//===- 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 "Driver.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 "llvm/Demangle/Demangle.h" #include "llvm/Support/Compiler.h" #include using namespace llvm; using namespace llvm::object; using namespace llvm::ELF; using namespace lld; using namespace lld::elf; static_assert(sizeof(SymbolUnion) <= 64, "SymbolUnion too large"); template struct AssertSymbol { static_assert(std::is_trivially_destructible(), "Symbol types must be trivially destructible"); static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small"); static_assert(alignof(T) <= alignof(SymbolUnion), "SymbolUnion not aligned enough"); }; LLVM_ATTRIBUTE_UNUSED static inline void assertSymbols() { AssertSymbol(); AssertSymbol(); AssertSymbol(); AssertSymbol(); AssertSymbol(); } // Returns a symbol for an error message. static std::string maybeDemangleSymbol(StringRef symName) { return elf::config->demangle ? demangle(symName.str()) : symName.str(); } std::string lld::toString(const elf::Symbol &sym) { StringRef name = sym.getName(); std::string ret = maybeDemangleSymbol(name); const char *suffix = sym.getVersionSuffix(); if (*suffix == '@') ret += suffix; return ret; } Defined *ElfSym::bss; 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::riscvGlobalPointer; Defined *ElfSym::relaIpltStart; Defined *ElfSym::relaIpltEnd; Defined *ElfSym::tlsModuleBase; SmallVector elf::symAux; 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); 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; // 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); if (d.isSection()) va -= addend; // 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.hasFlag(NEEDS_COPY))) 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::LazyKind: llvm_unreachable("lazy symbol reached writer"); 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 { return getSymVA(*this, addend) + addend; } uint64_t Symbol::getGotVA() const { if (gotInIgot) return in.igotPlt->getVA() + getGotPltOffset(); return in.got->getVA() + getGotOffset(); } uint64_t Symbol::getGotOffset() const { return getGotIdx() * target->gotEntrySize; } 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 getPltIdx() * target->gotEntrySize; return (getPltIdx() + target->gotPltHeaderEntriesNum) * target->gotEntrySize; } uint64_t Symbol::getPltVA() const { uint64_t outVA = isInIplt ? in.iplt->getVA() + getPltIdx() * target->ipltEntrySize : in.plt->getVA() + in.plt->headerSize + getPltIdx() * 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->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() { // Return if localized by a local: pattern in a version script. if (versionId == VER_NDX_LOCAL) return; StringRef s = getName(); size_t pos = s.find('@'); if (pos == StringRef::npos) return; StringRef verstr = s.substr(pos + 1); // Truncate the symbol name so that it doesn't include the version string. nameSize = pos; if (verstr.empty()) return; // 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::extract() const { if (file->lazy) { file->lazy = false; parseFile(file); } } uint8_t Symbol::computeBinding() const { auto v = visibility(); if ((v != STV_DEFAULT && v != STV_PROTECTED) || versionId == VER_NDX_LOCAL) return STB_LOCAL; if (binding == STB_GNU_UNIQUE && !config->gnuUnique) return STB_GLOBAL; return binding; } bool Symbol::includeInDynsym() const { 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 !(isUndefWeak() && config->noDynamicLinker); return exportDynamic || inDynamicList; } // Print out a log message for --trace-symbol. void elf::printTraceSymbol(const Symbol &sym, StringRef name) { 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 + name); } static void recordWhyExtract(const InputFile *reference, const InputFile &extracted, const Symbol &sym) { ctx.whyExtractRecords.emplace_back(toString(reference), &extracted, sym); } 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 (excluding those // demoted by demoteSymbols). // // Note, ld.bfd --symbol-ordering-file= does not warn on undefined symbols, // but we don't have to be compatible here. if (sym->isUndefined() && !cast(sym)->discardedSecIdx && 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()) { if (cast(sym)->discardedSecIdx) report(": unable to order discarded symbol: "); else 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->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() || sym.isPlaceholder()); // 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. -Bsymbolic-non-weak-functions is a non-weak subset of // -Bsymbolic-functions. if (config->symbolic || (config->bsymbolic == BsymbolicKind::NonWeak && sym.binding != STB_WEAK) || (config->bsymbolic == BsymbolicKind::Functions && sym.isFunc()) || (config->bsymbolic == BsymbolicKind::NonWeakFunctions && sym.isFunc() && sym.binding != STB_WEAK)) return sym.inDynamicList; return true; } // 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; // DSO symbols do not affect visibility in the output. if (!other.isShared() && other.visibility() != STV_DEFAULT) { uint8_t v = visibility(), ov = other.visibility(); setVisibility(v == STV_DEFAULT ? ov : std::min(v, ov)); } } void Symbol::resolve(const Undefined &other) { if (other.visibility() != STV_DEFAULT) { uint8_t v = visibility(), ov = other.visibility(); setVisibility(v == STV_DEFAULT ? ov : std::min(v, ov)); } // 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 (isPlaceholder() || (isShared() && other.visibility() != STV_DEFAULT) || (isUndefined() && other.binding != STB_WEAK && other.discardedSecIdx)) { other.overwrite(*this); return; } if (traced) printTraceSymbol(other, getName()); if (isLazy()) { // An undefined weak will not extract 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 // extracting 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; extract(); if (!config->whyExtract.empty()) recordWhyExtract(other.file, *file, *this); // 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. this->file needs to be saved because in the case of LTO it // may be reset to nullptr or be replaced with a file named lto.tmp. if (backref && !isWeak()) ctx.backwardReferences.try_emplace(this, std::make_pair(other.file, file)); return; } // Undefined symbols in a SharedFile do not change the binding. if (isa_and_nonnull(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; } } // Compare two symbols. Return true if the new symbol should win. bool Symbol::shouldReplace(const Defined &other) const { if (LLVM_UNLIKELY(isCommon())) { if (config->warnCommon) warn("common " + getName() + " is overridden"); return !other.isWeak(); } if (!isDefined()) return true; // Incoming STB_GLOBAL overrides STB_WEAK/STB_GNU_UNIQUE. -fgnu-unique changes // some vague linkage data in COMDAT from STB_WEAK to STB_GNU_UNIQUE. Treat // STB_GNU_UNIQUE like STB_WEAK so that we prefer the first among all // STB_WEAK/STB_GNU_UNIQUE copies. If we prefer an incoming STB_GNU_UNIQUE to // an existing STB_WEAK, there may be discarded section errors because the // selected copy may be in a non-prevailing COMDAT. return !isGlobal() && other.isGlobal(); } void elf::reportDuplicate(const Symbol &sym, const InputFile *newFile, InputSectionBase *errSec, uint64_t errOffset) { if (config->allowMultipleDefinition) return; // In glibc<2.32, crti.o has .gnu.linkonce.t.__x86.get_pc_thunk.bx, which // is sort of proto-comdat. There is actually no duplicate if we have // full support for .gnu.linkonce. const Defined *d = dyn_cast(&sym); if (!d || d->getName() == "__x86.get_pc_thunk.bx") return; // Allow absolute symbols with the same value for GNU ld compatibility. if (!d->section && !errSec && errOffset && d->value == errOffset) return; if (!d->section || !errSec) { errorOrWarn("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; errorOrWarn(msg); } void Symbol::checkDuplicate(const Defined &other) const { if (isDefined() && !isWeak() && !other.isWeak()) reportDuplicate(*this, other.file, dyn_cast_or_null(other.section), other.value); } void Symbol::resolve(const CommonSymbol &other) { if (other.exportDynamic) exportDynamic = true; if (other.visibility() != STV_DEFAULT) { uint8_t v = visibility(), ov = other.visibility(); setVisibility(v == STV_DEFAULT ? ov : std::min(v, ov)); } if (isDefined() && !isWeak()) { if (config->warnCommon) warn("common " + getName() + " is overridden"); return; } if (CommonSymbol *oldSym = dyn_cast(this)) { if (config->warnCommon) warn("multiple common of " + getName()); oldSym->alignment = std::max(oldSym->alignment, other.alignment); if (oldSym->size < other.size) { oldSym->file = other.file; oldSym->size = other.size; } return; } 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; other.overwrite(*this); if (size > cast(this)->size) cast(this)->size = size; } else { other.overwrite(*this); } } void Symbol::resolve(const Defined &other) { if (other.exportDynamic) exportDynamic = true; if (other.visibility() != STV_DEFAULT) { uint8_t v = visibility(), ov = other.visibility(); setVisibility(v == STV_DEFAULT ? ov : std::min(v, ov)); } if (shouldReplace(other)) other.overwrite(*this); } void Symbol::resolve(const LazySymbol &other) { if (isPlaceholder()) { other.overwrite(*this); return; } // For common objects, we want to look for global or weak definitions that // should be extracted as the canonical definition instead. if (LLVM_UNLIKELY(isCommon()) && elf::config->fortranCommon && other.file->shouldExtractForCommon(getName())) { ctx.backwardReferences.erase(this); other.overwrite(*this); other.extract(); return; } if (!isUndefined()) { // See the comment in resolveUndefined(). if (isDefined()) ctx.backwardReferences.erase(this); return; } // An undefined weak will not extract archive members. See comment on Lazy in // Symbols.h for the details. if (isWeak()) { uint8_t ty = type; other.overwrite(*this); type = ty; binding = STB_WEAK; return; } const InputFile *oldFile = file; other.extract(); if (!config->whyExtract.empty()) recordWhyExtract(oldFile, *file, *this); } void Symbol::resolve(const SharedSymbol &other) { exportDynamic = true; if (isPlaceholder()) { other.overwrite(*this); return; } 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; other.overwrite(*this); binding = bind; } else if (traced) printTraceSymbol(other, getName()); } void Defined::overwrite(Symbol &sym) const { if (isa_and_nonnull(sym.file)) sym.versionId = VER_NDX_GLOBAL; Symbol::overwrite(sym, DefinedKind); auto &s = static_cast(sym); s.value = value; s.size = size; s.section = section; }