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/MC/MCELFObjectWriter.h"
15 #include "llvm/ADT/STLExtras.h"
16 #include "llvm/ADT/SmallPtrSet.h"
17 #include "llvm/ADT/SmallString.h"
18 #include "llvm/ADT/StringMap.h"
19 #include "llvm/MC/MCAsmBackend.h"
20 #include "llvm/MC/MCAsmInfo.h"
21 #include "llvm/MC/MCAsmLayout.h"
22 #include "llvm/MC/MCAssembler.h"
23 #include "llvm/MC/MCContext.h"
24 #include "llvm/MC/MCELF.h"
25 #include "llvm/MC/MCELFSymbolFlags.h"
26 #include "llvm/MC/MCExpr.h"
27 #include "llvm/MC/MCFixupKindInfo.h"
28 #include "llvm/MC/MCObjectWriter.h"
29 #include "llvm/MC/MCSectionELF.h"
30 #include "llvm/MC/MCValue.h"
31 #include "llvm/MC/StringTableBuilder.h"
32 #include "llvm/Support/Compression.h"
33 #include "llvm/Support/Debug.h"
34 #include "llvm/Support/ELF.h"
35 #include "llvm/Support/Endian.h"
36 #include "llvm/Support/ErrorHandling.h"
41 #define DEBUG_TYPE "reloc-info"
44 class FragmentWriter {
48 FragmentWriter(bool IsLittleEndian);
49 template <typename T> void write(MCDataFragment &F, T Val);
52 typedef DenseMap<const MCSectionELF *, uint32_t> SectionIndexMapTy;
54 class ELFObjectWriter;
56 class SymbolTableWriter {
57 ELFObjectWriter &EWriter;
60 // indexes we are going to write to .symtab_shndx.
61 std::vector<uint32_t> ShndxIndexes;
63 // The numbel of symbols written so far.
66 void createSymtabShndx();
68 template <typename T> void write(T Value);
71 SymbolTableWriter(ELFObjectWriter &EWriter, bool Is64Bit);
73 void writeSymbol(uint32_t name, uint8_t info, uint64_t value, uint64_t size,
74 uint8_t other, uint32_t shndx, bool Reserved);
76 ArrayRef<uint32_t> getShndxIndexes() const { return ShndxIndexes; }
79 class ELFObjectWriter : public MCObjectWriter {
80 FragmentWriter FWriter;
84 static bool isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind);
85 static bool RelocNeedsGOT(MCSymbolRefExpr::VariantKind Variant);
86 static uint64_t SymbolValue(MCSymbolData &Data, const MCAsmLayout &Layout);
87 static bool isInSymtab(const MCAsmLayout &Layout, const MCSymbolData &Data,
88 bool Used, bool Renamed);
89 static bool isLocal(const MCSymbolData &Data, bool isUsedInReloc);
90 static bool IsELFMetaDataSection(const MCSectionData &SD);
92 void writeDataSectionData(MCAssembler &Asm, const MCAsmLayout &Layout,
93 const MCSectionData &SD);
95 /// Helper struct for containing some precomputed information on symbols.
96 struct ELFSymbolData {
97 MCSymbolData *SymbolData;
99 uint32_t SectionIndex;
102 // Support lexicographic sorting.
103 bool operator<(const ELFSymbolData &RHS) const {
104 unsigned LHSType = MCELF::GetType(*SymbolData);
105 unsigned RHSType = MCELF::GetType(*RHS.SymbolData);
106 if (LHSType == ELF::STT_SECTION && RHSType != ELF::STT_SECTION)
108 if (LHSType != ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
110 if (LHSType == ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
111 return SectionIndex < RHS.SectionIndex;
112 return Name < RHS.Name;
116 /// The target specific ELF writer instance.
117 std::unique_ptr<MCELFObjectTargetWriter> TargetObjectWriter;
119 SmallPtrSet<const MCSymbol *, 16> UsedInReloc;
120 SmallPtrSet<const MCSymbol *, 16> WeakrefUsedInReloc;
121 DenseMap<const MCSymbol *, const MCSymbol *> Renames;
123 llvm::DenseMap<const MCSectionData *, std::vector<ELFRelocationEntry>>
125 StringTableBuilder ShStrTabBuilder;
128 /// @name Symbol Table Data
131 StringTableBuilder StrTabBuilder;
132 std::vector<uint64_t> FileSymbolData;
133 std::vector<ELFSymbolData> LocalSymbolData;
134 std::vector<ELFSymbolData> ExternalSymbolData;
135 std::vector<ELFSymbolData> UndefinedSymbolData;
141 // This holds the symbol table index of the last local symbol.
142 unsigned LastLocalSymbolIndex;
143 // This holds the .strtab section index.
144 unsigned StringTableIndex;
145 // This holds the .symtab section index.
146 unsigned SymbolTableIndex;
148 unsigned ShstrtabIndex;
151 // TargetObjectWriter wrappers.
152 bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
153 bool hasRelocationAddend() const {
154 return TargetObjectWriter->hasRelocationAddend();
156 unsigned GetRelocType(const MCValue &Target, const MCFixup &Fixup,
157 bool IsPCRel) const {
158 return TargetObjectWriter->GetRelocType(Target, Fixup, IsPCRel);
162 ELFObjectWriter(MCELFObjectTargetWriter *MOTW, raw_pwrite_stream &OS,
164 : MCObjectWriter(OS, IsLittleEndian), FWriter(IsLittleEndian),
165 TargetObjectWriter(MOTW), NeedsGOT(false) {}
167 void reset() override {
169 WeakrefUsedInReloc.clear();
172 ShStrTabBuilder.clear();
173 StrTabBuilder.clear();
174 FileSymbolData.clear();
175 LocalSymbolData.clear();
176 ExternalSymbolData.clear();
177 UndefinedSymbolData.clear();
178 MCObjectWriter::reset();
181 ~ELFObjectWriter() override;
183 void WriteWord(uint64_t W) {
190 template <typename T> void write(T Val) {
192 support::endian::Writer<support::little>(OS).write(Val);
194 support::endian::Writer<support::big>(OS).write(Val);
197 template <typename T> void write(MCDataFragment &F, T Value) {
198 FWriter.write(F, Value);
201 void writeHeader(const MCAssembler &Asm);
203 void WriteSymbol(SymbolTableWriter &Writer, ELFSymbolData &MSD,
204 const MCAsmLayout &Layout);
206 // Start and end offset of each section
207 typedef std::vector<std::pair<uint64_t, uint64_t>> SectionOffsetsTy;
209 void WriteSymbolTable(MCAssembler &Asm, const MCAsmLayout &Layout,
210 std::vector<const MCSectionELF *> &Sections,
211 SectionOffsetsTy &SectionOffsets);
213 bool shouldRelocateWithSymbol(const MCAssembler &Asm,
214 const MCSymbolRefExpr *RefA,
215 const MCSymbolData *SD, uint64_t C,
216 unsigned Type) const;
218 void RecordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
219 const MCFragment *Fragment, const MCFixup &Fixup,
220 MCValue Target, bool &IsPCRel,
221 uint64_t &FixedValue) override;
223 uint64_t getSymbolIndexInSymbolTable(const MCAssembler &Asm,
226 // Map from a signature symbol to the group section index
227 typedef DenseMap<const MCSymbol *, unsigned> RevGroupMapTy;
229 /// Compute the symbol table data
231 /// \param Asm - The assembler.
232 /// \param SectionIndexMap - Maps a section to its index.
233 /// \param RevGroupMap - Maps a signature symbol to the group section.
234 void computeSymbolTable(MCAssembler &Asm, const MCAsmLayout &Layout,
235 const SectionIndexMapTy &SectionIndexMap,
236 const RevGroupMapTy &RevGroupMap);
238 void maybeAddToGroup(MCAssembler &Asm,
239 ArrayRef<const MCSectionELF *> Sections,
240 const RevGroupMapTy &RevGroupMap,
241 const MCSectionELF &Section, unsigned Index);
243 void computeIndexMap(MCAssembler &Asm,
244 std::vector<const MCSectionELF *> &Sections,
245 SectionIndexMapTy &SectionIndexMap,
246 const RevGroupMapTy &RevGroupMap);
248 void createRelocationSection(MCAssembler &Asm, const MCSectionData &SD);
250 void CompressDebugSections(MCAssembler &Asm, MCAsmLayout &Layout);
252 void WriteRelocations(MCAssembler &Asm, const MCAsmLayout &Layout);
255 createSectionHeaderStringTable(MCAssembler &Asm,
256 std::vector<const MCSectionELF *> &Sections);
257 void createStringTable(MCAssembler &Asm,
258 std::vector<const MCSectionELF *> &Sections);
260 // Create the sections that show up in the symbol table. Currently
261 // those are the .note.GNU-stack section and the group sections.
262 void createIndexedSections(MCAssembler &Asm, const MCAsmLayout &Layout,
263 RevGroupMapTy &RevGroupMap,
264 std::vector<const MCSectionELF *> &Sections,
265 SectionIndexMapTy &SectionIndexMap);
267 void ExecutePostLayoutBinding(MCAssembler &Asm,
268 const MCAsmLayout &Layout) override;
270 void writeSectionHeader(ArrayRef<const MCSectionELF *> Sections,
271 MCAssembler &Asm, const MCAsmLayout &Layout,
272 const SectionIndexMapTy &SectionIndexMap,
273 const SectionOffsetsTy &SectionOffsets);
275 void WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
276 uint64_t Address, uint64_t Offset,
277 uint64_t Size, uint32_t Link, uint32_t Info,
278 uint64_t Alignment, uint64_t EntrySize);
280 void WriteRelocationsFragment(const MCAssembler &Asm,
282 const MCSectionData *SD);
285 IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
286 const MCSymbolData &DataA,
287 const MCFragment &FB,
289 bool IsPCRel) const override;
291 bool isWeak(const MCSymbolData &SD) const override;
293 void WriteObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
294 void writeSection(MCAssembler &Asm,
295 const SectionIndexMapTy &SectionIndexMap,
296 uint32_t GroupSymbolIndex,
297 uint64_t Offset, uint64_t Size, uint64_t Alignment,
298 const MCSectionELF &Section);
302 FragmentWriter::FragmentWriter(bool IsLittleEndian)
303 : IsLittleEndian(IsLittleEndian) {}
305 template <typename T> void FragmentWriter::write(MCDataFragment &F, T Val) {
307 Val = support::endian::byte_swap<T, support::little>(Val);
309 Val = support::endian::byte_swap<T, support::big>(Val);
310 const char *Start = (const char *)&Val;
311 F.getContents().append(Start, Start + sizeof(T));
314 void SymbolTableWriter::createSymtabShndx() {
315 if (!ShndxIndexes.empty())
318 ShndxIndexes.resize(NumWritten);
321 template <typename T> void SymbolTableWriter::write(T Value) {
322 EWriter.write(Value);
325 SymbolTableWriter::SymbolTableWriter(ELFObjectWriter &EWriter, bool Is64Bit)
326 : EWriter(EWriter), Is64Bit(Is64Bit), NumWritten(0) {}
328 void SymbolTableWriter::writeSymbol(uint32_t name, uint8_t info, uint64_t value,
329 uint64_t size, uint8_t other,
330 uint32_t shndx, bool Reserved) {
331 bool LargeIndex = shndx >= ELF::SHN_LORESERVE && !Reserved;
336 if (!ShndxIndexes.empty()) {
338 ShndxIndexes.push_back(shndx);
340 ShndxIndexes.push_back(0);
343 uint16_t Index = LargeIndex ? uint16_t(ELF::SHN_XINDEX) : shndx;
346 write(name); // st_name
347 write(info); // st_info
348 write(other); // st_other
349 write(Index); // st_shndx
350 write(value); // st_value
351 write(size); // st_size
353 write(name); // st_name
354 write(uint32_t(value)); // st_value
355 write(uint32_t(size)); // st_size
356 write(info); // st_info
357 write(other); // st_other
358 write(Index); // st_shndx
364 bool ELFObjectWriter::isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind) {
365 const MCFixupKindInfo &FKI =
366 Asm.getBackend().getFixupKindInfo((MCFixupKind) Kind);
368 return FKI.Flags & MCFixupKindInfo::FKF_IsPCRel;
371 bool ELFObjectWriter::RelocNeedsGOT(MCSymbolRefExpr::VariantKind Variant) {
375 case MCSymbolRefExpr::VK_GOT:
376 case MCSymbolRefExpr::VK_PLT:
377 case MCSymbolRefExpr::VK_GOTPCREL:
378 case MCSymbolRefExpr::VK_GOTOFF:
379 case MCSymbolRefExpr::VK_TPOFF:
380 case MCSymbolRefExpr::VK_TLSGD:
381 case MCSymbolRefExpr::VK_GOTTPOFF:
382 case MCSymbolRefExpr::VK_INDNTPOFF:
383 case MCSymbolRefExpr::VK_NTPOFF:
384 case MCSymbolRefExpr::VK_GOTNTPOFF:
385 case MCSymbolRefExpr::VK_TLSLDM:
386 case MCSymbolRefExpr::VK_DTPOFF:
387 case MCSymbolRefExpr::VK_TLSLD:
392 ELFObjectWriter::~ELFObjectWriter()
395 // Emit the ELF header.
396 void ELFObjectWriter::writeHeader(const MCAssembler &Asm) {
402 // emitWord method behaves differently for ELF32 and ELF64, writing
403 // 4 bytes in the former and 8 in the latter.
405 WriteBytes(ELF::ElfMagic); // e_ident[EI_MAG0] to e_ident[EI_MAG3]
407 Write8(is64Bit() ? ELF::ELFCLASS64 : ELF::ELFCLASS32); // e_ident[EI_CLASS]
410 Write8(isLittleEndian() ? ELF::ELFDATA2LSB : ELF::ELFDATA2MSB);
412 Write8(ELF::EV_CURRENT); // e_ident[EI_VERSION]
414 Write8(TargetObjectWriter->getOSABI());
415 Write8(0); // e_ident[EI_ABIVERSION]
417 WriteZeros(ELF::EI_NIDENT - ELF::EI_PAD);
419 Write16(ELF::ET_REL); // e_type
421 Write16(TargetObjectWriter->getEMachine()); // e_machine = target
423 Write32(ELF::EV_CURRENT); // e_version
424 WriteWord(0); // e_entry, no entry point in .o file
425 WriteWord(0); // e_phoff, no program header for .o
426 WriteWord(0); // e_shoff = sec hdr table off in bytes
428 // e_flags = whatever the target wants
429 Write32(Asm.getELFHeaderEFlags());
431 // e_ehsize = ELF header size
432 Write16(is64Bit() ? sizeof(ELF::Elf64_Ehdr) : sizeof(ELF::Elf32_Ehdr));
434 Write16(0); // e_phentsize = prog header entry size
435 Write16(0); // e_phnum = # prog header entries = 0
437 // e_shentsize = Section header entry size
438 Write16(is64Bit() ? sizeof(ELF::Elf64_Shdr) : sizeof(ELF::Elf32_Shdr));
440 // e_shnum = # of section header ents
443 // e_shstrndx = Section # of '.shstrtab'
444 assert(ShstrtabIndex < ELF::SHN_LORESERVE);
445 Write16(ShstrtabIndex);
448 uint64_t ELFObjectWriter::SymbolValue(MCSymbolData &Data,
449 const MCAsmLayout &Layout) {
450 if (Data.isCommon() && Data.isExternal())
451 return Data.getCommonAlignment();
454 if (!Layout.getSymbolOffset(&Data, Res))
457 if (Layout.getAssembler().isThumbFunc(&Data.getSymbol()))
463 void ELFObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
464 const MCAsmLayout &Layout) {
465 // The presence of symbol versions causes undefined symbols and
466 // versions declared with @@@ to be renamed.
468 for (MCSymbolData &OriginalData : Asm.symbols()) {
469 const MCSymbol &Alias = OriginalData.getSymbol();
472 if (!Alias.isVariable())
474 auto *Ref = dyn_cast<MCSymbolRefExpr>(Alias.getVariableValue());
477 const MCSymbol &Symbol = Ref->getSymbol();
478 MCSymbolData &SD = Asm.getSymbolData(Symbol);
480 StringRef AliasName = Alias.getName();
481 size_t Pos = AliasName.find('@');
482 if (Pos == StringRef::npos)
485 // Aliases defined with .symvar copy the binding from the symbol they alias.
486 // This is the first place we are able to copy this information.
487 OriginalData.setExternal(SD.isExternal());
488 MCELF::SetBinding(OriginalData, MCELF::GetBinding(SD));
490 StringRef Rest = AliasName.substr(Pos);
491 if (!Symbol.isUndefined() && !Rest.startswith("@@@"))
494 // FIXME: produce a better error message.
495 if (Symbol.isUndefined() && Rest.startswith("@@") &&
496 !Rest.startswith("@@@"))
497 report_fatal_error("A @@ version cannot be undefined");
499 Renames.insert(std::make_pair(&Symbol, &Alias));
503 static uint8_t mergeTypeForSet(uint8_t origType, uint8_t newType) {
504 uint8_t Type = newType;
506 // Propagation rules:
507 // IFUNC > FUNC > OBJECT > NOTYPE
508 // TLS_OBJECT > OBJECT > NOTYPE
510 // dont let the new type degrade the old type
514 case ELF::STT_GNU_IFUNC:
515 if (Type == ELF::STT_FUNC || Type == ELF::STT_OBJECT ||
516 Type == ELF::STT_NOTYPE || Type == ELF::STT_TLS)
517 Type = ELF::STT_GNU_IFUNC;
520 if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
521 Type == ELF::STT_TLS)
522 Type = ELF::STT_FUNC;
524 case ELF::STT_OBJECT:
525 if (Type == ELF::STT_NOTYPE)
526 Type = ELF::STT_OBJECT;
529 if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
530 Type == ELF::STT_GNU_IFUNC || Type == ELF::STT_FUNC)
538 void ELFObjectWriter::WriteSymbol(SymbolTableWriter &Writer, ELFSymbolData &MSD,
539 const MCAsmLayout &Layout) {
540 MCSymbolData &OrigData = *MSD.SymbolData;
541 assert((!OrigData.getFragment() ||
542 (&OrigData.getFragment()->getParent()->getSection() ==
543 &OrigData.getSymbol().getSection())) &&
544 "The symbol's section doesn't match the fragment's symbol");
545 const MCSymbol *Base = Layout.getBaseSymbol(OrigData.getSymbol());
547 // This has to be in sync with when computeSymbolTable uses SHN_ABS or
549 bool IsReserved = !Base || OrigData.isCommon();
551 // Binding and Type share the same byte as upper and lower nibbles
552 uint8_t Binding = MCELF::GetBinding(OrigData);
553 uint8_t Type = MCELF::GetType(OrigData);
554 MCSymbolData *BaseSD = nullptr;
556 BaseSD = &Layout.getAssembler().getSymbolData(*Base);
557 Type = mergeTypeForSet(Type, MCELF::GetType(*BaseSD));
559 uint8_t Info = (Binding << ELF_STB_Shift) | (Type << ELF_STT_Shift);
561 // Other and Visibility share the same byte with Visibility using the lower
563 uint8_t Visibility = MCELF::GetVisibility(OrigData);
564 uint8_t Other = MCELF::getOther(OrigData) << (ELF_STO_Shift - ELF_STV_Shift);
567 uint64_t Value = SymbolValue(OrigData, Layout);
570 const MCExpr *ESize = OrigData.getSize();
572 ESize = BaseSD->getSize();
576 if (!ESize->evaluateKnownAbsolute(Res, Layout))
577 report_fatal_error("Size expression must be absolute.");
581 // Write out the symbol table entry
582 Writer.writeSymbol(MSD.StringIndex, Info, Value, Size, Other,
583 MSD.SectionIndex, IsReserved);
586 void ELFObjectWriter::WriteSymbolTable(
587 MCAssembler &Asm, const MCAsmLayout &Layout,
588 std::vector<const MCSectionELF *> &Sections,
589 SectionOffsetsTy &SectionOffsets) {
591 MCContext &Ctx = Asm.getContext();
593 unsigned EntrySize = is64Bit() ? ELF::SYMENTRY_SIZE64 : ELF::SYMENTRY_SIZE32;
596 const MCSectionELF *SymtabSection =
597 Ctx.getELFSection(".symtab", ELF::SHT_SYMTAB, 0, EntrySize, "");
598 MCSectionData &SymtabSD = Asm.getOrCreateSectionData(*SymtabSection);
599 SymtabSD.setAlignment(is64Bit() ? 8 : 4);
600 SymbolTableIndex = Sections.size() + 1;
601 Sections.push_back(SymtabSection);
603 // The string table must be emitted first because we need the index
604 // into the string table for all the symbol names.
606 SymbolTableWriter Writer(*this, is64Bit());
608 uint64_t Padding = OffsetToAlignment(OS.tell(), SymtabSD.getAlignment());
611 uint64_t SecStart = OS.tell();
613 // The first entry is the undefined symbol entry.
614 Writer.writeSymbol(0, 0, 0, 0, 0, 0, false);
616 for (unsigned i = 0, e = FileSymbolData.size(); i != e; ++i) {
617 Writer.writeSymbol(FileSymbolData[i], ELF::STT_FILE | ELF::STB_LOCAL, 0, 0,
618 ELF::STV_DEFAULT, ELF::SHN_ABS, true);
621 // Write the symbol table entries.
622 LastLocalSymbolIndex = FileSymbolData.size() + LocalSymbolData.size() + 1;
624 for (unsigned i = 0, e = LocalSymbolData.size(); i != e; ++i) {
625 ELFSymbolData &MSD = LocalSymbolData[i];
626 WriteSymbol(Writer, MSD, Layout);
629 for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i) {
630 ELFSymbolData &MSD = ExternalSymbolData[i];
631 MCSymbolData &Data = *MSD.SymbolData;
632 assert(((Data.getFlags() & ELF_STB_Global) ||
633 (Data.getFlags() & ELF_STB_Weak)) &&
634 "External symbol requires STB_GLOBAL or STB_WEAK flag");
635 WriteSymbol(Writer, MSD, Layout);
636 if (MCELF::GetBinding(Data) == ELF::STB_LOCAL)
637 LastLocalSymbolIndex++;
640 for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i) {
641 ELFSymbolData &MSD = UndefinedSymbolData[i];
642 MCSymbolData &Data = *MSD.SymbolData;
643 WriteSymbol(Writer, MSD, Layout);
644 if (MCELF::GetBinding(Data) == ELF::STB_LOCAL)
645 LastLocalSymbolIndex++;
648 uint64_t SecEnd = OS.tell();
649 SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
651 ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
652 if (ShndxIndexes.empty())
655 SecStart = OS.tell();
656 const MCSectionELF *SymtabShndxSection =
657 Ctx.getELFSection(".symtab_shndxr", ELF::SHT_SYMTAB_SHNDX, 0, 4, "");
658 Sections.push_back(SymtabShndxSection);
659 MCSectionData *SymtabShndxSD =
660 &Asm.getOrCreateSectionData(*SymtabShndxSection);
661 SymtabShndxSD->setAlignment(4);
662 for (uint32_t Index : ShndxIndexes)
665 SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
668 // It is always valid to create a relocation with a symbol. It is preferable
669 // to use a relocation with a section if that is possible. Using the section
670 // allows us to omit some local symbols from the symbol table.
671 bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
672 const MCSymbolRefExpr *RefA,
673 const MCSymbolData *SD,
675 unsigned Type) const {
676 // A PCRel relocation to an absolute value has no symbol (or section). We
677 // represent that with a relocation to a null section.
681 MCSymbolRefExpr::VariantKind Kind = RefA->getKind();
685 // The .odp creation emits a relocation against the symbol ".TOC." which
686 // create a R_PPC64_TOC relocation. However the relocation symbol name
687 // in final object creation should be NULL, since the symbol does not
688 // really exist, it is just the reference to TOC base for the current
689 // object file. Since the symbol is undefined, returning false results
690 // in a relocation with a null section which is the desired result.
691 case MCSymbolRefExpr::VK_PPC_TOCBASE:
694 // These VariantKind cause the relocation to refer to something other than
695 // the symbol itself, like a linker generated table. Since the address of
696 // symbol is not relevant, we cannot replace the symbol with the
697 // section and patch the difference in the addend.
698 case MCSymbolRefExpr::VK_GOT:
699 case MCSymbolRefExpr::VK_PLT:
700 case MCSymbolRefExpr::VK_GOTPCREL:
701 case MCSymbolRefExpr::VK_Mips_GOT:
702 case MCSymbolRefExpr::VK_PPC_GOT_LO:
703 case MCSymbolRefExpr::VK_PPC_GOT_HI:
704 case MCSymbolRefExpr::VK_PPC_GOT_HA:
708 // An undefined symbol is not in any section, so the relocation has to point
709 // to the symbol itself.
710 const MCSymbol &Sym = SD->getSymbol();
711 if (Sym.isUndefined())
714 unsigned Binding = MCELF::GetBinding(*SD);
717 llvm_unreachable("Invalid Binding");
721 // If the symbol is weak, it might be overridden by a symbol in another
722 // file. The relocation has to point to the symbol so that the linker
725 case ELF::STB_GLOBAL:
726 // Global ELF symbols can be preempted by the dynamic linker. The relocation
727 // has to point to the symbol for a reason analogous to the STB_WEAK case.
731 // If a relocation points to a mergeable section, we have to be careful.
732 // If the offset is zero, a relocation with the section will encode the
733 // same information. With a non-zero offset, the situation is different.
734 // For example, a relocation can point 42 bytes past the end of a string.
735 // If we change such a relocation to use the section, the linker would think
736 // that it pointed to another string and subtracting 42 at runtime will
737 // produce the wrong value.
738 auto &Sec = cast<MCSectionELF>(Sym.getSection());
739 unsigned Flags = Sec.getFlags();
740 if (Flags & ELF::SHF_MERGE) {
744 // It looks like gold has a bug (http://sourceware.org/PR16794) and can
745 // only handle section relocations to mergeable sections if using RELA.
746 if (!hasRelocationAddend())
750 // Most TLS relocations use a got, so they need the symbol. Even those that
751 // are just an offset (@tpoff), require a symbol in gold versions before
752 // 5efeedf61e4fe720fd3e9a08e6c91c10abb66d42 (2014-09-26) which fixed
753 // http://sourceware.org/PR16773.
754 if (Flags & ELF::SHF_TLS)
757 // If the symbol is a thumb function the final relocation must set the lowest
758 // bit. With a symbol that is done by just having the symbol have that bit
759 // set, so we would lose the bit if we relocated with the section.
760 // FIXME: We could use the section but add the bit to the relocation value.
761 if (Asm.isThumbFunc(&Sym))
764 if (TargetObjectWriter->needsRelocateWithSymbol(*SD, Type))
769 static const MCSymbol *getWeakRef(const MCSymbolRefExpr &Ref) {
770 const MCSymbol &Sym = Ref.getSymbol();
772 if (Ref.getKind() == MCSymbolRefExpr::VK_WEAKREF)
775 if (!Sym.isVariable())
778 const MCExpr *Expr = Sym.getVariableValue();
779 const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr);
783 if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
784 return &Inner->getSymbol();
788 // True if the assembler knows nothing about the final value of the symbol.
789 // This doesn't cover the comdat issues, since in those cases the assembler
790 // can at least know that all symbols in the section will move together.
791 static bool isWeak(const MCSymbolData &D) {
792 if (MCELF::GetType(D) == ELF::STT_GNU_IFUNC)
795 switch (MCELF::GetBinding(D)) {
797 llvm_unreachable("Unknown binding");
800 case ELF::STB_GLOBAL:
803 case ELF::STB_GNU_UNIQUE:
808 void ELFObjectWriter::RecordRelocation(MCAssembler &Asm,
809 const MCAsmLayout &Layout,
810 const MCFragment *Fragment,
811 const MCFixup &Fixup, MCValue Target,
812 bool &IsPCRel, uint64_t &FixedValue) {
813 const MCSectionData *FixupSection = Fragment->getParent();
814 uint64_t C = Target.getConstant();
815 uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
817 if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
818 assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
819 "Should not have constructed this");
821 // Let A, B and C being the components of Target and R be the location of
822 // the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
823 // If it is pcrel, we want to compute (A - B + C - R).
825 // In general, ELF has no relocations for -B. It can only represent (A + C)
826 // or (A + C - R). If B = R + K and the relocation is not pcrel, we can
827 // replace B to implement it: (A - R - K + C)
829 Asm.getContext().FatalError(
831 "No relocation available to represent this relative expression");
833 const MCSymbol &SymB = RefB->getSymbol();
835 if (SymB.isUndefined())
836 Asm.getContext().FatalError(
838 Twine("symbol '") + SymB.getName() +
839 "' can not be undefined in a subtraction expression");
841 assert(!SymB.isAbsolute() && "Should have been folded");
842 const MCSection &SecB = SymB.getSection();
843 if (&SecB != &FixupSection->getSection())
844 Asm.getContext().FatalError(
845 Fixup.getLoc(), "Cannot represent a difference across sections");
847 const MCSymbolData &SymBD = Asm.getSymbolData(SymB);
849 Asm.getContext().FatalError(
850 Fixup.getLoc(), "Cannot represent a subtraction with a weak symbol");
852 uint64_t SymBOffset = Layout.getSymbolOffset(&SymBD);
853 uint64_t K = SymBOffset - FixupOffset;
858 // We either rejected the fixup or folded B into C at this point.
859 const MCSymbolRefExpr *RefA = Target.getSymA();
860 const MCSymbol *SymA = RefA ? &RefA->getSymbol() : nullptr;
861 const MCSymbolData *SymAD = SymA ? &Asm.getSymbolData(*SymA) : nullptr;
863 unsigned Type = GetRelocType(Target, Fixup, IsPCRel);
864 bool RelocateWithSymbol = shouldRelocateWithSymbol(Asm, RefA, SymAD, C, Type);
865 if (!RelocateWithSymbol && SymA && !SymA->isUndefined())
866 C += Layout.getSymbolOffset(SymAD);
869 if (hasRelocationAddend()) {
876 // FIXME: What is this!?!?
877 MCSymbolRefExpr::VariantKind Modifier =
878 RefA ? RefA->getKind() : MCSymbolRefExpr::VK_None;
879 if (RelocNeedsGOT(Modifier))
882 if (!RelocateWithSymbol) {
883 const MCSection *SecA =
884 (SymA && !SymA->isUndefined()) ? &SymA->getSection() : nullptr;
885 auto *ELFSec = cast_or_null<MCSectionELF>(SecA);
886 MCSymbol *SectionSymbol =
887 ELFSec ? Asm.getContext().getOrCreateSectionSymbol(*ELFSec)
889 ELFRelocationEntry Rec(FixupOffset, SectionSymbol, Type, Addend);
890 Relocations[FixupSection].push_back(Rec);
895 if (const MCSymbol *R = Renames.lookup(SymA))
898 if (const MCSymbol *WeakRef = getWeakRef(*RefA))
899 WeakrefUsedInReloc.insert(WeakRef);
901 UsedInReloc.insert(SymA);
903 ELFRelocationEntry Rec(FixupOffset, SymA, Type, Addend);
904 Relocations[FixupSection].push_back(Rec);
910 ELFObjectWriter::getSymbolIndexInSymbolTable(const MCAssembler &Asm,
912 const MCSymbolData &SD = Asm.getSymbolData(*S);
913 return SD.getIndex();
916 bool ELFObjectWriter::isInSymtab(const MCAsmLayout &Layout,
917 const MCSymbolData &Data, bool Used,
919 const MCSymbol &Symbol = Data.getSymbol();
920 if (Symbol.isVariable()) {
921 const MCExpr *Expr = Symbol.getVariableValue();
922 if (const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr)) {
923 if (Ref->getKind() == MCSymbolRefExpr::VK_WEAKREF)
934 if (Symbol.getName() == "_GLOBAL_OFFSET_TABLE_")
937 if (Symbol.isVariable()) {
938 const MCSymbol *Base = Layout.getBaseSymbol(Symbol);
939 if (Base && Base->isUndefined())
943 bool IsGlobal = MCELF::GetBinding(Data) == ELF::STB_GLOBAL;
944 if (!Symbol.isVariable() && Symbol.isUndefined() && !IsGlobal)
947 if (Symbol.isTemporary())
953 bool ELFObjectWriter::isLocal(const MCSymbolData &Data, bool isUsedInReloc) {
954 if (Data.isExternal())
957 const MCSymbol &Symbol = Data.getSymbol();
958 if (Symbol.isDefined())
967 void ELFObjectWriter::maybeAddToGroup(MCAssembler &Asm,
968 ArrayRef<const MCSectionELF *> Sections,
969 const RevGroupMapTy &RevGroupMap,
970 const MCSectionELF &Section,
972 const MCSymbol *Sym = Section.getGroup();
975 const MCSectionELF *Group = Sections[RevGroupMap.lookup(Sym) - 1];
976 MCSectionData &Data = Asm.getOrCreateSectionData(*Group);
977 // FIXME: we could use the previous fragment
978 MCDataFragment *F = new MCDataFragment(&Data);
982 void ELFObjectWriter::computeIndexMap(
983 MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections,
984 SectionIndexMapTy &SectionIndexMap, const RevGroupMapTy &RevGroupMap) {
985 for (const MCSectionData &SD : Asm) {
986 const MCSectionELF &Section =
987 static_cast<const MCSectionELF &>(SD.getSection());
988 if (Section.getType() == ELF::SHT_GROUP)
990 Sections.push_back(&Section);
991 unsigned Index = Sections.size();
992 SectionIndexMap[&Section] = Index;
993 maybeAddToGroup(Asm, Sections, RevGroupMap, Section, Index);
994 createRelocationSection(Asm, SD);
998 void ELFObjectWriter::computeSymbolTable(
999 MCAssembler &Asm, const MCAsmLayout &Layout,
1000 const SectionIndexMapTy &SectionIndexMap,
1001 const RevGroupMapTy &RevGroupMap) {
1002 // FIXME: Is this the correct place to do this?
1003 // FIXME: Why is an undefined reference to _GLOBAL_OFFSET_TABLE_ needed?
1005 StringRef Name = "_GLOBAL_OFFSET_TABLE_";
1006 MCSymbol *Sym = Asm.getContext().GetOrCreateSymbol(Name);
1007 MCSymbolData &Data = Asm.getOrCreateSymbolData(*Sym);
1008 Data.setExternal(true);
1009 MCELF::SetBinding(Data, ELF::STB_GLOBAL);
1012 // Add the data for the symbols.
1013 for (MCSymbolData &SD : Asm.symbols()) {
1014 const MCSymbol &Symbol = SD.getSymbol();
1016 bool Used = UsedInReloc.count(&Symbol);
1017 bool WeakrefUsed = WeakrefUsedInReloc.count(&Symbol);
1018 bool isSignature = RevGroupMap.count(&Symbol);
1020 if (!isInSymtab(Layout, SD,
1021 Used || WeakrefUsed || isSignature,
1022 Renames.count(&Symbol)))
1026 MSD.SymbolData = &SD;
1027 const MCSymbol *BaseSymbol = Layout.getBaseSymbol(Symbol);
1029 // Undefined symbols are global, but this is the first place we
1030 // are able to set it.
1031 bool Local = isLocal(SD, Used);
1032 if (!Local && MCELF::GetBinding(SD) == ELF::STB_LOCAL) {
1034 MCSymbolData &BaseData = Asm.getSymbolData(*BaseSymbol);
1035 MCELF::SetBinding(SD, ELF::STB_GLOBAL);
1036 MCELF::SetBinding(BaseData, ELF::STB_GLOBAL);
1040 MSD.SectionIndex = ELF::SHN_ABS;
1041 } else if (SD.isCommon()) {
1043 MSD.SectionIndex = ELF::SHN_COMMON;
1044 } else if (BaseSymbol->isUndefined()) {
1045 if (isSignature && !Used)
1046 MSD.SectionIndex = RevGroupMap.lookup(&Symbol);
1048 MSD.SectionIndex = ELF::SHN_UNDEF;
1049 if (!Used && WeakrefUsed)
1050 MCELF::SetBinding(SD, ELF::STB_WEAK);
1052 const MCSectionELF &Section =
1053 static_cast<const MCSectionELF&>(BaseSymbol->getSection());
1054 MSD.SectionIndex = SectionIndexMap.lookup(&Section);
1055 assert(MSD.SectionIndex && "Invalid section index!");
1058 // The @@@ in symbol version is replaced with @ in undefined symbols and @@
1061 // FIXME: All name handling should be done before we get to the writer,
1062 // including dealing with GNU-style version suffixes. Fixing this isn't
1065 // We thus have to be careful to not perform the symbol version replacement
1068 // The ELF format is used on Windows by the MCJIT engine. Thus, on
1069 // Windows, the ELFObjectWriter can encounter symbols mangled using the MS
1070 // Visual Studio C++ name mangling scheme. Symbols mangled using the MSVC
1071 // C++ name mangling can legally have "@@@" as a sub-string. In that case,
1072 // the EFLObjectWriter should not interpret the "@@@" sub-string as
1073 // specifying GNU-style symbol versioning. The ELFObjectWriter therefore
1074 // checks for the MSVC C++ name mangling prefix which is either "?", "@?",
1075 // "__imp_?" or "__imp_@?".
1077 // It would have been interesting to perform the MS mangling prefix check
1078 // only when the target triple is of the form *-pc-windows-elf. But, it
1079 // seems that this information is not easily accessible from the
1081 StringRef Name = Symbol.getName();
1082 if (!Name.startswith("?") && !Name.startswith("@?") &&
1083 !Name.startswith("__imp_?") && !Name.startswith("__imp_@?")) {
1084 // This symbol isn't following the MSVC C++ name mangling convention. We
1085 // can thus safely interpret the @@@ in symbol names as specifying symbol
1087 SmallString<32> Buf;
1088 size_t Pos = Name.find("@@@");
1089 if (Pos != StringRef::npos) {
1090 Buf += Name.substr(0, Pos);
1091 unsigned Skip = MSD.SectionIndex == ELF::SHN_UNDEF ? 2 : 1;
1092 Buf += Name.substr(Pos + Skip);
1097 // Sections have their own string table
1098 if (MCELF::GetType(SD) != ELF::STT_SECTION)
1099 MSD.Name = StrTabBuilder.add(Name);
1101 if (MSD.SectionIndex == ELF::SHN_UNDEF)
1102 UndefinedSymbolData.push_back(MSD);
1104 LocalSymbolData.push_back(MSD);
1106 ExternalSymbolData.push_back(MSD);
1109 for (auto i = Asm.file_names_begin(), e = Asm.file_names_end(); i != e; ++i)
1110 StrTabBuilder.add(*i);
1112 StrTabBuilder.finalize(StringTableBuilder::ELF);
1114 for (auto i = Asm.file_names_begin(), e = Asm.file_names_end(); i != e; ++i)
1115 FileSymbolData.push_back(StrTabBuilder.getOffset(*i));
1117 for (ELFSymbolData &MSD : LocalSymbolData)
1118 MSD.StringIndex = MCELF::GetType(*MSD.SymbolData) == ELF::STT_SECTION
1120 : StrTabBuilder.getOffset(MSD.Name);
1121 for (ELFSymbolData &MSD : ExternalSymbolData)
1122 MSD.StringIndex = StrTabBuilder.getOffset(MSD.Name);
1123 for (ELFSymbolData& MSD : UndefinedSymbolData)
1124 MSD.StringIndex = StrTabBuilder.getOffset(MSD.Name);
1126 // Symbols are required to be in lexicographic order.
1127 array_pod_sort(LocalSymbolData.begin(), LocalSymbolData.end());
1128 array_pod_sort(ExternalSymbolData.begin(), ExternalSymbolData.end());
1129 array_pod_sort(UndefinedSymbolData.begin(), UndefinedSymbolData.end());
1131 // Set the symbol indices. Local symbols must come before all other
1132 // symbols with non-local bindings.
1133 unsigned Index = FileSymbolData.size() + 1;
1134 for (unsigned i = 0, e = LocalSymbolData.size(); i != e; ++i)
1135 LocalSymbolData[i].SymbolData->setIndex(Index++);
1137 for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i)
1138 ExternalSymbolData[i].SymbolData->setIndex(Index++);
1139 for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i)
1140 UndefinedSymbolData[i].SymbolData->setIndex(Index++);
1143 void ELFObjectWriter::createRelocationSection(MCAssembler &Asm,
1144 const MCSectionData &SD) {
1145 if (Relocations[&SD].empty())
1148 MCContext &Ctx = Asm.getContext();
1149 const MCSectionELF &Section =
1150 static_cast<const MCSectionELF &>(SD.getSection());
1152 const StringRef SectionName = Section.getSectionName();
1153 std::string RelaSectionName = hasRelocationAddend() ? ".rela" : ".rel";
1154 RelaSectionName += SectionName;
1157 if (hasRelocationAddend())
1158 EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rela) : sizeof(ELF::Elf32_Rela);
1160 EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rel) : sizeof(ELF::Elf32_Rel);
1163 if (Section.getFlags() & ELF::SHF_GROUP)
1164 Flags = ELF::SHF_GROUP;
1166 const MCSectionELF *RelaSection = Ctx.createELFRelSection(
1167 RelaSectionName, hasRelocationAddend() ? ELF::SHT_RELA : ELF::SHT_REL,
1168 Flags, EntrySize, Section.getGroup(), &Section);
1169 Asm.getOrCreateSectionData(*RelaSection);
1172 static SmallVector<char, 128>
1173 getUncompressedData(const MCAsmLayout &Layout,
1174 MCSectionData::FragmentListType &Fragments) {
1175 SmallVector<char, 128> UncompressedData;
1176 for (const MCFragment &F : Fragments) {
1177 const SmallVectorImpl<char> *Contents;
1178 switch (F.getKind()) {
1179 case MCFragment::FT_Data:
1180 Contents = &cast<MCDataFragment>(F).getContents();
1182 case MCFragment::FT_Dwarf:
1183 Contents = &cast<MCDwarfLineAddrFragment>(F).getContents();
1185 case MCFragment::FT_DwarfFrame:
1186 Contents = &cast<MCDwarfCallFrameFragment>(F).getContents();
1190 "Not expecting any other fragment types in a debug_* section");
1192 UncompressedData.append(Contents->begin(), Contents->end());
1194 return UncompressedData;
1197 // Include the debug info compression header:
1198 // "ZLIB" followed by 8 bytes representing the uncompressed size of the section,
1199 // useful for consumers to preallocate a buffer to decompress into.
1201 prependCompressionHeader(uint64_t Size,
1202 SmallVectorImpl<char> &CompressedContents) {
1203 const StringRef Magic = "ZLIB";
1204 if (Size <= Magic.size() + sizeof(Size) + CompressedContents.size())
1206 if (sys::IsLittleEndianHost)
1207 sys::swapByteOrder(Size);
1208 CompressedContents.insert(CompressedContents.begin(),
1209 Magic.size() + sizeof(Size), 0);
1210 std::copy(Magic.begin(), Magic.end(), CompressedContents.begin());
1211 std::copy(reinterpret_cast<char *>(&Size),
1212 reinterpret_cast<char *>(&Size + 1),
1213 CompressedContents.begin() + Magic.size());
1217 // Return a single fragment containing the compressed contents of the whole
1218 // section. Null if the section was not compressed for any reason.
1219 static std::unique_ptr<MCDataFragment>
1220 getCompressedFragment(const MCAsmLayout &Layout,
1221 MCSectionData::FragmentListType &Fragments) {
1222 std::unique_ptr<MCDataFragment> CompressedFragment(new MCDataFragment());
1224 // Gather the uncompressed data from all the fragments, recording the
1225 // alignment fragment, if seen, and any fixups.
1226 SmallVector<char, 128> UncompressedData =
1227 getUncompressedData(Layout, Fragments);
1229 SmallVectorImpl<char> &CompressedContents = CompressedFragment->getContents();
1231 zlib::Status Success = zlib::compress(
1232 StringRef(UncompressedData.data(), UncompressedData.size()),
1233 CompressedContents);
1234 if (Success != zlib::StatusOK)
1237 if (!prependCompressionHeader(UncompressedData.size(), CompressedContents))
1240 return CompressedFragment;
1243 typedef DenseMap<const MCSectionData *, std::vector<MCSymbolData *>>
1246 static void UpdateSymbols(const MCAsmLayout &Layout,
1247 const std::vector<MCSymbolData *> &Symbols,
1248 MCFragment &NewFragment) {
1249 for (MCSymbolData *Sym : Symbols) {
1250 Sym->setOffset(Sym->getOffset() +
1251 Layout.getFragmentOffset(Sym->getFragment()));
1252 Sym->setFragment(&NewFragment);
1256 static void CompressDebugSection(MCAssembler &Asm, MCAsmLayout &Layout,
1257 const DefiningSymbolMap &DefiningSymbols,
1258 const MCSectionELF &Section,
1259 MCSectionData &SD) {
1260 StringRef SectionName = Section.getSectionName();
1261 MCSectionData::FragmentListType &Fragments = SD.getFragmentList();
1263 std::unique_ptr<MCDataFragment> CompressedFragment =
1264 getCompressedFragment(Layout, Fragments);
1266 // Leave the section as-is if the fragments could not be compressed.
1267 if (!CompressedFragment)
1270 // Update the fragment+offsets of any symbols referring to fragments in this
1271 // section to refer to the new fragment.
1272 auto I = DefiningSymbols.find(&SD);
1273 if (I != DefiningSymbols.end())
1274 UpdateSymbols(Layout, I->second, *CompressedFragment);
1276 // Invalidate the layout for the whole section since it will have new and
1277 // different fragments now.
1278 Layout.invalidateFragmentsFrom(&Fragments.front());
1281 // Complete the initialization of the new fragment
1282 CompressedFragment->setParent(&SD);
1283 CompressedFragment->setLayoutOrder(0);
1284 Fragments.push_back(CompressedFragment.release());
1286 // Rename from .debug_* to .zdebug_*
1287 Asm.getContext().renameELFSection(&Section,
1288 (".z" + SectionName.drop_front(1)).str());
1291 void ELFObjectWriter::CompressDebugSections(MCAssembler &Asm,
1292 MCAsmLayout &Layout) {
1293 if (!Asm.getContext().getAsmInfo()->compressDebugSections())
1296 DefiningSymbolMap DefiningSymbols;
1298 for (MCSymbolData &SD : Asm.symbols())
1299 if (MCFragment *F = SD.getFragment())
1300 DefiningSymbols[F->getParent()].push_back(&SD);
1302 for (MCSectionData &SD : Asm) {
1303 const MCSectionELF &Section =
1304 static_cast<const MCSectionELF &>(SD.getSection());
1305 StringRef SectionName = Section.getSectionName();
1307 // Compressing debug_frame requires handling alignment fragments which is
1308 // more work (possibly generalizing MCAssembler.cpp:writeFragment to allow
1309 // for writing to arbitrary buffers) for little benefit.
1310 if (!SectionName.startswith(".debug_") || SectionName == ".debug_frame")
1313 CompressDebugSection(Asm, Layout, DefiningSymbols, Section, SD);
1317 void ELFObjectWriter::WriteRelocations(MCAssembler &Asm,
1318 const MCAsmLayout &Layout) {
1319 for (MCAssembler::iterator it = Asm.begin(), ie = Asm.end(); it != ie; ++it) {
1320 MCSectionData &RelSD = *it;
1321 const MCSectionELF &RelSection =
1322 static_cast<const MCSectionELF &>(RelSD.getSection());
1324 unsigned Type = RelSection.getType();
1325 if (Type != ELF::SHT_REL && Type != ELF::SHT_RELA)
1328 const MCSectionELF *Section = RelSection.getAssociatedSection();
1329 MCSectionData &SD = Asm.getOrCreateSectionData(*Section);
1330 RelSD.setAlignment(is64Bit() ? 8 : 4);
1332 MCDataFragment *F = new MCDataFragment(&RelSD);
1333 WriteRelocationsFragment(Asm, F, &SD);
1337 void ELFObjectWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type,
1338 uint64_t Flags, uint64_t Address,
1339 uint64_t Offset, uint64_t Size,
1340 uint32_t Link, uint32_t Info,
1342 uint64_t EntrySize) {
1343 Write32(Name); // sh_name: index into string table
1344 Write32(Type); // sh_type
1345 WriteWord(Flags); // sh_flags
1346 WriteWord(Address); // sh_addr
1347 WriteWord(Offset); // sh_offset
1348 WriteWord(Size); // sh_size
1349 Write32(Link); // sh_link
1350 Write32(Info); // sh_info
1351 WriteWord(Alignment); // sh_addralign
1352 WriteWord(EntrySize); // sh_entsize
1355 void ELFObjectWriter::WriteRelocationsFragment(const MCAssembler &Asm,
1357 const MCSectionData *SD) {
1358 std::vector<ELFRelocationEntry> &Relocs = Relocations[SD];
1360 // Sort the relocation entries. Most targets just sort by Offset, but some
1361 // (e.g., MIPS) have additional constraints.
1362 TargetObjectWriter->sortRelocs(Asm, Relocs);
1364 for (unsigned i = 0, e = Relocs.size(); i != e; ++i) {
1365 const ELFRelocationEntry &Entry = Relocs[e - i - 1];
1367 Entry.Symbol ? getSymbolIndexInSymbolTable(Asm, Entry.Symbol) : 0;
1370 write(*F, Entry.Offset);
1371 if (TargetObjectWriter->isN64()) {
1372 write(*F, uint32_t(Index));
1374 write(*F, TargetObjectWriter->getRSsym(Entry.Type));
1375 write(*F, TargetObjectWriter->getRType3(Entry.Type));
1376 write(*F, TargetObjectWriter->getRType2(Entry.Type));
1377 write(*F, TargetObjectWriter->getRType(Entry.Type));
1379 struct ELF::Elf64_Rela ERE64;
1380 ERE64.setSymbolAndType(Index, Entry.Type);
1381 write(*F, ERE64.r_info);
1383 if (hasRelocationAddend())
1384 write(*F, Entry.Addend);
1386 write(*F, uint32_t(Entry.Offset));
1388 struct ELF::Elf32_Rela ERE32;
1389 ERE32.setSymbolAndType(Index, Entry.Type);
1390 write(*F, ERE32.r_info);
1392 if (hasRelocationAddend())
1393 write(*F, uint32_t(Entry.Addend));
1398 void ELFObjectWriter::createSectionHeaderStringTable(
1399 MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections) {
1400 const MCSectionELF *ShstrtabSection = Sections[ShstrtabIndex - 1];
1402 Asm.getOrCreateSectionData(*ShstrtabSection);
1404 for (MCSectionData &SD : Asm) {
1405 const MCSectionELF &Section =
1406 static_cast<const MCSectionELF &>(SD.getSection());
1407 ShStrTabBuilder.add(Section.getSectionName());
1409 ShStrTabBuilder.finalize(StringTableBuilder::ELF);
1410 OS << ShStrTabBuilder.data();
1413 void ELFObjectWriter::createStringTable(
1414 MCAssembler &Asm, std::vector<const MCSectionELF *> &Sections) {
1415 MCContext &Ctx = Asm.getContext();
1416 const MCSectionELF *StrtabSection =
1417 Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
1418 Asm.getOrCreateSectionData(*StrtabSection);
1419 Sections.push_back(StrtabSection);
1420 StringTableIndex = Sections.size();
1421 OS << StrTabBuilder.data();
1424 void ELFObjectWriter::createIndexedSections(
1425 MCAssembler &Asm, const MCAsmLayout &Layout, RevGroupMapTy &RevGroupMap,
1426 std::vector<const MCSectionELF *> &Sections,
1427 SectionIndexMapTy &SectionIndexMap) {
1428 MCContext &Ctx = Asm.getContext();
1430 const MCSectionELF *ShstrtabSection =
1431 Ctx.getELFSection(".shstrtab", ELF::SHT_STRTAB, 0);
1432 Sections.push_back(ShstrtabSection);
1433 ShstrtabIndex = Sections.size();
1434 assert(ShstrtabIndex == 1);
1437 for (const MCSectionData &SD : Asm) {
1438 const MCSectionELF &Section =
1439 static_cast<const MCSectionELF &>(SD.getSection());
1440 if (!(Section.getFlags() & ELF::SHF_GROUP))
1443 const MCSymbol *SignatureSymbol = Section.getGroup();
1444 Asm.getOrCreateSymbolData(*SignatureSymbol);
1445 unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
1447 const MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
1448 Sections.push_back(Group);
1449 GroupIdx = Sections.size();
1451 MCSectionData &Data = Asm.getOrCreateSectionData(*Group);
1452 Data.setAlignment(4);
1453 MCDataFragment *F = new MCDataFragment(&Data);
1454 write(*F, uint32_t(ELF::GRP_COMDAT));
1458 computeIndexMap(Asm, Sections, SectionIndexMap, RevGroupMap);
1461 void ELFObjectWriter::writeSection(MCAssembler &Asm,
1462 const SectionIndexMapTy &SectionIndexMap,
1463 uint32_t GroupSymbolIndex,
1464 uint64_t Offset, uint64_t Size,
1466 const MCSectionELF &Section) {
1467 uint64_t sh_link = 0;
1468 uint64_t sh_info = 0;
1470 switch(Section.getType()) {
1475 case ELF::SHT_DYNAMIC:
1476 sh_link = ShStrTabBuilder.getOffset(Section.getSectionName());
1480 case ELF::SHT_RELA: {
1481 sh_link = SymbolTableIndex;
1482 assert(sh_link && ".symtab not found");
1483 const MCSectionELF *InfoSection = Section.getAssociatedSection();
1484 sh_info = SectionIndexMap.lookup(InfoSection);
1488 case ELF::SHT_SYMTAB:
1489 case ELF::SHT_DYNSYM:
1490 sh_link = StringTableIndex;
1491 sh_info = LastLocalSymbolIndex;
1494 case ELF::SHT_SYMTAB_SHNDX:
1495 sh_link = SymbolTableIndex;
1498 case ELF::SHT_GROUP:
1499 sh_link = SymbolTableIndex;
1500 sh_info = GroupSymbolIndex;
1504 if (TargetObjectWriter->getEMachine() == ELF::EM_ARM &&
1505 Section.getType() == ELF::SHT_ARM_EXIDX)
1506 sh_link = SectionIndexMap.lookup(Section.getAssociatedSection());
1508 WriteSecHdrEntry(ShStrTabBuilder.getOffset(Section.getSectionName()),
1510 Section.getFlags(), 0, Offset, Size, sh_link, sh_info,
1511 Alignment, Section.getEntrySize());
1514 bool ELFObjectWriter::IsELFMetaDataSection(const MCSectionData &SD) {
1515 return SD.getOrdinal() == ~UINT32_C(0) &&
1516 !SD.getSection().isVirtualSection();
1519 void ELFObjectWriter::writeDataSectionData(MCAssembler &Asm,
1520 const MCAsmLayout &Layout,
1521 const MCSectionData &SD) {
1522 if (IsELFMetaDataSection(SD)) {
1523 for (MCSectionData::const_iterator i = SD.begin(), e = SD.end(); i != e;
1525 const MCFragment &F = *i;
1526 assert(F.getKind() == MCFragment::FT_Data);
1527 WriteBytes(cast<MCDataFragment>(F).getContents());
1530 Asm.writeSectionData(&SD, Layout);
1534 void ELFObjectWriter::writeSectionHeader(
1535 ArrayRef<const MCSectionELF *> Sections, MCAssembler &Asm,
1536 const MCAsmLayout &Layout, const SectionIndexMapTy &SectionIndexMap,
1537 const SectionOffsetsTy &SectionOffsets) {
1538 const unsigned NumSections = Asm.size();
1540 // Null section first.
1541 uint64_t FirstSectionSize =
1542 (NumSections + 1) >= ELF::SHN_LORESERVE ? NumSections + 1 : 0;
1543 WriteSecHdrEntry(0, 0, 0, 0, 0, FirstSectionSize, 0, 0, 0, 0);
1545 for (unsigned i = 0; i < NumSections; ++i) {
1546 const MCSectionELF &Section = *Sections[i];
1547 const MCSectionData &SD = Asm.getOrCreateSectionData(Section);
1548 uint32_t GroupSymbolIndex;
1549 if (Section.getType() != ELF::SHT_GROUP)
1550 GroupSymbolIndex = 0;
1552 GroupSymbolIndex = getSymbolIndexInSymbolTable(Asm, Section.getGroup());
1554 const std::pair<uint64_t, uint64_t> &Offsets = SectionOffsets[i];
1555 uint64_t Size = Section.getType() == ELF::SHT_NOBITS
1556 ? Layout.getSectionAddressSize(&SD)
1557 : Offsets.second - Offsets.first;
1559 writeSection(Asm, SectionIndexMap, GroupSymbolIndex, Offsets.first, Size,
1560 SD.getAlignment(), Section);
1564 void ELFObjectWriter::WriteObject(MCAssembler &Asm,
1565 const MCAsmLayout &Layout) {
1566 RevGroupMapTy RevGroupMap;
1567 SectionIndexMapTy SectionIndexMap;
1569 CompressDebugSections(Asm, const_cast<MCAsmLayout &>(Layout));
1570 std::vector<const MCSectionELF *> Sections;
1571 createIndexedSections(Asm, Layout, RevGroupMap, Sections, SectionIndexMap);
1573 // Compute symbol table information.
1574 computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap);
1576 WriteRelocations(Asm, Layout);
1579 SectionOffsetsTy SectionOffsets;
1581 // Write out the ELF header ...
1584 // ... then the sections ...
1585 SectionOffsets.push_back(std::make_pair(0, 0));
1586 for (auto I = ++Sections.begin(), E = Sections.end(); I != E; ++I) {
1587 const MCSectionData &SD = Asm.getOrCreateSectionData(**I);
1588 uint64_t Padding = OffsetToAlignment(OS.tell(), SD.getAlignment());
1589 WriteZeros(Padding);
1591 // Remember the offset into the file for this section.
1592 uint64_t SecStart = OS.tell();
1593 writeDataSectionData(Asm, Layout, SD);
1594 uint64_t SecEnd = OS.tell();
1595 SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
1598 WriteSymbolTable(Asm, Layout, Sections, SectionOffsets);
1601 uint64_t SecStart = OS.tell();
1602 createStringTable(Asm, Sections);
1603 uint64_t SecEnd = OS.tell();
1604 SectionOffsets.push_back(std::make_pair(SecStart, SecEnd));
1608 uint64_t SecStart = OS.tell();
1609 createSectionHeaderStringTable(Asm, Sections);
1610 uint64_t SecEnd = OS.tell();
1611 SectionOffsets[0] = std::make_pair(SecStart, SecEnd);
1614 uint64_t NaturalAlignment = is64Bit() ? 8 : 4;
1615 uint64_t Padding = OffsetToAlignment(OS.tell(), NaturalAlignment);
1616 WriteZeros(Padding);
1618 const unsigned SectionHeaderOffset = OS.tell();
1620 // ... then the section header table ...
1621 writeSectionHeader(Sections, Asm, Layout, SectionIndexMap, SectionOffsets);
1623 uint16_t NumSections = (Sections.size() + 1 >= ELF::SHN_LORESERVE)
1625 : Sections.size() + 1;
1626 if (sys::IsLittleEndianHost != IsLittleEndian)
1627 sys::swapByteOrder(NumSections);
1628 unsigned NumSectionsOffset;
1631 uint64_t Val = SectionHeaderOffset;
1632 if (sys::IsLittleEndianHost != IsLittleEndian)
1633 sys::swapByteOrder(Val);
1634 OS.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
1635 offsetof(ELF::Elf64_Ehdr, e_shoff));
1636 NumSectionsOffset = offsetof(ELF::Elf64_Ehdr, e_shnum);
1638 uint32_t Val = SectionHeaderOffset;
1639 if (sys::IsLittleEndianHost != IsLittleEndian)
1640 sys::swapByteOrder(Val);
1641 OS.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
1642 offsetof(ELF::Elf32_Ehdr, e_shoff));
1643 NumSectionsOffset = offsetof(ELF::Elf32_Ehdr, e_shnum);
1645 OS.pwrite(reinterpret_cast<char *>(&NumSections), sizeof(NumSections),
1649 bool ELFObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
1650 const MCAssembler &Asm, const MCSymbolData &DataA, const MCFragment &FB,
1651 bool InSet, bool IsPCRel) const {
1654 if (::isWeak(DataA))
1657 return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(Asm, DataA, FB,
1661 bool ELFObjectWriter::isWeak(const MCSymbolData &SD) const {
1665 // It is invalid to replace a reference to a global in a comdat
1666 // with a reference to a local since out of comdat references
1667 // to a local are forbidden.
1668 // We could try to return false for more cases, like the reference
1669 // being in the same comdat or Sym being an alias to another global,
1670 // but it is not clear if it is worth the effort.
1671 if (MCELF::GetBinding(SD) != ELF::STB_GLOBAL)
1674 const MCSymbol &Sym = SD.getSymbol();
1675 if (!Sym.isInSection())
1678 const auto &Sec = cast<MCSectionELF>(Sym.getSection());
1679 return Sec.getGroup();
1682 MCObjectWriter *llvm::createELFObjectWriter(MCELFObjectTargetWriter *MOTW,
1683 raw_pwrite_stream &OS,
1684 bool IsLittleEndian) {
1685 return new ELFObjectWriter(MOTW, OS, IsLittleEndian);