//===- Symbols.cpp --------------------------------------------------------===// // // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. // See https://llvm.org/LICENSE.txt for license information. // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception // //===----------------------------------------------------------------------===// #include "Symbols.h" #include "InputFiles.h" #include "InputSection.h" #include "OutputSections.h" #include "SyntheticSections.h" #include "Target.h" #include "Writer.h" #include "lld/Common/ErrorHandler.h" #include "lld/Common/Strings.h" #include "llvm/ADT/STLExtras.h" #include "llvm/Support/FileSystem.h" #include "llvm/Support/Path.h" #include using namespace llvm; using namespace llvm::object; using namespace llvm::ELF; using namespace lld; using namespace lld::elf; // Returns a symbol for an error message. static std::string demangle(StringRef symName) { if (elf::config->demangle) return demangleItanium(symName); return std::string(symName); } std::string lld::toString(const elf::Symbol &sym) { StringRef name = sym.getName(); std::string ret = demangle(name); // If sym has a non-default version, its name may have been truncated at '@' // by Symbol::parseSymbolVersion(). Add the trailing part. This check is safe // because every symbol name ends with '\0'. if (name.data()[name.size()] == '@') ret += name.data() + name.size(); return ret; } std::string lld::toELFString(const Archive::Symbol &b) { return demangle(b.getName()); } Defined *ElfSym::bss; Defined *ElfSym::data; Defined *ElfSym::etext1; Defined *ElfSym::etext2; Defined *ElfSym::edata1; Defined *ElfSym::edata2; Defined *ElfSym::end1; Defined *ElfSym::end2; Defined *ElfSym::globalOffsetTable; Defined *ElfSym::mipsGp; Defined *ElfSym::mipsGpDisp; Defined *ElfSym::mipsLocalGp; Defined *ElfSym::relaIpltStart; Defined *ElfSym::relaIpltEnd; Defined *ElfSym::riscvGlobalPointer; Defined *ElfSym::tlsModuleBase; DenseMap elf::backwardReferences; static uint64_t getSymVA(const Symbol &sym, int64_t &addend) { switch (sym.kind()) { case Symbol::DefinedKind: { auto &d = cast(sym); SectionBase *isec = d.section; // This is an absolute symbol. if (!isec) return d.value; assert(isec != &InputSection::discarded); isec = isec->repl; uint64_t offset = d.value; // An object in an SHF_MERGE section might be referenced via a // section symbol (as a hack for reducing the number of local // symbols). // Depending on the addend, the reference via a section symbol // refers to a different object in the merge section. // Since the objects in the merge section are not necessarily // contiguous in the output, the addend can thus affect the final // VA in a non-linear way. // To make this work, we incorporate the addend into the section // offset (and zero out the addend for later processing) so that // we find the right object in the section. if (d.isSection()) { offset += addend; addend = 0; } // In the typical case, this is actually very simple and boils // down to adding together 3 numbers: // 1. The address of the output section. // 2. The offset of the input section within the output section. // 3. The offset within the input section (this addition happens // inside InputSection::getOffset). // // If you understand the data structures involved with this next // line (and how they get built), then you have a pretty good // understanding of the linker. uint64_t va = isec->getVA(offset); // MIPS relocatable files can mix regular and microMIPS code. // Linker needs to distinguish such code. To do so microMIPS // symbols has the `STO_MIPS_MICROMIPS` flag in the `st_other` // field. Unfortunately, the `MIPS::relocate()` method has // a symbol value only. To pass type of the symbol (regular/microMIPS) // to that routine as well as other places where we write // a symbol value as-is (.dynamic section, `Elf_Ehdr::e_entry` // field etc) do the same trick as compiler uses to mark microMIPS // for CPU - set the less-significant bit. if (config->emachine == EM_MIPS && isMicroMips() && ((sym.stOther & STO_MIPS_MICROMIPS) || sym.needsPltAddr)) va |= 1; if (d.isTls() && !config->relocatable) { // Use the address of the TLS segment's first section rather than the // segment's address, because segment addresses aren't initialized until // after sections are finalized. (e.g. Measuring the size of .rela.dyn // for Android relocation packing requires knowing TLS symbol addresses // during section finalization.) if (!Out::tlsPhdr || !Out::tlsPhdr->firstSec) fatal(toString(d.file) + " has an STT_TLS symbol but doesn't have an SHF_TLS section"); return va - Out::tlsPhdr->firstSec->addr; } return va; } case Symbol::SharedKind: case Symbol::UndefinedKind: return 0; case Symbol::LazyArchiveKind: case Symbol::LazyObjectKind: assert(sym.isUsedInRegularObj && "lazy symbol reached writer"); return 0; case Symbol::CommonKind: llvm_unreachable("common symbol reached writer"); case Symbol::PlaceholderKind: llvm_unreachable("placeholder symbol reached writer"); } llvm_unreachable("invalid symbol kind"); } uint64_t Symbol::getVA(int64_t addend) const { uint64_t outVA = getSymVA(*this, addend); return outVA + addend; } uint64_t Symbol::getGotVA() const { if (gotInIgot) return in.igotPlt->getVA() + getGotPltOffset(); return in.got->getVA() + getGotOffset(); } uint64_t Symbol::getGotOffset() const { return gotIndex * config->wordsize; } uint64_t Symbol::getGotPltVA() const { if (isInIplt) return in.igotPlt->getVA() + getGotPltOffset(); return in.gotPlt->getVA() + getGotPltOffset(); } uint64_t Symbol::getGotPltOffset() const { if (isInIplt) return pltIndex * config->wordsize; return (pltIndex + target->gotPltHeaderEntriesNum) * config->wordsize; } uint64_t Symbol::getPltVA() const { uint64_t outVA = isInIplt ? in.iplt->getVA() + pltIndex * target->ipltEntrySize : in.plt->getVA() + in.plt->headerSize + pltIndex * target->pltEntrySize; // While linking microMIPS code PLT code are always microMIPS // code. Set the less-significant bit to track that fact. // See detailed comment in the `getSymVA` function. if (config->emachine == EM_MIPS && isMicroMips()) outVA |= 1; return outVA; } uint64_t Symbol::getSize() const { if (const auto *dr = dyn_cast(this)) return dr->size; return cast(this)->size; } OutputSection *Symbol::getOutputSection() const { if (auto *s = dyn_cast(this)) { if (auto *sec = s->section) return sec->repl->getOutputSection(); return nullptr; } return nullptr; } // If a symbol name contains '@', the characters after that is // a symbol version name. This function parses that. void Symbol::parseSymbolVersion() { StringRef s = getName(); size_t pos = s.find('@'); if (pos == 0 || pos == StringRef::npos) return; StringRef verstr = s.substr(pos + 1); if (verstr.empty()) return; // Truncate the symbol name so that it doesn't include the version string. nameSize = pos; // If this is not in this DSO, it is not a definition. if (!isDefined()) return; // '@@' in a symbol name means the default version. // It is usually the most recent one. bool isDefault = (verstr[0] == '@'); if (isDefault) verstr = verstr.substr(1); for (const VersionDefinition &ver : namedVersionDefs()) { if (ver.name != verstr) continue; if (isDefault) versionId = ver.id; else versionId = ver.id | VERSYM_HIDDEN; return; } // It is an error if the specified version is not defined. // Usually version script is not provided when linking executable, // but we may still want to override a versioned symbol from DSO, // so we do not report error in this case. We also do not error // if the symbol has a local version as it won't be in the dynamic // symbol table. if (config->shared && versionId != VER_NDX_LOCAL) error(toString(file) + ": symbol " + s + " has undefined version " + verstr); } void Symbol::fetch() const { if (auto *sym = dyn_cast(this)) { cast(sym->file)->fetch(sym->sym); return; } if (auto *sym = dyn_cast(this)) { dyn_cast(sym->file)->fetch(); return; } llvm_unreachable("Symbol::fetch() is called on a non-lazy symbol"); } MemoryBufferRef LazyArchive::getMemberBuffer() { Archive::Child c = CHECK(sym.getMember(), "could not get the member for symbol " + toELFString(sym)); return CHECK(c.getMemoryBufferRef(), "could not get the buffer for the member defining symbol " + toELFString(sym)); } uint8_t Symbol::computeBinding() const { if (config->relocatable) return binding; if ((visibility != STV_DEFAULT && visibility != STV_PROTECTED) || (versionId == VER_NDX_LOCAL && isDefined())) return STB_LOCAL; if (!config->gnuUnique && binding == STB_GNU_UNIQUE) return STB_GLOBAL; return binding; } bool Symbol::includeInDynsym() const { if (!config->hasDynSymTab) return false; if (computeBinding() == STB_LOCAL) return false; if (!isDefined() && !isCommon()) // This should unconditionally return true, unfortunately glibc -static-pie // expects undefined weak symbols not to exist in .dynsym, e.g. // __pthread_mutex_lock reference in _dl_add_to_namespace_list, // __pthread_initialize_minimal reference in csu/libc-start.c. return !(config->noDynamicLinker && isUndefWeak()); return exportDynamic || inDynamicList; } // Print out a log message for --trace-symbol. void elf::printTraceSymbol(const Symbol *sym) { std::string s; if (sym->isUndefined()) s = ": reference to "; else if (sym->isLazy()) s = ": lazy definition of "; else if (sym->isShared()) s = ": shared definition of "; else if (sym->isCommon()) s = ": common definition of "; else s = ": definition of "; message(toString(sym->file) + s + sym->getName()); } void elf::maybeWarnUnorderableSymbol(const Symbol *sym) { if (!config->warnSymbolOrdering) return; // If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning // is emitted. It makes sense to not warn on undefined symbols. // // Note, ld.bfd --symbol-ordering-file= does not warn on undefined symbols, // but we don't have to be compatible here. if (sym->isUndefined() && config->unresolvedSymbols == UnresolvedPolicy::Ignore) return; const InputFile *file = sym->file; auto *d = dyn_cast(sym); auto report = [&](StringRef s) { warn(toString(file) + s + sym->getName()); }; if (sym->isUndefined()) report(": unable to order undefined symbol: "); else if (sym->isShared()) report(": unable to order shared symbol: "); else if (d && !d->section) report(": unable to order absolute symbol: "); else if (d && isa(d->section)) report(": unable to order synthetic symbol: "); else if (d && !d->section->repl->isLive()) report(": unable to order discarded symbol: "); } // Returns true if a symbol can be replaced at load-time by a symbol // with the same name defined in other ELF executable or DSO. bool elf::computeIsPreemptible(const Symbol &sym) { assert(!sym.isLocal()); // Only symbols with default visibility that appear in dynsym can be // preempted. Symbols with protected visibility cannot be preempted. if (!sym.includeInDynsym() || sym.visibility != STV_DEFAULT) return false; // At this point copy relocations have not been created yet, so any // symbol that is not defined locally is preemptible. if (!sym.isDefined()) return true; if (!config->shared) return false; // If -Bsymbolic or --dynamic-list is specified, or -Bsymbolic-functions is // specified and the symbol is STT_FUNC, the symbol is preemptible iff it is // in the dynamic list. if (config->symbolic || (config->bsymbolicFunctions && sym.isFunc())) return sym.inDynamicList; return true; } void elf::reportBackrefs() { for (auto &it : backwardReferences) { const Symbol &sym = *it.first; warn("backward reference detected: " + sym.getName() + " in " + toString(it.second) + " refers to " + toString(sym.file)); } } static uint8_t getMinVisibility(uint8_t va, uint8_t vb) { if (va == STV_DEFAULT) return vb; if (vb == STV_DEFAULT) return va; return std::min(va, vb); } // Merge symbol properties. // // When we have many symbols of the same name, we choose one of them, // and that's the result of symbol resolution. However, symbols that // were not chosen still affect some symbol properties. void Symbol::mergeProperties(const Symbol &other) { if (other.exportDynamic) exportDynamic = true; if (other.isUsedInRegularObj) isUsedInRegularObj = true; // DSO symbols do not affect visibility in the output. if (!other.isShared()) visibility = getMinVisibility(visibility, other.visibility); } void Symbol::resolve(const Symbol &other) { mergeProperties(other); if (isPlaceholder()) { replace(other); return; } switch (other.kind()) { case Symbol::UndefinedKind: resolveUndefined(cast(other)); break; case Symbol::CommonKind: resolveCommon(cast(other)); break; case Symbol::DefinedKind: resolveDefined(cast(other)); break; case Symbol::LazyArchiveKind: resolveLazy(cast(other)); break; case Symbol::LazyObjectKind: resolveLazy(cast(other)); break; case Symbol::SharedKind: resolveShared(cast(other)); break; case Symbol::PlaceholderKind: llvm_unreachable("bad symbol kind"); } } void Symbol::resolveUndefined(const Undefined &other) { // An undefined symbol with non default visibility must be satisfied // in the same DSO. // // If this is a non-weak defined symbol in a discarded section, override the // existing undefined symbol for better error message later. if ((isShared() && other.visibility != STV_DEFAULT) || (isUndefined() && other.binding != STB_WEAK && other.discardedSecIdx)) { replace(other); return; } if (traced) printTraceSymbol(&other); if (isLazy()) { // An undefined weak will not fetch archive members. See comment on Lazy in // Symbols.h for the details. if (other.binding == STB_WEAK) { binding = STB_WEAK; type = other.type; return; } // Do extra check for --warn-backrefs. // // --warn-backrefs is an option to prevent an undefined reference from // fetching an archive member written earlier in the command line. It can be // used to keep compatibility with GNU linkers to some degree. // I'll explain the feature and why you may find it useful in this comment. // // lld's symbol resolution semantics is more relaxed than traditional Unix // linkers. For example, // // ld.lld foo.a bar.o // // succeeds even if bar.o contains an undefined symbol that has to be // resolved by some object file in foo.a. Traditional Unix linkers don't // allow this kind of backward reference, as they visit each file only once // from left to right in the command line while resolving all undefined // symbols at the moment of visiting. // // In the above case, since there's no undefined symbol when a linker visits // foo.a, no files are pulled out from foo.a, and because the linker forgets // about foo.a after visiting, it can't resolve undefined symbols in bar.o // that could have been resolved otherwise. // // That lld accepts more relaxed form means that (besides it'd make more // sense) you can accidentally write a command line or a build file that // works only with lld, even if you have a plan to distribute it to wider // users who may be using GNU linkers. With --warn-backrefs, you can detect // a library order that doesn't work with other Unix linkers. // // The option is also useful to detect cyclic dependencies between static // archives. Again, lld accepts // // ld.lld foo.a bar.a // // even if foo.a and bar.a depend on each other. With --warn-backrefs, it is // handled as an error. // // Here is how the option works. We assign a group ID to each file. A file // with a smaller group ID can pull out object files from an archive file // with an equal or greater group ID. Otherwise, it is a reverse dependency // and an error. // // A file outside --{start,end}-group gets a fresh ID when instantiated. All // files within the same --{start,end}-group get the same group ID. E.g. // // ld.lld A B --start-group C D --end-group E // // A forms group 0. B form group 1. C and D (including their member object // files) form group 2. E forms group 3. I think that you can see how this // group assignment rule simulates the traditional linker's semantics. bool backref = config->warnBackrefs && other.file && file->groupId < other.file->groupId; if (backref) { // Some libraries have known problems and can cause noise. Filter them out // with --warn-backrefs-exclude=. StringRef name = !file->archiveName.empty() ? file->archiveName : file->getName(); for (const llvm::GlobPattern &pat : config->warnBackrefsExclude) if (pat.match(name)) { backref = false; break; } } fetch(); // We don't report backward references to weak symbols as they can be // overridden later. // // A traditional linker does not error for -ldef1 -lref -ldef2 (linking // sandwich), where def2 may or may not be the same as def1. We don't want // to warn for this case, so dismiss the warning if we see a subsequent lazy // definition. if (backref && !isWeak()) backwardReferences.try_emplace(this, other.file); return; } // Undefined symbols in a SharedFile do not change the binding. if (dyn_cast_or_null(other.file)) return; if (isUndefined() || isShared()) { // The binding will be weak if there is at least one reference and all are // weak. The binding has one opportunity to change to weak: if the first // reference is weak. if (other.binding != STB_WEAK || !referenced) binding = other.binding; } } // Using .symver foo,foo@@VER unfortunately creates two symbols: foo and // foo@@VER. We want to effectively ignore foo, so give precedence to // foo@@VER. // FIXME: If users can transition to using // .symver foo,foo@@@VER // we can delete this hack. static int compareVersion(StringRef a, StringRef b) { bool x = a.contains("@@"); bool y = b.contains("@@"); if (!x && y) return 1; if (x && !y) return -1; return 0; } // Compare two symbols. Return 1 if the new symbol should win, -1 if // the new symbol should lose, or 0 if there is a conflict. int Symbol::compare(const Symbol *other) const { assert(other->isDefined() || other->isCommon()); if (!isDefined() && !isCommon()) return 1; if (int cmp = compareVersion(getName(), other->getName())) return cmp; if (other->isWeak()) return -1; if (isWeak()) return 1; if (isCommon() && other->isCommon()) { if (config->warnCommon) warn("multiple common of " + getName()); return 0; } if (isCommon()) { if (config->warnCommon) warn("common " + getName() + " is overridden"); return 1; } if (other->isCommon()) { if (config->warnCommon) warn("common " + getName() + " is overridden"); return -1; } auto *oldSym = cast(this); auto *newSym = cast(other); if (dyn_cast_or_null(other->file)) return 0; if (!oldSym->section && !newSym->section && oldSym->value == newSym->value && newSym->binding == STB_GLOBAL) return -1; return 0; } static void reportDuplicate(Symbol *sym, InputFile *newFile, InputSectionBase *errSec, uint64_t errOffset) { if (config->allowMultipleDefinition) return; Defined *d = cast(sym); if (!d->section || !errSec) { error("duplicate symbol: " + toString(*sym) + "\n>>> defined in " + toString(sym->file) + "\n>>> defined in " + toString(newFile)); return; } // Construct and print an error message in the form of: // // ld.lld: error: duplicate symbol: foo // >>> defined at bar.c:30 // >>> bar.o (/home/alice/src/bar.o) // >>> defined at baz.c:563 // >>> baz.o in archive libbaz.a auto *sec1 = cast(d->section); std::string src1 = sec1->getSrcMsg(*sym, d->value); std::string obj1 = sec1->getObjMsg(d->value); std::string src2 = errSec->getSrcMsg(*sym, errOffset); std::string obj2 = errSec->getObjMsg(errOffset); std::string msg = "duplicate symbol: " + toString(*sym) + "\n>>> defined at "; if (!src1.empty()) msg += src1 + "\n>>> "; msg += obj1 + "\n>>> defined at "; if (!src2.empty()) msg += src2 + "\n>>> "; msg += obj2; error(msg); } void Symbol::resolveCommon(const CommonSymbol &other) { int cmp = compare(&other); if (cmp < 0) return; if (cmp > 0) { if (auto *s = dyn_cast(this)) { // Increase st_size if the shared symbol has a larger st_size. The shared // symbol may be created from common symbols. The fact that some object // files were linked into a shared object first should not change the // regular rule that picks the largest st_size. uint64_t size = s->size; replace(other); if (size > cast(this)->size) cast(this)->size = size; } else { replace(other); } return; } CommonSymbol *oldSym = cast(this); oldSym->alignment = std::max(oldSym->alignment, other.alignment); if (oldSym->size < other.size) { oldSym->file = other.file; oldSym->size = other.size; } } void Symbol::resolveDefined(const Defined &other) { int cmp = compare(&other); if (cmp > 0) replace(other); else if (cmp == 0) reportDuplicate(this, other.file, dyn_cast_or_null(other.section), other.value); } template void Symbol::resolveLazy(const LazyT &other) { if (!isUndefined()) { // See the comment in resolveUndefined(). if (isDefined()) backwardReferences.erase(this); return; } // An undefined weak will not fetch archive members. See comment on Lazy in // Symbols.h for the details. if (isWeak()) { uint8_t ty = type; replace(other); type = ty; binding = STB_WEAK; return; } other.fetch(); } void Symbol::resolveShared(const SharedSymbol &other) { if (isCommon()) { // See the comment in resolveCommon() above. if (other.size > cast(this)->size) cast(this)->size = other.size; return; } if (visibility == STV_DEFAULT && (isUndefined() || isLazy())) { // An undefined symbol with non default visibility must be satisfied // in the same DSO. uint8_t bind = binding; replace(other); binding = bind; } else if (traced) printTraceSymbol(&other); }