeSymbolTable uses SHN_ABS or
522 bool IsReserved = !Base || Symbol.isCommon();
524 // Binding and Type share the same byte as upper and lower nibbles
525 uint8_t Binding = Symbol.getBinding();
526 uint8_t Type = Symbol.getType();
528 Type = mergeTypeForSet(Type, Base->getType());
530 uint8_t Info = (Binding << 4) | Type;
532 // Other and Visibility share the same byte with Visibility using the lower
534 uint8_t Visibility = Symbol.getVisibility();
535 uint8_t Other = Symbol.getOther() | Visibility;
537 uint64_t Value = SymbolValue(*MSD.Symbol, Layout);
540 const MCExpr *ESize = MSD.Symbol->getSize();
542 ESize = Base->getSize();
546 if (!ESize->evaluateKnownAbsolute(Res, Layout))
547 report_fatal_error("Size expression must be absolute.");
551 // Write out the symbol table entry
552 Writer.writeSymbol(StringIndex, Info, Value, Size, Other, MSD.SectionIndex,
556 // True if the assembler knows nothing about the final value of the symbol.
557 // This doesn't cover the comdat issues, since in those cases the assembler
558 // can at least know that all symbols in the section will move together.
559 static bool isWeak(const MCSymbolELF &Sym) {
560 if (Sym.getType() == ELF::STT_GNU_IFUNC)
563 switch (Sym.getBinding()) {
565 llvm_unreachable("Unknown binding");
568 case ELF::STB_GLOBAL:
571 case ELF::STB_GNU_UNIQUE:
576 bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
577 bool Used, bool Renamed) {
578 if (Symbol.isVariable()) {
579 const MCExpr *Expr = Symbol.getVariableValue();
580 if (const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr)) {
581 if (Ref->getKind() == MCSymbolRefExpr::VK_WEAKREF)
592 if (Symbol.isVariable() && Symbol.isUndefined()) {
593 // FIXME: this is here just to diagnose the case of a var = commmon_sym.
594 Layout.getBaseSymbol(Symbol);
598 if (Symbol.isUndefined() && !Symbol.isBindingSet())
601 if (Symbol.isTemporary())
604 if (Symbol.getType() == ELF::STT_SECTION)
610 void ELFWriter::computeSymbolTable(
611 MCAssembler &Asm, const MCAsmLayout &Layout,
612 const SectionIndexMapTy &SectionIndexMap, const RevGroupMapTy &RevGroupMap,
613 SectionOffsetsTy &SectionOffsets) {
614 MCContext &Ctx = Asm.getContext();
615 SymbolTableWriter Writer(*this, is64Bit());
618 unsigned EntrySize = is64Bit() ? ELF::SYMENTRY_SIZE64 : ELF::SYMENTRY_SIZE32;
619 MCSectionELF *SymtabSection =
620 Ctx.getELFSection(".symtab", ELF::SHT_SYMTAB, 0, EntrySize, "");
621 SymtabSection->setAlignment(is64Bit() ? 8 : 4);
622 SymbolTableIndex = addToSectionTable(SymtabSection);
624 align(SymtabSection->getAlignment());
625 uint64_t SecStart = W.OS.tell();
627 // The first entry is the undefined symbol entry.
628 Writer.writeSymbol(0, 0, 0, 0, 0, 0, false);
630 std::vector<ELFSymbolData> LocalSymbolData;
631 std::vector<ELFSymbolData> ExternalSymbolData;
633 // Add the data for the symbols.
634 bool HasLargeSectionIndex = false;
635 for (const MCSymbol &S : Asm.symbols()) {
636 const auto &Symbol = cast<MCSymbolELF>(S);
637 bool Used = Symbol.isUsedInReloc();
638 bool WeakrefUsed = Symbol.isWeakrefUsedInReloc();
639 bool isSignature = Symbol.isSignature();
641 if (!isInSymtab(Layout, Symbol, Used || WeakrefUsed || isSignature,
642 OWriter.Renames.count(&Symbol)))
645 if (Symbol.isTemporary() && Symbol.isUndefined()) {
646 Ctx.reportError(SMLoc(), "Undefined temporary symbol");
651 MSD.Symbol = cast<MCSymbolELF>(&Symbol);
653 bool Local = Symbol.getBinding() == ELF::STB_LOCAL;
654 assert(Local || !Symbol.isTemporary());
656 if (Symbol.isAbsolute()) {
657 MSD.SectionIndex = ELF::SHN_ABS;
658 } else if (Symbol.isCommon()) {
660 MSD.SectionIndex = ELF::SHN_COMMON;
661 } else if (Symbol.isUndefined()) {
662 if (isSignature && !Used) {
663 MSD.SectionIndex = RevGroupMap.lookup(&Symbol);
664 if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
665 HasLargeSectionIndex = true;
667 MSD.SectionIndex = ELF::SHN_UNDEF;
670 const MCSectionELF &Section =
671 static_cast<const MCSectionELF &>(Symbol.getSection());
673 // We may end up with a situation when section symbol is technically
674 // defined, but should not be. That happens because we explicitly
675 // pre-create few .debug_* sections to have accessors.
676 // And if these sections were not really defined in the code, but were
677 // referenced, we simply error out.
678 if (!Section.isRegistered()) {
679 assert(static_cast<const MCSymbolELF &>(Symbol).getType() ==
681 Ctx.reportError(SMLoc(),
682 "Undefined section reference: " + Symbol.getName());
686 if (Mode == NonDwoOnly && isDwoSection(Section))
688 MSD.SectionIndex = SectionIndexMap.lookup(&Section);
689 assert(MSD.SectionIndex && "Invalid section index!");
690 if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
691 HasLargeSectionIndex = true;
694 StringRef Name = Symbol.getName();
696 // Sections have their own string table
697 if (Symbol.getType() != ELF::STT_SECTION) {
699 StrTabBuilder.add(Name);
703 LocalSymbolData.push_back(MSD);
705 ExternalSymbolData.push_back(MSD);
708 // This holds the .symtab_shndx section index.
709 unsigned SymtabShndxSectionIndex = 0;
711 if (HasLargeSectionIndex) {
712 MCSectionELF *SymtabShndxSection =
713 Ctx.getELFSection(".symtab_shndxr", ELF::SHT_SYMTAB_SHNDX, 0, 4, "");
714 SymtabShndxSectionIndex = addToSectionTable(SymtabShndxSection);
715 SymtabShndxSection->setAlignment(4);
718 ArrayRef<std::string> FileNames = Asm.getFileNames();
719 for (const std::string &Name : FileNames)
720 StrTabBuilder.add(Name);
722 StrTabBuilder.finalize();
724 // File symbols are emitted first and handled separately from normal symbols,
725 // i.e. a non-STT_FILE symbol with the same name may appear.
726 for (const std::string &Name : FileNames)
727 Writer.writeSymbol(StrTabBuilder.getOffset(Name),
728 ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
731 // Symbols are required to be in lexicographic order.
732 array_pod_sort(LocalSymbolData.begin(), LocalSymbolData.end());
733 array_pod_sort(ExternalSymbolData.begin(), ExternalSymbolData.end());
735 // Set the symbol indices. Local symbols must come before all other
736 // symbols with non-local bindings.
737 unsigned Index = FileNames.size() + 1;
739 for (ELFSymbolData &MSD : LocalSymbolData) {
740 unsigned StringIndex = MSD.Symbol->getType() == ELF::STT_SECTION
742 : StrTabBuilder.getOffset(MSD.Name);
743 MSD.Symbol->setIndex(Index++);
744 writeSymbol(Writer, StringIndex, MSD, Layout);
747 // Write the symbol table entries.
748 LastLocalSymbolIndex = Index;
750 for (ELFSymbolData &MSD : ExternalSymbolData) {
751 unsigned StringIndex = StrTabBuilder.getOffset(MSD.Name);
752 MSD.Symbol->setIndex(Index++);
753 writeSymbol(Writer, StringIndex, MSD, Layout);
754 assert(MSD.Symbol->getBinding() != ELF::STB_LOCAL);
757 uint64_t SecEnd = W.OS.tell();
758 SectionOffsets[SymtabSection] = std::make_pair(SecStart, SecEnd);
760 ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
761 if (ShndxIndexes.empty()) {
762 assert(SymtabShndxSectionIndex == 0);
765 assert(SymtabShndxSectionIndex != 0);
767 SecStart = W.OS.tell();
768 const MCSectionELF *SymtabShndxSection =
769 SectionTable[SymtabShndxSectionIndex - 1];
770 for (uint32_t Index : ShndxIndexes)
772 SecEnd = W.OS.tell();
773 SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
776 void ELFWriter::writeAddrsigSection() {
777 for (const MCSymbol *Sym : OWriter.AddrsigSyms)
778 encodeULEB128(Sym->getIndex(), W.OS);
781 MCSectionELF *ELFWriter::createRelocationSection(MCContext &Ctx,
782 const MCSectionELF &Sec) {
783 if (OWriter.Relocations[&Sec].empty())
786 const StringRef SectionName = Sec.getSectionName();
787 std::string RelaSectionName = hasRelocationAddend() ? ".rela" : ".rel";
788 RelaSectionName += SectionName;
791 if (hasRelocationAddend())
792 EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rela) : sizeof(ELF::Elf32_Rela);
794 EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rel) : sizeof(ELF::Elf32_Rel);
797 if (Sec.getFlags() & ELF::SHF_GROUP)
798 Flags = ELF::SHF_GROUP;
800 MCSectionELF *RelaSection = Ctx.createELFRelSection(
801 RelaSectionName, hasRelocationAddend() ? ELF::SHT_RELA : ELF::SHT_REL,
802 Flags, EntrySize, Sec.getGroup(), &Sec);
803 RelaSection->setAlignment(is64Bit() ? 8 : 4);
807 // Include the debug info compression header.
808 bool ELFWriter::maybeWriteCompression(
809 uint64_t Size, SmallVectorImpl<char> &CompressedContents, bool ZLibStyle,
810 unsigned Alignment) {
813 is64Bit() ? sizeof(ELF::Elf32_Chdr) : sizeof(ELF::Elf64_Chdr);
814 if (Size <= HdrSize + CompressedContents.size())
816 // Platform specific header is followed by compressed data.
818 // Write Elf64_Chdr header.
819 write(static_cast<ELF::Elf64_Word>(ELF::ELFCOMPRESS_ZLIB));
820 write(static_cast<ELF::Elf64_Word>(0)); // ch_reserved field.
821 write(static_cast<ELF::Elf64_Xword>(Size));
822 write(static_cast<ELF::Elf64_Xword>(Alignment));
824 // Write Elf32_Chdr header otherwise.
825 write(static_cast<ELF::Elf32_Word>(ELF::ELFCOMPRESS_ZLIB));
826 write(static_cast<ELF::Elf32_Word>(Size));
827 write(static_cast<ELF::Elf32_Word>(Alignment));
832 // "ZLIB" followed by 8 bytes representing the uncompressed size of the section,
833 // useful for consumers to preallocate a buffer to decompress into.
834 const StringRef Magic = "ZLIB";
835 if (Size <= Magic.size() + sizeof(Size) + CompressedContents.size())
838 support::endian::write(W.OS, Size, support::big);
842 void ELFWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
843 const MCAsmLayout &Layout) {
844 MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
845 StringRef SectionName = Section.getSectionName();
847 auto &MC = Asm.getContext();
848 const auto &MAI = MC.getAsmInfo();
850 // Compressing debug_frame requires handling alignment fragments which is
851 // more work (possibly generalizing MCAssembler.cpp:writeFragment to allow
852 // for writing to arbitrary buffers) for little benefit.
853 bool CompressionEnabled =
854 MAI->compressDebugSections() != DebugCompressionType::None;
855 if (!CompressionEnabled || !SectionName.startswith(".debug_") ||
856 SectionName == ".debug_frame") {
857 Asm.writeSectionData(W.OS, &Section, Layout);
861 assert((MAI->compressDebugSections() == DebugCompressionType::Z ||
862 MAI->compressDebugSections() == DebugCompressionType::GNU) &&
863 "expected zlib or zlib-gnu style compression");
865 SmallVector<char, 128> UncompressedData;
866 raw_svector_ostream VecOS(UncompressedData);
867 Asm.writeSectionData(VecOS, &Section, Layout);
869 SmallVector<char, 128> CompressedContents;
870 if (Error E = zlib::compress(
871 StringRef(UncompressedData.data(), UncompressedData.size()),
872 CompressedContents)) {
873 consumeError(std::move(E));
874 W.OS << UncompressedData;
878 bool ZlibStyle = MAI->compressDebugSections() == DebugCompressionType::Z;
879 if (!maybeWriteCompression(UncompressedData.size(), CompressedContents,
880 ZlibStyle, Sec.getAlignment())) {
881 W.OS << UncompressedData;
886 // Set the compressed flag. That is zlib style.
887 Section.setFlags(Section.getFlags() | ELF::SHF_COMPRESSED);
889 // Add "z" prefix to section name. This is zlib-gnu style.
890 MC.renameELFSection(&Section, (".z" + SectionName.drop_front(1)).str());
891 W.OS << CompressedContents;
894 void ELFWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
895 uint64_t Address, uint64_t Offset,
896 uint64_t Size, uint32_t Link, uint32_t Info,
897 uint64_t Alignment, uint64_t EntrySize) {
898 W.write<uint32_t>(Name); // sh_name: index into string table
899 W.write<uint32_t>(Type); // sh_type
900 WriteWord(Flags); // sh_flags
901 WriteWord(Address); // sh_addr
902 WriteWord(Offset); // sh_offset
903 WriteWord(Size); // sh_size
904 W.write<uint32_t>(Link); // sh_link
905 W.write<uint32_t>(Info); // sh_info
906 WriteWord(Alignment); // sh_addralign
907 WriteWord(EntrySize); // sh_entsize
910 void ELFWriter::writeRelocations(const MCAssembler &Asm,
911 const MCSectionELF &Sec) {
912 std::vector<ELFRelocationEntry> &Relocs = OWriter.Relocations[&Sec];
914 // We record relocations by pushing to the end of a vector. Reverse the vector
915 // to get the relocations in the order they were created.
916 // In most cases that is not important, but it can be for special sections
917 // (.eh_frame) or specific relocations (TLS optimizations on SystemZ).
918 std::reverse(Relocs.begin(), Relocs.end());
920 // Sort the relocation entries. MIPS needs this.
921 OWriter.TargetObjectWriter->sortRelocs(Asm, Relocs);
923 for (unsigned i = 0, e = Relocs.size(); i != e; ++i) {
924 const ELFRelocationEntry &Entry = Relocs[e - i - 1];
925 unsigned Index = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
929 if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
930 write(uint32_t(Index));
932 write(OWriter.TargetObjectWriter->getRSsym(Entry.Type));
933 write(OWriter.TargetObjectWriter->getRType3(Entry.Type));
934 write(OWriter.TargetObjectWriter->getRType2(Entry.Type));
935 write(OWriter.TargetObjectWriter->getRType(Entry.Type));
937 struct ELF::Elf64_Rela ERE64;
938 ERE64.setSymbolAndType(Index, Entry.Type);
941 if (hasRelocationAddend())
944 write(uint32_t(Entry.Offset));
946 struct ELF::Elf32_Rela ERE32;
947 ERE32.setSymbolAndType(Index, Entry.Type);
950 if (hasRelocationAddend())
951 write(uint32_t(Entry.Addend));
953 if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
955 OWriter.TargetObjectWriter->getRType2(Entry.Type)) {
956 write(uint32_t(Entry.Offset));
958 ERE32.setSymbolAndType(0, RType);
963 OWriter.TargetObjectWriter->getRType3(Entry.Type)) {
964 write(uint32_t(Entry.Offset));
966 ERE32.setSymbolAndType(0, RType);
975 const MCSectionELF *ELFWriter::createStringTable(MCContext &Ctx) {
976 const MCSectionELF *StrtabSection = SectionTable[StringTableIndex - 1];
977 StrTabBuilder.write(W.OS);
978 return StrtabSection;
981 void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
982 uint32_t GroupSymbolIndex, uint64_t Offset,
983 uint64_t Size, const MCSectionELF &Section) {
984 uint64_t sh_link = 0;
985 uint64_t sh_info = 0;
987 switch(Section.getType()) {
992 case ELF::SHT_DYNAMIC:
993 llvm_unreachable("SHT_DYNAMIC in a relocatable object");
996 case ELF::SHT_RELA: {
997 sh_link = SymbolTableIndex;
998 assert(sh_link && ".symtab not found");
999 const MCSection *InfoSection = Section.getAssociatedSection();
1000 sh_info = SectionIndexMap.lookup(cast<MCSectionELF>(InfoSection));
1004 case ELF::SHT_SYMTAB:
1005 sh_link = StringTableIndex;
1006 sh_info = LastLocalSymbolIndex;
1009 case ELF::SHT_SYMTAB_SHNDX:
1010 case ELF::SHT_LLVM_CALL_GRAPH_PROFILE:
1011 case ELF::SHT_LLVM_ADDRSIG:
1012 sh_link = SymbolTableIndex;
1015 case ELF::SHT_GROUP:
1016 sh_link = SymbolTableIndex;
1017 sh_info = GroupSymbolIndex;
1021 if (Section.getFlags() & ELF::SHF_LINK_ORDER) {
1022 const MCSymbol *Sym = Section.getAssociatedSymbol();
1023 const MCSectionELF *Sec = cast<MCSectionELF>(&Sym->getSection());
1024 sh_link = SectionIndexMap.lookup(Sec);
1027 WriteSecHdrEntry(StrTabBuilder.getOffset(Section.getSectionName()),
1028 Section.getType(), Section.getFlags(), 0, Offset, Size,
1029 sh_link, sh_info, Section.getAlignment(),
1030 Section.getEntrySize());
1033 void ELFWriter::writeSectionHeader(
1034 const MCAsmLayout &Layout, const SectionIndexMapTy &SectionIndexMap,
1035 const SectionOffsetsTy &SectionOffsets) {
1036 const unsigned NumSections = SectionTable.size();
1038 // Null section first.
1039 uint64_t FirstSectionSize =
1040 (NumSections + 1) >= ELF::SHN_LORESERVE ? NumSections + 1 : 0;
1041 WriteSecHdrEntry(0, 0, 0, 0, 0, FirstSectionSize, 0, 0, 0, 0);
1043 for (const MCSectionELF *Section : SectionTable) {
1044 uint32_t GroupSymbolIndex;
1045 unsigned Type = Section->getType();
1046 if (Type != ELF::SHT_GROUP)
1047 GroupSymbolIndex = 0;
1049 GroupSymbolIndex = Section->getGroup()->getIndex();
1051 const std::pair<uint64_t, uint64_t> &Offsets =
1052 SectionOffsets.find(Section)->second;
1054 if (Type == ELF::SHT_NOBITS)
1055 Size = Layout.getSectionAddressSize(Section);
1057 Size = Offsets.second - Offsets.first;
1059 writeSection(SectionIndexMap, GroupSymbolIndex, Offsets.first, Size,
1064 uint64_t ELFWriter::writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) {
1065 uint64_t StartOffset = W.OS.tell();
1067 MCContext &Ctx = Asm.getContext();
1068 MCSectionELF *StrtabSection =
1069 Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
1070 StringTableIndex = addToSectionTable(StrtabSection);
1072 RevGroupMapTy RevGroupMap;
1073 SectionIndexMapTy SectionIndexMap;
1075 std::map<const MCSymbol *, std::vector<const MCSectionELF *>> GroupMembers;
1077 // Write out the ELF header ...
1080 // ... then the sections ...
1081 SectionOffsetsTy SectionOffsets;
1082 std::vector<MCSectionELF *> Groups;
1083 std::vector<MCSectionELF *> Relocations;
1084 for (MCSection &Sec : Asm) {
1085 MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
1086 if (Mode == NonDwoOnly && isDwoSection(Section))
1088 if (Mode == DwoOnly && !isDwoSection(Section))
1091 align(Section.getAlignment());
1093 // Remember the offset into the file for this section.
1094 uint64_t SecStart = W.OS.tell();
1096 const MCSymbolELF *SignatureSymbol = Section.getGroup();
1097 writeSectionData(Asm, Section, Layout);
1099 uint64_t SecEnd = W.OS.tell();
1100 SectionOffsets[&Section] = std::make_pair(SecStart, SecEnd);
1102 MCSectionELF *RelSection = createRelocationSection(Ctx, Section);
1104 if (SignatureSymbol) {
1105 Asm.registerSymbol(*SignatureSymbol);
1106 unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
1108 MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
1109 GroupIdx = addToSectionTable(Group);
1110 Group->setAlignment(4);
1111 Groups.push_back(Group);
1113 std::vector<const MCSectionELF *> &Members =
1114 GroupMembers[SignatureSymbol];
1115 Members.push_back(&Section);
1117 Members.push_back(RelSection);
1120 SectionIndexMap[&Section] = addToSectionTable(&Section);
1122 SectionIndexMap[RelSection] = addToSectionTable(RelSection);
1123 Relocations.push_back(RelSection);
1126 OWriter.TargetObjectWriter->addTargetSectionFlags(Ctx, Section);
1129 MCSectionELF *CGProfileSection = nullptr;
1130 if (!Asm.CGProfile.empty()) {
1131 CGProfileSection = Ctx.getELFSection(".llvm.call-graph-profile",
1132 ELF::SHT_LLVM_CALL_GRAPH_PROFILE,
1133 ELF::SHF_EXCLUDE, 16, "");
1134 SectionIndexMap[CGProfileSection] = addToSectionTable(CGProfileSection);
1137 for (MCSectionELF *Group : Groups) {
1138 align(Group->getAlignment());
1140 // Remember the offset into the file for this section.
1141 uint64_t SecStart = W.OS.tell();
1143 const MCSymbol *SignatureSymbol = Group->getGroup();
1144 assert(SignatureSymbol);
1145 write(uint32_t(ELF::GRP_COMDAT));
1146 for (const MCSectionELF *Member : GroupMembers[SignatureSymbol]) {
1147 uint32_t SecIndex = SectionIndexMap.lookup(Member);
1151 uint64_t SecEnd = W.OS.tell();
1152 SectionOffsets[Group] = std::make_pair(SecStart, SecEnd);
1155 if (Mode == DwoOnly) {
1156 // dwo files don't have symbol tables or relocations, but they do have
1158 StrTabBuilder.finalize();
1160 MCSectionELF *AddrsigSection;
1161 if (OWriter.EmitAddrsigSection) {
1162 AddrsigSection = Ctx.getELFSection(".llvm_addrsig", ELF::SHT_LLVM_ADDRSIG,
1164 addToSectionTable(AddrsigSection);
1167 // Compute symbol table information.
1168 computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap,
1171 for (MCSectionELF *RelSection : Relocations) {
1172 align(RelSection->getAlignment());
1174 // Remember the offset into the file for this section.
1175 uint64_t SecStart = W.OS.tell();
1177 writeRelocations(Asm,
1178 cast<MCSectionELF>(*RelSection->getAssociatedSection()));
1180 uint64_t SecEnd = W.OS.tell();
1181 SectionOffsets[RelSection] = std::make_pair(SecStart, SecEnd);
1184 if (OWriter.EmitAddrsigSection) {
1185 uint64_t SecStart = W.OS.tell();
1186 writeAddrsigSection();
1187 uint64_t SecEnd = W.OS.tell();
1188 SectionOffsets[AddrsigSection] = std::make_pair(SecStart, SecEnd);
1192 if (CGProfileSection) {
1193 uint64_t SecStart = W.OS.tell();
1194 for (const MCAssembler::CGProfileEntry &CGPE : Asm.CGProfile) {
1195 W.write<uint32_t>(CGPE.From->getSymbol().getIndex());
1196 W.write<uint32_t>(CGPE.To->getSymbol().getIndex());
1197 W.write<uint64_t>(CGPE.Count);
1199 uint64_t SecEnd = W.OS.tell();
1200 SectionOffsets[CGProfileSection] = std::make_pair(SecStart, SecEnd);
1204 uint64_t SecStart = W.OS.tell();
1205 const MCSectionELF *Sec = createStringTable(Ctx);
1206 uint64_t SecEnd = W.OS.tell();
1207 SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
1210 uint64_t NaturalAlignment = is64Bit() ? 8 : 4;
1211 align(NaturalAlignment);
1213 const uint64_t SectionHeaderOffset = W.OS.tell();
1215 // ... then the section header table ...
1216 writeSectionHeader(Layout, SectionIndexMap, SectionOffsets);
1218 uint16_t NumSections = support::endian::byte_swap<uint16_t>(
1219 (SectionTable.size() + 1 >= ELF::SHN_LORESERVE) ? (uint16_t)ELF::SHN_UNDEF
1220 : SectionTable.size() + 1,
1222 unsigned NumSectionsOffset;
1224 auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
1227 support::endian::byte_swap<uint64_t>(SectionHeaderOffset, W.Endian);
1228 Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
1229 offsetof(ELF::Elf64_Ehdr, e_shoff));
1230 NumSectionsOffset = offsetof(ELF::Elf64_Ehdr, e_shnum);
1233 support::endian::byte_swap<uint32_t>(SectionHeaderOffset, W.Endian);
1234 Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
1235 offsetof(ELF::Elf32_Ehdr, e_shoff));
1236 NumSectionsOffset = offsetof(ELF::Elf32_Ehdr, e_shnum);
1238 Stream.pwrite(reinterpret_cast<char *>(&NumSections), sizeof(NumSections),
1241 return W.OS.tell() - StartOffset;
1244 bool ELFObjectWriter::hasRelocationAddend() const {
1245 return TargetObjectWriter->hasRelocationAddend();
1248 void ELFObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
1249 const MCAsmLayout &Layout) {
1250 // The presence of symbol versions causes undefined symbols and
1251 // versions declared with @@@ to be renamed.
1252 for (const std::pair<StringRef, const MCSymbol *> &P : Asm.Symvers) {
1253 StringRef AliasName = P.first;
1254 const auto &Symbol = cast<MCSymbolELF>(*P.second);
1255 size_t Pos = AliasName.find('@');
1256 assert(Pos != StringRef::npos);
1258 StringRef Prefix = AliasName.substr(0, Pos);
1259 StringRef Rest = AliasName.substr(Pos);
1260 StringRef Tail = Rest;
1261 if (Rest.startswith("@@@"))
1262 Tail = Rest.substr(Symbol.isUndefined() ? 2 : 1);
1265 cast<MCSymbolELF>(Asm.getContext().getOrCreateSymbol(Prefix + Tail));
1266 Asm.registerSymbol(*Alias);
1267 const MCExpr *Value = MCSymbolRefExpr::create(&Symbol, Asm.getContext());
1268 Alias->setVariableValue(Value);
1270 // Aliases defined with .symvar copy the binding from the symbol they alias.
1271 // This is the first place we are able to copy this information.
1272 Alias->setExternal(Symbol.isExternal());
1273 Alias->setBinding(Symbol.getBinding());
1275 if (!Symbol.isUndefined() && !Rest.startswith("@@@"))
1278 // FIXME: Get source locations for these errors or diagnose them earlier.
1279 if (Symbol.isUndefined() && Rest.startswith("@@") &&
1280 !Rest.startswith("@@@")) {
1281 Asm.getContext().reportError(SMLoc(), "versioned symbol " + AliasName +
1282 " must be defined");
1286 if (Renames.count(&Symbol) && Renames[&Symbol] != Alias) {
1287 Asm.getContext().reportError(
1288 SMLoc(), llvm::Twine("multiple symbol versions defined for ") +
1293 Renames.insert(std::make_pair(&Symbol, Alias));
1296 for (const MCSymbol *&Sym : AddrsigSyms) {
1297 if (const MCSymbol *R = Renames.lookup(cast<MCSymbolELF>(Sym)))
1299 if (Sym->isInSection() && Sym->getName().startswith(".L"))
1300 Sym = Sym->getSection().getBeginSymbol();
1301 Sym->setUsedInReloc();
1305 // It is always valid to create a relocation with a symbol. It is preferable
1306 // to use a relocation with a section if that is possible. Using the section
1307 // allows us to omit some local symbols from the symbol table.
1308 bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
1309 const MCSymbolRefExpr *RefA,
1310 const MCSymbolELF *Sym,
1312 unsigned Type) const {
1313 // A PCRel relocation to an absolute value has no symbol (or section). We
1314 // represent that with a relocation to a null section.
1318 MCSymbolRefExpr::VariantKind Kind = RefA->getKind();
1322 // The .odp creation emits a relocation against the symbol ".TOC." which
1323 // create a R_PPC64_TOC relocation. However the relocation symbol name
1324 // in final object creation should be NULL, since the symbol does not
1325 // really exist, it is just the reference to TOC base for the current
1326 // object file. Since the symbol is undefined, returning false results
1327 // in a relocation with a null section which is the desired result.
1328 case MCSymbolRefExpr::VK_PPC_TOCBASE:
1331 // These VariantKind cause the relocation to refer to something other than
1332 // the symbol itself, like a linker generated table. Since the address of
1333 // symbol is not relevant, we cannot replace the symbol with the
1334 // section and patch the difference in the addend.
1335 case MCSymbolRefExpr::VK_GOT:
1336 case MCSymbolRefExpr::VK_PLT:
1337 case MCSymbolRefExpr::VK_GOTPCREL:
1338 case MCSymbolRefExpr::VK_PPC_GOT_LO:
1339 case MCSymbolRefExpr::VK_PPC_GOT_HI:
1340 case MCSymbolRefExpr::VK_PPC_GOT_HA:
1344 // An undefined symbol is not in any section, so the relocation has to point
1345 // to the symbol itself.
1346 assert(Sym && "Expected a symbol");
1347 if (Sym->isUndefined())
1350 unsigned Binding = Sym->getBinding();
1353 llvm_unreachable("Invalid Binding");
1354 case ELF::STB_LOCAL:
1357 // If the symbol is weak, it might be overridden by a symbol in another
1358 // file. The relocation has to point to the symbol so that the linker
1361 case ELF::STB_GLOBAL:
1362 // Global ELF symbols can be preempted by the dynamic linker. The relocation
1363 // has to point to the symbol for a reason analogous to the STB_WEAK case.
1367 // If a relocation points to a mergeable section, we have to be careful.
1368 // If the offset is zero, a relocation with the section will encode the
1369 // same information. With a non-zero offset, the situation is different.
1370 // For example, a relocation can point 42 bytes past the end of a string.
1371 // If we change such a relocation to use the section, the linker would think
1372 // that it pointed to another string and subtracting 42 at runtime will
1373 // produce the wrong value.
1374 if (Sym->isInSection()) {
1375 auto &Sec = cast<MCSectionELF>(Sym->getSection());
1376 unsigned Flags = Sec.getFlags();
1377 if (Flags & ELF::SHF_MERGE) {
1381 // It looks like gold has a bug (http://sourceware.org/PR16794) and can
1382 // only handle section relocations to mergeable sections if using RELA.
1383 if (!hasRelocationAddend())
1387 // Most TLS relocations use a got, so they need the symbol. Even those that
1388 // are just an offset (@tpoff), require a symbol in gold versions before
1389 // 5efeedf61e4fe720fd3e9a08e6c91c10abb66d42 (2014-09-26) which fixed
1390 // http://sourceware.org/PR16773.
1391 if (Flags & ELF::SHF_TLS)
1395 // If the symbol is a thumb function the final relocation must set the lowest
1396 // bit. With a symbol that is done by just having the symbol have that bit
1397 // set, so we would lose the bit if we relocated with the section.
1398 // FIXME: We could use the section but add the bit to the relocation value.
1399 if (Asm.isThumbFunc(Sym))
1402 if (TargetObjectWriter->needsRelocateWithSymbol(*Sym, Type))
1407 void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
1408 const MCAsmLayout &Layout,
1409 const MCFragment *Fragment,
1410 const MCFixup &Fixup, MCValue Target,
1411 uint64_t &FixedValue) {
1412 MCAsmBackend &Backend = Asm.getBackend();
1413 bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
1414 MCFixupKindInfo::FKF_IsPCRel;
1415 const MCSectionELF &FixupSection = cast<MCSectionELF>(*Fragment->getParent());
1416 uint64_t C = Target.getConstant();
1417 uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
1418 MCContext &Ctx = Asm.getContext();
1420 if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
1421 // Let A, B and C being the components of Target and R be the location of
1422 // the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
1423 // If it is pcrel, we want to compute (A - B + C - R).
1425 // In general, ELF has no relocations for -B. It can only represent (A + C)
1426 // or (A + C - R). If B = R + K and the relocation is not pcrel, we can
1427 // replace B to implement it: (A - R - K + C)
1431 "No relocation available to represent this relative expression");
1435 const auto &SymB = cast<MCSymbolELF>(RefB->getSymbol());
1437 if (SymB.isUndefined()) {
1438 Ctx.reportError(Fixup.getLoc(),
1439 Twine("symbol '") + SymB.getName() +
1440 "' can not be undefined in a subtraction expression");
1444 assert(!SymB.isAbsolute() && "Should have been folded");
1445 const MCSection &SecB = SymB.getSection();
1446 if (&SecB != &FixupSection) {
1447 Ctx.reportError(Fixup.getLoc(),
1448 "Cannot represent a difference across sections");
1452 uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
1453 uint64_t K = SymBOffset - FixupOffset;
1458 // We either rejected the fixup or folded B into C at this point.
1459 const MCSymbolRefExpr *RefA = Target.getSymA();
1460 const auto *SymA = RefA ? cast<MCSymbolELF>(&RefA->getSymbol()) : nullptr;
1462 bool ViaWeakRef = false;
1463 if (SymA && SymA->isVariable()) {
1464 const MCExpr *Expr = SymA->getVariableValue();
1465 if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
1466 if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF) {
1467 SymA = cast<MCSymbolELF>(&Inner->getSymbol());
1473 unsigned Type = TargetObjectWriter->getRelocType(Ctx, Target, Fixup, IsPCRel);
1474 uint64_t OriginalC = C;
1475 bool RelocateWithSymbol = shouldRelocateWithSymbol(Asm, RefA, SymA, C, Type);
1476 if (!RelocateWithSymbol && SymA && !SymA->isUndefined())
1477 C += Layout.getSymbolOffset(*SymA);
1479 uint64_t Addend = 0;
1480 if (hasRelocationAddend()) {
1487 const MCSectionELF *SecA = (SymA && SymA->isInSection())
1488 ? cast<MCSectionELF>(&SymA->getSection())
1490 if (!checkRelocation(Ctx, Fixup.getLoc(), &FixupSection, SecA))
1493 if (!RelocateWithSymbol) {
1494 const auto *SectionSymbol =
1495 SecA ? cast<MCSymbolELF>(SecA->getBeginSymbol()) : nullptr;
1497 SectionSymbol->setUsedInReloc();
1498 ELFRelocationEntry Rec(FixupOffset, SectionSymbol, Type, Addend, SymA,
1500 Relocations[&FixupSection].push_back(Rec);
1504 const auto *RenamedSymA = SymA;
1506 if (const MCSymbolELF *R = Renames.lookup(SymA))
1510 RenamedSymA->setIsWeakrefUsedInReloc();
1512 RenamedSymA->setUsedInReloc();
1514 ELFRelocationEntry Rec(FixupOffset, RenamedSymA, Type, Addend, SymA,
1516 Relocations[&FixupSection].push_back(Rec);
1519 bool ELFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
1520 const MCAssembler &Asm, const MCSymbol &SA, const MCFragment &FB,
1521 bool InSet, bool IsPCRel) const {
1522 const auto &SymA = cast<MCSymbolELF>(SA);
1528 return MCObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(Asm, SymA, FB,
1532 std::unique_ptr<MCObjectWriter>
1533 llvm::createELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
1534 raw_pwrite_stream &OS, bool IsLittleEndian) {
1535 return llvm::make_unique<ELFSingleObjectWriter>(std::move(MOTW), OS,
1539 std::unique_ptr<MCObjectWriter>
1540 llvm::createELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
1541 raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS,
1542 bool IsLittleEndian) {
1543 return llvm::make_unique<ELFDwoObjectWriter>(std::move(MOTW), OS, DwoOS,