1 //===- lib/MC/ELFObjectWriter.cpp - ELF File Writer -----------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements ELF object file writer information.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/ADT/ArrayRef.h"
14 #include "llvm/ADT/DenseMap.h"
15 #include "llvm/ADT/STLExtras.h"
16 #include "llvm/ADT/SmallString.h"
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/ADT/StringRef.h"
19 #include "llvm/ADT/Twine.h"
20 #include "llvm/BinaryFormat/ELF.h"
21 #include "llvm/MC/MCAsmBackend.h"
22 #include "llvm/MC/MCAsmInfo.h"
23 #include "llvm/MC/MCAsmLayout.h"
24 #include "llvm/MC/MCAssembler.h"
25 #include "llvm/MC/MCContext.h"
26 #include "llvm/MC/MCELFObjectWriter.h"
27 #include "llvm/MC/MCExpr.h"
28 #include "llvm/MC/MCFixup.h"
29 #include "llvm/MC/MCFixupKindInfo.h"
30 #include "llvm/MC/MCFragment.h"
31 #include "llvm/MC/MCObjectFileInfo.h"
32 #include "llvm/MC/MCObjectWriter.h"
33 #include "llvm/MC/MCSection.h"
34 #include "llvm/MC/MCSectionELF.h"
35 #include "llvm/MC/MCSymbol.h"
36 #include "llvm/MC/MCSymbolELF.h"
37 #include "llvm/MC/MCValue.h"
38 #include "llvm/MC/StringTableBuilder.h"
39 #include "llvm/Support/Alignment.h"
40 #include "llvm/Support/Allocator.h"
41 #include "llvm/Support/Casting.h"
42 #include "llvm/Support/Compression.h"
43 #include "llvm/Support/EndianStream.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"
53 #include <algorithm>
54 #include <cassert>
55 #include <cstddef>
56 #include <cstdint>
57 #include <map>
58 #include <memory>
59 #include <string>
60 #include <utility>
61 #include <vector>
62
63 using namespace llvm;
64
65 #undef DEBUG_TYPE
66 #define DEBUG_TYPE "reloc-info"
67
68 namespace {
69
70 using SectionIndexMapTy = DenseMap<const MCSectionELF *, uint32_t>;
71
72 class ELFObjectWriter;
73 struct ELFWriter;
74
isDwoSection(const MCSectionELF & Sec)75 bool isDwoSection(const MCSectionELF &Sec) {
76 return Sec.getName().endswith(".dwo");
77 }
78
79 class SymbolTableWriter {
80 ELFWriter &EWriter;
81 bool Is64Bit;
82
83 // indexes we are going to write to .symtab_shndx.
84 std::vector<uint32_t> ShndxIndexes;
85
86 // The numbel of symbols written so far.
87 unsigned NumWritten;
88
89 void createSymtabShndx();
90
91 template <typename T> void write(T Value);
92
93 public:
94 SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit);
95
96 void writeSymbol(uint32_t name, uint8_t info, uint64_t value, uint64_t size,
97 uint8_t other, uint32_t shndx, bool Reserved);
98
getShndxIndexes() const99 ArrayRef<uint32_t> getShndxIndexes() const { return ShndxIndexes; }
100 };
101
102 struct ELFWriter {
103 ELFObjectWriter &OWriter;
104 support::endian::Writer W;
105
106 enum DwoMode {
107 AllSections,
108 NonDwoOnly,
109 DwoOnly,
110 } Mode;
111
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);
115
116 /// Helper struct for containing some precomputed information on symbols.
117 struct ELFSymbolData {
118 const MCSymbolELF *Symbol;
119 uint32_t SectionIndex;
120 StringRef Name;
121
122 // Support lexicographic sorting.
operator <__anon4fb147720111::ELFWriter::ELFSymbolData123 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)
127 return false;
128 if (LHSType != ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
129 return true;
130 if (LHSType == ELF::STT_SECTION && RHSType == ELF::STT_SECTION)
131 return SectionIndex < RHS.SectionIndex;
132 return Name < RHS.Name;
133 }
134 };
135
136 /// @}
137 /// @name Symbol Table Data
138 /// @{
139
140 StringTableBuilder StrTabBuilder{StringTableBuilder::ELF};
141
142 /// @}
143
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;
150
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);
154
155 // TargetObjectWriter wrappers.
156 bool is64Bit() const;
157 bool hasRelocationAddend() const;
158
159 void align(unsigned Alignment);
160
161 bool maybeWriteCompression(uint64_t Size,
162 SmallVectorImpl<char> &CompressedContents,
163 bool ZLibStyle, unsigned Alignment);
164
165 public:
ELFWriter__anon4fb147720111::ELFWriter166 ELFWriter(ELFObjectWriter &OWriter, raw_pwrite_stream &OS,
167 bool IsLittleEndian, DwoMode Mode)
168 : OWriter(OWriter),
169 W(OS, IsLittleEndian ? support::little : support::big), Mode(Mode) {}
170
WriteWord__anon4fb147720111::ELFWriter171 void WriteWord(uint64_t Word) {
172 if (is64Bit())
173 W.write<uint64_t>(Word);
174 else
175 W.write<uint32_t>(Word);
176 }
177
write__anon4fb147720111::ELFWriter178 template <typename T> void write(T Val) {
179 W.write(Val);
180 }
181
182 void writeHeader(const MCAssembler &Asm);
183
184 void writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
185 ELFSymbolData &MSD, const MCAsmLayout &Layout);
186
187 // Start and end offset of each section
188 using SectionOffsetsTy =
189 std::map<const MCSectionELF *, std::pair<uint64_t, uint64_t>>;
190
191 // Map from a signature symbol to the group section index
192 using RevGroupMapTy = DenseMap<const MCSymbol *, unsigned>;
193
194 /// Compute the symbol table data
195 ///
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);
203
204 void writeAddrsigSection();
205
206 MCSectionELF *createRelocationSection(MCContext &Ctx,
207 const MCSectionELF &Sec);
208
209 const MCSectionELF *createStringTable(MCContext &Ctx);
210
211 void writeSectionHeader(const MCAsmLayout &Layout,
212 const SectionIndexMapTy &SectionIndexMap,
213 const SectionOffsetsTy &SectionOffsets);
214
215 void writeSectionData(const MCAssembler &Asm, MCSection &Sec,
216 const MCAsmLayout &Layout);
217
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,
221 uint64_t EntrySize);
222
223 void writeRelocations(const MCAssembler &Asm, const MCSectionELF &Sec);
224
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);
229 };
230
231 class ELFObjectWriter : public MCObjectWriter {
232 /// The target specific ELF writer instance.
233 std::unique_ptr<MCELFObjectTargetWriter> TargetObjectWriter;
234
235 DenseMap<const MCSectionELF *, std::vector<ELFRelocationEntry>> Relocations;
236
237 DenseMap<const MCSymbolELF *, const MCSymbolELF *> Renames;
238
239 bool EmitAddrsigSection = false;
240 std::vector<const MCSymbol *> AddrsigSyms;
241
242 bool hasRelocationAddend() const;
243
244 bool shouldRelocateWithSymbol(const MCAssembler &Asm,
245 const MCSymbolRefExpr *RefA,
246 const MCSymbolELF *Sym, uint64_t C,
247 unsigned Type) const;
248
249 public:
ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW)250 ELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW)
251 : TargetObjectWriter(std::move(MOTW)) {}
252
reset()253 void reset() override {
254 Relocations.clear();
255 Renames.clear();
256 MCObjectWriter::reset();
257 }
258
259 bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
260 const MCSymbol &SymA,
261 const MCFragment &FB, bool InSet,
262 bool IsPCRel) const override;
263
checkRelocation(MCContext & Ctx,SMLoc Loc,const MCSectionELF * From,const MCSectionELF * To)264 virtual bool checkRelocation(MCContext &Ctx, SMLoc Loc,
265 const MCSectionELF *From,
266 const MCSectionELF *To) {
267 return true;
268 }
269
270 void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
271 const MCFragment *Fragment, const MCFixup &Fixup,
272 MCValue Target, uint64_t &FixedValue) override;
273
274 void executePostLayoutBinding(MCAssembler &Asm,
275 const MCAsmLayout &Layout) override;
276
emitAddrsigSection()277 void emitAddrsigSection() override { EmitAddrsigSection = true; }
addAddrsigSymbol(const MCSymbol * Sym)278 void addAddrsigSymbol(const MCSymbol *Sym) override {
279 AddrsigSyms.push_back(Sym);
280 }
281
282 friend struct ELFWriter;
283 };
284
285 class ELFSingleObjectWriter : public ELFObjectWriter {
286 raw_pwrite_stream &OS;
287 bool IsLittleEndian;
288
289 public:
ELFSingleObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,raw_pwrite_stream & OS,bool IsLittleEndian)290 ELFSingleObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
291 raw_pwrite_stream &OS, bool IsLittleEndian)
292 : ELFObjectWriter(std::move(MOTW)), OS(OS),
293 IsLittleEndian(IsLittleEndian) {}
294
writeObject(MCAssembler & Asm,const MCAsmLayout & Layout)295 uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override {
296 return ELFWriter(*this, OS, IsLittleEndian, ELFWriter::AllSections)
297 .writeObject(Asm, Layout);
298 }
299
300 friend struct ELFWriter;
301 };
302
303 class ELFDwoObjectWriter : public ELFObjectWriter {
304 raw_pwrite_stream &OS, &DwoOS;
305 bool IsLittleEndian;
306
307 public:
ELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,raw_pwrite_stream & OS,raw_pwrite_stream & DwoOS,bool IsLittleEndian)308 ELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
309 raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS,
310 bool IsLittleEndian)
311 : ELFObjectWriter(std::move(MOTW)), OS(OS), DwoOS(DwoOS),
312 IsLittleEndian(IsLittleEndian) {}
313
checkRelocation(MCContext & Ctx,SMLoc Loc,const MCSectionELF * From,const MCSectionELF * To)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");
319 return false;
320 }
321 if (To && isDwoSection(*To)) {
322 Ctx.reportError(Loc, "A relocation may not refer to a dwo section");
323 return false;
324 }
325 return true;
326 }
327
writeObject(MCAssembler & Asm,const MCAsmLayout & Layout)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);
333 return Size;
334 }
335 };
336
337 } // end anonymous namespace
338
align(unsigned Alignment)339 void ELFWriter::align(unsigned Alignment) {
340 uint64_t Padding = offsetToAlignment(W.OS.tell(), Align(Alignment));
341 W.OS.write_zeros(Padding);
342 }
343
addToSectionTable(const MCSectionELF * Sec)344 unsigned ELFWriter::addToSectionTable(const MCSectionELF *Sec) {
345 SectionTable.push_back(Sec);
346 StrTabBuilder.add(Sec->getName());
347 return SectionTable.size();
348 }
349
createSymtabShndx()350 void SymbolTableWriter::createSymtabShndx() {
351 if (!ShndxIndexes.empty())
352 return;
353
354 ShndxIndexes.resize(NumWritten);
355 }
356
write(T Value)357 template <typename T> void SymbolTableWriter::write(T Value) {
358 EWriter.write(Value);
359 }
360
SymbolTableWriter(ELFWriter & EWriter,bool Is64Bit)361 SymbolTableWriter::SymbolTableWriter(ELFWriter &EWriter, bool Is64Bit)
362 : EWriter(EWriter), Is64Bit(Is64Bit), NumWritten(0) {}
363
writeSymbol(uint32_t name,uint8_t info,uint64_t value,uint64_t size,uint8_t other,uint32_t shndx,bool Reserved)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;
368
369 if (LargeIndex)
370 createSymtabShndx();
371
372 if (!ShndxIndexes.empty()) {
373 if (LargeIndex)
374 ShndxIndexes.push_back(shndx);
375 else
376 ShndxIndexes.push_back(0);
377 }
378
379 uint16_t Index = LargeIndex ? uint16_t(ELF::SHN_XINDEX) : shndx;
380
381 if (Is64Bit) {
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
388 } else {
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
395 }
396
397 ++NumWritten;
398 }
399
is64Bit() const400 bool ELFWriter::is64Bit() const {
401 return OWriter.TargetObjectWriter->is64Bit();
402 }
403
hasRelocationAddend() const404 bool ELFWriter::hasRelocationAddend() const {
405 return OWriter.hasRelocationAddend();
406 }
407
408 // Emit the ELF header.
writeHeader(const MCAssembler & Asm)409 void ELFWriter::writeHeader(const MCAssembler &Asm) {
410 // ELF Header
411 // ----------
412 //
413 // Note
414 // ----
415 // emitWord method behaves differently for ELF32 and ELF64, writing
416 // 4 bytes in the former and 8 in the latter.
417
418 W.OS << ELF::ElfMagic; // e_ident[EI_MAG0] to e_ident[EI_MAG3]
419
420 W.OS << char(is64Bit() ? ELF::ELFCLASS64 : ELF::ELFCLASS32); // e_ident[EI_CLASS]
421
422 // e_ident[EI_DATA]
423 W.OS << char(W.Endian == support::little ? ELF::ELFDATA2LSB
424 : ELF::ELFDATA2MSB);
425
426 W.OS << char(ELF::EV_CURRENT); // e_ident[EI_VERSION]
427 // e_ident[EI_OSABI]
428 W.OS << char(OWriter.TargetObjectWriter->getOSABI());
429 // e_ident[EI_ABIVERSION]
430 W.OS << char(OWriter.TargetObjectWriter->getABIVersion());
431
432 W.OS.write_zeros(ELF::EI_NIDENT - ELF::EI_PAD);
433
434 W.write<uint16_t>(ELF::ET_REL); // e_type
435
436 W.write<uint16_t>(OWriter.TargetObjectWriter->getEMachine()); // e_machine = target
437
438 W.write<uint32_t>(ELF::EV_CURRENT); // e_version
439 WriteWord(0); // e_entry, no entry point in .o file
440 WriteWord(0); // e_phoff, no program header for .o
441 WriteWord(0); // e_shoff = sec hdr table off in bytes
442
443 // e_flags = whatever the target wants
444 W.write<uint32_t>(Asm.getELFHeaderEFlags());
445
446 // e_ehsize = ELF header size
447 W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Ehdr)
448 : sizeof(ELF::Elf32_Ehdr));
449
450 W.write<uint16_t>(0); // e_phentsize = prog header entry size
451 W.write<uint16_t>(0); // e_phnum = # prog header entries = 0
452
453 // e_shentsize = Section header entry size
454 W.write<uint16_t>(is64Bit() ? sizeof(ELF::Elf64_Shdr)
455 : sizeof(ELF::Elf32_Shdr));
456
457 // e_shnum = # of section header ents
458 W.write<uint16_t>(0);
459
460 // e_shstrndx = Section # of '.shstrtab'
461 assert(StringTableIndex < ELF::SHN_LORESERVE);
462 W.write<uint16_t>(StringTableIndex);
463 }
464
SymbolValue(const MCSymbol & Sym,const MCAsmLayout & Layout)465 uint64_t ELFWriter::SymbolValue(const MCSymbol &Sym,
466 const MCAsmLayout &Layout) {
467 if (Sym.isCommon())
468 return Sym.getCommonAlignment();
469
470 uint64_t Res;
471 if (!Layout.getSymbolOffset(Sym, Res))
472 return 0;
473
474 if (Layout.getAssembler().isThumbFunc(&Sym))
475 Res |= 1;
476
477 return Res;
478 }
479
mergeTypeForSet(uint8_t origType,uint8_t newType)480 static uint8_t mergeTypeForSet(uint8_t origType, uint8_t newType) {
481 uint8_t Type = newType;
482
483 // Propagation rules:
484 // IFUNC > FUNC > OBJECT > NOTYPE
485 // TLS_OBJECT > OBJECT > NOTYPE
486 //
487 // dont let the new type degrade the old type
488 switch (origType) {
489 default:
490 break;
491 case ELF::STT_GNU_IFUNC:
492 if (Type == ELF::STT_FUNC || Type == ELF::STT_OBJECT ||
493 Type == ELF::STT_NOTYPE || Type == ELF::STT_TLS)
494 Type = ELF::STT_GNU_IFUNC;
495 break;
496 case ELF::STT_FUNC:
497 if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
498 Type == ELF::STT_TLS)
499 Type = ELF::STT_FUNC;
500 break;
501 case ELF::STT_OBJECT:
502 if (Type == ELF::STT_NOTYPE)
503 Type = ELF::STT_OBJECT;
504 break;
505 case ELF::STT_TLS:
506 if (Type == ELF::STT_OBJECT || Type == ELF::STT_NOTYPE ||
507 Type == ELF::STT_GNU_IFUNC || Type == ELF::STT_FUNC)
508 Type = ELF::STT_TLS;
509 break;
510 }
511
512 return Type;
513 }
514
isIFunc(const MCSymbolELF * Symbol)515 static bool isIFunc(const MCSymbolELF *Symbol) {
516 while (Symbol->getType() != ELF::STT_GNU_IFUNC) {
517 const MCSymbolRefExpr *Value;
518 if (!Symbol->isVariable() ||
519 !(Value = dyn_cast<MCSymbolRefExpr>(Symbol->getVariableValue())) ||
520 Value->getKind() != MCSymbolRefExpr::VK_None ||
521 mergeTypeForSet(Symbol->getType(), ELF::STT_GNU_IFUNC) != ELF::STT_GNU_IFUNC)
522 return false;
523 Symbol = &cast<MCSymbolELF>(Value->getSymbol());
524 }
525 return true;
526 }
527
writeSymbol(SymbolTableWriter & Writer,uint32_t StringIndex,ELFSymbolData & MSD,const MCAsmLayout & Layout)528 void ELFWriter::writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
529 ELFSymbolData &MSD, const MCAsmLayout &Layout) {
530 const auto &Symbol = cast<MCSymbolELF>(*MSD.Symbol);
531 const MCSymbolELF *Base =
532 cast_or_null<MCSymbolELF>(Layout.getBaseSymbol(Symbol));
533
534 // This has to be in sync with when computeSymbolTable uses SHN_ABS or
535 // SHN_COMMON.
536 bool IsReserved = !Base || Symbol.isCommon();
537
538 // Binding and Type share the same byte as upper and lower nibbles
539 uint8_t Binding = Symbol.getBinding();
540 uint8_t Type = Symbol.getType();
541 if (isIFunc(&Symbol))
542 Type = ELF::STT_GNU_IFUNC;
543 if (Base) {
544 Type = mergeTypeForSet(Type, Base->getType());
545 }
546 uint8_t Info = (Binding << 4) | Type;
547
548 // Other and Visibility share the same byte with Visibility using the lower
549 // 2 bits
550 uint8_t Visibility = Symbol.getVisibility();
551 uint8_t Other = Symbol.getOther() | Visibility;
552
553 uint64_t Value = SymbolValue(*MSD.Symbol, Layout);
554 uint64_t Size = 0;
555
556 const MCExpr *ESize = MSD.Symbol->getSize();
557 if (!ESize && Base)
558 ESize = Base->getSize();
559
560 if (ESize) {
561 int64_t Res;
562 if (!ESize->evaluateKnownAbsolute(Res, Layout))
563 report_fatal_error("Size expression must be absolute.");
564 Size = Res;
565 }
566
567 // Write out the symbol table entry
568 Writer.writeSymbol(StringIndex, Info, Value, Size, Other, MSD.SectionIndex,
569 IsReserved);
570 }
571
isInSymtab(const MCAsmLayout & Layout,const MCSymbolELF & Symbol,bool Used,bool Renamed)572 bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
573 bool Used, bool Renamed) {
574 if (Symbol.isVariable()) {
575 const MCExpr *Expr = Symbol.getVariableValue();
576 // Target Expressions that are always inlined do not appear in the symtab
577 if (const auto *T = dyn_cast<MCTargetExpr>(Expr))
578 if (T->inlineAssignedExpr())
579 return false;
580 if (const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr)) {
581 if (Ref->getKind() == MCSymbolRefExpr::VK_WEAKREF)
582 return false;
583 }
584 }
585
586 if (Used)
587 return true;
588
589 if (Renamed)
590 return false;
591
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);
595 return false;
596 }
597
598 if (Symbol.isTemporary())
599 return false;
600
601 if (Symbol.getType() == ELF::STT_SECTION)
602 return false;
603
604 return true;
605 }
606
computeSymbolTable(MCAssembler & Asm,const MCAsmLayout & Layout,const SectionIndexMapTy & SectionIndexMap,const RevGroupMapTy & RevGroupMap,SectionOffsetsTy & SectionOffsets)607 void ELFWriter::computeSymbolTable(
608 MCAssembler &Asm, const MCAsmLayout &Layout,
609 const SectionIndexMapTy &SectionIndexMap, const RevGroupMapTy &RevGroupMap,
610 SectionOffsetsTy &SectionOffsets) {
611 MCContext &Ctx = Asm.getContext();
612 SymbolTableWriter Writer(*this, is64Bit());
613
614 // Symbol table
615 unsigned EntrySize = is64Bit() ? ELF::SYMENTRY_SIZE64 : ELF::SYMENTRY_SIZE32;
616 MCSectionELF *SymtabSection =
617 Ctx.getELFSection(".symtab", ELF::SHT_SYMTAB, 0, EntrySize, "");
618 SymtabSection->setAlignment(is64Bit() ? Align(8) : Align(4));
619 SymbolTableIndex = addToSectionTable(SymtabSection);
620
621 align(SymtabSection->getAlignment());
622 uint64_t SecStart = W.OS.tell();
623
624 // The first entry is the undefined symbol entry.
625 Writer.writeSymbol(0, 0, 0, 0, 0, 0, false);
626
627 std::vector<ELFSymbolData> LocalSymbolData;
628 std::vector<ELFSymbolData> ExternalSymbolData;
629
630 // Add the data for the symbols.
631 bool HasLargeSectionIndex = false;
632 for (const MCSymbol &S : Asm.symbols()) {
633 const auto &Symbol = cast<MCSymbolELF>(S);
634 bool Used = Symbol.isUsedInReloc();
635 bool WeakrefUsed = Symbol.isWeakrefUsedInReloc();
636 bool isSignature = Symbol.isSignature();
637
638 if (!isInSymtab(Layout, Symbol, Used || WeakrefUsed || isSignature,
639 OWriter.Renames.count(&Symbol)))
640 continue;
641
642 if (Symbol.isTemporary() && Symbol.isUndefined()) {
643 Ctx.reportError(SMLoc(), "Undefined temporary symbol " + Symbol.getName());
644 continue;
645 }
646
647 ELFSymbolData MSD;
648 MSD.Symbol = cast<MCSymbolELF>(&Symbol);
649
650 bool Local = Symbol.getBinding() == ELF::STB_LOCAL;
651 assert(Local || !Symbol.isTemporary());
652
653 if (Symbol.isAbsolute()) {
654 MSD.SectionIndex = ELF::SHN_ABS;
655 } else if (Symbol.isCommon()) {
656 if (Symbol.isTargetCommon()) {
657 MSD.SectionIndex = Symbol.getIndex();
658 } else {
659 assert(!Local);
660 MSD.SectionIndex = ELF::SHN_COMMON;
661 }
662 } else if (Symbol.isUndefined()) {
663 if (isSignature && !Used) {
664 MSD.SectionIndex = RevGroupMap.lookup(&Symbol);
665 if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
666 HasLargeSectionIndex = true;
667 } else {
668 MSD.SectionIndex = ELF::SHN_UNDEF;
669 }
670 } else {
671 const MCSectionELF &Section =
672 static_cast<const MCSectionELF &>(Symbol.getSection());
673
674 // We may end up with a situation when section symbol is technically
675 // defined, but should not be. That happens because we explicitly
676 // pre-create few .debug_* sections to have accessors.
677 // And if these sections were not really defined in the code, but were
678 // referenced, we simply error out.
679 if (!Section.isRegistered()) {
680 assert(static_cast<const MCSymbolELF &>(Symbol).getType() ==
681 ELF::STT_SECTION);
682 Ctx.reportError(SMLoc(),
683 "Undefined section reference: " + Symbol.getName());
684 continue;
685 }
686
687 if (Mode == NonDwoOnly && isDwoSection(Section))
688 continue;
689 MSD.SectionIndex = SectionIndexMap.lookup(&Section);
690 assert(MSD.SectionIndex && "Invalid section index!");
691 if (MSD.SectionIndex >= ELF::SHN_LORESERVE)
692 HasLargeSectionIndex = true;
693 }
694
695 StringRef Name = Symbol.getName();
696
697 // Sections have their own string table
698 if (Symbol.getType() != ELF::STT_SECTION) {
699 MSD.Name = Name;
700 StrTabBuilder.add(Name);
701 }
702
703 if (Local)
704 LocalSymbolData.push_back(MSD);
705 else
706 ExternalSymbolData.push_back(MSD);
707 }
708
709 // This holds the .symtab_shndx section index.
710 unsigned SymtabShndxSectionIndex = 0;
711
712 if (HasLargeSectionIndex) {
713 MCSectionELF *SymtabShndxSection =
714 Ctx.getELFSection(".symtab_shndx", ELF::SHT_SYMTAB_SHNDX, 0, 4, "");
715 SymtabShndxSectionIndex = addToSectionTable(SymtabShndxSection);
716 SymtabShndxSection->setAlignment(Align(4));
717 }
718
719 ArrayRef<std::string> FileNames = Asm.getFileNames();
720 for (const std::string &Name : FileNames)
721 StrTabBuilder.add(Name);
722
723 StrTabBuilder.finalize();
724
725 // File symbols are emitted first and handled separately from normal symbols,
726 // i.e. a non-STT_FILE symbol with the same name may appear.
727 for (const std::string &Name : FileNames)
728 Writer.writeSymbol(StrTabBuilder.getOffset(Name),
729 ELF::STT_FILE | ELF::STB_LOCAL, 0, 0, ELF::STV_DEFAULT,
730 ELF::SHN_ABS, true);
731
732 // Symbols are required to be in lexicographic order.
733 array_pod_sort(LocalSymbolData.begin(), LocalSymbolData.end());
734 array_pod_sort(ExternalSymbolData.begin(), ExternalSymbolData.end());
735
736 // Set the symbol indices. Local symbols must come before all other
737 // symbols with non-local bindings.
738 unsigned Index = FileNames.size() + 1;
739
740 for (ELFSymbolData &MSD : LocalSymbolData) {
741 unsigned StringIndex = MSD.Symbol->getType() == ELF::STT_SECTION
742 ? 0
743 : StrTabBuilder.getOffset(MSD.Name);
744 MSD.Symbol->setIndex(Index++);
745 writeSymbol(Writer, StringIndex, MSD, Layout);
746 }
747
748 // Write the symbol table entries.
749 LastLocalSymbolIndex = Index;
750
751 for (ELFSymbolData &MSD : ExternalSymbolData) {
752 unsigned StringIndex = StrTabBuilder.getOffset(MSD.Name);
753 MSD.Symbol->setIndex(Index++);
754 writeSymbol(Writer, StringIndex, MSD, Layout);
755 assert(MSD.Symbol->getBinding() != ELF::STB_LOCAL);
756 }
757
758 uint64_t SecEnd = W.OS.tell();
759 SectionOffsets[SymtabSection] = std::make_pair(SecStart, SecEnd);
760
761 ArrayRef<uint32_t> ShndxIndexes = Writer.getShndxIndexes();
762 if (ShndxIndexes.empty()) {
763 assert(SymtabShndxSectionIndex == 0);
764 return;
765 }
766 assert(SymtabShndxSectionIndex != 0);
767
768 SecStart = W.OS.tell();
769 const MCSectionELF *SymtabShndxSection =
770 SectionTable[SymtabShndxSectionIndex - 1];
771 for (uint32_t Index : ShndxIndexes)
772 write(Index);
773 SecEnd = W.OS.tell();
774 SectionOffsets[SymtabShndxSection] = std::make_pair(SecStart, SecEnd);
775 }
776
writeAddrsigSection()777 void ELFWriter::writeAddrsigSection() {
778 for (const MCSymbol *Sym : OWriter.AddrsigSyms)
779 encodeULEB128(Sym->getIndex(), W.OS);
780 }
781
createRelocationSection(MCContext & Ctx,const MCSectionELF & Sec)782 MCSectionELF *ELFWriter::createRelocationSection(MCContext &Ctx,
783 const MCSectionELF &Sec) {
784 if (OWriter.Relocations[&Sec].empty())
785 return nullptr;
786
787 const StringRef SectionName = Sec.getName();
788 std::string RelaSectionName = hasRelocationAddend() ? ".rela" : ".rel";
789 RelaSectionName += SectionName;
790
791 unsigned EntrySize;
792 if (hasRelocationAddend())
793 EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rela) : sizeof(ELF::Elf32_Rela);
794 else
795 EntrySize = is64Bit() ? sizeof(ELF::Elf64_Rel) : sizeof(ELF::Elf32_Rel);
796
797 unsigned Flags = 0;
798 if (Sec.getFlags() & ELF::SHF_GROUP)
799 Flags = ELF::SHF_GROUP;
800
801 MCSectionELF *RelaSection = Ctx.createELFRelSection(
802 RelaSectionName, hasRelocationAddend() ? ELF::SHT_RELA : ELF::SHT_REL,
803 Flags, EntrySize, Sec.getGroup(), &Sec);
804 RelaSection->setAlignment(is64Bit() ? Align(8) : Align(4));
805 return RelaSection;
806 }
807
808 // Include the debug info compression header.
maybeWriteCompression(uint64_t Size,SmallVectorImpl<char> & CompressedContents,bool ZLibStyle,unsigned Alignment)809 bool ELFWriter::maybeWriteCompression(
810 uint64_t Size, SmallVectorImpl<char> &CompressedContents, bool ZLibStyle,
811 unsigned Alignment) {
812 if (ZLibStyle) {
813 uint64_t HdrSize =
814 is64Bit() ? sizeof(ELF::Elf32_Chdr) : sizeof(ELF::Elf64_Chdr);
815 if (Size <= HdrSize + CompressedContents.size())
816 return false;
817 // Platform specific header is followed by compressed data.
818 if (is64Bit()) {
819 // Write Elf64_Chdr header.
820 write(static_cast<ELF::Elf64_Word>(ELF::ELFCOMPRESS_ZLIB));
821 write(static_cast<ELF::Elf64_Word>(0)); // ch_reserved field.
822 write(static_cast<ELF::Elf64_Xword>(Size));
823 write(static_cast<ELF::Elf64_Xword>(Alignment));
824 } else {
825 // Write Elf32_Chdr header otherwise.
826 write(static_cast<ELF::Elf32_Word>(ELF::ELFCOMPRESS_ZLIB));
827 write(static_cast<ELF::Elf32_Word>(Size));
828 write(static_cast<ELF::Elf32_Word>(Alignment));
829 }
830 return true;
831 }
832
833 // "ZLIB" followed by 8 bytes representing the uncompressed size of the section,
834 // useful for consumers to preallocate a buffer to decompress into.
835 const StringRef Magic = "ZLIB";
836 if (Size <= Magic.size() + sizeof(Size) + CompressedContents.size())
837 return false;
838 W.OS << Magic;
839 support::endian::write(W.OS, Size, support::big);
840 return true;
841 }
842
writeSectionData(const MCAssembler & Asm,MCSection & Sec,const MCAsmLayout & Layout)843 void ELFWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
844 const MCAsmLayout &Layout) {
845 MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
846 StringRef SectionName = Section.getName();
847
848 auto &MC = Asm.getContext();
849 const auto &MAI = MC.getAsmInfo();
850
851 // Compressing debug_frame requires handling alignment fragments which is
852 // more work (possibly generalizing MCAssembler.cpp:writeFragment to allow
853 // for writing to arbitrary buffers) for little benefit.
854 bool CompressionEnabled =
855 MAI->compressDebugSections() != DebugCompressionType::None;
856 if (!CompressionEnabled || !SectionName.startswith(".debug_") ||
857 SectionName == ".debug_frame") {
858 Asm.writeSectionData(W.OS, &Section, Layout);
859 return;
860 }
861
862 assert((MAI->compressDebugSections() == DebugCompressionType::Z ||
863 MAI->compressDebugSections() == DebugCompressionType::GNU) &&
864 "expected zlib or zlib-gnu style compression");
865
866 SmallVector<char, 128> UncompressedData;
867 raw_svector_ostream VecOS(UncompressedData);
868 Asm.writeSectionData(VecOS, &Section, Layout);
869
870 SmallVector<char, 128> CompressedContents;
871 if (Error E = zlib::compress(
872 StringRef(UncompressedData.data(), UncompressedData.size()),
873 CompressedContents)) {
874 consumeError(std::move(E));
875 W.OS << UncompressedData;
876 return;
877 }
878
879 bool ZlibStyle = MAI->compressDebugSections() == DebugCompressionType::Z;
880 if (!maybeWriteCompression(UncompressedData.size(), CompressedContents,
881 ZlibStyle, Sec.getAlignment())) {
882 W.OS << UncompressedData;
883 return;
884 }
885
886 if (ZlibStyle) {
887 // Set the compressed flag. That is zlib style.
888 Section.setFlags(Section.getFlags() | ELF::SHF_COMPRESSED);
889 // Alignment field should reflect the requirements of
890 // the compressed section header.
891 Section.setAlignment(is64Bit() ? Align(8) : Align(4));
892 } else {
893 // Add "z" prefix to section name. This is zlib-gnu style.
894 MC.renameELFSection(&Section, (".z" + SectionName.drop_front(1)).str());
895 }
896 W.OS << CompressedContents;
897 }
898
WriteSecHdrEntry(uint32_t Name,uint32_t Type,uint64_t Flags,uint64_t Address,uint64_t Offset,uint64_t Size,uint32_t Link,uint32_t Info,uint64_t Alignment,uint64_t EntrySize)899 void ELFWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type, uint64_t Flags,
900 uint64_t Address, uint64_t Offset,
901 uint64_t Size, uint32_t Link, uint32_t Info,
902 uint64_t Alignment, uint64_t EntrySize) {
903 W.write<uint32_t>(Name); // sh_name: index into string table
904 W.write<uint32_t>(Type); // sh_type
905 WriteWord(Flags); // sh_flags
906 WriteWord(Address); // sh_addr
907 WriteWord(Offset); // sh_offset
908 WriteWord(Size); // sh_size
909 W.write<uint32_t>(Link); // sh_link
910 W.write<uint32_t>(Info); // sh_info
911 WriteWord(Alignment); // sh_addralign
912 WriteWord(EntrySize); // sh_entsize
913 }
914
writeRelocations(const MCAssembler & Asm,const MCSectionELF & Sec)915 void ELFWriter::writeRelocations(const MCAssembler &Asm,
916 const MCSectionELF &Sec) {
917 std::vector<ELFRelocationEntry> &Relocs = OWriter.Relocations[&Sec];
918
919 // We record relocations by pushing to the end of a vector. Reverse the vector
920 // to get the relocations in the order they were created.
921 // In most cases that is not important, but it can be for special sections
922 // (.eh_frame) or specific relocations (TLS optimizations on SystemZ).
923 std::reverse(Relocs.begin(), Relocs.end());
924
925 // Sort the relocation entries. MIPS needs this.
926 OWriter.TargetObjectWriter->sortRelocs(Asm, Relocs);
927
928 for (unsigned i = 0, e = Relocs.size(); i != e; ++i) {
929 const ELFRelocationEntry &Entry = Relocs[e - i - 1];
930 unsigned Index = Entry.Symbol ? Entry.Symbol->getIndex() : 0;
931
932 if (is64Bit()) {
933 write(Entry.Offset);
934 if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
935 write(uint32_t(Index));
936
937 write(OWriter.TargetObjectWriter->getRSsym(Entry.Type));
938 write(OWriter.TargetObjectWriter->getRType3(Entry.Type));
939 write(OWriter.TargetObjectWriter->getRType2(Entry.Type));
940 write(OWriter.TargetObjectWriter->getRType(Entry.Type));
941 } else {
942 struct ELF::Elf64_Rela ERE64;
943 ERE64.setSymbolAndType(Index, Entry.Type);
944 write(ERE64.r_info);
945 }
946 if (hasRelocationAddend())
947 write(Entry.Addend);
948 } else {
949 write(uint32_t(Entry.Offset));
950
951 struct ELF::Elf32_Rela ERE32;
952 ERE32.setSymbolAndType(Index, Entry.Type);
953 write(ERE32.r_info);
954
955 if (hasRelocationAddend())
956 write(uint32_t(Entry.Addend));
957
958 if (OWriter.TargetObjectWriter->getEMachine() == ELF::EM_MIPS) {
959 if (uint32_t RType =
960 OWriter.TargetObjectWriter->getRType2(Entry.Type)) {
961 write(uint32_t(Entry.Offset));
962
963 ERE32.setSymbolAndType(0, RType);
964 write(ERE32.r_info);
965 write(uint32_t(0));
966 }
967 if (uint32_t RType =
968 OWriter.TargetObjectWriter->getRType3(Entry.Type)) {
969 write(uint32_t(Entry.Offset));
970
971 ERE32.setSymbolAndType(0, RType);
972 write(ERE32.r_info);
973 write(uint32_t(0));
974 }
975 }
976 }
977 }
978 }
979
createStringTable(MCContext & Ctx)980 const MCSectionELF *ELFWriter::createStringTable(MCContext &Ctx) {
981 const MCSectionELF *StrtabSection = SectionTable[StringTableIndex - 1];
982 StrTabBuilder.write(W.OS);
983 return StrtabSection;
984 }
985
writeSection(const SectionIndexMapTy & SectionIndexMap,uint32_t GroupSymbolIndex,uint64_t Offset,uint64_t Size,const MCSectionELF & Section)986 void ELFWriter::writeSection(const SectionIndexMapTy &SectionIndexMap,
987 uint32_t GroupSymbolIndex, uint64_t Offset,
988 uint64_t Size, const MCSectionELF &Section) {
989 uint64_t sh_link = 0;
990 uint64_t sh_info = 0;
991
992 switch(Section.getType()) {
993 default:
994 // Nothing to do.
995 break;
996
997 case ELF::SHT_DYNAMIC:
998 llvm_unreachable("SHT_DYNAMIC in a relocatable object");
999
1000 case ELF::SHT_REL:
1001 case ELF::SHT_RELA: {
1002 sh_link = SymbolTableIndex;
1003 assert(sh_link && ".symtab not found");
1004 const MCSection *InfoSection = Section.getLinkedToSection();
1005 sh_info = SectionIndexMap.lookup(cast<MCSectionELF>(InfoSection));
1006 break;
1007 }
1008
1009 case ELF::SHT_SYMTAB:
1010 sh_link = StringTableIndex;
1011 sh_info = LastLocalSymbolIndex;
1012 break;
1013
1014 case ELF::SHT_SYMTAB_SHNDX:
1015 case ELF::SHT_LLVM_CALL_GRAPH_PROFILE:
1016 case ELF::SHT_LLVM_ADDRSIG:
1017 sh_link = SymbolTableIndex;
1018 break;
1019
1020 case ELF::SHT_GROUP:
1021 sh_link = SymbolTableIndex;
1022 sh_info = GroupSymbolIndex;
1023 break;
1024 }
1025
1026 if (Section.getFlags() & ELF::SHF_LINK_ORDER) {
1027 // If the value in the associated metadata is not a definition, Sym will be
1028 // undefined. Represent this with sh_link=0.
1029 const MCSymbol *Sym = Section.getLinkedToSymbol();
1030 if (Sym && Sym->isInSection()) {
1031 const MCSectionELF *Sec = cast<MCSectionELF>(&Sym->getSection());
1032 sh_link = SectionIndexMap.lookup(Sec);
1033 }
1034 }
1035
1036 WriteSecHdrEntry(StrTabBuilder.getOffset(Section.getName()),
1037 Section.getType(), Section.getFlags(), 0, Offset, Size,
1038 sh_link, sh_info, Section.getAlignment(),
1039 Section.getEntrySize());
1040 }
1041
writeSectionHeader(const MCAsmLayout & Layout,const SectionIndexMapTy & SectionIndexMap,const SectionOffsetsTy & SectionOffsets)1042 void ELFWriter::writeSectionHeader(
1043 const MCAsmLayout &Layout, const SectionIndexMapTy &SectionIndexMap,
1044 const SectionOffsetsTy &SectionOffsets) {
1045 const unsigned NumSections = SectionTable.size();
1046
1047 // Null section first.
1048 uint64_t FirstSectionSize =
1049 (NumSections + 1) >= ELF::SHN_LORESERVE ? NumSections + 1 : 0;
1050 WriteSecHdrEntry(0, 0, 0, 0, 0, FirstSectionSize, 0, 0, 0, 0);
1051
1052 for (const MCSectionELF *Section : SectionTable) {
1053 uint32_t GroupSymbolIndex;
1054 unsigned Type = Section->getType();
1055 if (Type != ELF::SHT_GROUP)
1056 GroupSymbolIndex = 0;
1057 else
1058 GroupSymbolIndex = Section->getGroup()->getIndex();
1059
1060 const std::pair<uint64_t, uint64_t> &Offsets =
1061 SectionOffsets.find(Section)->second;
1062 uint64_t Size;
1063 if (Type == ELF::SHT_NOBITS)
1064 Size = Layout.getSectionAddressSize(Section);
1065 else
1066 Size = Offsets.second - Offsets.first;
1067
1068 writeSection(SectionIndexMap, GroupSymbolIndex, Offsets.first, Size,
1069 *Section);
1070 }
1071 }
1072
writeObject(MCAssembler & Asm,const MCAsmLayout & Layout)1073 uint64_t ELFWriter::writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) {
1074 uint64_t StartOffset = W.OS.tell();
1075
1076 MCContext &Ctx = Asm.getContext();
1077 MCSectionELF *StrtabSection =
1078 Ctx.getELFSection(".strtab", ELF::SHT_STRTAB, 0);
1079 StringTableIndex = addToSectionTable(StrtabSection);
1080
1081 RevGroupMapTy RevGroupMap;
1082 SectionIndexMapTy SectionIndexMap;
1083
1084 std::map<const MCSymbol *, std::vector<const MCSectionELF *>> GroupMembers;
1085
1086 // Write out the ELF header ...
1087 writeHeader(Asm);
1088
1089 // ... then the sections ...
1090 SectionOffsetsTy SectionOffsets;
1091 std::vector<MCSectionELF *> Groups;
1092 std::vector<MCSectionELF *> Relocations;
1093 for (MCSection &Sec : Asm) {
1094 MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
1095 if (Mode == NonDwoOnly && isDwoSection(Section))
1096 continue;
1097 if (Mode == DwoOnly && !isDwoSection(Section))
1098 continue;
1099
1100 align(Section.getAlignment());
1101
1102 // Remember the offset into the file for this section.
1103 uint64_t SecStart = W.OS.tell();
1104
1105 const MCSymbolELF *SignatureSymbol = Section.getGroup();
1106 writeSectionData(Asm, Section, Layout);
1107
1108 uint64_t SecEnd = W.OS.tell();
1109 SectionOffsets[&Section] = std::make_pair(SecStart, SecEnd);
1110
1111 MCSectionELF *RelSection = createRelocationSection(Ctx, Section);
1112
1113 if (SignatureSymbol) {
1114 Asm.registerSymbol(*SignatureSymbol);
1115 unsigned &GroupIdx = RevGroupMap[SignatureSymbol];
1116 if (!GroupIdx) {
1117 MCSectionELF *Group = Ctx.createELFGroupSection(SignatureSymbol);
1118 GroupIdx = addToSectionTable(Group);
1119 Group->setAlignment(Align(4));
1120 Groups.push_back(Group);
1121 }
1122 std::vector<const MCSectionELF *> &Members =
1123 GroupMembers[SignatureSymbol];
1124 Members.push_back(&Section);
1125 if (RelSection)
1126 Members.push_back(RelSection);
1127 }
1128
1129 SectionIndexMap[&Section] = addToSectionTable(&Section);
1130 if (RelSection) {
1131 SectionIndexMap[RelSection] = addToSectionTable(RelSection);
1132 Relocations.push_back(RelSection);
1133 }
1134
1135 OWriter.TargetObjectWriter->addTargetSectionFlags(Ctx, Section);
1136 }
1137
1138 MCSectionELF *CGProfileSection = nullptr;
1139 if (!Asm.CGProfile.empty()) {
1140 CGProfileSection = Ctx.getELFSection(".llvm.call-graph-profile",
1141 ELF::SHT_LLVM_CALL_GRAPH_PROFILE,
1142 ELF::SHF_EXCLUDE, 16, "");
1143 SectionIndexMap[CGProfileSection] = addToSectionTable(CGProfileSection);
1144 }
1145
1146 for (MCSectionELF *Group : Groups) {
1147 align(Group->getAlignment());
1148
1149 // Remember the offset into the file for this section.
1150 uint64_t SecStart = W.OS.tell();
1151
1152 const MCSymbol *SignatureSymbol = Group->getGroup();
1153 assert(SignatureSymbol);
1154 write(uint32_t(ELF::GRP_COMDAT));
1155 for (const MCSectionELF *Member : GroupMembers[SignatureSymbol]) {
1156 uint32_t SecIndex = SectionIndexMap.lookup(Member);
1157 write(SecIndex);
1158 }
1159
1160 uint64_t SecEnd = W.OS.tell();
1161 SectionOffsets[Group] = std::make_pair(SecStart, SecEnd);
1162 }
1163
1164 if (Mode == DwoOnly) {
1165 // dwo files don't have symbol tables or relocations, but they do have
1166 // string tables.
1167 StrTabBuilder.finalize();
1168 } else {
1169 MCSectionELF *AddrsigSection;
1170 if (OWriter.EmitAddrsigSection) {
1171 AddrsigSection = Ctx.getELFSection(".llvm_addrsig", ELF::SHT_LLVM_ADDRSIG,
1172 ELF::SHF_EXCLUDE);
1173 addToSectionTable(AddrsigSection);
1174 }
1175
1176 // Compute symbol table information.
1177 computeSymbolTable(Asm, Layout, SectionIndexMap, RevGroupMap,
1178 SectionOffsets);
1179
1180 for (MCSectionELF *RelSection : Relocations) {
1181 align(RelSection->getAlignment());
1182
1183 // Remember the offset into the file for this section.
1184 uint64_t SecStart = W.OS.tell();
1185
1186 writeRelocations(Asm,
1187 cast<MCSectionELF>(*RelSection->getLinkedToSection()));
1188
1189 uint64_t SecEnd = W.OS.tell();
1190 SectionOffsets[RelSection] = std::make_pair(SecStart, SecEnd);
1191 }
1192
1193 if (OWriter.EmitAddrsigSection) {
1194 uint64_t SecStart = W.OS.tell();
1195 writeAddrsigSection();
1196 uint64_t SecEnd = W.OS.tell();
1197 SectionOffsets[AddrsigSection] = std::make_pair(SecStart, SecEnd);
1198 }
1199 }
1200
1201 if (CGProfileSection) {
1202 uint64_t SecStart = W.OS.tell();
1203 for (const MCAssembler::CGProfileEntry &CGPE : Asm.CGProfile) {
1204 W.write<uint32_t>(CGPE.From->getSymbol().getIndex());
1205 W.write<uint32_t>(CGPE.To->getSymbol().getIndex());
1206 W.write<uint64_t>(CGPE.Count);
1207 }
1208 uint64_t SecEnd = W.OS.tell();
1209 SectionOffsets[CGProfileSection] = std::make_pair(SecStart, SecEnd);
1210 }
1211
1212 {
1213 uint64_t SecStart = W.OS.tell();
1214 const MCSectionELF *Sec = createStringTable(Ctx);
1215 uint64_t SecEnd = W.OS.tell();
1216 SectionOffsets[Sec] = std::make_pair(SecStart, SecEnd);
1217 }
1218
1219 uint64_t NaturalAlignment = is64Bit() ? 8 : 4;
1220 align(NaturalAlignment);
1221
1222 const uint64_t SectionHeaderOffset = W.OS.tell();
1223
1224 // ... then the section header table ...
1225 writeSectionHeader(Layout, SectionIndexMap, SectionOffsets);
1226
1227 uint16_t NumSections = support::endian::byte_swap<uint16_t>(
1228 (SectionTable.size() + 1 >= ELF::SHN_LORESERVE) ? (uint16_t)ELF::SHN_UNDEF
1229 : SectionTable.size() + 1,
1230 W.Endian);
1231 unsigned NumSectionsOffset;
1232
1233 auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
1234 if (is64Bit()) {
1235 uint64_t Val =
1236 support::endian::byte_swap<uint64_t>(SectionHeaderOffset, W.Endian);
1237 Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
1238 offsetof(ELF::Elf64_Ehdr, e_shoff));
1239 NumSectionsOffset = offsetof(ELF::Elf64_Ehdr, e_shnum);
1240 } else {
1241 uint32_t Val =
1242 support::endian::byte_swap<uint32_t>(SectionHeaderOffset, W.Endian);
1243 Stream.pwrite(reinterpret_cast<char *>(&Val), sizeof(Val),
1244 offsetof(ELF::Elf32_Ehdr, e_shoff));
1245 NumSectionsOffset = offsetof(ELF::Elf32_Ehdr, e_shnum);
1246 }
1247 Stream.pwrite(reinterpret_cast<char *>(&NumSections), sizeof(NumSections),
1248 NumSectionsOffset);
1249
1250 return W.OS.tell() - StartOffset;
1251 }
1252
hasRelocationAddend() const1253 bool ELFObjectWriter::hasRelocationAddend() const {
1254 return TargetObjectWriter->hasRelocationAddend();
1255 }
1256
executePostLayoutBinding(MCAssembler & Asm,const MCAsmLayout & Layout)1257 void ELFObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
1258 const MCAsmLayout &Layout) {
1259 // The presence of symbol versions causes undefined symbols and
1260 // versions declared with @@@ to be renamed.
1261 for (const MCAssembler::Symver &S : Asm.Symvers) {
1262 StringRef AliasName = S.Name;
1263 const auto &Symbol = cast<MCSymbolELF>(*S.Sym);
1264 size_t Pos = AliasName.find('@');
1265 assert(Pos != StringRef::npos);
1266
1267 StringRef Prefix = AliasName.substr(0, Pos);
1268 StringRef Rest = AliasName.substr(Pos);
1269 StringRef Tail = Rest;
1270 if (Rest.startswith("@@@"))
1271 Tail = Rest.substr(Symbol.isUndefined() ? 2 : 1);
1272
1273 auto *Alias =
1274 cast<MCSymbolELF>(Asm.getContext().getOrCreateSymbol(Prefix + Tail));
1275 Asm.registerSymbol(*Alias);
1276 const MCExpr *Value = MCSymbolRefExpr::create(&Symbol, Asm.getContext());
1277 Alias->setVariableValue(Value);
1278
1279 // Aliases defined with .symvar copy the binding from the symbol they alias.
1280 // This is the first place we are able to copy this information.
1281 Alias->setBinding(Symbol.getBinding());
1282 Alias->setVisibility(Symbol.getVisibility());
1283 Alias->setOther(Symbol.getOther());
1284
1285 if (!Symbol.isUndefined() && !Rest.startswith("@@@"))
1286 continue;
1287
1288 if (Symbol.isUndefined() && Rest.startswith("@@") &&
1289 !Rest.startswith("@@@")) {
1290 Asm.getContext().reportError(S.Loc, "default version symbol " +
1291 AliasName + " must be defined");
1292 continue;
1293 }
1294
1295 if (Renames.count(&Symbol) && Renames[&Symbol] != Alias) {
1296 Asm.getContext().reportError(S.Loc, Twine("multiple versions for ") +
1297 Symbol.getName());
1298 continue;
1299 }
1300
1301 Renames.insert(std::make_pair(&Symbol, Alias));
1302 }
1303
1304 for (const MCSymbol *&Sym : AddrsigSyms) {
1305 if (const MCSymbol *R = Renames.lookup(cast<MCSymbolELF>(Sym)))
1306 Sym = R;
1307 if (Sym->isInSection() && Sym->getName().startswith(".L"))
1308 Sym = Sym->getSection().getBeginSymbol();
1309 Sym->setUsedInReloc();
1310 }
1311 }
1312
1313 // It is always valid to create a relocation with a symbol. It is preferable
1314 // to use a relocation with a section if that is possible. Using the section
1315 // allows us to omit some local symbols from the symbol table.
shouldRelocateWithSymbol(const MCAssembler & Asm,const MCSymbolRefExpr * RefA,const MCSymbolELF * Sym,uint64_t C,unsigned Type) const1316 bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
1317 const MCSymbolRefExpr *RefA,
1318 const MCSymbolELF *Sym,
1319 uint64_t C,
1320 unsigned Type) const {
1321 // A PCRel relocation to an absolute value has no symbol (or section). We
1322 // represent that with a relocation to a null section.
1323 if (!RefA)
1324 return false;
1325
1326 MCSymbolRefExpr::VariantKind Kind = RefA->getKind();
1327 switch (Kind) {
1328 default:
1329 break;
1330 // The .odp creation emits a relocation against the symbol ".TOC." which
1331 // create a R_PPC64_TOC relocation. However the relocation symbol name
1332 // in final object creation should be NULL, since the symbol does not
1333 // really exist, it is just the reference to TOC base for the current
1334 // object file. Since the symbol is undefined, returning false results
1335 // in a relocation with a null section which is the desired result.
1336 case MCSymbolRefExpr::VK_PPC_TOCBASE:
1337 return false;
1338
1339 // These VariantKind cause the relocation to refer to something other than
1340 // the symbol itself, like a linker generated table. Since the address of
1341 // symbol is not relevant, we cannot replace the symbol with the
1342 // section and patch the difference in the addend.
1343 case MCSymbolRefExpr::VK_GOT:
1344 case MCSymbolRefExpr::VK_PLT:
1345 case MCSymbolRefExpr::VK_GOTPCREL:
1346 case MCSymbolRefExpr::VK_PPC_GOT_LO:
1347 case MCSymbolRefExpr::VK_PPC_GOT_HI:
1348 case MCSymbolRefExpr::VK_PPC_GOT_HA:
1349 return true;
1350 }
1351
1352 // An undefined symbol is not in any section, so the relocation has to point
1353 // to the symbol itself.
1354 assert(Sym && "Expected a symbol");
1355 if (Sym->isUndefined())
1356 return true;
1357
1358 unsigned Binding = Sym->getBinding();
1359 switch(Binding) {
1360 default:
1361 llvm_unreachable("Invalid Binding");
1362 case ELF::STB_LOCAL:
1363 break;
1364 case ELF::STB_WEAK:
1365 // If the symbol is weak, it might be overridden by a symbol in another
1366 // file. The relocation has to point to the symbol so that the linker
1367 // can update it.
1368 return true;
1369 case ELF::STB_GLOBAL:
1370 // Global ELF symbols can be preempted by the dynamic linker. The relocation
1371 // has to point to the symbol for a reason analogous to the STB_WEAK case.
1372 return true;
1373 }
1374
1375 // Keep symbol type for a local ifunc because it may result in an IRELATIVE
1376 // reloc that the dynamic loader will use to resolve the address at startup
1377 // time.
1378 if (Sym->getType() == ELF::STT_GNU_IFUNC)
1379 return true;
1380
1381 // If a relocation points to a mergeable section, we have to be careful.
1382 // If the offset is zero, a relocation with the section will encode the
1383 // same information. With a non-zero offset, the situation is different.
1384 // For example, a relocation can point 42 bytes past the end of a string.
1385 // If we change such a relocation to use the section, the linker would think
1386 // that it pointed to another string and subtracting 42 at runtime will
1387 // produce the wrong value.
1388 if (Sym->isInSection()) {
1389 auto &Sec = cast<MCSectionELF>(Sym->getSection());
1390 unsigned Flags = Sec.getFlags();
1391 if (Flags & ELF::SHF_MERGE) {
1392 if (C != 0)
1393 return true;
1394
1395 // gold<2.34 incorrectly ignored the addend for R_386_GOTOFF (9)
1396 // (http://sourceware.org/PR16794).
1397 if (TargetObjectWriter->getEMachine() == ELF::EM_386 &&
1398 Type == ELF::R_386_GOTOFF)
1399 return true;
1400
1401 // ld.lld handles R_MIPS_HI16/R_MIPS_LO16 separately, not as a whole, so
1402 // it doesn't know that an R_MIPS_HI16 with implicit addend 1 and an
1403 // R_MIPS_LO16 with implicit addend -32768 represents 32768, which is in
1404 // range of a MergeInputSection. We could introduce a new RelExpr member
1405 // (like R_RISCV_PC_INDIRECT for R_RISCV_PCREL_HI20 / R_RISCV_PCREL_LO12)
1406 // but the complexity is unnecessary given that GNU as keeps the original
1407 // symbol for this case as well.
1408 if (TargetObjectWriter->getEMachine() == ELF::EM_MIPS &&
1409 !hasRelocationAddend())
1410 return true;
1411 }
1412
1413 // Most TLS relocations use a got, so they need the symbol. Even those that
1414 // are just an offset (@tpoff), require a symbol in gold versions before
1415 // 5efeedf61e4fe720fd3e9a08e6c91c10abb66d42 (2014-09-26) which fixed
1416 // http://sourceware.org/PR16773.
1417 if (Flags & ELF::SHF_TLS)
1418 return true;
1419 }
1420
1421 // If the symbol is a thumb function the final relocation must set the lowest
1422 // bit. With a symbol that is done by just having the symbol have that bit
1423 // set, so we would lose the bit if we relocated with the section.
1424 // FIXME: We could use the section but add the bit to the relocation value.
1425 if (Asm.isThumbFunc(Sym))
1426 return true;
1427
1428 if (TargetObjectWriter->needsRelocateWithSymbol(*Sym, Type))
1429 return true;
1430 return false;
1431 }
1432
recordRelocation(MCAssembler & Asm,const MCAsmLayout & Layout,const MCFragment * Fragment,const MCFixup & Fixup,MCValue Target,uint64_t & FixedValue)1433 void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
1434 const MCAsmLayout &Layout,
1435 const MCFragment *Fragment,
1436 const MCFixup &Fixup, MCValue Target,
1437 uint64_t &FixedValue) {
1438 MCAsmBackend &Backend = Asm.getBackend();
1439 bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
1440 MCFixupKindInfo::FKF_IsPCRel;
1441 const MCSectionELF &FixupSection = cast<MCSectionELF>(*Fragment->getParent());
1442 uint64_t C = Target.getConstant();
1443 uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
1444 MCContext &Ctx = Asm.getContext();
1445
1446 if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
1447 const auto &SymB = cast<MCSymbolELF>(RefB->getSymbol());
1448 if (SymB.isUndefined()) {
1449 Ctx.reportError(Fixup.getLoc(),
1450 Twine("symbol '") + SymB.getName() +
1451 "' can not be undefined in a subtraction expression");
1452 return;
1453 }
1454
1455 assert(!SymB.isAbsolute() && "Should have been folded");
1456 const MCSection &SecB = SymB.getSection();
1457 if (&SecB != &FixupSection) {
1458 Ctx.reportError(Fixup.getLoc(),
1459 "Cannot represent a difference across sections");
1460 return;
1461 }
1462
1463 assert(!IsPCRel && "should have been folded");
1464 IsPCRel = true;
1465 C += FixupOffset - Layout.getSymbolOffset(SymB);
1466 }
1467
1468 // We either rejected the fixup or folded B into C at this point.
1469 const MCSymbolRefExpr *RefA = Target.getSymA();
1470 const auto *SymA = RefA ? cast<MCSymbolELF>(&RefA->getSymbol()) : nullptr;
1471
1472 bool ViaWeakRef = false;
1473 if (SymA && SymA->isVariable()) {
1474 const MCExpr *Expr = SymA->getVariableValue();
1475 if (const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr)) {
1476 if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF) {
1477 SymA = cast<MCSymbolELF>(&Inner->getSymbol());
1478 ViaWeakRef = true;
1479 }
1480 }
1481 }
1482
1483 const MCSectionELF *SecA = (SymA && SymA->isInSection())
1484 ? cast<MCSectionELF>(&SymA->getSection())
1485 : nullptr;
1486 if (!checkRelocation(Ctx, Fixup.getLoc(), &FixupSection, SecA))
1487 return;
1488
1489 unsigned Type = TargetObjectWriter->getRelocType(Ctx, Target, Fixup, IsPCRel);
1490 bool RelocateWithSymbol = shouldRelocateWithSymbol(Asm, RefA, SymA, C, Type);
1491 uint64_t Addend = 0;
1492
1493 FixedValue = !RelocateWithSymbol && SymA && !SymA->isUndefined()
1494 ? C + Layout.getSymbolOffset(*SymA)
1495 : C;
1496 if (hasRelocationAddend()) {
1497 Addend = FixedValue;
1498 FixedValue = 0;
1499 }
1500
1501 if (!RelocateWithSymbol) {
1502 const auto *SectionSymbol =
1503 SecA ? cast<MCSymbolELF>(SecA->getBeginSymbol()) : nullptr;
1504 if (SectionSymbol)
1505 SectionSymbol->setUsedInReloc();
1506 ELFRelocationEntry Rec(FixupOffset, SectionSymbol, Type, Addend, SymA, C);
1507 Relocations[&FixupSection].push_back(Rec);
1508 return;
1509 }
1510
1511 const MCSymbolELF *RenamedSymA = SymA;
1512 if (SymA) {
1513 if (const MCSymbolELF *R = Renames.lookup(SymA))
1514 RenamedSymA = R;
1515
1516 if (ViaWeakRef)
1517 RenamedSymA->setIsWeakrefUsedInReloc();
1518 else
1519 RenamedSymA->setUsedInReloc();
1520 }
1521 ELFRelocationEntry Rec(FixupOffset, RenamedSymA, Type, Addend, SymA, C);
1522 Relocations[&FixupSection].push_back(Rec);
1523 }
1524
isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler & Asm,const MCSymbol & SA,const MCFragment & FB,bool InSet,bool IsPCRel) const1525 bool ELFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
1526 const MCAssembler &Asm, const MCSymbol &SA, const MCFragment &FB,
1527 bool InSet, bool IsPCRel) const {
1528 const auto &SymA = cast<MCSymbolELF>(SA);
1529 if (IsPCRel) {
1530 assert(!InSet);
1531 if (SymA.getBinding() != ELF::STB_LOCAL ||
1532 SymA.getType() == ELF::STT_GNU_IFUNC)
1533 return false;
1534 }
1535 return MCObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(Asm, SymA, FB,
1536 InSet, IsPCRel);
1537 }
1538
1539 std::unique_ptr<MCObjectWriter>
createELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,raw_pwrite_stream & OS,bool IsLittleEndian)1540 llvm::createELFObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
1541 raw_pwrite_stream &OS, bool IsLittleEndian) {
1542 return std::make_unique<ELFSingleObjectWriter>(std::move(MOTW), OS,
1543 IsLittleEndian);
1544 }
1545
1546 std::unique_ptr<MCObjectWriter>
createELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,raw_pwrite_stream & OS,raw_pwrite_stream & DwoOS,bool IsLittleEndian)1547 llvm::createELFDwoObjectWriter(std::unique_ptr<MCELFObjectTargetWriter> MOTW,
1548 raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS,
1549 bool IsLittleEndian) {
1550 return std::make_unique<ELFDwoObjectWriter>(std::move(MOTW), OS, DwoOS,
1551 IsLittleEndian);
1552 }
1553