1 //===- Writer.cpp ---------------------------------------------------------===//
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
12 #include "LinkerScript.h"
14 #include "OutputSections.h"
15 #include "Relocations.h"
17 #include "SymbolTable.h"
18 #include "SyntheticSections.h"
20 #include "llvm/ADT/StringMap.h"
21 #include "llvm/ADT/StringSwitch.h"
22 #include "llvm/Support/FileOutputBuffer.h"
23 #include "llvm/Support/FileSystem.h"
24 #include "llvm/Support/raw_ostream.h"
29 using namespace llvm::ELF;
30 using namespace llvm::object;
31 using namespace llvm::support;
32 using namespace llvm::support::endian;
35 using namespace lld::elf;
38 // The writer writes a SymbolTable result to a file.
39 template <class ELFT> class Writer {
41 typedef typename ELFT::uint uintX_t;
42 typedef typename ELFT::Shdr Elf_Shdr;
43 typedef typename ELFT::Ehdr Elf_Ehdr;
44 typedef typename ELFT::Phdr Elf_Phdr;
45 typedef typename ELFT::Sym Elf_Sym;
46 typedef typename ELFT::SymRange Elf_Sym_Range;
47 typedef typename ELFT::Rela Elf_Rela;
51 void createSyntheticSections();
52 void copyLocalSymbols();
53 void addReservedSymbols();
54 void addInputSec(InputSectionBase<ELFT> *S);
55 void createSections();
56 void forEachRelSec(std::function<void(InputSectionBase<ELFT> &)> Fn);
58 void finalizeSections();
59 void addPredefinedSections();
61 std::vector<PhdrEntry> createPhdrs();
62 void removeEmptyPTLoad();
63 void addPtArmExid(std::vector<PhdrEntry> &Phdrs);
64 void assignAddresses();
65 void assignFileOffsets();
66 void assignFileOffsetsBinary();
69 void fixSectionAlignments();
70 void fixAbsoluteSymbols();
74 void writeSectionsBinary();
77 std::unique_ptr<FileOutputBuffer> Buffer;
79 std::vector<OutputSectionBase *> OutputSections;
80 OutputSectionFactory<ELFT> Factory;
82 void addRelIpltSymbols();
83 void addStartEndSymbols();
84 void addStartStopSymbols(OutputSectionBase *Sec);
85 uintX_t getEntryAddr();
86 OutputSectionBase *findSection(StringRef Name);
88 std::vector<PhdrEntry> Phdrs;
91 uintX_t SectionHeaderOff;
92 bool AllocateHeader = true;
94 } // anonymous namespace
96 StringRef elf::getOutputSectionName(StringRef Name) {
97 if (Config->Relocatable)
101 {".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.",
102 ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
103 ".gcc_except_table.", ".tdata.", ".ARM.exidx."}) {
104 StringRef Prefix = V.drop_back();
105 if (Name.startswith(V) || Name == Prefix)
109 // CommonSection is identified as "COMMON" in linker scripts.
110 // By default, it should go to .bss section.
111 if (Name == "COMMON")
114 // ".zdebug_" is a prefix for ZLIB-compressed sections.
115 // Because we decompressed input sections, we want to remove 'z'.
116 if (Name.startswith(".zdebug_"))
117 return Saver.save(Twine(".") + Name.substr(2));
121 template <class ELFT> void elf::reportDiscarded(InputSectionBase<ELFT> *IS) {
122 if (!Config->PrintGcSections)
124 errs() << "removing unused section from '" << IS->Name << "' in file '"
125 << IS->getFile()->getName() << "'\n";
128 template <class ELFT> static bool needsInterpSection() {
129 return !Symtab<ELFT>::X->getSharedFiles().empty() &&
130 !Config->DynamicLinker.empty() &&
131 !Script<ELFT>::X->ignoreInterpSection();
134 template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
136 template <class ELFT> void Writer<ELFT>::removeEmptyPTLoad() {
137 auto I = std::remove_if(Phdrs.begin(), Phdrs.end(), [&](const PhdrEntry &P) {
138 if (P.p_type != PT_LOAD)
142 uintX_t Size = P.Last->Addr + P.Last->Size - P.First->Addr;
145 Phdrs.erase(I, Phdrs.end());
148 // The main function of the writer.
149 template <class ELFT> void Writer<ELFT>::run() {
150 // Create linker-synthesized sections such as .got or .plt.
151 // Such sections are of type input section.
152 createSyntheticSections();
154 // We need to create some reserved symbols such as _end. Create them.
155 if (!Config->Relocatable)
156 addReservedSymbols();
158 // Some architectures use small displacements for jump instructions.
159 // It is linker's responsibility to create thunks containing long
160 // jump instructions if jump targets are too far. Create thunks.
161 if (Target->NeedsThunks)
162 forEachRelSec(createThunks<ELFT>);
164 // Create output sections.
165 Script<ELFT>::X->OutputSections = &OutputSections;
166 if (ScriptConfig->HasSections) {
167 // If linker script contains SECTIONS commands, let it create sections.
168 Script<ELFT>::X->processCommands(Factory);
170 // Linker scripts may have left some input sections unassigned.
171 // Assign such sections using the default rule.
172 Script<ELFT>::X->addOrphanSections(Factory);
174 // If linker script does not contain SECTIONS commands, create
175 // output sections by default rules. We still need to give the
176 // linker script a chance to run, because it might contain
177 // non-SECTIONS commands such as ASSERT.
179 Script<ELFT>::X->processCommands(Factory);
182 if (Config->Discard != DiscardPolicy::All)
185 // Now that we have a complete set of output sections. This function
186 // completes section contents. For example, we need to add strings
187 // to the string table, and add entries to .got and .plt.
188 // finalizeSections does that.
193 if (Config->Relocatable) {
196 if (ScriptConfig->HasSections) {
197 Script<ELFT>::X->assignAddresses(Phdrs);
199 fixSectionAlignments();
203 // Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a
204 // 0 sized region. This has to be done late since only after assignAddresses
205 // we know the size of the sections.
208 if (!Config->OFormatBinary)
211 assignFileOffsetsBinary();
214 fixAbsoluteSymbols();
217 // Write the result down to a file.
221 if (!Config->OFormatBinary) {
225 writeSectionsBinary();
228 // Backfill .note.gnu.build-id section content. This is done at last
229 // because the content is usually a hash value of the entire output file.
234 if (auto EC = Buffer->commit())
235 error(EC, "failed to write to the output file");
237 // Flush the output streams and exit immediately. A full shutdown
238 // is a good test that we are keeping track of all allocated memory,
239 // but actually freeing it is a waste of time in a regular linker run.
240 if (Config->ExitEarly)
244 // Initialize Out<ELFT> members.
245 template <class ELFT> void Writer<ELFT>::createSyntheticSections() {
246 // Initialize all pointers with NULL. This is needed because
247 // you can call lld::elf::main more than once as a library.
248 memset(&Out<ELFT>::First, 0, sizeof(Out<ELFT>));
250 // Create singleton output sections.
252 make<OutputSection<ELFT>>(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
253 In<ELFT>::DynStrTab = make<StringTableSection<ELFT>>(".dynstr", true);
254 In<ELFT>::Dynamic = make<DynamicSection<ELFT>>();
255 Out<ELFT>::EhFrame = make<EhOutputSection<ELFT>>();
256 In<ELFT>::RelaDyn = make<RelocationSection<ELFT>>(
257 Config->Rela ? ".rela.dyn" : ".rel.dyn", Config->ZCombreloc);
258 In<ELFT>::ShStrTab = make<StringTableSection<ELFT>>(".shstrtab", false);
260 Out<ELFT>::ElfHeader = make<OutputSectionBase>("", 0, SHF_ALLOC);
261 Out<ELFT>::ElfHeader->Size = sizeof(Elf_Ehdr);
262 Out<ELFT>::ProgramHeaders = make<OutputSectionBase>("", 0, SHF_ALLOC);
263 Out<ELFT>::ProgramHeaders->updateAlignment(sizeof(uintX_t));
265 if (needsInterpSection<ELFT>()) {
266 In<ELFT>::Interp = createInterpSection<ELFT>();
267 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Interp);
269 In<ELFT>::Interp = nullptr;
272 if (!Config->Relocatable)
273 Symtab<ELFT>::X->Sections.push_back(createCommentSection<ELFT>());
275 if (Config->Strip != StripPolicy::All) {
276 In<ELFT>::StrTab = make<StringTableSection<ELFT>>(".strtab", false);
277 In<ELFT>::SymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::StrTab);
280 if (Config->BuildId != BuildIdKind::None) {
281 In<ELFT>::BuildId = make<BuildIdSection<ELFT>>();
282 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::BuildId);
285 InputSection<ELFT> *Common = createCommonSection<ELFT>();
286 if (!Common->Data.empty()) {
287 In<ELFT>::Common = Common;
288 Symtab<ELFT>::X->Sections.push_back(Common);
291 // Add MIPS-specific sections.
292 bool HasDynSymTab = !Symtab<ELFT>::X->getSharedFiles().empty() || Config->Pic;
293 if (Config->EMachine == EM_MIPS) {
294 if (!Config->Shared && HasDynSymTab) {
295 In<ELFT>::MipsRldMap = make<MipsRldMapSection<ELFT>>();
296 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::MipsRldMap);
298 if (auto *Sec = MipsAbiFlagsSection<ELFT>::create())
299 Symtab<ELFT>::X->Sections.push_back(Sec);
300 if (auto *Sec = MipsOptionsSection<ELFT>::create())
301 Symtab<ELFT>::X->Sections.push_back(Sec);
302 if (auto *Sec = MipsReginfoSection<ELFT>::create())
303 Symtab<ELFT>::X->Sections.push_back(Sec);
307 In<ELFT>::DynSymTab = make<SymbolTableSection<ELFT>>(*In<ELFT>::DynStrTab);
308 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::DynSymTab);
310 In<ELFT>::VerSym = make<VersionTableSection<ELFT>>();
311 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::VerSym);
313 if (!Config->VersionDefinitions.empty()) {
314 In<ELFT>::VerDef = make<VersionDefinitionSection<ELFT>>();
315 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::VerDef);
318 In<ELFT>::VerNeed = make<VersionNeedSection<ELFT>>();
319 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::VerNeed);
321 if (Config->GnuHash) {
322 In<ELFT>::GnuHashTab = make<GnuHashTableSection<ELFT>>();
323 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::GnuHashTab);
326 if (Config->SysvHash) {
327 In<ELFT>::HashTab = make<HashTableSection<ELFT>>();
328 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::HashTab);
331 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Dynamic);
332 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::DynStrTab);
333 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::RelaDyn);
336 // Add .got. MIPS' .got is so different from the other archs,
337 // it has its own class.
338 if (Config->EMachine == EM_MIPS) {
339 In<ELFT>::MipsGot = make<MipsGotSection<ELFT>>();
340 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::MipsGot);
342 In<ELFT>::Got = make<GotSection<ELFT>>();
343 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Got);
346 In<ELFT>::GotPlt = make<GotPltSection<ELFT>>();
347 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::GotPlt);
348 In<ELFT>::IgotPlt = make<IgotPltSection<ELFT>>();
349 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::IgotPlt);
351 if (Config->GdbIndex) {
352 In<ELFT>::GdbIndex = make<GdbIndexSection<ELFT>>();
353 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::GdbIndex);
356 // We always need to add rel[a].plt to output if it has entries.
357 // Even for static linking it can contain R_[*]_IRELATIVE relocations.
358 In<ELFT>::RelaPlt = make<RelocationSection<ELFT>>(
359 Config->Rela ? ".rela.plt" : ".rel.plt", false /*Sort*/);
360 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::RelaPlt);
362 // The RelaIplt immediately follows .rel.plt (.rel.dyn for ARM) to ensure
363 // that the IRelative relocations are processed last by the dynamic loader
364 In<ELFT>::RelaIplt = make<RelocationSection<ELFT>>(
365 (Config->EMachine == EM_ARM) ? ".rel.dyn" : In<ELFT>::RelaPlt->Name,
367 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::RelaIplt);
369 In<ELFT>::Plt = make<PltSection<ELFT>>();
370 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Plt);
371 In<ELFT>::Iplt = make<IpltSection<ELFT>>();
372 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::Iplt);
374 if (Config->EhFrameHdr) {
375 In<ELFT>::EhFrameHdr = make<EhFrameHeader<ELFT>>();
376 Symtab<ELFT>::X->Sections.push_back(In<ELFT>::EhFrameHdr);
380 template <class ELFT>
381 static bool shouldKeepInSymtab(InputSectionBase<ELFT> *Sec, StringRef SymName,
382 const SymbolBody &B) {
386 // We keep sections in symtab for relocatable output.
388 return Config->Relocatable;
390 // If sym references a section in a discarded group, don't keep it.
391 if (Sec == &InputSection<ELFT>::Discarded)
394 if (Config->Discard == DiscardPolicy::None)
397 // In ELF assembly .L symbols are normally discarded by the assembler.
398 // If the assembler fails to do so, the linker discards them if
399 // * --discard-locals is used.
400 // * The symbol is in a SHF_MERGE section, which is normally the reason for
401 // the assembler keeping the .L symbol.
402 if (!SymName.startswith(".L") && !SymName.empty())
405 if (Config->Discard == DiscardPolicy::Locals)
408 return !Sec || !(Sec->Flags & SHF_MERGE);
411 template <class ELFT> static bool includeInSymtab(const SymbolBody &B) {
412 if (!B.isLocal() && !B.symbol()->IsUsedInRegularObj)
415 // If --retain-symbols-file is given, we'll keep only symbols listed in that
417 if (Config->Discard == DiscardPolicy::RetainFile &&
418 !Config->RetainSymbolsFile.count(B.getName()))
421 if (auto *D = dyn_cast<DefinedRegular<ELFT>>(&B)) {
422 // Always include absolute symbols.
425 // Exclude symbols pointing to garbage-collected sections.
426 if (!D->Section->Live)
428 if (auto *S = dyn_cast<MergeInputSection<ELFT>>(D->Section))
429 if (!S->getSectionPiece(D->Value)->Live)
435 // Local symbols are not in the linker's symbol table. This function scans
436 // each object file's symbol table to copy local symbols to the output.
437 template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
438 if (!In<ELFT>::SymTab)
440 for (elf::ObjectFile<ELFT> *F : Symtab<ELFT>::X->getObjectFiles()) {
441 for (SymbolBody *B : F->getLocalSymbols()) {
444 ": broken object: getLocalSymbols returns a non-local symbol");
445 auto *DR = dyn_cast<DefinedRegular<ELFT>>(B);
447 // No reason to keep local undefined symbol in symtab.
450 if (!includeInSymtab<ELFT>(*B))
453 InputSectionBase<ELFT> *Sec = DR->Section;
454 if (!shouldKeepInSymtab<ELFT>(Sec, B->getName(), *B))
456 ++In<ELFT>::SymTab->NumLocals;
457 if (Config->Relocatable)
458 B->DynsymIndex = In<ELFT>::SymTab->NumLocals;
459 F->KeptLocalSyms.push_back(std::make_pair(
460 DR, In<ELFT>::SymTab->StrTabSec.addString(B->getName())));
465 // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections that
466 // we would like to make sure appear is a specific order to maximize their
467 // coverage by a single signed 16-bit offset from the TOC base pointer.
468 // Conversely, the special .tocbss section should be first among all SHT_NOBITS
469 // sections. This will put it next to the loaded special PPC64 sections (and,
470 // thus, within reach of the TOC base pointer).
471 static int getPPC64SectionRank(StringRef SectionName) {
472 return StringSwitch<int>(SectionName)
474 .Case(".branch_lt", 2)
481 template <class ELFT> bool elf::isRelroSection(const OutputSectionBase *Sec) {
484 uint64_t Flags = Sec->Flags;
485 if (!(Flags & SHF_ALLOC) || !(Flags & SHF_WRITE))
489 uint32_t Type = Sec->Type;
490 if (Type == SHT_INIT_ARRAY || Type == SHT_FINI_ARRAY ||
491 Type == SHT_PREINIT_ARRAY)
493 if (Sec == In<ELFT>::GotPlt->OutSec)
495 if (Sec == In<ELFT>::Dynamic->OutSec)
497 if (In<ELFT>::Got && Sec == In<ELFT>::Got->OutSec)
499 if (In<ELFT>::MipsGot && Sec == In<ELFT>::MipsGot->OutSec)
501 StringRef S = Sec->getName();
502 return S == ".data.rel.ro" || S == ".ctors" || S == ".dtors" || S == ".jcr" ||
503 S == ".eh_frame" || S == ".openbsd.randomdata";
506 template <class ELFT>
507 static bool compareSectionsNonScript(const OutputSectionBase *A,
508 const OutputSectionBase *B) {
509 // Put .interp first because some loaders want to see that section
510 // on the first page of the executable file when loaded into memory.
511 bool AIsInterp = A->getName() == ".interp";
512 bool BIsInterp = B->getName() == ".interp";
513 if (AIsInterp != BIsInterp)
516 // Allocatable sections go first to reduce the total PT_LOAD size and
517 // so debug info doesn't change addresses in actual code.
518 bool AIsAlloc = A->Flags & SHF_ALLOC;
519 bool BIsAlloc = B->Flags & SHF_ALLOC;
520 if (AIsAlloc != BIsAlloc)
523 // We don't have any special requirements for the relative order of two non
524 // allocatable sections.
528 // We want to put section specified by -T option first, so we
529 // can start assigning VA starting from them later.
530 auto AAddrSetI = Config->SectionStartMap.find(A->getName());
531 auto BAddrSetI = Config->SectionStartMap.find(B->getName());
532 bool AHasAddrSet = AAddrSetI != Config->SectionStartMap.end();
533 bool BHasAddrSet = BAddrSetI != Config->SectionStartMap.end();
534 if (AHasAddrSet != BHasAddrSet)
537 return AAddrSetI->second < BAddrSetI->second;
539 // We want the read only sections first so that they go in the PT_LOAD
540 // covering the program headers at the start of the file.
541 bool AIsWritable = A->Flags & SHF_WRITE;
542 bool BIsWritable = B->Flags & SHF_WRITE;
543 if (AIsWritable != BIsWritable)
546 if (!Config->SingleRoRx) {
547 // For a corresponding reason, put non exec sections first (the program
548 // header PT_LOAD is not executable).
549 // We only do that if we are not using linker scripts, since with linker
550 // scripts ro and rx sections are in the same PT_LOAD, so their relative
551 // order is not important. The same applies for -no-rosegment.
552 bool AIsExec = A->Flags & SHF_EXECINSTR;
553 bool BIsExec = B->Flags & SHF_EXECINSTR;
554 if (AIsExec != BIsExec)
558 // If we got here we know that both A and B are in the same PT_LOAD.
560 // The TLS initialization block needs to be a single contiguous block in a R/W
561 // PT_LOAD, so stick TLS sections directly before R/W sections. The TLS NOBITS
562 // sections are placed here as they don't take up virtual address space in the
564 bool AIsTls = A->Flags & SHF_TLS;
565 bool BIsTls = B->Flags & SHF_TLS;
566 if (AIsTls != BIsTls)
569 // The next requirement we have is to put nobits sections last. The
570 // reason is that the only thing the dynamic linker will see about
571 // them is a p_memsz that is larger than p_filesz. Seeing that it
572 // zeros the end of the PT_LOAD, so that has to correspond to the
574 bool AIsNoBits = A->Type == SHT_NOBITS;
575 bool BIsNoBits = B->Type == SHT_NOBITS;
576 if (AIsNoBits != BIsNoBits)
579 // We place RelRo section before plain r/w ones.
580 bool AIsRelRo = isRelroSection<ELFT>(A);
581 bool BIsRelRo = isRelroSection<ELFT>(B);
582 if (AIsRelRo != BIsRelRo)
585 // Some architectures have additional ordering restrictions for sections
586 // within the same PT_LOAD.
587 if (Config->EMachine == EM_PPC64)
588 return getPPC64SectionRank(A->getName()) <
589 getPPC64SectionRank(B->getName());
594 // Output section ordering is determined by this function.
595 template <class ELFT>
596 static bool compareSections(const OutputSectionBase *A,
597 const OutputSectionBase *B) {
598 // For now, put sections mentioned in a linker script first.
599 int AIndex = Script<ELFT>::X->getSectionIndex(A->getName());
600 int BIndex = Script<ELFT>::X->getSectionIndex(B->getName());
601 bool AInScript = AIndex != INT_MAX;
602 bool BInScript = BIndex != INT_MAX;
603 if (AInScript != BInScript)
605 // If both are in the script, use that order.
607 return AIndex < BIndex;
609 return compareSectionsNonScript<ELFT>(A, B);
612 // Program header entry
613 PhdrEntry::PhdrEntry(unsigned Type, unsigned Flags) {
618 void PhdrEntry::add(OutputSectionBase *Sec) {
622 p_align = std::max(p_align, Sec->Addralign);
623 if (p_type == PT_LOAD)
624 Sec->FirstInPtLoad = First;
627 template <class ELFT>
628 static void addOptionalSynthetic(StringRef Name, OutputSectionBase *Sec,
629 typename ELFT::uint Val,
630 uint8_t StOther = STV_HIDDEN) {
631 if (SymbolBody *S = Symtab<ELFT>::X->find(Name))
632 if (S->isUndefined() || S->isShared())
633 Symtab<ELFT>::X->addSynthetic(Name, Sec, Val, StOther);
636 template <class ELFT>
637 static Symbol *addRegular(StringRef Name, InputSectionBase<ELFT> *Sec,
638 typename ELFT::uint Value) {
639 // The linker generated symbols are added as STB_WEAK to allow user defined
640 // ones to override them.
641 return Symtab<ELFT>::X->addRegular(Name, STV_HIDDEN, STT_NOTYPE, Value,
642 /*Size=*/0, STB_WEAK, Sec,
646 template <class ELFT>
647 static Symbol *addOptionalRegular(StringRef Name, InputSectionBase<ELFT> *IS,
648 typename ELFT::uint Value) {
649 SymbolBody *S = Symtab<ELFT>::X->find(Name);
652 if (!S->isUndefined() && !S->isShared())
654 return addRegular(Name, IS, Value);
657 // The beginning and the ending of .rel[a].plt section are marked
658 // with __rel[a]_iplt_{start,end} symbols if it is a statically linked
659 // executable. The runtime needs these symbols in order to resolve
660 // all IRELATIVE relocs on startup. For dynamic executables, we don't
661 // need these symbols, since IRELATIVE relocs are resolved through GOT
662 // and PLT. For details, see http://www.airs.com/blog/archives/403.
663 template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
664 if (In<ELFT>::DynSymTab)
666 StringRef S = Config->Rela ? "__rela_iplt_start" : "__rel_iplt_start";
667 addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, 0);
669 S = Config->Rela ? "__rela_iplt_end" : "__rel_iplt_end";
670 addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, -1);
673 // The linker is expected to define some symbols depending on
674 // the linking result. This function defines such symbols.
675 template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
676 if (Config->EMachine == EM_MIPS) {
677 // Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
678 // so that it points to an absolute address which by default is relative
679 // to GOT. Default offset is 0x7ff0.
680 // See "Global Data Symbols" in Chapter 6 in the following document:
681 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
682 ElfSym<ELFT>::MipsGp =
683 Symtab<ELFT>::X->addAbsolute("_gp", STV_HIDDEN, STB_LOCAL);
685 // On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
686 // start of function and 'gp' pointer into GOT. To simplify relocation
687 // calculation we assign _gp value to it and calculate corresponding
688 // relocations as relative to this value.
689 if (Symtab<ELFT>::X->find("_gp_disp"))
690 ElfSym<ELFT>::MipsGpDisp =
691 Symtab<ELFT>::X->addAbsolute("_gp_disp", STV_HIDDEN, STB_LOCAL);
693 // The __gnu_local_gp is a magic symbol equal to the current value of 'gp'
694 // pointer. This symbol is used in the code generated by .cpload pseudo-op
695 // in case of using -mno-shared option.
696 // https://sourceware.org/ml/binutils/2004-12/msg00094.html
697 if (Symtab<ELFT>::X->find("__gnu_local_gp"))
698 ElfSym<ELFT>::MipsLocalGp =
699 Symtab<ELFT>::X->addAbsolute("__gnu_local_gp", STV_HIDDEN, STB_LOCAL);
702 // In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol
703 // is magical and is used to produce a R_386_GOTPC relocation.
704 // The R_386_GOTPC relocation value doesn't actually depend on the
705 // symbol value, so it could use an index of STN_UNDEF which, according
706 // to the spec, means the symbol value is 0.
707 // Unfortunately both gas and MC keep the _GLOBAL_OFFSET_TABLE_ symbol in
709 // The situation is even stranger on x86_64 where the assembly doesn't
710 // need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
711 // an undefined symbol in the .o files.
712 // Given that the symbol is effectively unused, we just create a dummy
713 // hidden one to avoid the undefined symbol error.
714 Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");
716 // __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
717 // static linking the linker is required to optimize away any references to
718 // __tls_get_addr, so it's not defined anywhere. Create a hidden definition
719 // to avoid the undefined symbol error. As usual special cases are ARM and
720 // MIPS - the libc for these targets defines __tls_get_addr itself because
721 // there are no TLS optimizations for these targets.
722 if (!In<ELFT>::DynSymTab &&
723 (Config->EMachine != EM_MIPS && Config->EMachine != EM_ARM))
724 Symtab<ELFT>::X->addIgnored("__tls_get_addr");
726 // If linker script do layout we do not need to create any standart symbols.
727 if (ScriptConfig->HasSections)
730 ElfSym<ELFT>::EhdrStart = Symtab<ELFT>::X->addIgnored("__ehdr_start");
732 auto Define = [this](StringRef S, DefinedRegular<ELFT> *&Sym1,
733 DefinedRegular<ELFT> *&Sym2) {
734 Sym1 = Symtab<ELFT>::X->addIgnored(S, STV_DEFAULT);
736 // The name without the underscore is not a reserved name,
737 // so it is defined only when there is a reference against it.
738 assert(S.startswith("_"));
740 if (SymbolBody *B = Symtab<ELFT>::X->find(S))
741 if (B->isUndefined())
742 Sym2 = Symtab<ELFT>::X->addAbsolute(S, STV_DEFAULT);
745 Define("_end", ElfSym<ELFT>::End, ElfSym<ELFT>::End2);
746 Define("_etext", ElfSym<ELFT>::Etext, ElfSym<ELFT>::Etext2);
747 Define("_edata", ElfSym<ELFT>::Edata, ElfSym<ELFT>::Edata2);
750 // Sort input sections by section name suffixes for
751 // __attribute__((init_priority(N))).
752 template <class ELFT> static void sortInitFini(OutputSectionBase *S) {
754 reinterpret_cast<OutputSection<ELFT> *>(S)->sortInitFini();
757 // Sort input sections by the special rule for .ctors and .dtors.
758 template <class ELFT> static void sortCtorsDtors(OutputSectionBase *S) {
760 reinterpret_cast<OutputSection<ELFT> *>(S)->sortCtorsDtors();
763 // Sort input sections using the list provided by --symbol-ordering-file.
764 template <class ELFT>
765 static void sortBySymbolsOrder(ArrayRef<OutputSectionBase *> OutputSections) {
766 if (Config->SymbolOrderingFile.empty())
769 // Build a map from symbols to their priorities. Symbols that didn't
770 // appear in the symbol ordering file have the lowest priority 0.
771 // All explicitly mentioned symbols have negative (higher) priorities.
772 DenseMap<StringRef, int> SymbolOrder;
773 int Priority = -Config->SymbolOrderingFile.size();
774 for (StringRef S : Config->SymbolOrderingFile)
775 SymbolOrder.insert({S, Priority++});
777 // Build a map from sections to their priorities.
778 DenseMap<InputSectionBase<ELFT> *, int> SectionOrder;
779 for (elf::ObjectFile<ELFT> *File : Symtab<ELFT>::X->getObjectFiles()) {
780 for (SymbolBody *Body : File->getSymbols()) {
781 auto *D = dyn_cast<DefinedRegular<ELFT>>(Body);
782 if (!D || !D->Section)
784 int &Priority = SectionOrder[D->Section];
785 Priority = std::min(Priority, SymbolOrder.lookup(D->getName()));
789 // Sort sections by priority.
790 for (OutputSectionBase *Base : OutputSections)
791 if (auto *Sec = dyn_cast<OutputSection<ELFT>>(Base))
792 Sec->sort([&](InputSection<ELFT> *S) { return SectionOrder.lookup(S); });
795 template <class ELFT>
796 void Writer<ELFT>::forEachRelSec(
797 std::function<void(InputSectionBase<ELFT> &)> Fn) {
798 for (InputSectionBase<ELFT> *IS : Symtab<ELFT>::X->Sections) {
801 // Scan all relocations. Each relocation goes through a series
802 // of tests to determine if it needs special treatment, such as
803 // creating GOT, PLT, copy relocations, etc.
804 // Note that relocations for non-alloc sections are directly
805 // processed by InputSection::relocateNonAlloc.
806 if (!(IS->Flags & SHF_ALLOC))
808 if (isa<InputSection<ELFT>>(IS) || isa<EhInputSection<ELFT>>(IS))
813 template <class ELFT>
814 void Writer<ELFT>::addInputSec(InputSectionBase<ELFT> *IS) {
822 OutputSectionBase *Sec;
824 StringRef OutsecName = getOutputSectionName(IS->Name);
825 std::tie(Sec, IsNew) = Factory.create(IS, OutsecName);
827 OutputSections.push_back(Sec);
831 template <class ELFT> void Writer<ELFT>::createSections() {
832 for (InputSectionBase<ELFT> *IS : Symtab<ELFT>::X->Sections)
835 sortBySymbolsOrder<ELFT>(OutputSections);
836 sortInitFini<ELFT>(findSection(".init_array"));
837 sortInitFini<ELFT>(findSection(".fini_array"));
838 sortCtorsDtors<ELFT>(findSection(".ctors"));
839 sortCtorsDtors<ELFT>(findSection(".dtors"));
841 for (OutputSectionBase *Sec : OutputSections)
842 Sec->assignOffsets();
845 template <class ELFT>
846 static bool canSharePtLoad(const OutputSectionBase &S1,
847 const OutputSectionBase &S2) {
848 if (!(S1.Flags & SHF_ALLOC) || !(S2.Flags & SHF_ALLOC))
851 bool S1IsWrite = S1.Flags & SHF_WRITE;
852 bool S2IsWrite = S2.Flags & SHF_WRITE;
853 if (S1IsWrite != S2IsWrite)
857 return true; // RO and RX share a PT_LOAD with linker scripts.
858 return (S1.Flags & SHF_EXECINSTR) == (S2.Flags & SHF_EXECINSTR);
861 template <class ELFT> void Writer<ELFT>::sortSections() {
862 // Don't sort if using -r. It is not necessary and we want to preserve the
863 // relative order for SHF_LINK_ORDER sections.
864 if (Config->Relocatable)
866 if (!ScriptConfig->HasSections) {
867 std::stable_sort(OutputSections.begin(), OutputSections.end(),
868 compareSectionsNonScript<ELFT>);
871 Script<ELFT>::X->adjustSectionsBeforeSorting();
873 // The order of the sections in the script is arbitrary and may not agree with
874 // compareSectionsNonScript. This means that we cannot easily define a
875 // strict weak ordering. To see why, consider a comparison of a section in the
876 // script and one not in the script. We have a two simple options:
877 // * Make them equivalent (a is not less than b, and b is not less than a).
878 // The problem is then that equivalence has to be transitive and we can
879 // have sections a, b and c with only b in a script and a less than c
880 // which breaks this property.
881 // * Use compareSectionsNonScript. Given that the script order doesn't have
882 // to match, we can end up with sections a, b, c, d where b and c are in the
883 // script and c is compareSectionsNonScript less than b. In which case d
884 // can be equivalent to c, a to b and d < a. As a concrete example:
885 // .a (rx) # not in script
886 // .b (rx) # in script
887 // .c (ro) # in script
888 // .d (ro) # not in script
890 // The way we define an order then is:
891 // * First put script sections at the start and sort the script and
892 // non-script sections independently.
893 // * Move each non-script section to its preferred position. We try
894 // to put each section in the last position where it it can share
897 std::stable_sort(OutputSections.begin(), OutputSections.end(),
898 compareSections<ELFT>);
900 auto I = OutputSections.begin();
901 auto E = OutputSections.end();
903 std::find_if(OutputSections.begin(), E, [](OutputSectionBase *S) {
904 return Script<ELFT>::X->getSectionIndex(S->getName()) == INT_MAX;
906 while (NonScriptI != E) {
907 auto BestPos = std::max_element(
908 I, NonScriptI, [&](OutputSectionBase *&A, OutputSectionBase *&B) {
909 bool ACanSharePtLoad = canSharePtLoad<ELFT>(**NonScriptI, *A);
910 bool BCanSharePtLoad = canSharePtLoad<ELFT>(**NonScriptI, *B);
911 if (ACanSharePtLoad != BCanSharePtLoad)
912 return BCanSharePtLoad;
914 bool ACmp = compareSectionsNonScript<ELFT>(*NonScriptI, A);
915 bool BCmp = compareSectionsNonScript<ELFT>(*NonScriptI, B);
917 return BCmp; // FIXME: missing test
919 size_t PosA = &A - &OutputSections[0];
920 size_t PosB = &B - &OutputSections[0];
921 return ACmp ? PosA > PosB : PosA < PosB;
924 // max_element only returns NonScriptI if the range is empty. If the range
925 // is not empty we should consider moving the the element forward one
927 if (BestPos != NonScriptI &&
928 !compareSectionsNonScript<ELFT>(*NonScriptI, *BestPos))
930 std::rotate(BestPos, NonScriptI, NonScriptI + 1);
934 Script<ELFT>::X->adjustSectionsAfterSorting();
937 template <class ELFT>
939 finalizeSynthetic(const std::vector<SyntheticSection<ELFT> *> &Sections) {
940 for (SyntheticSection<ELFT> *SS : Sections)
941 if (SS && SS->OutSec && !SS->empty()) {
943 SS->OutSec->Size = 0;
944 SS->OutSec->assignOffsets();
948 // We need to add input synthetic sections early in createSyntheticSections()
949 // to make them visible from linkescript side. But not all sections are always
950 // required to be in output. For example we don't need dynamic section content
951 // sometimes. This function filters out such unused sections from output.
952 template <class ELFT>
953 static void removeUnusedSyntheticSections(std::vector<OutputSectionBase *> &V) {
954 // Input synthetic sections are placed after all regular ones. We iterate over
955 // them all and exit at first non-synthetic.
956 for (InputSectionBase<ELFT> *S : llvm::reverse(Symtab<ELFT>::X->Sections)) {
957 SyntheticSection<ELFT> *SS = dyn_cast<SyntheticSection<ELFT>>(S);
960 if (!SS->empty() || !SS->OutSec)
963 OutputSection<ELFT> *OutSec = cast<OutputSection<ELFT>>(SS->OutSec);
964 OutSec->Sections.erase(
965 std::find(OutSec->Sections.begin(), OutSec->Sections.end(), SS));
966 // If there is no other sections in output section, remove it from output.
967 if (OutSec->Sections.empty())
968 V.erase(std::find(V.begin(), V.end(), OutSec));
972 // Create output section objects and add them to OutputSections.
973 template <class ELFT> void Writer<ELFT>::finalizeSections() {
974 Out<ELFT>::DebugInfo = findSection(".debug_info");
975 Out<ELFT>::PreinitArray = findSection(".preinit_array");
976 Out<ELFT>::InitArray = findSection(".init_array");
977 Out<ELFT>::FiniArray = findSection(".fini_array");
979 // The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
980 // symbols for sections, so that the runtime can get the start and end
981 // addresses of each section by section name. Add such symbols.
982 if (!Config->Relocatable) {
983 addStartEndSymbols();
984 for (OutputSectionBase *Sec : OutputSections)
985 addStartStopSymbols(Sec);
988 // Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
989 // It should be okay as no one seems to care about the type.
990 // Even the author of gold doesn't remember why gold behaves that way.
991 // https://sourceware.org/ml/binutils/2002-03/msg00360.html
992 if (In<ELFT>::DynSymTab)
993 addRegular("_DYNAMIC", In<ELFT>::Dynamic, 0);
995 // Define __rel[a]_iplt_{start,end} symbols if needed.
998 if (!Out<ELFT>::EhFrame->empty()) {
999 OutputSections.push_back(Out<ELFT>::EhFrame);
1000 Out<ELFT>::EhFrame->finalize();
1003 // Scan relocations. This must be done after every symbol is declared so that
1004 // we can correctly decide if a dynamic relocation is needed.
1005 forEachRelSec(scanRelocations<ELFT>);
1007 // Now that we have defined all possible symbols including linker-
1008 // synthesized ones. Visit all symbols to give the finishing touches.
1009 for (Symbol *S : Symtab<ELFT>::X->getSymbols()) {
1010 SymbolBody *Body = S->body();
1012 if (!includeInSymtab<ELFT>(*Body))
1014 if (In<ELFT>::SymTab)
1015 In<ELFT>::SymTab->addSymbol(Body);
1017 if (In<ELFT>::DynSymTab && S->includeInDynsym()) {
1018 In<ELFT>::DynSymTab->addSymbol(Body);
1019 if (auto *SS = dyn_cast<SharedSymbol<ELFT>>(Body))
1020 if (SS->file()->isNeeded())
1021 In<ELFT>::VerNeed->addSymbol(SS);
1025 // Do not proceed if there was an undefined symbol.
1029 // So far we have added sections from input object files.
1030 // This function adds linker-created Out<ELFT>::* sections.
1031 addPredefinedSections();
1032 removeUnusedSyntheticSections<ELFT>(OutputSections);
1037 for (OutputSectionBase *Sec : OutputSections) {
1038 Sec->SectionIndex = I++;
1039 Sec->ShName = In<ELFT>::ShStrTab->addString(Sec->getName());
1042 // Binary and relocatable output does not have PHDRS.
1043 // The headers have to be created before finalize as that can influence the
1044 // image base and the dynamic section on mips includes the image base.
1045 if (!Config->Relocatable && !Config->OFormatBinary) {
1046 Phdrs = Script<ELFT>::X->hasPhdrsCommands() ? Script<ELFT>::X->createPhdrs()
1048 addPtArmExid(Phdrs);
1052 // Fill other section headers. The dynamic table is finalized
1053 // at the end because some tags like RELSZ depend on result
1054 // of finalizing other sections.
1055 for (OutputSectionBase *Sec : OutputSections)
1058 // Dynamic section must be the last one in this list and dynamic
1059 // symbol table section (DynSymTab) must be the first one.
1060 finalizeSynthetic<ELFT>(
1061 {In<ELFT>::DynSymTab, In<ELFT>::GnuHashTab, In<ELFT>::HashTab,
1062 In<ELFT>::SymTab, In<ELFT>::ShStrTab, In<ELFT>::StrTab,
1063 In<ELFT>::VerDef, In<ELFT>::DynStrTab, In<ELFT>::GdbIndex,
1064 In<ELFT>::Got, In<ELFT>::MipsGot, In<ELFT>::IgotPlt,
1065 In<ELFT>::GotPlt, In<ELFT>::RelaDyn, In<ELFT>::RelaIplt,
1066 In<ELFT>::RelaPlt, In<ELFT>::Plt, In<ELFT>::Iplt,
1067 In<ELFT>::Plt, In<ELFT>::EhFrameHdr, In<ELFT>::VerSym,
1068 In<ELFT>::VerNeed, In<ELFT>::Dynamic});
1071 template <class ELFT> void Writer<ELFT>::addPredefinedSections() {
1072 if (Out<ELFT>::Bss->Size > 0)
1073 OutputSections.push_back(Out<ELFT>::Bss);
1075 auto OS = dyn_cast_or_null<OutputSection<ELFT>>(findSection(".ARM.exidx"));
1076 if (OS && !OS->Sections.empty() && !Config->Relocatable)
1077 OS->addSection(make<ARMExidxSentinelSection<ELFT>>());
1079 addInputSec(In<ELFT>::SymTab);
1080 addInputSec(In<ELFT>::ShStrTab);
1081 addInputSec(In<ELFT>::StrTab);
1084 // The linker is expected to define SECNAME_start and SECNAME_end
1085 // symbols for a few sections. This function defines them.
1086 template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
1087 auto Define = [&](StringRef Start, StringRef End, OutputSectionBase *OS) {
1088 // These symbols resolve to the image base if the section does not exist.
1089 // A special value -1 indicates end of the section.
1090 addOptionalSynthetic<ELFT>(Start, OS, 0);
1091 addOptionalSynthetic<ELFT>(End, OS, OS ? -1 : 0);
1094 Define("__preinit_array_start", "__preinit_array_end",
1095 Out<ELFT>::PreinitArray);
1096 Define("__init_array_start", "__init_array_end", Out<ELFT>::InitArray);
1097 Define("__fini_array_start", "__fini_array_end", Out<ELFT>::FiniArray);
1099 if (OutputSectionBase *Sec = findSection(".ARM.exidx"))
1100 Define("__exidx_start", "__exidx_end", Sec);
1103 // If a section name is valid as a C identifier (which is rare because of
1104 // the leading '.'), linkers are expected to define __start_<secname> and
1105 // __stop_<secname> symbols. They are at beginning and end of the section,
1106 // respectively. This is not requested by the ELF standard, but GNU ld and
1107 // gold provide the feature, and used by many programs.
1108 template <class ELFT>
1109 void Writer<ELFT>::addStartStopSymbols(OutputSectionBase *Sec) {
1110 StringRef S = Sec->getName();
1111 if (!isValidCIdentifier(S))
1113 addOptionalSynthetic<ELFT>(Saver.save("__start_" + S), Sec, 0, STV_DEFAULT);
1114 addOptionalSynthetic<ELFT>(Saver.save("__stop_" + S), Sec, -1, STV_DEFAULT);
1117 template <class ELFT>
1118 OutputSectionBase *Writer<ELFT>::findSection(StringRef Name) {
1119 for (OutputSectionBase *Sec : OutputSections)
1120 if (Sec->getName() == Name)
1125 template <class ELFT> static bool needsPtLoad(OutputSectionBase *Sec) {
1126 if (!(Sec->Flags & SHF_ALLOC))
1129 // Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
1130 // responsible for allocating space for them, not the PT_LOAD that
1131 // contains the TLS initialization image.
1132 if (Sec->Flags & SHF_TLS && Sec->Type == SHT_NOBITS)
1137 // Linker scripts are responsible for aligning addresses. Unfortunately, most
1138 // linker scripts are designed for creating two PT_LOADs only, one RX and one
1139 // RW. This means that there is no alignment in the RO to RX transition and we
1140 // cannot create a PT_LOAD there.
1141 template <class ELFT>
1142 static typename ELFT::uint computeFlags(typename ELFT::uint F) {
1144 return PF_R | PF_W | PF_X;
1145 if (Config->SingleRoRx && !(F & PF_W))
1150 // Decide which program headers to create and which sections to include in each
1152 template <class ELFT> std::vector<PhdrEntry> Writer<ELFT>::createPhdrs() {
1153 std::vector<PhdrEntry> Ret;
1154 auto AddHdr = [&](unsigned Type, unsigned Flags) -> PhdrEntry * {
1155 Ret.emplace_back(Type, Flags);
1159 // The first phdr entry is PT_PHDR which describes the program header itself.
1160 PhdrEntry &Hdr = *AddHdr(PT_PHDR, PF_R);
1161 Hdr.add(Out<ELFT>::ProgramHeaders);
1163 // PT_INTERP must be the second entry if exists.
1164 if (OutputSectionBase *Sec = findSection(".interp")) {
1165 PhdrEntry &Hdr = *AddHdr(PT_INTERP, Sec->getPhdrFlags());
1169 // Add the first PT_LOAD segment for regular output sections.
1170 uintX_t Flags = computeFlags<ELFT>(PF_R);
1171 PhdrEntry *Load = AddHdr(PT_LOAD, Flags);
1173 PhdrEntry TlsHdr(PT_TLS, PF_R);
1174 PhdrEntry RelRo(PT_GNU_RELRO, PF_R);
1175 PhdrEntry Note(PT_NOTE, PF_R);
1176 for (OutputSectionBase *Sec : OutputSections) {
1177 if (!(Sec->Flags & SHF_ALLOC))
1180 // If we meet TLS section then we create TLS header
1181 // and put all TLS sections inside for further use when
1182 // assign addresses.
1183 if (Sec->Flags & SHF_TLS)
1186 if (!needsPtLoad<ELFT>(Sec))
1189 // Segments are contiguous memory regions that has the same attributes
1190 // (e.g. executable or writable). There is one phdr for each segment.
1191 // Therefore, we need to create a new phdr when the next section has
1192 // different flags or is loaded at a discontiguous address using AT linker
1194 uintX_t NewFlags = computeFlags<ELFT>(Sec->getPhdrFlags());
1195 if (Script<ELFT>::X->hasLMA(Sec->getName()) || Flags != NewFlags) {
1196 Load = AddHdr(PT_LOAD, NewFlags);
1202 if (isRelroSection<ELFT>(Sec))
1204 if (Sec->Type == SHT_NOTE)
1208 // Add the TLS segment unless it's empty.
1210 Ret.push_back(std::move(TlsHdr));
1212 // Add an entry for .dynamic.
1213 if (In<ELFT>::DynSymTab) {
1215 *AddHdr(PT_DYNAMIC, In<ELFT>::Dynamic->OutSec->getPhdrFlags());
1216 H.add(In<ELFT>::Dynamic->OutSec);
1219 // PT_GNU_RELRO includes all sections that should be marked as
1220 // read-only by dynamic linker after proccessing relocations.
1222 Ret.push_back(std::move(RelRo));
1224 // PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
1225 if (!Out<ELFT>::EhFrame->empty() && In<ELFT>::EhFrameHdr) {
1227 *AddHdr(PT_GNU_EH_FRAME, In<ELFT>::EhFrameHdr->OutSec->getPhdrFlags());
1228 Hdr.add(In<ELFT>::EhFrameHdr->OutSec);
1231 // PT_OPENBSD_RANDOMIZE specifies the location and size of a part of the
1232 // memory image of the program that must be filled with random data before any
1233 // code in the object is executed.
1234 if (OutputSectionBase *Sec = findSection(".openbsd.randomdata")) {
1235 PhdrEntry &Hdr = *AddHdr(PT_OPENBSD_RANDOMIZE, Sec->getPhdrFlags());
1239 // PT_GNU_STACK is a special section to tell the loader to make the
1240 // pages for the stack non-executable.
1241 if (!Config->ZExecstack) {
1242 PhdrEntry &Hdr = *AddHdr(PT_GNU_STACK, PF_R | PF_W);
1243 if (Config->ZStackSize != uint64_t(-1))
1244 Hdr.p_memsz = Config->ZStackSize;
1247 // PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
1248 // is expected to perform W^X violations, such as calling mprotect(2) or
1249 // mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
1251 if (Config->ZWxneeded)
1252 AddHdr(PT_OPENBSD_WXNEEDED, PF_X);
1255 Ret.push_back(std::move(Note));
1259 template <class ELFT>
1260 void Writer<ELFT>::addPtArmExid(std::vector<PhdrEntry> &Phdrs) {
1261 if (Config->EMachine != EM_ARM)
1263 auto I = std::find_if(
1264 OutputSections.begin(), OutputSections.end(),
1265 [](OutputSectionBase *Sec) { return Sec->Type == SHT_ARM_EXIDX; });
1266 if (I == OutputSections.end())
1269 // PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
1270 PhdrEntry ARMExidx(PT_ARM_EXIDX, PF_R);
1272 Phdrs.push_back(ARMExidx);
1275 // The first section of each PT_LOAD and the first section after PT_GNU_RELRO
1276 // have to be page aligned so that the dynamic linker can set the permissions.
1277 template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
1278 for (const PhdrEntry &P : Phdrs)
1279 if (P.p_type == PT_LOAD && P.First)
1280 P.First->PageAlign = true;
1282 for (const PhdrEntry &P : Phdrs) {
1283 if (P.p_type != PT_GNU_RELRO)
1285 // Find the first section after PT_GNU_RELRO. If it is in a PT_LOAD we
1286 // have to align it to a page.
1287 auto End = OutputSections.end();
1288 auto I = std::find(OutputSections.begin(), End, P.Last);
1289 if (I == End || (I + 1) == End)
1291 OutputSectionBase *Sec = *(I + 1);
1292 if (needsPtLoad<ELFT>(Sec))
1293 Sec->PageAlign = true;
1297 template <class ELFT>
1298 void elf::allocateHeaders(MutableArrayRef<PhdrEntry> Phdrs,
1299 ArrayRef<OutputSectionBase *> OutputSections) {
1301 std::find_if(Phdrs.begin(), Phdrs.end(),
1302 [](const PhdrEntry &E) { return E.p_type == PT_LOAD; });
1303 if (FirstPTLoad == Phdrs.end())
1305 if (FirstPTLoad->First)
1306 for (OutputSectionBase *Sec : OutputSections)
1307 if (Sec->FirstInPtLoad == FirstPTLoad->First)
1308 Sec->FirstInPtLoad = Out<ELFT>::ElfHeader;
1309 FirstPTLoad->First = Out<ELFT>::ElfHeader;
1310 if (!FirstPTLoad->Last)
1311 FirstPTLoad->Last = Out<ELFT>::ProgramHeaders;
1314 // We should set file offsets and VAs for elf header and program headers
1315 // sections. These are special, we do not include them into output sections
1316 // list, but have them to simplify the code.
1317 template <class ELFT> void Writer<ELFT>::fixHeaders() {
1318 Out<ELFT>::ProgramHeaders->Size = sizeof(Elf_Phdr) * Phdrs.size();
1319 // If the script has SECTIONS, assignAddresses will compute the values.
1320 if (ScriptConfig->HasSections)
1323 uintX_t HeaderSize = getHeaderSize<ELFT>();
1324 // When -T<section> option is specified, lower the base to make room for those
1326 if (!Config->SectionStartMap.empty()) {
1328 for (const auto &P : Config->SectionStartMap)
1329 Min = std::min(Min, P.second);
1330 if (HeaderSize < Min)
1333 AllocateHeader = false;
1334 if (Min < Config->ImageBase)
1335 Config->ImageBase = alignDown(Min, Config->MaxPageSize);
1339 allocateHeaders<ELFT>(Phdrs, OutputSections);
1341 uintX_t BaseVA = Config->ImageBase;
1342 Out<ELFT>::ElfHeader->Addr = BaseVA;
1343 Out<ELFT>::ProgramHeaders->Addr = BaseVA + Out<ELFT>::ElfHeader->Size;
1346 // Assign VAs (addresses at run-time) to output sections.
1347 template <class ELFT> void Writer<ELFT>::assignAddresses() {
1348 uintX_t VA = Config->ImageBase;
1350 VA += getHeaderSize<ELFT>();
1351 uintX_t ThreadBssOffset = 0;
1352 for (OutputSectionBase *Sec : OutputSections) {
1353 uintX_t Alignment = Sec->Addralign;
1355 Alignment = std::max<uintX_t>(Alignment, Config->MaxPageSize);
1357 auto I = Config->SectionStartMap.find(Sec->getName());
1358 if (I != Config->SectionStartMap.end())
1361 // We only assign VAs to allocated sections.
1362 if (needsPtLoad<ELFT>(Sec)) {
1363 VA = alignTo(VA, Alignment);
1366 } else if (Sec->Flags & SHF_TLS && Sec->Type == SHT_NOBITS) {
1367 uintX_t TVA = VA + ThreadBssOffset;
1368 TVA = alignTo(TVA, Alignment);
1370 ThreadBssOffset = TVA - VA + Sec->Size;
1375 // Adjusts the file alignment for a given output section and returns
1376 // its new file offset. The file offset must be the same with its
1377 // virtual address (modulo the page size) so that the loader can load
1378 // executables without any address adjustment.
1379 template <class ELFT, class uintX_t>
1380 static uintX_t getFileAlignment(uintX_t Off, OutputSectionBase *Sec) {
1381 OutputSectionBase *First = Sec->FirstInPtLoad;
1382 // If the section is not in a PT_LOAD, we just have to align it.
1384 return alignTo(Off, Sec->Addralign);
1386 // The first section in a PT_LOAD has to have congruent offset and address
1387 // module the page size.
1389 return alignTo(Off, Config->MaxPageSize, Sec->Addr);
1391 // If two sections share the same PT_LOAD the file offset is calculated
1392 // using this formula: Off2 = Off1 + (VA2 - VA1).
1393 return First->Offset + Sec->Addr - First->Addr;
1396 template <class ELFT, class uintX_t>
1397 void setOffset(OutputSectionBase *Sec, uintX_t &Off) {
1398 if (Sec->Type == SHT_NOBITS) {
1403 Off = getFileAlignment<ELFT>(Off, Sec);
1408 template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
1410 for (OutputSectionBase *Sec : OutputSections)
1411 if (Sec->Flags & SHF_ALLOC)
1412 setOffset<ELFT>(Sec, Off);
1413 FileSize = alignTo(Off, sizeof(uintX_t));
1416 // Assign file offsets to output sections.
1417 template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
1419 setOffset<ELFT>(Out<ELFT>::ElfHeader, Off);
1420 setOffset<ELFT>(Out<ELFT>::ProgramHeaders, Off);
1422 for (OutputSectionBase *Sec : OutputSections)
1423 setOffset<ELFT>(Sec, Off);
1425 SectionHeaderOff = alignTo(Off, sizeof(uintX_t));
1426 FileSize = SectionHeaderOff + (OutputSections.size() + 1) * sizeof(Elf_Shdr);
1429 // Finalize the program headers. We call this function after we assign
1430 // file offsets and VAs to all sections.
1431 template <class ELFT> void Writer<ELFT>::setPhdrs() {
1432 for (PhdrEntry &P : Phdrs) {
1433 OutputSectionBase *First = P.First;
1434 OutputSectionBase *Last = P.Last;
1436 P.p_filesz = Last->Offset - First->Offset;
1437 if (Last->Type != SHT_NOBITS)
1438 P.p_filesz += Last->Size;
1439 P.p_memsz = Last->Addr + Last->Size - First->Addr;
1440 P.p_offset = First->Offset;
1441 P.p_vaddr = First->Addr;
1443 P.p_paddr = First->getLMA();
1445 if (P.p_type == PT_LOAD)
1446 P.p_align = Config->MaxPageSize;
1447 else if (P.p_type == PT_GNU_RELRO) {
1449 // The glibc dynamic loader rounds the size down, so we need to round up
1450 // to protect the last page. This is a no-op on FreeBSD which always
1452 P.p_memsz = alignTo(P.p_memsz, Config->MaxPageSize);
1455 // The TLS pointer goes after PT_TLS. At least glibc will align it,
1456 // so round up the size to make sure the offsets are correct.
1457 if (P.p_type == PT_TLS) {
1458 Out<ELFT>::TlsPhdr = &P;
1460 P.p_memsz = alignTo(P.p_memsz, P.p_align);
1465 // The entry point address is chosen in the following ways.
1467 // 1. the '-e' entry command-line option;
1468 // 2. the ENTRY(symbol) command in a linker control script;
1469 // 3. the value of the symbol start, if present;
1470 // 4. the address of the first byte of the .text section, if present;
1471 // 5. the address 0.
1472 template <class ELFT> typename ELFT::uint Writer<ELFT>::getEntryAddr() {
1473 // Case 1, 2 or 3. As a special case, if the symbol is actually
1474 // a number, we'll use that number as an address.
1475 if (SymbolBody *B = Symtab<ELFT>::X->find(Config->Entry))
1476 return B->getVA<ELFT>();
1478 if (!Config->Entry.getAsInteger(0, Addr))
1482 if (OutputSectionBase *Sec = findSection(".text")) {
1483 if (Config->WarnMissingEntry)
1484 warn("cannot find entry symbol " + Config->Entry + "; defaulting to 0x" +
1485 utohexstr(Sec->Addr));
1490 if (Config->WarnMissingEntry)
1491 warn("cannot find entry symbol " + Config->Entry +
1492 "; not setting start address");
1496 template <class ELFT> static uint8_t getELFEncoding() {
1497 if (ELFT::TargetEndianness == llvm::support::little)
1502 static uint16_t getELFType() {
1505 if (Config->Relocatable)
1510 // This function is called after we have assigned address and size
1511 // to each section. This function fixes some predefined absolute
1512 // symbol values that depend on section address and size.
1513 template <class ELFT> void Writer<ELFT>::fixAbsoluteSymbols() {
1514 // __ehdr_start is the location of program headers.
1515 if (ElfSym<ELFT>::EhdrStart)
1516 ElfSym<ELFT>::EhdrStart->Value = Out<ELFT>::ProgramHeaders->Addr;
1518 auto Set = [](DefinedRegular<ELFT> *S1, DefinedRegular<ELFT> *S2, uintX_t V) {
1525 // _etext is the first location after the last read-only loadable segment.
1526 // _edata is the first location after the last read-write loadable segment.
1527 // _end is the first location after the uninitialized data region.
1528 for (PhdrEntry &P : Phdrs) {
1529 if (P.p_type != PT_LOAD)
1531 Set(ElfSym<ELFT>::End, ElfSym<ELFT>::End2, P.p_vaddr + P.p_memsz);
1533 uintX_t Val = P.p_vaddr + P.p_filesz;
1534 if (P.p_flags & PF_W)
1535 Set(ElfSym<ELFT>::Edata, ElfSym<ELFT>::Edata2, Val);
1537 Set(ElfSym<ELFT>::Etext, ElfSym<ELFT>::Etext2, Val);
1540 // Setup MIPS _gp_disp/__gnu_local_gp symbols which should
1541 // be equal to the _gp symbol's value.
1542 if (Config->EMachine == EM_MIPS) {
1543 if (!ElfSym<ELFT>::MipsGp->Value) {
1544 // Find GP-relative section with the lowest address
1545 // and use this address to calculate default _gp value.
1547 for (const OutputSectionBase * OS : OutputSections)
1548 if ((OS->Flags & SHF_MIPS_GPREL) && OS->Addr < Gp)
1550 if (Gp != (uintX_t)-1)
1551 ElfSym<ELFT>::MipsGp->Value = Gp + 0x7ff0;
1553 if (ElfSym<ELFT>::MipsGpDisp)
1554 ElfSym<ELFT>::MipsGpDisp->Value = ElfSym<ELFT>::MipsGp->Value;
1555 if (ElfSym<ELFT>::MipsLocalGp)
1556 ElfSym<ELFT>::MipsLocalGp->Value = ElfSym<ELFT>::MipsGp->Value;
1560 template <class ELFT> void Writer<ELFT>::writeHeader() {
1561 uint8_t *Buf = Buffer->getBufferStart();
1562 memcpy(Buf, "\177ELF", 4);
1564 // Write the ELF header.
1565 auto *EHdr = reinterpret_cast<Elf_Ehdr *>(Buf);
1566 EHdr->e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
1567 EHdr->e_ident[EI_DATA] = getELFEncoding<ELFT>();
1568 EHdr->e_ident[EI_VERSION] = EV_CURRENT;
1569 EHdr->e_ident[EI_OSABI] = Config->OSABI;
1570 EHdr->e_type = getELFType();
1571 EHdr->e_machine = Config->EMachine;
1572 EHdr->e_version = EV_CURRENT;
1573 EHdr->e_entry = getEntryAddr();
1574 EHdr->e_shoff = SectionHeaderOff;
1575 EHdr->e_ehsize = sizeof(Elf_Ehdr);
1576 EHdr->e_phnum = Phdrs.size();
1577 EHdr->e_shentsize = sizeof(Elf_Shdr);
1578 EHdr->e_shnum = OutputSections.size() + 1;
1579 EHdr->e_shstrndx = In<ELFT>::ShStrTab->OutSec->SectionIndex;
1581 if (Config->EMachine == EM_ARM)
1582 // We don't currently use any features incompatible with EF_ARM_EABI_VER5,
1583 // but we don't have any firm guarantees of conformance. Linux AArch64
1584 // kernels (as of 2016) require an EABI version to be set.
1585 EHdr->e_flags = EF_ARM_EABI_VER5;
1586 else if (Config->EMachine == EM_MIPS)
1587 EHdr->e_flags = getMipsEFlags<ELFT>();
1589 if (!Config->Relocatable) {
1590 EHdr->e_phoff = sizeof(Elf_Ehdr);
1591 EHdr->e_phentsize = sizeof(Elf_Phdr);
1594 // Write the program header table.
1595 auto *HBuf = reinterpret_cast<Elf_Phdr *>(Buf + EHdr->e_phoff);
1596 for (PhdrEntry &P : Phdrs) {
1597 HBuf->p_type = P.p_type;
1598 HBuf->p_flags = P.p_flags;
1599 HBuf->p_offset = P.p_offset;
1600 HBuf->p_vaddr = P.p_vaddr;
1601 HBuf->p_paddr = P.p_paddr;
1602 HBuf->p_filesz = P.p_filesz;
1603 HBuf->p_memsz = P.p_memsz;
1604 HBuf->p_align = P.p_align;
1608 // Write the section header table. Note that the first table entry is null.
1609 auto *SHdrs = reinterpret_cast<Elf_Shdr *>(Buf + EHdr->e_shoff);
1610 for (OutputSectionBase *Sec : OutputSections)
1611 Sec->writeHeaderTo<ELFT>(++SHdrs);
1614 // Removes a given file asynchronously. This is a performance hack,
1615 // so remove this when operating systems are improved.
1617 // On Linux (and probably on other Unix-like systems), unlink(2) is a
1618 // noticeably slow system call. As of 2016, unlink takes 250
1619 // milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
1621 // To create a new result file, we first remove existing file. So, if
1622 // you repeatedly link a 1 GB program in a regular compile-link-debug
1623 // cycle, every cycle wastes 250 milliseconds only to remove a file.
1624 // Since LLD can link a 1 GB binary in about 5 seconds, that waste
1627 // This function spawns a background thread to call unlink.
1628 // The calling thread returns almost immediately.
1629 static void unlinkAsync(StringRef Path) {
1630 if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
1633 // First, rename Path to avoid race condition. We cannot remove
1634 // Path from a different thread because we are now going to create
1635 // Path as a new file. If we do that in a different thread, the new
1636 // thread can remove the new file.
1637 SmallString<128> TempPath;
1638 if (auto EC = sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
1639 fatal(EC, "createUniqueFile failed");
1640 if (auto EC = sys::fs::rename(Path, TempPath))
1641 fatal(EC, "rename failed");
1643 // Remove TempPath in background.
1644 std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
1647 // Open a result file.
1648 template <class ELFT> void Writer<ELFT>::openFile() {
1649 unlinkAsync(Config->OutputFile);
1650 ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
1651 FileOutputBuffer::create(Config->OutputFile, FileSize,
1652 FileOutputBuffer::F_executable);
1654 if (auto EC = BufferOrErr.getError())
1655 error(EC, "failed to open " + Config->OutputFile);
1657 Buffer = std::move(*BufferOrErr);
1660 template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
1661 uint8_t *Buf = Buffer->getBufferStart();
1662 for (OutputSectionBase *Sec : OutputSections)
1663 if (Sec->Flags & SHF_ALLOC)
1664 Sec->writeTo(Buf + Sec->Offset);
1667 // Write section contents to a mmap'ed file.
1668 template <class ELFT> void Writer<ELFT>::writeSections() {
1669 uint8_t *Buf = Buffer->getBufferStart();
1671 // PPC64 needs to process relocations in the .opd section
1672 // before processing relocations in code-containing sections.
1673 Out<ELFT>::Opd = findSection(".opd");
1674 if (Out<ELFT>::Opd) {
1675 Out<ELFT>::OpdBuf = Buf + Out<ELFT>::Opd->Offset;
1676 Out<ELFT>::Opd->writeTo(Buf + Out<ELFT>::Opd->Offset);
1679 OutputSectionBase *EhFrameHdr =
1680 In<ELFT>::EhFrameHdr ? In<ELFT>::EhFrameHdr->OutSec : nullptr;
1681 for (OutputSectionBase *Sec : OutputSections)
1682 if (Sec != Out<ELFT>::Opd && Sec != EhFrameHdr)
1683 Sec->writeTo(Buf + Sec->Offset);
1685 // The .eh_frame_hdr depends on .eh_frame section contents, therefore
1686 // it should be written after .eh_frame is written.
1687 if (!Out<ELFT>::EhFrame->empty() && EhFrameHdr)
1688 EhFrameHdr->writeTo(Buf + EhFrameHdr->Offset);
1691 template <class ELFT> void Writer<ELFT>::writeBuildId() {
1692 if (!In<ELFT>::BuildId || !In<ELFT>::BuildId->OutSec)
1695 // Compute a hash of all sections of the output file.
1696 uint8_t *Start = Buffer->getBufferStart();
1697 uint8_t *End = Start + FileSize;
1698 In<ELFT>::BuildId->writeBuildId({Start, End});
1701 template void elf::writeResult<ELF32LE>();
1702 template void elf::writeResult<ELF32BE>();
1703 template void elf::writeResult<ELF64LE>();
1704 template void elf::writeResult<ELF64BE>();
1706 template void elf::allocateHeaders<ELF32LE>(MutableArrayRef<PhdrEntry>,
1707 ArrayRef<OutputSectionBase *>);
1708 template void elf::allocateHeaders<ELF32BE>(MutableArrayRef<PhdrEntry>,
1709 ArrayRef<OutputSectionBase *>);
1710 template void elf::allocateHeaders<ELF64LE>(MutableArrayRef<PhdrEntry>,
1711 ArrayRef<OutputSectionBase *>);
1712 template void elf::allocateHeaders<ELF64BE>(MutableArrayRef<PhdrEntry>,
1713 ArrayRef<OutputSectionBase *>);
1715 template bool elf::isRelroSection<ELF32LE>(const OutputSectionBase *);
1716 template bool elf::isRelroSection<ELF32BE>(const OutputSectionBase *);
1717 template bool elf::isRelroSection<ELF64LE>(const OutputSectionBase *);
1718 template bool elf::isRelroSection<ELF64BE>(const OutputSectionBase *);
1720 template void elf::reportDiscarded<ELF32LE>(InputSectionBase<ELF32LE> *);
1721 template void elf::reportDiscarded<ELF32BE>(InputSectionBase<ELF32BE> *);
1722 template void elf::reportDiscarded<ELF64LE>(InputSectionBase<ELF64LE> *);
1723 template void elf::reportDiscarded<ELF64BE>(InputSectionBase<ELF64BE> *);