//===- Writer.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 "Writer.h" #include "AArch64ErrataFix.h" #include "ARMErrataFix.h" #include "CallGraphSort.h" #include "Config.h" #include "InputFiles.h" #include "LinkerScript.h" #include "MapFile.h" #include "OutputSections.h" #include "Relocations.h" #include "SymbolTable.h" #include "Symbols.h" #include "SyntheticSections.h" #include "Target.h" #include "lld/Common/Arrays.h" #include "lld/Common/CommonLinkerContext.h" #include "lld/Common/Filesystem.h" #include "lld/Common/Strings.h" #include "llvm/ADT/STLExtras.h" #include "llvm/ADT/StringMap.h" #include "llvm/Support/BLAKE3.h" #include "llvm/Support/Parallel.h" #include "llvm/Support/RandomNumberGenerator.h" #include "llvm/Support/TimeProfiler.h" #include "llvm/Support/xxhash.h" #include #define DEBUG_TYPE "lld" using namespace llvm; using namespace llvm::ELF; using namespace llvm::object; using namespace llvm::support; using namespace llvm::support::endian; using namespace lld; using namespace lld::elf; namespace { // The writer writes a SymbolTable result to a file. template class Writer { public: LLVM_ELF_IMPORT_TYPES_ELFT(ELFT) Writer() : buffer(errorHandler().outputBuffer) {} void run(); private: void addSectionSymbols(); void sortSections(); void resolveShfLinkOrder(); void finalizeAddressDependentContent(); void optimizeBasicBlockJumps(); void sortInputSections(); void sortOrphanSections(); void finalizeSections(); void checkExecuteOnly(); void setReservedSymbolSections(); SmallVector createPhdrs(Partition &part); void addPhdrForSection(Partition &part, unsigned shType, unsigned pType, unsigned pFlags); void assignFileOffsets(); void assignFileOffsetsBinary(); void setPhdrs(Partition &part); void checkSections(); void fixSectionAlignments(); void openFile(); void writeTrapInstr(); void writeHeader(); void writeSections(); void writeSectionsBinary(); void writeBuildId(); std::unique_ptr &buffer; void addRelIpltSymbols(); void addStartEndSymbols(); void addStartStopSymbols(OutputSection &osec); uint64_t fileSize; uint64_t sectionHeaderOff; }; } // anonymous namespace template void elf::writeResult() { Writer().run(); } static void removeEmptyPTLoad(SmallVector &phdrs) { auto it = std::stable_partition( phdrs.begin(), phdrs.end(), [&](const PhdrEntry *p) { if (p->p_type != PT_LOAD) return true; if (!p->firstSec) return false; uint64_t size = p->lastSec->addr + p->lastSec->size - p->firstSec->addr; return size != 0; }); // Clear OutputSection::ptLoad for sections contained in removed // segments. DenseSet removed(it, phdrs.end()); for (OutputSection *sec : outputSections) if (removed.count(sec->ptLoad)) sec->ptLoad = nullptr; phdrs.erase(it, phdrs.end()); } void elf::copySectionsIntoPartitions() { SmallVector newSections; const size_t ehSize = ctx.ehInputSections.size(); for (unsigned part = 2; part != partitions.size() + 1; ++part) { for (InputSectionBase *s : ctx.inputSections) { if (!(s->flags & SHF_ALLOC) || !s->isLive() || s->type != SHT_NOTE) continue; auto *copy = make(cast(*s)); copy->partition = part; newSections.push_back(copy); } for (size_t i = 0; i != ehSize; ++i) { assert(ctx.ehInputSections[i]->isLive()); auto *copy = make(*ctx.ehInputSections[i]); copy->partition = part; ctx.ehInputSections.push_back(copy); } } ctx.inputSections.insert(ctx.inputSections.end(), newSections.begin(), newSections.end()); } static Defined *addOptionalRegular(StringRef name, SectionBase *sec, uint64_t val, uint8_t stOther = STV_HIDDEN) { Symbol *s = symtab.find(name); if (!s || s->isDefined() || s->isCommon()) return nullptr; s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL, stOther, STT_NOTYPE, val, /*size=*/0, sec}); s->isUsedInRegularObj = true; return cast(s); } // The linker is expected to define some symbols depending on // the linking result. This function defines such symbols. void elf::addReservedSymbols() { if (config->emachine == EM_MIPS) { auto addAbsolute = [](StringRef name) { Symbol *sym = symtab.addSymbol(Defined{ctx.internalFile, name, STB_GLOBAL, STV_HIDDEN, STT_NOTYPE, 0, 0, nullptr}); sym->isUsedInRegularObj = true; return cast(sym); }; // Define _gp for MIPS. st_value of _gp symbol will be updated by Writer // so that it points to an absolute address which by default is relative // to GOT. Default offset is 0x7ff0. // See "Global Data Symbols" in Chapter 6 in the following document: // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf ElfSym::mipsGp = addAbsolute("_gp"); // On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between // start of function and 'gp' pointer into GOT. if (symtab.find("_gp_disp")) ElfSym::mipsGpDisp = addAbsolute("_gp_disp"); // The __gnu_local_gp is a magic symbol equal to the current value of 'gp' // pointer. This symbol is used in the code generated by .cpload pseudo-op // in case of using -mno-shared option. // https://sourceware.org/ml/binutils/2004-12/msg00094.html if (symtab.find("__gnu_local_gp")) ElfSym::mipsLocalGp = addAbsolute("__gnu_local_gp"); } else if (config->emachine == EM_PPC) { // glibc *crt1.o has a undefined reference to _SDA_BASE_. Since we don't // support Small Data Area, define it arbitrarily as 0. addOptionalRegular("_SDA_BASE_", nullptr, 0, STV_HIDDEN); } else if (config->emachine == EM_PPC64) { addPPC64SaveRestore(); } // The Power Architecture 64-bit v2 ABI defines a TableOfContents (TOC) which // combines the typical ELF GOT with the small data sections. It commonly // includes .got .toc .sdata .sbss. The .TOC. symbol replaces both // _GLOBAL_OFFSET_TABLE_ and _SDA_BASE_ from the 32-bit ABI. It is used to // represent the TOC base which is offset by 0x8000 bytes from the start of // the .got section. // We do not allow _GLOBAL_OFFSET_TABLE_ to be defined by input objects as the // correctness of some relocations depends on its value. StringRef gotSymName = (config->emachine == EM_PPC64) ? ".TOC." : "_GLOBAL_OFFSET_TABLE_"; if (Symbol *s = symtab.find(gotSymName)) { if (s->isDefined()) { error(toString(s->file) + " cannot redefine linker defined symbol '" + gotSymName + "'"); return; } uint64_t gotOff = 0; if (config->emachine == EM_PPC64) gotOff = 0x8000; s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL, STV_HIDDEN, STT_NOTYPE, gotOff, /*size=*/0, Out::elfHeader}); ElfSym::globalOffsetTable = cast(s); } // __ehdr_start is the location of ELF file headers. Note that we define // this symbol unconditionally even when using a linker script, which // differs from the behavior implemented by GNU linker which only define // this symbol if ELF headers are in the memory mapped segment. addOptionalRegular("__ehdr_start", Out::elfHeader, 0, STV_HIDDEN); // __executable_start is not documented, but the expectation of at // least the Android libc is that it points to the ELF header. addOptionalRegular("__executable_start", Out::elfHeader, 0, STV_HIDDEN); // __dso_handle symbol is passed to cxa_finalize as a marker to identify // each DSO. The address of the symbol doesn't matter as long as they are // different in different DSOs, so we chose the start address of the DSO. addOptionalRegular("__dso_handle", Out::elfHeader, 0, STV_HIDDEN); // If linker script do layout we do not need to create any standard symbols. if (script->hasSectionsCommand) return; auto add = [](StringRef s, int64_t pos) { return addOptionalRegular(s, Out::elfHeader, pos, STV_DEFAULT); }; ElfSym::bss = add("__bss_start", 0); ElfSym::end1 = add("end", -1); ElfSym::end2 = add("_end", -1); ElfSym::etext1 = add("etext", -1); ElfSym::etext2 = add("_etext", -1); ElfSym::edata1 = add("edata", -1); ElfSym::edata2 = add("_edata", -1); } static void demoteDefined(Defined &sym, DenseMap &map) { if (map.empty()) for (auto [i, sec] : llvm::enumerate(sym.file->getSections())) map.try_emplace(sec, i); // Change WEAK to GLOBAL so that if a scanned relocation references sym, // maybeReportUndefined will report an error. uint8_t binding = sym.isWeak() ? uint8_t(STB_GLOBAL) : sym.binding; Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type, /*discardedSecIdx=*/map.lookup(sym.section)) .overwrite(sym); // Eliminate from the symbol table, otherwise we would leave an undefined // symbol if the symbol is unreferenced in the absence of GC. sym.isUsedInRegularObj = false; } // If all references to a DSO happen to be weak, the DSO is not added to // DT_NEEDED. If that happens, replace ShardSymbol with Undefined to avoid // dangling references to an unneeded DSO. Use a weak binding to avoid // --no-allow-shlib-undefined diagnostics. Similarly, demote lazy symbols. // // In addition, demote symbols defined in discarded sections, so that // references to /DISCARD/ discarded symbols will lead to errors. static void demoteSymbolsAndComputeIsPreemptible() { llvm::TimeTraceScope timeScope("Demote symbols"); DenseMap> sectionIndexMap; for (Symbol *sym : symtab.getSymbols()) { if (auto *d = dyn_cast(sym)) { if (d->section && !d->section->isLive()) demoteDefined(*d, sectionIndexMap[d->file]); } else { auto *s = dyn_cast(sym); if (sym->isLazy() || (s && !cast(s->file)->isNeeded)) { uint8_t binding = sym->isLazy() ? sym->binding : uint8_t(STB_WEAK); Undefined(ctx.internalFile, sym->getName(), binding, sym->stOther, sym->type) .overwrite(*sym); sym->versionId = VER_NDX_GLOBAL; } } if (config->hasDynSymTab) sym->isPreemptible = computeIsPreemptible(*sym); } } static OutputSection *findSection(StringRef name, unsigned partition = 1) { for (SectionCommand *cmd : script->sectionCommands) if (auto *osd = dyn_cast(cmd)) if (osd->osec.name == name && osd->osec.partition == partition) return &osd->osec; return nullptr; } // The main function of the writer. template void Writer::run() { // Now that we have a complete set of output sections. This function // completes section contents. For example, we need to add strings // to the string table, and add entries to .got and .plt. // finalizeSections does that. finalizeSections(); checkExecuteOnly(); // If --compressed-debug-sections is specified, compress .debug_* sections. // Do it right now because it changes the size of output sections. for (OutputSection *sec : outputSections) sec->maybeCompress(); if (script->hasSectionsCommand) script->allocateHeaders(mainPart->phdrs); // Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a // 0 sized region. This has to be done late since only after assignAddresses // we know the size of the sections. for (Partition &part : partitions) removeEmptyPTLoad(part.phdrs); if (!config->oFormatBinary) assignFileOffsets(); else assignFileOffsetsBinary(); for (Partition &part : partitions) setPhdrs(part); // Handle --print-map(-M)/--Map and --cref. Dump them before checkSections() // because the files may be useful in case checkSections() or openFile() // fails, for example, due to an erroneous file size. writeMapAndCref(); // Handle --print-memory-usage option. if (config->printMemoryUsage) script->printMemoryUsage(lld::outs()); if (config->checkSections) checkSections(); // It does not make sense try to open the file if we have error already. if (errorCount()) return; { llvm::TimeTraceScope timeScope("Write output file"); // Write the result down to a file. openFile(); if (errorCount()) return; if (!config->oFormatBinary) { if (config->zSeparate != SeparateSegmentKind::None) writeTrapInstr(); writeHeader(); writeSections(); } else { writeSectionsBinary(); } // Backfill .note.gnu.build-id section content. This is done at last // because the content is usually a hash value of the entire output file. writeBuildId(); if (errorCount()) return; if (auto e = buffer->commit()) fatal("failed to write output '" + buffer->getPath() + "': " + toString(std::move(e))); if (!config->cmseOutputLib.empty()) writeARMCmseImportLib(); } } template static void markUsedLocalSymbolsImpl(ObjFile *file, llvm::ArrayRef rels) { for (const RelTy &rel : rels) { Symbol &sym = file->getRelocTargetSym(rel); if (sym.isLocal()) sym.used = true; } } // The function ensures that the "used" field of local symbols reflects the fact // that the symbol is used in a relocation from a live section. template static void markUsedLocalSymbols() { // With --gc-sections, the field is already filled. // See MarkLive::resolveReloc(). if (config->gcSections) return; for (ELFFileBase *file : ctx.objectFiles) { ObjFile *f = cast>(file); for (InputSectionBase *s : f->getSections()) { InputSection *isec = dyn_cast_or_null(s); if (!isec) continue; if (isec->type == SHT_REL) { markUsedLocalSymbolsImpl(f, isec->getDataAs()); } else if (isec->type == SHT_RELA) { markUsedLocalSymbolsImpl(f, isec->getDataAs()); } else if (isec->type == SHT_CREL) { // The is64=true variant also works with ELF32 since only the r_symidx // member is used. for (Elf_Crel_Impl r : RelocsCrel(isec->content_)) { Symbol &sym = file->getSymbol(r.r_symidx); if (sym.isLocal()) sym.used = true; } } } } } static bool shouldKeepInSymtab(const Defined &sym) { if (sym.isSection()) return false; // If --emit-reloc or -r is given, preserve symbols referenced by relocations // from live sections. if (sym.used && config->copyRelocs) return true; // Exclude local symbols pointing to .ARM.exidx sections. // They are probably mapping symbols "$d", which are optional for these // sections. After merging the .ARM.exidx sections, some of these symbols // may become dangling. The easiest way to avoid the issue is not to add // them to the symbol table from the beginning. if (config->emachine == EM_ARM && sym.section && sym.section->type == SHT_ARM_EXIDX) return false; if (config->discard == DiscardPolicy::None) return true; if (config->discard == DiscardPolicy::All) return false; // In ELF assembly .L symbols are normally discarded by the assembler. // If the assembler fails to do so, the linker discards them if // * --discard-locals is used. // * The symbol is in a SHF_MERGE section, which is normally the reason for // the assembler keeping the .L symbol. if (sym.getName().starts_with(".L") && (config->discard == DiscardPolicy::Locals || (sym.section && (sym.section->flags & SHF_MERGE)))) return false; return true; } bool lld::elf::includeInSymtab(const Symbol &b) { if (auto *d = dyn_cast(&b)) { // Always include absolute symbols. SectionBase *sec = d->section; if (!sec) return true; assert(sec->isLive()); if (auto *s = dyn_cast(sec)) return s->getSectionPiece(d->value).live; return true; } return b.used || !config->gcSections; } // Scan local symbols to: // // - demote symbols defined relative to /DISCARD/ discarded input sections so // that relocations referencing them will lead to errors. // - copy eligible symbols to .symTab static void demoteAndCopyLocalSymbols() { llvm::TimeTraceScope timeScope("Add local symbols"); for (ELFFileBase *file : ctx.objectFiles) { DenseMap sectionIndexMap; for (Symbol *b : file->getLocalSymbols()) { assert(b->isLocal() && "should have been caught in initializeSymbols()"); auto *dr = dyn_cast(b); if (!dr) continue; if (dr->section && !dr->section->isLive()) demoteDefined(*dr, sectionIndexMap); else if (in.symTab && includeInSymtab(*b) && shouldKeepInSymtab(*dr)) in.symTab->addSymbol(b); } } } // Create a section symbol for each output section so that we can represent // relocations that point to the section. If we know that no relocation is // referring to a section (that happens if the section is a synthetic one), we // don't create a section symbol for that section. template void Writer::addSectionSymbols() { for (SectionCommand *cmd : script->sectionCommands) { auto *osd = dyn_cast(cmd); if (!osd) continue; OutputSection &osec = osd->osec; InputSectionBase *isec = nullptr; // Iterate over all input sections and add a STT_SECTION symbol if any input // section may be a relocation target. for (SectionCommand *cmd : osec.commands) { auto *isd = dyn_cast(cmd); if (!isd) continue; for (InputSectionBase *s : isd->sections) { // Relocations are not using REL[A] section symbols. if (isStaticRelSecType(s->type)) continue; // Unlike other synthetic sections, mergeable output sections contain // data copied from input sections, and there may be a relocation // pointing to its contents if -r or --emit-reloc is given. if (isa(s) && !(s->flags & SHF_MERGE)) continue; isec = s; break; } } if (!isec) continue; // Set the symbol to be relative to the output section so that its st_value // equals the output section address. Note, there may be a gap between the // start of the output section and isec. in.symTab->addSymbol(makeDefined(isec->file, "", STB_LOCAL, /*stOther=*/0, STT_SECTION, /*value=*/0, /*size=*/0, &osec)); } } // Today's loaders have a feature to make segments read-only after // processing dynamic relocations to enhance security. PT_GNU_RELRO // is defined for that. // // This function returns true if a section needs to be put into a // PT_GNU_RELRO segment. static bool isRelroSection(const OutputSection *sec) { if (!config->zRelro) return false; if (sec->relro) return true; uint64_t flags = sec->flags; // Non-allocatable or non-writable sections don't need RELRO because // they are not writable or not even mapped to memory in the first place. // RELRO is for sections that are essentially read-only but need to // be writable only at process startup to allow dynamic linker to // apply relocations. if (!(flags & SHF_ALLOC) || !(flags & SHF_WRITE)) return false; // Once initialized, TLS data segments are used as data templates // for a thread-local storage. For each new thread, runtime // allocates memory for a TLS and copy templates there. No thread // are supposed to use templates directly. Thus, it can be in RELRO. if (flags & SHF_TLS) return true; // .init_array, .preinit_array and .fini_array contain pointers to // functions that are executed on process startup or exit. These // pointers are set by the static linker, and they are not expected // to change at runtime. But if you are an attacker, you could do // interesting things by manipulating pointers in .fini_array, for // example. So they are put into RELRO. uint32_t type = sec->type; if (type == SHT_INIT_ARRAY || type == SHT_FINI_ARRAY || type == SHT_PREINIT_ARRAY) return true; // .got contains pointers to external symbols. They are resolved by // the dynamic linker when a module is loaded into memory, and after // that they are not expected to change. So, it can be in RELRO. if (in.got && sec == in.got->getParent()) return true; // .toc is a GOT-ish section for PowerPC64. Their contents are accessed // through r2 register, which is reserved for that purpose. Since r2 is used // for accessing .got as well, .got and .toc need to be close enough in the // virtual address space. Usually, .toc comes just after .got. Since we place // .got into RELRO, .toc needs to be placed into RELRO too. if (sec->name == ".toc") return true; // .got.plt contains pointers to external function symbols. They are // by default resolved lazily, so we usually cannot put it into RELRO. // However, if "-z now" is given, the lazy symbol resolution is // disabled, which enables us to put it into RELRO. if (sec == in.gotPlt->getParent()) return config->zNow; if (in.relroPadding && sec == in.relroPadding->getParent()) return true; // .dynamic section contains data for the dynamic linker, and // there's no need to write to it at runtime, so it's better to put // it into RELRO. if (sec->name == ".dynamic") return true; // Sections with some special names are put into RELRO. This is a // bit unfortunate because section names shouldn't be significant in // ELF in spirit. But in reality many linker features depend on // magic section names. StringRef s = sec->name; bool abiAgnostic = s == ".data.rel.ro" || s == ".bss.rel.ro" || s == ".ctors" || s == ".dtors" || s == ".jcr" || s == ".eh_frame" || s == ".fini_array" || s == ".init_array" || s == ".preinit_array"; bool abiSpecific = config->osabi == ELFOSABI_OPENBSD && s == ".openbsd.randomdata"; return abiAgnostic || abiSpecific; } // We compute a rank for each section. The rank indicates where the // section should be placed in the file. Instead of using simple // numbers (0,1,2...), we use a series of flags. One for each decision // point when placing the section. // Using flags has two key properties: // * It is easy to check if a give branch was taken. // * It is easy two see how similar two ranks are (see getRankProximity). enum RankFlags { RF_NOT_ADDR_SET = 1 << 27, RF_NOT_ALLOC = 1 << 26, RF_PARTITION = 1 << 18, // Partition number (8 bits) RF_LARGE_ALT = 1 << 15, RF_WRITE = 1 << 14, RF_EXEC_WRITE = 1 << 13, RF_EXEC = 1 << 12, RF_RODATA = 1 << 11, RF_LARGE = 1 << 10, RF_NOT_RELRO = 1 << 9, RF_NOT_TLS = 1 << 8, RF_BSS = 1 << 7, }; unsigned elf::getSectionRank(OutputSection &osec) { unsigned rank = osec.partition * RF_PARTITION; // We want to put section specified by -T option first, so we // can start assigning VA starting from them later. if (config->sectionStartMap.count(osec.name)) return rank; rank |= RF_NOT_ADDR_SET; // Allocatable sections go first to reduce the total PT_LOAD size and // so debug info doesn't change addresses in actual code. if (!(osec.flags & SHF_ALLOC)) return rank | RF_NOT_ALLOC; // Sort sections based on their access permission in the following // order: R, RX, RXW, RW(RELRO), RW(non-RELRO). // // Read-only sections come first such that they go in the PT_LOAD covering the // program headers at the start of the file. // // The layout for writable sections is PT_LOAD(PT_GNU_RELRO(.data.rel.ro // .bss.rel.ro) | .data .bss), where | marks where page alignment happens. // An alternative ordering is PT_LOAD(.data | PT_GNU_RELRO( .data.rel.ro // .bss.rel.ro) | .bss), but it may waste more bytes due to 2 alignment // places. bool isExec = osec.flags & SHF_EXECINSTR; bool isWrite = osec.flags & SHF_WRITE; if (!isWrite && !isExec) { // Among PROGBITS sections, place .lrodata further from .text. // For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This // layout has one extra PT_LOAD, but alleviates relocation overflow // pressure for absolute relocations referencing small data from -fno-pic // relocatable files. if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64) rank |= config->zLrodataAfterBss ? RF_LARGE_ALT : 0; else rank |= config->zLrodataAfterBss ? 0 : RF_LARGE; if (osec.type == SHT_LLVM_PART_EHDR) ; else if (osec.type == SHT_LLVM_PART_PHDR) rank |= 1; else if (osec.name == ".interp") rank |= 2; // Put .note sections at the beginning so that they are likely to be // included in a truncate core file. In particular, .note.gnu.build-id, if // available, can identify the object file. else if (osec.type == SHT_NOTE) rank |= 3; // Make PROGBITS sections (e.g .rodata .eh_frame) closer to .text to // alleviate relocation overflow pressure. Large special sections such as // .dynstr and .dynsym can be away from .text. else if (osec.type != SHT_PROGBITS) rank |= 4; else rank |= RF_RODATA; } else if (isExec) { rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC; } else { rank |= RF_WRITE; // The TLS initialization block needs to be a single contiguous block. Place // TLS sections directly before the other RELRO sections. if (!(osec.flags & SHF_TLS)) rank |= RF_NOT_TLS; if (isRelroSection(&osec)) osec.relro = true; else rank |= RF_NOT_RELRO; // Place .ldata and .lbss after .bss. Making .bss closer to .text // alleviates relocation overflow pressure. // For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss. // .bss/.lbss being adjacent reuses the NOBITS size optimization. if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64) { rank |= config->zLrodataAfterBss ? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT) : RF_LARGE; } } // Within TLS sections, or within other RelRo sections, or within non-RelRo // sections, place non-NOBITS sections first. if (osec.type == SHT_NOBITS) rank |= RF_BSS; // Some architectures have additional ordering restrictions for sections // within the same PT_LOAD. if (config->emachine == EM_PPC64) { // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections // that we would like to make sure appear is a specific order to maximize // their coverage by a single signed 16-bit offset from the TOC base // pointer. StringRef name = osec.name; if (name == ".got") rank |= 1; else if (name == ".toc") rank |= 2; } if (config->emachine == EM_MIPS) { if (osec.name != ".got") rank |= 1; // All sections with SHF_MIPS_GPREL flag should be grouped together // because data in these sections is addressable with a gp relative address. if (osec.flags & SHF_MIPS_GPREL) rank |= 2; } if (config->emachine == EM_RISCV) { // .sdata and .sbss are placed closer to make GP relaxation more profitable // and match GNU ld. StringRef name = osec.name; if (name == ".sdata" || (osec.type == SHT_NOBITS && name != ".sbss")) rank |= 1; } return rank; } static bool compareSections(const SectionCommand *aCmd, const SectionCommand *bCmd) { const OutputSection *a = &cast(aCmd)->osec; const OutputSection *b = &cast(bCmd)->osec; if (a->sortRank != b->sortRank) return a->sortRank < b->sortRank; if (!(a->sortRank & RF_NOT_ADDR_SET)) return config->sectionStartMap.lookup(a->name) < config->sectionStartMap.lookup(b->name); return false; } void PhdrEntry::add(OutputSection *sec) { lastSec = sec; if (!firstSec) firstSec = sec; p_align = std::max(p_align, sec->addralign); if (p_type == PT_LOAD) sec->ptLoad = this; } // A statically linked position-dependent executable should only contain // IRELATIVE relocations and no other dynamic relocations. Encapsulation symbols // __rel[a]_iplt_{start,end} will be defined for .rel[a].dyn, to be // processed by the libc runtime. Other executables or DSOs use dynamic tags // instead. template void Writer::addRelIpltSymbols() { if (config->isPic) return; // __rela_iplt_{start,end} are initially defined relative to dummy section 0. // We'll override Out::elfHeader with relaDyn later when we are sure that // .rela.dyn will be present in the output. std::string name = config->isRela ? "__rela_iplt_start" : "__rel_iplt_start"; ElfSym::relaIpltStart = addOptionalRegular(name, Out::elfHeader, 0, STV_HIDDEN); name.replace(name.size() - 5, 5, "end"); ElfSym::relaIpltEnd = addOptionalRegular(name, Out::elfHeader, 0, STV_HIDDEN); } // This function generates assignments for predefined symbols (e.g. _end or // _etext) and inserts them into the commands sequence to be processed at the // appropriate time. This ensures that the value is going to be correct by the // time any references to these symbols are processed and is equivalent to // defining these symbols explicitly in the linker script. template void Writer::setReservedSymbolSections() { if (ElfSym::globalOffsetTable) { // The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually // to the start of the .got or .got.plt section. InputSection *sec = in.gotPlt.get(); if (!target->gotBaseSymInGotPlt) sec = in.mipsGot ? cast(in.mipsGot.get()) : cast(in.got.get()); ElfSym::globalOffsetTable->section = sec; } // .rela_iplt_{start,end} mark the start and the end of .rel[a].dyn. if (ElfSym::relaIpltStart && mainPart->relaDyn->isNeeded()) { ElfSym::relaIpltStart->section = mainPart->relaDyn.get(); ElfSym::relaIpltEnd->section = mainPart->relaDyn.get(); ElfSym::relaIpltEnd->value = mainPart->relaDyn->getSize(); } PhdrEntry *last = nullptr; OutputSection *lastRO = nullptr; auto isLarge = [](OutputSection *osec) { return config->emachine == EM_X86_64 && osec->flags & SHF_X86_64_LARGE; }; for (Partition &part : partitions) { for (PhdrEntry *p : part.phdrs) { if (p->p_type != PT_LOAD) continue; last = p; if (!(p->p_flags & PF_W) && p->lastSec && !isLarge(p->lastSec)) lastRO = p->lastSec; } } if (lastRO) { // _etext is the first location after the last read-only loadable segment // that does not contain large sections. if (ElfSym::etext1) ElfSym::etext1->section = lastRO; if (ElfSym::etext2) ElfSym::etext2->section = lastRO; } if (last) { // _edata points to the end of the last non-large mapped initialized // section. OutputSection *edata = nullptr; for (OutputSection *os : outputSections) { if (os->type != SHT_NOBITS && !isLarge(os)) edata = os; if (os == last->lastSec) break; } if (ElfSym::edata1) ElfSym::edata1->section = edata; if (ElfSym::edata2) ElfSym::edata2->section = edata; // _end is the first location after the uninitialized data region. if (ElfSym::end1) ElfSym::end1->section = last->lastSec; if (ElfSym::end2) ElfSym::end2->section = last->lastSec; } if (ElfSym::bss) { // On RISC-V, set __bss_start to the start of .sbss if present. OutputSection *sbss = config->emachine == EM_RISCV ? findSection(".sbss") : nullptr; ElfSym::bss->section = sbss ? sbss : findSection(".bss"); } // Setup MIPS _gp_disp/__gnu_local_gp symbols which should // be equal to the _gp symbol's value. if (ElfSym::mipsGp) { // Find GP-relative section with the lowest address // and use this address to calculate default _gp value. for (OutputSection *os : outputSections) { if (os->flags & SHF_MIPS_GPREL) { ElfSym::mipsGp->section = os; ElfSym::mipsGp->value = 0x7ff0; break; } } } } // We want to find how similar two ranks are. // The more branches in getSectionRank that match, the more similar they are. // Since each branch corresponds to a bit flag, we can just use // countLeadingZeros. static int getRankProximity(OutputSection *a, SectionCommand *b) { auto *osd = dyn_cast(b); return (osd && osd->osec.hasInputSections) ? llvm::countl_zero(a->sortRank ^ osd->osec.sortRank) : -1; } // When placing orphan sections, we want to place them after symbol assignments // so that an orphan after // begin_foo = .; // foo : { *(foo) } // end_foo = .; // doesn't break the intended meaning of the begin/end symbols. // We don't want to go over sections since findOrphanPos is the // one in charge of deciding the order of the sections. // We don't want to go over changes to '.', since doing so in // rx_sec : { *(rx_sec) } // . = ALIGN(0x1000); // /* The RW PT_LOAD starts here*/ // rw_sec : { *(rw_sec) } // would mean that the RW PT_LOAD would become unaligned. static bool shouldSkip(SectionCommand *cmd) { if (auto *assign = dyn_cast(cmd)) return assign->name != "."; return false; } // We want to place orphan sections so that they share as much // characteristics with their neighbors as possible. For example, if // both are rw, or both are tls. static SmallVectorImpl::iterator findOrphanPos(SmallVectorImpl::iterator b, SmallVectorImpl::iterator e) { // Place non-alloc orphan sections at the end. This matches how we assign file // offsets to non-alloc sections. OutputSection *sec = &cast(*e)->osec; if (!(sec->flags & SHF_ALLOC)) return e; // As a special case, place .relro_padding before the SymbolAssignment using // DATA_SEGMENT_RELRO_END, if present. if (in.relroPadding && sec == in.relroPadding->getParent()) { auto i = std::find_if(b, e, [=](SectionCommand *a) { if (auto *assign = dyn_cast(a)) return assign->dataSegmentRelroEnd; return false; }); if (i != e) return i; } // Find the most similar output section as the anchor. Rank Proximity is a // value in the range [-1, 32] where [0, 32] indicates potential anchors (0: // least similar; 32: identical). -1 means not an anchor. // // In the event of proximity ties, we select the first or last section // depending on whether the orphan's rank is smaller. int maxP = 0; auto i = e; for (auto j = b; j != e; ++j) { int p = getRankProximity(sec, *j); if (p > maxP || (p == maxP && cast(*j)->osec.sortRank <= sec->sortRank)) { maxP = p; i = j; } } if (i == e) return e; auto isOutputSecWithInputSections = [](SectionCommand *cmd) { auto *osd = dyn_cast(cmd); return osd && osd->osec.hasInputSections; }; // Then, scan backward or forward through the script for a suitable insertion // point. If i's rank is larger, the orphan section can be placed before i. // // However, don't do this if custom program headers are defined. Otherwise, // adding the orphan to a previous segment can change its flags, for example, // making a read-only segment writable. If memory regions are defined, an // orphan section should continue the same region as the found section to // better resemble the behavior of GNU ld. bool mustAfter = script->hasPhdrsCommands() || !script->memoryRegions.empty(); if (cast(*i)->osec.sortRank <= sec->sortRank || mustAfter) { for (auto j = ++i; j != e; ++j) { if (!isOutputSecWithInputSections(*j)) continue; if (getRankProximity(sec, *j) != maxP) break; i = j + 1; } } else { for (; i != b; --i) if (isOutputSecWithInputSections(i[-1])) break; } // As a special case, if the orphan section is the last section, put // it at the very end, past any other commands. // This matches bfd's behavior and is convenient when the linker script fully // specifies the start of the file, but doesn't care about the end (the non // alloc sections for example). if (std::find_if(i, e, isOutputSecWithInputSections) == e) return e; while (i != e && shouldSkip(*i)) ++i; return i; } // Adds random priorities to sections not already in the map. static void maybeShuffle(DenseMap &order) { if (config->shuffleSections.empty()) return; SmallVector matched, sections = ctx.inputSections; matched.reserve(sections.size()); for (const auto &patAndSeed : config->shuffleSections) { matched.clear(); for (InputSectionBase *sec : sections) if (patAndSeed.first.match(sec->name)) matched.push_back(sec); const uint32_t seed = patAndSeed.second; if (seed == UINT32_MAX) { // If --shuffle-sections =-1, reverse the section order. The // section order is stable even if the number of sections changes. This is // useful to catch issues like static initialization order fiasco // reliably. std::reverse(matched.begin(), matched.end()); } else { std::mt19937 g(seed ? seed : std::random_device()()); llvm::shuffle(matched.begin(), matched.end(), g); } size_t i = 0; for (InputSectionBase *&sec : sections) if (patAndSeed.first.match(sec->name)) sec = matched[i++]; } // Existing priorities are < 0, so use priorities >= 0 for the missing // sections. int prio = 0; for (InputSectionBase *sec : sections) { if (order.try_emplace(sec, prio).second) ++prio; } } // Builds section order for handling --symbol-ordering-file. static DenseMap buildSectionOrder() { DenseMap sectionOrder; // Use the rarely used option --call-graph-ordering-file to sort sections. if (!config->callGraphProfile.empty()) return computeCallGraphProfileOrder(); if (config->symbolOrderingFile.empty()) return sectionOrder; struct SymbolOrderEntry { int priority; bool present; }; // Build a map from symbols to their priorities. Symbols that didn't // appear in the symbol ordering file have the lowest priority 0. // All explicitly mentioned symbols have negative (higher) priorities. DenseMap symbolOrder; int priority = -config->symbolOrderingFile.size(); for (StringRef s : config->symbolOrderingFile) symbolOrder.insert({CachedHashStringRef(s), {priority++, false}}); // Build a map from sections to their priorities. auto addSym = [&](Symbol &sym) { auto it = symbolOrder.find(CachedHashStringRef(sym.getName())); if (it == symbolOrder.end()) return; SymbolOrderEntry &ent = it->second; ent.present = true; maybeWarnUnorderableSymbol(&sym); if (auto *d = dyn_cast(&sym)) { if (auto *sec = dyn_cast_or_null(d->section)) { int &priority = sectionOrder[cast(sec)]; priority = std::min(priority, ent.priority); } } }; // We want both global and local symbols. We get the global ones from the // symbol table and iterate the object files for the local ones. for (Symbol *sym : symtab.getSymbols()) addSym(*sym); for (ELFFileBase *file : ctx.objectFiles) for (Symbol *sym : file->getLocalSymbols()) addSym(*sym); if (config->warnSymbolOrdering) for (auto orderEntry : symbolOrder) if (!orderEntry.second.present) warn("symbol ordering file: no such symbol: " + orderEntry.first.val()); return sectionOrder; } // Sorts the sections in ISD according to the provided section order. static void sortISDBySectionOrder(InputSectionDescription *isd, const DenseMap &order, bool executableOutputSection) { SmallVector unorderedSections; SmallVector, 0> orderedSections; uint64_t unorderedSize = 0; uint64_t totalSize = 0; for (InputSection *isec : isd->sections) { if (executableOutputSection) totalSize += isec->getSize(); auto i = order.find(isec); if (i == order.end()) { unorderedSections.push_back(isec); unorderedSize += isec->getSize(); continue; } orderedSections.push_back({isec, i->second}); } llvm::sort(orderedSections, llvm::less_second()); // Find an insertion point for the ordered section list in the unordered // section list. On targets with limited-range branches, this is the mid-point // of the unordered section list. This decreases the likelihood that a range // extension thunk will be needed to enter or exit the ordered region. If the // ordered section list is a list of hot functions, we can generally expect // the ordered functions to be called more often than the unordered functions, // making it more likely that any particular call will be within range, and // therefore reducing the number of thunks required. // // For example, imagine that you have 8MB of hot code and 32MB of cold code. // If the layout is: // // 8MB hot // 32MB cold // // only the first 8-16MB of the cold code (depending on which hot function it // is actually calling) can call the hot code without a range extension thunk. // However, if we use this layout: // // 16MB cold // 8MB hot // 16MB cold // // both the last 8-16MB of the first block of cold code and the first 8-16MB // of the second block of cold code can call the hot code without a thunk. So // we effectively double the amount of code that could potentially call into // the hot code without a thunk. // // The above is not necessary if total size of input sections in this "isd" // is small. Note that we assume all input sections are executable if the // output section is executable (which is not always true but supposed to // cover most cases). size_t insPt = 0; if (executableOutputSection && !orderedSections.empty() && target->getThunkSectionSpacing() && totalSize >= target->getThunkSectionSpacing()) { uint64_t unorderedPos = 0; for (; insPt != unorderedSections.size(); ++insPt) { unorderedPos += unorderedSections[insPt]->getSize(); if (unorderedPos > unorderedSize / 2) break; } } isd->sections.clear(); for (InputSection *isec : ArrayRef(unorderedSections).slice(0, insPt)) isd->sections.push_back(isec); for (std::pair p : orderedSections) isd->sections.push_back(p.first); for (InputSection *isec : ArrayRef(unorderedSections).slice(insPt)) isd->sections.push_back(isec); } static void sortSection(OutputSection &osec, const DenseMap &order) { StringRef name = osec.name; // Never sort these. if (name == ".init" || name == ".fini") return; // Sort input sections by priority using the list provided by // --symbol-ordering-file or --shuffle-sections=. This is a least significant // digit radix sort. The sections may be sorted stably again by a more // significant key. if (!order.empty()) for (SectionCommand *b : osec.commands) if (auto *isd = dyn_cast(b)) sortISDBySectionOrder(isd, order, osec.flags & SHF_EXECINSTR); if (script->hasSectionsCommand) return; if (name == ".init_array" || name == ".fini_array") { osec.sortInitFini(); } else if (name == ".ctors" || name == ".dtors") { osec.sortCtorsDtors(); } else if (config->emachine == EM_PPC64 && name == ".toc") { // .toc is allocated just after .got and is accessed using GOT-relative // relocations. Object files compiled with small code model have an // addressable range of [.got, .got + 0xFFFC] for GOT-relative relocations. // To reduce the risk of relocation overflow, .toc contents are sorted so // that sections having smaller relocation offsets are at beginning of .toc assert(osec.commands.size() == 1); auto *isd = cast(osec.commands[0]); llvm::stable_sort(isd->sections, [](const InputSection *a, const InputSection *b) -> bool { return a->file->ppc64SmallCodeModelTocRelocs && !b->file->ppc64SmallCodeModelTocRelocs; }); } } // If no layout was provided by linker script, we want to apply default // sorting for special input sections. This also handles --symbol-ordering-file. template void Writer::sortInputSections() { // Build the order once since it is expensive. DenseMap order = buildSectionOrder(); maybeShuffle(order); for (SectionCommand *cmd : script->sectionCommands) if (auto *osd = dyn_cast(cmd)) sortSection(osd->osec, order); } template void Writer::sortSections() { llvm::TimeTraceScope timeScope("Sort sections"); // Don't sort if using -r. It is not necessary and we want to preserve the // relative order for SHF_LINK_ORDER sections. if (config->relocatable) { script->adjustOutputSections(); return; } sortInputSections(); for (SectionCommand *cmd : script->sectionCommands) if (auto *osd = dyn_cast(cmd)) osd->osec.sortRank = getSectionRank(osd->osec); if (!script->hasSectionsCommand) { // OutputDescs are mostly contiguous, but may be interleaved with // SymbolAssignments in the presence of INSERT commands. auto mid = std::stable_partition( script->sectionCommands.begin(), script->sectionCommands.end(), [](SectionCommand *cmd) { return isa(cmd); }); std::stable_sort(script->sectionCommands.begin(), mid, compareSections); } // Process INSERT commands and update output section attributes. From this // point onwards the order of script->sectionCommands is fixed. script->processInsertCommands(); script->adjustOutputSections(); if (script->hasSectionsCommand) sortOrphanSections(); script->adjustSectionsAfterSorting(); } template void Writer::sortOrphanSections() { // Orphan sections are sections present in the input files which are // not explicitly placed into the output file by the linker script. // // The sections in the linker script are already in the correct // order. We have to figuere out where to insert the orphan // sections. // // The order of the sections in the script is arbitrary and may not agree with // compareSections. This means that we cannot easily define a strict weak // ordering. To see why, consider a comparison of a section in the script and // one not in the script. We have a two simple options: // * Make them equivalent (a is not less than b, and b is not less than a). // The problem is then that equivalence has to be transitive and we can // have sections a, b and c with only b in a script and a less than c // which breaks this property. // * Use compareSectionsNonScript. Given that the script order doesn't have // to match, we can end up with sections a, b, c, d where b and c are in the // script and c is compareSectionsNonScript less than b. In which case d // can be equivalent to c, a to b and d < a. As a concrete example: // .a (rx) # not in script // .b (rx) # in script // .c (ro) # in script // .d (ro) # not in script // // The way we define an order then is: // * Sort only the orphan sections. They are in the end right now. // * Move each orphan section to its preferred position. We try // to put each section in the last position where it can share // a PT_LOAD. // // There is some ambiguity as to where exactly a new entry should be // inserted, because Commands contains not only output section // commands but also other types of commands such as symbol assignment // expressions. There's no correct answer here due to the lack of the // formal specification of the linker script. We use heuristics to // determine whether a new output command should be added before or // after another commands. For the details, look at shouldSkip // function. auto i = script->sectionCommands.begin(); auto e = script->sectionCommands.end(); auto nonScriptI = std::find_if(i, e, [](SectionCommand *cmd) { if (auto *osd = dyn_cast(cmd)) return osd->osec.sectionIndex == UINT32_MAX; return false; }); // Sort the orphan sections. std::stable_sort(nonScriptI, e, compareSections); // As a horrible special case, skip the first . assignment if it is before any // section. We do this because it is common to set a load address by starting // the script with ". = 0xabcd" and the expectation is that every section is // after that. auto firstSectionOrDotAssignment = std::find_if(i, e, [](SectionCommand *cmd) { return !shouldSkip(cmd); }); if (firstSectionOrDotAssignment != e && isa(**firstSectionOrDotAssignment)) ++firstSectionOrDotAssignment; i = firstSectionOrDotAssignment; while (nonScriptI != e) { auto pos = findOrphanPos(i, nonScriptI); OutputSection *orphan = &cast(*nonScriptI)->osec; // As an optimization, find all sections with the same sort rank // and insert them with one rotate. unsigned rank = orphan->sortRank; auto end = std::find_if(nonScriptI + 1, e, [=](SectionCommand *cmd) { return cast(cmd)->osec.sortRank != rank; }); std::rotate(pos, nonScriptI, end); nonScriptI = end; } } static bool compareByFilePosition(InputSection *a, InputSection *b) { InputSection *la = a->flags & SHF_LINK_ORDER ? a->getLinkOrderDep() : nullptr; InputSection *lb = b->flags & SHF_LINK_ORDER ? b->getLinkOrderDep() : nullptr; // SHF_LINK_ORDER sections with non-zero sh_link are ordered before // non-SHF_LINK_ORDER sections and SHF_LINK_ORDER sections with zero sh_link. if (!la || !lb) return la && !lb; OutputSection *aOut = la->getParent(); OutputSection *bOut = lb->getParent(); if (aOut == bOut) return la->outSecOff < lb->outSecOff; if (aOut->addr == bOut->addr) return aOut->sectionIndex < bOut->sectionIndex; return aOut->addr < bOut->addr; } template void Writer::resolveShfLinkOrder() { llvm::TimeTraceScope timeScope("Resolve SHF_LINK_ORDER"); for (OutputSection *sec : outputSections) { if (!(sec->flags & SHF_LINK_ORDER)) continue; // The ARM.exidx section use SHF_LINK_ORDER, but we have consolidated // this processing inside the ARMExidxsyntheticsection::finalizeContents(). if (!config->relocatable && config->emachine == EM_ARM && sec->type == SHT_ARM_EXIDX) continue; // Link order may be distributed across several InputSectionDescriptions. // Sorting is performed separately. SmallVector scriptSections; SmallVector sections; for (SectionCommand *cmd : sec->commands) { auto *isd = dyn_cast(cmd); if (!isd) continue; bool hasLinkOrder = false; scriptSections.clear(); sections.clear(); for (InputSection *&isec : isd->sections) { if (isec->flags & SHF_LINK_ORDER) { InputSection *link = isec->getLinkOrderDep(); if (link && !link->getParent()) error(toString(isec) + ": sh_link points to discarded section " + toString(link)); hasLinkOrder = true; } scriptSections.push_back(&isec); sections.push_back(isec); } if (hasLinkOrder && errorCount() == 0) { llvm::stable_sort(sections, compareByFilePosition); for (int i = 0, n = sections.size(); i != n; ++i) *scriptSections[i] = sections[i]; } } } } static void finalizeSynthetic(SyntheticSection *sec) { if (sec && sec->isNeeded() && sec->getParent()) { llvm::TimeTraceScope timeScope("Finalize synthetic sections", sec->name); sec->finalizeContents(); } } // We need to generate and finalize the content that depends on the address of // InputSections. As the generation of the content may also alter InputSection // addresses we must converge to a fixed point. We do that here. See the comment // in Writer::finalizeSections(). template void Writer::finalizeAddressDependentContent() { llvm::TimeTraceScope timeScope("Finalize address dependent content"); ThunkCreator tc; AArch64Err843419Patcher a64p; ARMErr657417Patcher a32p; script->assignAddresses(); // .ARM.exidx and SHF_LINK_ORDER do not require precise addresses, but they // do require the relative addresses of OutputSections because linker scripts // can assign Virtual Addresses to OutputSections that are not monotonically // increasing. Anything here must be repeatable, since spilling may change // section order. const auto finalizeOrderDependentContent = [this] { for (Partition &part : partitions) finalizeSynthetic(part.armExidx.get()); resolveShfLinkOrder(); }; finalizeOrderDependentContent(); // Converts call x@GDPLT to call __tls_get_addr if (config->emachine == EM_HEXAGON) hexagonTLSSymbolUpdate(outputSections); uint32_t pass = 0, assignPasses = 0; for (;;) { bool changed = target->needsThunks ? tc.createThunks(pass, outputSections) : target->relaxOnce(pass); bool spilled = script->spillSections(); changed |= spilled; ++pass; // With Thunk Size much smaller than branch range we expect to // converge quickly; if we get to 30 something has gone wrong. if (changed && pass >= 30) { error(target->needsThunks ? "thunk creation not converged" : "relaxation not converged"); break; } if (config->fixCortexA53Errata843419) { if (changed) script->assignAddresses(); changed |= a64p.createFixes(); } if (config->fixCortexA8) { if (changed) script->assignAddresses(); changed |= a32p.createFixes(); } finalizeSynthetic(in.got.get()); if (in.mipsGot) in.mipsGot->updateAllocSize(); for (Partition &part : partitions) { // The R_AARCH64_AUTH_RELATIVE has a smaller addend field as bits [63:32] // encode the signing schema. We've put relocations in .relr.auth.dyn // during RelocationScanner::processAux, but the target VA for some of // them might be wider than 32 bits. We can only know the final VA at this // point, so move relocations with large values from .relr.auth.dyn to // .rela.dyn. See also AArch64::relocate. if (part.relrAuthDyn) { auto it = llvm::remove_if( part.relrAuthDyn->relocs, [&part](const RelativeReloc &elem) { const Relocation &reloc = elem.inputSec->relocs()[elem.relocIdx]; if (isInt<32>(reloc.sym->getVA(reloc.addend))) return false; part.relaDyn->addReloc({R_AARCH64_AUTH_RELATIVE, elem.inputSec, reloc.offset, DynamicReloc::AddendOnlyWithTargetVA, *reloc.sym, reloc.addend, R_ABS}); return true; }); changed |= (it != part.relrAuthDyn->relocs.end()); part.relrAuthDyn->relocs.erase(it, part.relrAuthDyn->relocs.end()); } if (part.relaDyn) changed |= part.relaDyn->updateAllocSize(); if (part.relrDyn) changed |= part.relrDyn->updateAllocSize(); if (part.relrAuthDyn) changed |= part.relrAuthDyn->updateAllocSize(); if (part.memtagGlobalDescriptors) changed |= part.memtagGlobalDescriptors->updateAllocSize(); } std::pair changes = script->assignAddresses(); if (!changed) { // Some symbols may be dependent on section addresses. When we break the // loop, the symbol values are finalized because a previous // assignAddresses() finalized section addresses. if (!changes.first && !changes.second) break; if (++assignPasses == 5) { if (changes.first) errorOrWarn("address (0x" + Twine::utohexstr(changes.first->addr) + ") of section '" + changes.first->name + "' does not converge"); if (changes.second) errorOrWarn("assignment to symbol " + toString(*changes.second) + " does not converge"); break; } } else if (spilled) { // Spilling can change relative section order. finalizeOrderDependentContent(); } } if (!config->relocatable) target->finalizeRelax(pass); if (config->relocatable) for (OutputSection *sec : outputSections) sec->addr = 0; // If addrExpr is set, the address may not be a multiple of the alignment. // Warn because this is error-prone. for (SectionCommand *cmd : script->sectionCommands) if (auto *osd = dyn_cast(cmd)) { OutputSection *osec = &osd->osec; if (osec->addr % osec->addralign != 0) warn("address (0x" + Twine::utohexstr(osec->addr) + ") of section " + osec->name + " is not a multiple of alignment (" + Twine(osec->addralign) + ")"); } // Sizes are no longer allowed to grow, so all allowable spills have been // taken. Remove any leftover potential spills. script->erasePotentialSpillSections(); } // If Input Sections have been shrunk (basic block sections) then // update symbol values and sizes associated with these sections. With basic // block sections, input sections can shrink when the jump instructions at // the end of the section are relaxed. static void fixSymbolsAfterShrinking() { for (InputFile *File : ctx.objectFiles) { parallelForEach(File->getSymbols(), [&](Symbol *Sym) { auto *def = dyn_cast(Sym); if (!def) return; const SectionBase *sec = def->section; if (!sec) return; const InputSectionBase *inputSec = dyn_cast(sec); if (!inputSec || !inputSec->bytesDropped) return; const size_t OldSize = inputSec->content().size(); const size_t NewSize = OldSize - inputSec->bytesDropped; if (def->value > NewSize && def->value <= OldSize) { LLVM_DEBUG(llvm::dbgs() << "Moving symbol " << Sym->getName() << " from " << def->value << " to " << def->value - inputSec->bytesDropped << " bytes\n"); def->value -= inputSec->bytesDropped; return; } if (def->value + def->size > NewSize && def->value <= OldSize && def->value + def->size <= OldSize) { LLVM_DEBUG(llvm::dbgs() << "Shrinking symbol " << Sym->getName() << " from " << def->size << " to " << def->size - inputSec->bytesDropped << " bytes\n"); def->size -= inputSec->bytesDropped; } }); } } // If basic block sections exist, there are opportunities to delete fall thru // jumps and shrink jump instructions after basic block reordering. This // relaxation pass does that. It is only enabled when --optimize-bb-jumps // option is used. template void Writer::optimizeBasicBlockJumps() { assert(config->optimizeBBJumps); SmallVector storage; script->assignAddresses(); // For every output section that has executable input sections, this // does the following: // 1. Deletes all direct jump instructions in input sections that // jump to the following section as it is not required. // 2. If there are two consecutive jump instructions, it checks // if they can be flipped and one can be deleted. for (OutputSection *osec : outputSections) { if (!(osec->flags & SHF_EXECINSTR)) continue; ArrayRef sections = getInputSections(*osec, storage); size_t numDeleted = 0; // Delete all fall through jump instructions. Also, check if two // consecutive jump instructions can be flipped so that a fall // through jmp instruction can be deleted. for (size_t i = 0, e = sections.size(); i != e; ++i) { InputSection *next = i + 1 < sections.size() ? sections[i + 1] : nullptr; InputSection &sec = *sections[i]; numDeleted += target->deleteFallThruJmpInsn(sec, sec.file, next); } if (numDeleted > 0) { script->assignAddresses(); LLVM_DEBUG(llvm::dbgs() << "Removing " << numDeleted << " fall through jumps\n"); } } fixSymbolsAfterShrinking(); for (OutputSection *osec : outputSections) for (InputSection *is : getInputSections(*osec, storage)) is->trim(); } // In order to allow users to manipulate linker-synthesized sections, // we had to add synthetic sections to the input section list early, // even before we make decisions whether they are needed. This allows // users to write scripts like this: ".mygot : { .got }". // // Doing it has an unintended side effects. If it turns out that we // don't need a .got (for example) at all because there's no // relocation that needs a .got, we don't want to emit .got. // // To deal with the above problem, this function is called after // scanRelocations is called to remove synthetic sections that turn // out to be empty. static void removeUnusedSyntheticSections() { // All input synthetic sections that can be empty are placed after // all regular ones. Reverse iterate to find the first synthetic section // after a non-synthetic one which will be our starting point. auto start = llvm::find_if(llvm::reverse(ctx.inputSections), [](InputSectionBase *s) { return !isa(s); }).base(); // Remove unused synthetic sections from ctx.inputSections; DenseSet unused; auto end = std::remove_if(start, ctx.inputSections.end(), [&](InputSectionBase *s) { auto *sec = cast(s); if (sec->getParent() && sec->isNeeded()) return false; // .relr.auth.dyn relocations may be moved to .rela.dyn in // finalizeAddressDependentContent, making .rela.dyn no longer empty. // Conservatively keep .rela.dyn. .relr.auth.dyn can be made empty, but // we would fail to remove it here. if (config->emachine == EM_AARCH64 && config->relrPackDynRelocs) if (auto *relSec = dyn_cast(sec)) if (relSec == mainPart->relaDyn.get()) return false; unused.insert(sec); return true; }); ctx.inputSections.erase(end, ctx.inputSections.end()); // Remove unused synthetic sections from the corresponding input section // description and orphanSections. for (auto *sec : unused) if (OutputSection *osec = cast(sec)->getParent()) for (SectionCommand *cmd : osec->commands) if (auto *isd = dyn_cast(cmd)) llvm::erase_if(isd->sections, [&](InputSection *isec) { return unused.count(isec); }); llvm::erase_if(script->orphanSections, [&](const InputSectionBase *sec) { return unused.count(sec); }); } // Create output section objects and add them to OutputSections. template void Writer::finalizeSections() { if (!config->relocatable) { Out::preinitArray = findSection(".preinit_array"); Out::initArray = findSection(".init_array"); Out::finiArray = findSection(".fini_array"); // The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop // symbols for sections, so that the runtime can get the start and end // addresses of each section by section name. Add such symbols. addStartEndSymbols(); for (SectionCommand *cmd : script->sectionCommands) if (auto *osd = dyn_cast(cmd)) addStartStopSymbols(osd->osec); // Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type. // It should be okay as no one seems to care about the type. // Even the author of gold doesn't remember why gold behaves that way. // https://sourceware.org/ml/binutils/2002-03/msg00360.html if (mainPart->dynamic->parent) { Symbol *s = symtab.addSymbol(Defined{ ctx.internalFile, "_DYNAMIC", STB_WEAK, STV_HIDDEN, STT_NOTYPE, /*value=*/0, /*size=*/0, mainPart->dynamic.get()}); s->isUsedInRegularObj = true; } // Define __rel[a]_iplt_{start,end} symbols if needed. addRelIpltSymbols(); // RISC-V's gp can address +/- 2 KiB, set it to .sdata + 0x800. This symbol // should only be defined in an executable. If .sdata does not exist, its // value/section does not matter but it has to be relative, so set its // st_shndx arbitrarily to 1 (Out::elfHeader). if (config->emachine == EM_RISCV) { ElfSym::riscvGlobalPointer = nullptr; if (!config->shared) { OutputSection *sec = findSection(".sdata"); addOptionalRegular( "__global_pointer$", sec ? sec : Out::elfHeader, 0x800, STV_DEFAULT); // Set riscvGlobalPointer to be used by the optional global pointer // relaxation. if (config->relaxGP) { Symbol *s = symtab.find("__global_pointer$"); if (s && s->isDefined()) ElfSym::riscvGlobalPointer = cast(s); } } } if (config->emachine == EM_386 || config->emachine == EM_X86_64) { // On targets that support TLSDESC, _TLS_MODULE_BASE_ is defined in such a // way that: // // 1) Without relaxation: it produces a dynamic TLSDESC relocation that // computes 0. // 2) With LD->LE relaxation: _TLS_MODULE_BASE_@tpoff = 0 (lowest address // in the TLS block). // // 2) is special cased in @tpoff computation. To satisfy 1), we define it // as an absolute symbol of zero. This is different from GNU linkers which // define _TLS_MODULE_BASE_ relative to the first TLS section. Symbol *s = symtab.find("_TLS_MODULE_BASE_"); if (s && s->isUndefined()) { s->resolve(Defined{ctx.internalFile, StringRef(), STB_GLOBAL, STV_HIDDEN, STT_TLS, /*value=*/0, 0, /*section=*/nullptr}); ElfSym::tlsModuleBase = cast(s); } } // This responsible for splitting up .eh_frame section into // pieces. The relocation scan uses those pieces, so this has to be // earlier. { llvm::TimeTraceScope timeScope("Finalize .eh_frame"); for (Partition &part : partitions) finalizeSynthetic(part.ehFrame.get()); } } demoteSymbolsAndComputeIsPreemptible(); if (config->copyRelocs && config->discard != DiscardPolicy::None) markUsedLocalSymbols(); demoteAndCopyLocalSymbols(); if (config->copyRelocs) addSectionSymbols(); // Change values of linker-script-defined symbols from placeholders (assigned // by declareSymbols) to actual definitions. script->processSymbolAssignments(); if (!config->relocatable) { llvm::TimeTraceScope timeScope("Scan relocations"); // Scan relocations. This must be done after every symbol is declared so // that we can correctly decide if a dynamic relocation is needed. This is // called after processSymbolAssignments() because it needs to know whether // a linker-script-defined symbol is absolute. ppc64noTocRelax.clear(); scanRelocations(); reportUndefinedSymbols(); postScanRelocations(); if (in.plt && in.plt->isNeeded()) in.plt->addSymbols(); if (in.iplt && in.iplt->isNeeded()) in.iplt->addSymbols(); if (config->unresolvedSymbolsInShlib != UnresolvedPolicy::Ignore) { auto diagnose = config->unresolvedSymbolsInShlib == UnresolvedPolicy::ReportError ? errorOrWarn : warn; // Error on undefined symbols in a shared object, if all of its DT_NEEDED // entries are seen. These cases would otherwise lead to runtime errors // reported by the dynamic linker. // // ld.bfd traces all DT_NEEDED to emulate the logic of the dynamic linker // to catch more cases. That is too much for us. Our approach resembles // the one used in ld.gold, achieves a good balance to be useful but not // too smart. // // If a DSO reference is resolved by a SharedSymbol, but the SharedSymbol // is overridden by a hidden visibility Defined (which is later discarded // due to GC), don't report the diagnostic. However, this may indicate an // unintended SharedSymbol. for (SharedFile *file : ctx.sharedFiles) { bool allNeededIsKnown = llvm::all_of(file->dtNeeded, [&](StringRef needed) { return symtab.soNames.count(CachedHashStringRef(needed)); }); if (!allNeededIsKnown) continue; for (Symbol *sym : file->requiredSymbols) { if (sym->dsoDefined) continue; if (sym->isUndefined() && !sym->isWeak()) { diagnose("undefined reference: " + toString(*sym) + "\n>>> referenced by " + toString(file) + " (disallowed by --no-allow-shlib-undefined)"); } else if (sym->isDefined() && sym->computeBinding() == STB_LOCAL) { diagnose("non-exported symbol '" + toString(*sym) + "' in '" + toString(sym->file) + "' is referenced by DSO '" + toString(file) + "'"); } } } } } { llvm::TimeTraceScope timeScope("Add symbols to symtabs"); // Now that we have defined all possible global symbols including linker- // synthesized ones. Visit all symbols to give the finishing touches. for (Symbol *sym : symtab.getSymbols()) { if (!sym->isUsedInRegularObj || !includeInSymtab(*sym)) continue; if (!config->relocatable) sym->binding = sym->computeBinding(); if (in.symTab) in.symTab->addSymbol(sym); if (sym->includeInDynsym()) { partitions[sym->partition - 1].dynSymTab->addSymbol(sym); if (auto *file = dyn_cast_or_null(sym->file)) if (file->isNeeded && !sym->isUndefined()) addVerneed(sym); } } // We also need to scan the dynamic relocation tables of the other // partitions and add any referenced symbols to the partition's dynsym. for (Partition &part : MutableArrayRef(partitions).slice(1)) { DenseSet syms; for (const SymbolTableEntry &e : part.dynSymTab->getSymbols()) syms.insert(e.sym); for (DynamicReloc &reloc : part.relaDyn->relocs) if (reloc.sym && reloc.needsDynSymIndex() && syms.insert(reloc.sym).second) part.dynSymTab->addSymbol(reloc.sym); } } if (in.mipsGot) in.mipsGot->build(); removeUnusedSyntheticSections(); script->diagnoseOrphanHandling(); script->diagnoseMissingSGSectionAddress(); sortSections(); // Create a list of OutputSections, assign sectionIndex, and populate // in.shStrTab. for (SectionCommand *cmd : script->sectionCommands) if (auto *osd = dyn_cast(cmd)) { OutputSection *osec = &osd->osec; outputSections.push_back(osec); osec->sectionIndex = outputSections.size(); osec->shName = in.shStrTab->addString(osec->name); } // Prefer command line supplied address over other constraints. for (OutputSection *sec : outputSections) { auto i = config->sectionStartMap.find(sec->name); if (i != config->sectionStartMap.end()) sec->addrExpr = [=] { return i->second; }; } // With the outputSections available check for GDPLT relocations // and add __tls_get_addr symbol if needed. if (config->emachine == EM_HEXAGON && hexagonNeedsTLSSymbol(outputSections)) { Symbol *sym = symtab.addSymbol(Undefined{ctx.internalFile, "__tls_get_addr", STB_GLOBAL, STV_DEFAULT, STT_NOTYPE}); sym->isPreemptible = true; partitions[0].dynSymTab->addSymbol(sym); } // This is a bit of a hack. A value of 0 means undef, so we set it // to 1 to make __ehdr_start defined. The section number is not // particularly relevant. Out::elfHeader->sectionIndex = 1; Out::elfHeader->size = sizeof(typename ELFT::Ehdr); // Binary and relocatable output does not have PHDRS. // The headers have to be created before finalize as that can influence the // image base and the dynamic section on mips includes the image base. if (!config->relocatable && !config->oFormatBinary) { for (Partition &part : partitions) { part.phdrs = script->hasPhdrsCommands() ? script->createPhdrs() : createPhdrs(part); if (config->emachine == EM_ARM) { // PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME addPhdrForSection(part, SHT_ARM_EXIDX, PT_ARM_EXIDX, PF_R); } if (config->emachine == EM_MIPS) { // Add separate segments for MIPS-specific sections. addPhdrForSection(part, SHT_MIPS_REGINFO, PT_MIPS_REGINFO, PF_R); addPhdrForSection(part, SHT_MIPS_OPTIONS, PT_MIPS_OPTIONS, PF_R); addPhdrForSection(part, SHT_MIPS_ABIFLAGS, PT_MIPS_ABIFLAGS, PF_R); } #if 0 // XXX: This stops elftoolchain strip adjusting .riscv.attributes, // leaving large holes in binaries. if (config->emachine == EM_RISCV) addPhdrForSection(part, SHT_RISCV_ATTRIBUTES, PT_RISCV_ATTRIBUTES, PF_R); #endif } Out::programHeaders->size = sizeof(Elf_Phdr) * mainPart->phdrs.size(); // Find the TLS segment. This happens before the section layout loop so that // Android relocation packing can look up TLS symbol addresses. We only need // to care about the main partition here because all TLS symbols were moved // to the main partition (see MarkLive.cpp). for (PhdrEntry *p : mainPart->phdrs) if (p->p_type == PT_TLS) Out::tlsPhdr = p; } // Some symbols are defined in term of program headers. Now that we // have the headers, we can find out which sections they point to. setReservedSymbolSections(); if (script->noCrossRefs.size()) { llvm::TimeTraceScope timeScope("Check NOCROSSREFS"); checkNoCrossRefs(); } { llvm::TimeTraceScope timeScope("Finalize synthetic sections"); finalizeSynthetic(in.bss.get()); finalizeSynthetic(in.bssRelRo.get()); finalizeSynthetic(in.symTabShndx.get()); finalizeSynthetic(in.shStrTab.get()); finalizeSynthetic(in.strTab.get()); finalizeSynthetic(in.got.get()); finalizeSynthetic(in.mipsGot.get()); finalizeSynthetic(in.igotPlt.get()); finalizeSynthetic(in.gotPlt.get()); finalizeSynthetic(in.relaPlt.get()); finalizeSynthetic(in.plt.get()); finalizeSynthetic(in.iplt.get()); finalizeSynthetic(in.ppc32Got2.get()); finalizeSynthetic(in.partIndex.get()); // Dynamic section must be the last one in this list and dynamic // symbol table section (dynSymTab) must be the first one. for (Partition &part : partitions) { if (part.relaDyn) { part.relaDyn->mergeRels(); // Compute DT_RELACOUNT to be used by part.dynamic. part.relaDyn->partitionRels(); finalizeSynthetic(part.relaDyn.get()); } if (part.relrDyn) { part.relrDyn->mergeRels(); finalizeSynthetic(part.relrDyn.get()); } if (part.relrAuthDyn) { part.relrAuthDyn->mergeRels(); finalizeSynthetic(part.relrAuthDyn.get()); } finalizeSynthetic(part.dynSymTab.get()); finalizeSynthetic(part.gnuHashTab.get()); finalizeSynthetic(part.hashTab.get()); finalizeSynthetic(part.verDef.get()); finalizeSynthetic(part.ehFrameHdr.get()); finalizeSynthetic(part.verSym.get()); finalizeSynthetic(part.verNeed.get()); finalizeSynthetic(part.dynamic.get()); } } if (!script->hasSectionsCommand && !config->relocatable) fixSectionAlignments(); // This is used to: // 1) Create "thunks": // Jump instructions in many ISAs have small displacements, and therefore // they cannot jump to arbitrary addresses in memory. For example, RISC-V // JAL instruction can target only +-1 MiB from PC. It is a linker's // responsibility to create and insert small pieces of code between // sections to extend the ranges if jump targets are out of range. Such // code pieces are called "thunks". // // We add thunks at this stage. We couldn't do this before this point // because this is the earliest point where we know sizes of sections and // their layouts (that are needed to determine if jump targets are in // range). // // 2) Update the sections. We need to generate content that depends on the // address of InputSections. For example, MIPS GOT section content or // android packed relocations sections content. // // 3) Assign the final values for the linker script symbols. Linker scripts // sometimes using forward symbol declarations. We want to set the correct // values. They also might change after adding the thunks. finalizeAddressDependentContent(); // All information needed for OutputSection part of Map file is available. if (errorCount()) return; { llvm::TimeTraceScope timeScope("Finalize synthetic sections"); // finalizeAddressDependentContent may have added local symbols to the // static symbol table. finalizeSynthetic(in.symTab.get()); finalizeSynthetic(in.debugNames.get()); finalizeSynthetic(in.ppc64LongBranchTarget.get()); finalizeSynthetic(in.armCmseSGSection.get()); } // Relaxation to delete inter-basic block jumps created by basic block // sections. Run after in.symTab is finalized as optimizeBasicBlockJumps // can relax jump instructions based on symbol offset. if (config->optimizeBBJumps) optimizeBasicBlockJumps(); // Fill other section headers. The dynamic table is finalized // at the end because some tags like RELSZ depend on result // of finalizing other sections. for (OutputSection *sec : outputSections) sec->finalize(); script->checkFinalScriptConditions(); if (config->emachine == EM_ARM && !config->isLE && config->armBe8) { addArmInputSectionMappingSymbols(); sortArmMappingSymbols(); } } // Ensure data sections are not mixed with executable sections when // --execute-only is used. --execute-only make pages executable but not // readable. template void Writer::checkExecuteOnly() { if (!config->executeOnly) return; SmallVector storage; for (OutputSection *osec : outputSections) if (osec->flags & SHF_EXECINSTR) for (InputSection *isec : getInputSections(*osec, storage)) if (!(isec->flags & SHF_EXECINSTR)) error("cannot place " + toString(isec) + " into " + toString(osec->name) + ": --execute-only does not support intermingling data and code"); } // The linker is expected to define SECNAME_start and SECNAME_end // symbols for a few sections. This function defines them. template void Writer::addStartEndSymbols() { // If the associated output section does not exist, there is ambiguity as to // how we define _start and _end symbols for an init/fini section. Users // expect no "undefined symbol" linker errors and loaders expect equal // st_value but do not particularly care whether the symbols are defined or // not. We retain the output section so that the section indexes will be // correct. auto define = [=](StringRef start, StringRef end, OutputSection *os) { if (os) { Defined *startSym = addOptionalRegular(start, os, 0); Defined *stopSym = addOptionalRegular(end, os, -1); if (startSym || stopSym) os->usedInExpression = true; } else { addOptionalRegular(start, Out::elfHeader, 0); addOptionalRegular(end, Out::elfHeader, 0); } }; define("__preinit_array_start", "__preinit_array_end", Out::preinitArray); define("__init_array_start", "__init_array_end", Out::initArray); define("__fini_array_start", "__fini_array_end", Out::finiArray); // As a special case, don't unnecessarily retain .ARM.exidx, which would // create an empty PT_ARM_EXIDX. if (OutputSection *sec = findSection(".ARM.exidx")) define("__exidx_start", "__exidx_end", sec); } // If a section name is valid as a C identifier (which is rare because of // the leading '.'), linkers are expected to define __start_ and // __stop_ symbols. They are at beginning and end of the section, // respectively. This is not requested by the ELF standard, but GNU ld and // gold provide the feature, and used by many programs. template void Writer::addStartStopSymbols(OutputSection &osec) { StringRef s = osec.name; if (!isValidCIdentifier(s)) return; Defined *startSym = addOptionalRegular(saver().save("__start_" + s), &osec, 0, config->zStartStopVisibility); Defined *stopSym = addOptionalRegular(saver().save("__stop_" + s), &osec, -1, config->zStartStopVisibility); if (startSym || stopSym) osec.usedInExpression = true; } static bool needsPtLoad(OutputSection *sec) { if (!(sec->flags & SHF_ALLOC)) return false; // Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is // responsible for allocating space for them, not the PT_LOAD that // contains the TLS initialization image. if ((sec->flags & SHF_TLS) && sec->type == SHT_NOBITS) return false; return true; } // Adjust phdr flags according to certain options. static uint64_t computeFlags(uint64_t flags) { if (config->omagic) return PF_R | PF_W | PF_X; if (config->executeOnly && (flags & PF_X)) return flags & ~PF_R; return flags; } // Decide which program headers to create and which sections to include in each // one. template SmallVector Writer::createPhdrs(Partition &part) { SmallVector ret; auto addHdr = [&](unsigned type, unsigned flags) -> PhdrEntry * { ret.push_back(make(type, flags)); return ret.back(); }; unsigned partNo = part.getNumber(); bool isMain = partNo == 1; // Add the first PT_LOAD segment for regular output sections. uint64_t flags = computeFlags(PF_R); PhdrEntry *load = nullptr; // nmagic or omagic output does not have PT_PHDR, PT_INTERP, or the readonly // PT_LOAD. if (!config->nmagic && !config->omagic) { // The first phdr entry is PT_PHDR which describes the program header // itself. if (isMain) addHdr(PT_PHDR, PF_R)->add(Out::programHeaders); else addHdr(PT_PHDR, PF_R)->add(part.programHeaders->getParent()); // PT_INTERP must be the second entry if exists. if (OutputSection *cmd = findSection(".interp", partNo)) addHdr(PT_INTERP, cmd->getPhdrFlags())->add(cmd); // Add the headers. We will remove them if they don't fit. // In the other partitions the headers are ordinary sections, so they don't // need to be added here. if (isMain) { load = addHdr(PT_LOAD, flags); load->add(Out::elfHeader); load->add(Out::programHeaders); } } // PT_GNU_RELRO includes all sections that should be marked as // read-only by dynamic linker after processing relocations. // Current dynamic loaders only support one PT_GNU_RELRO PHDR, give // an error message if more than one PT_GNU_RELRO PHDR is required. PhdrEntry *relRo = make(PT_GNU_RELRO, PF_R); bool inRelroPhdr = false; OutputSection *relroEnd = nullptr; for (OutputSection *sec : outputSections) { if (sec->partition != partNo || !needsPtLoad(sec)) continue; if (isRelroSection(sec)) { inRelroPhdr = true; if (!relroEnd) relRo->add(sec); else error("section: " + sec->name + " is not contiguous with other relro" + " sections"); } else if (inRelroPhdr) { inRelroPhdr = false; relroEnd = sec; } } relRo->p_align = 1; for (OutputSection *sec : outputSections) { if (!needsPtLoad(sec)) continue; // Normally, sections in partitions other than the current partition are // ignored. But partition number 255 is a special case: it contains the // partition end marker (.part.end). It needs to be added to the main // partition so that a segment is created for it in the main partition, // which will cause the dynamic loader to reserve space for the other // partitions. if (sec->partition != partNo) { if (isMain && sec->partition == 255) addHdr(PT_LOAD, computeFlags(sec->getPhdrFlags()))->add(sec); continue; } // Segments are contiguous memory regions that has the same attributes // (e.g. executable or writable). There is one phdr for each segment. // Therefore, we need to create a new phdr when the next section has // incompatible flags or is loaded at a discontiguous address or memory // region using AT or AT> linker script command, respectively. // // As an exception, we don't create a separate load segment for the ELF // headers, even if the first "real" output has an AT or AT> attribute. // // In addition, NOBITS sections should only be placed at the end of a LOAD // segment (since it's represented as p_filesz < p_memsz). If we have a // not-NOBITS section after a NOBITS, we create a new LOAD for the latter // even if flags match, so as not to require actually writing the // supposed-to-be-NOBITS section to the output file. (However, we cannot do // so when hasSectionsCommand, since we cannot introduce the extra alignment // needed to create a new LOAD) uint64_t newFlags = computeFlags(sec->getPhdrFlags()); // When --no-rosegment is specified, RO and RX sections are compatible. uint32_t incompatible = flags ^ newFlags; if (config->singleRoRx && !(newFlags & PF_W)) incompatible &= ~PF_X; if (incompatible) load = nullptr; bool sameLMARegion = load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion; if (load && sec != relroEnd && sec->memRegion == load->firstSec->memRegion && (sameLMARegion || load->lastSec == Out::programHeaders) && (script->hasSectionsCommand || sec->type == SHT_NOBITS || load->lastSec->type != SHT_NOBITS)) { load->p_flags |= newFlags; } else { load = addHdr(PT_LOAD, newFlags); flags = newFlags; } load->add(sec); } // Add a TLS segment if any. PhdrEntry *tlsHdr = make(PT_TLS, PF_R); for (OutputSection *sec : outputSections) if (sec->partition == partNo && sec->flags & SHF_TLS) tlsHdr->add(sec); if (tlsHdr->firstSec) ret.push_back(tlsHdr); // Add an entry for .dynamic. if (OutputSection *sec = part.dynamic->getParent()) addHdr(PT_DYNAMIC, sec->getPhdrFlags())->add(sec); if (relRo->firstSec) ret.push_back(relRo); // PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr. if (part.ehFrame->isNeeded() && part.ehFrameHdr && part.ehFrame->getParent() && part.ehFrameHdr->getParent()) addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags()) ->add(part.ehFrameHdr->getParent()); if (config->osabi == ELFOSABI_OPENBSD) { // PT_OPENBSD_MUTABLE makes the dynamic linker fill the segment with // zero data, like bss, but it can be treated differently. if (OutputSection *cmd = findSection(".openbsd.mutable", partNo)) addHdr(PT_OPENBSD_MUTABLE, cmd->getPhdrFlags())->add(cmd); // PT_OPENBSD_RANDOMIZE makes the dynamic linker fill the segment // with random data. if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo)) addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd); // PT_OPENBSD_SYSCALLS makes the kernel and dynamic linker register // system call sites. if (OutputSection *cmd = findSection(".openbsd.syscalls", partNo)) addHdr(PT_OPENBSD_SYSCALLS, cmd->getPhdrFlags())->add(cmd); } if (config->zGnustack != GnuStackKind::None) { // PT_GNU_STACK is a special section to tell the loader to make the // pages for the stack non-executable. If you really want an executable // stack, you can pass -z execstack, but that's not recommended for // security reasons. unsigned perm = PF_R | PF_W; if (config->zGnustack == GnuStackKind::Exec) perm |= PF_X; addHdr(PT_GNU_STACK, perm)->p_memsz = config->zStackSize; } // PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable // is expected to perform W^X violations, such as calling mprotect(2) or // mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on // OpenBSD. if (config->zWxneeded) addHdr(PT_OPENBSD_WXNEEDED, PF_X); if (OutputSection *cmd = findSection(".note.gnu.property", partNo)) addHdr(PT_GNU_PROPERTY, PF_R)->add(cmd); // Create one PT_NOTE per a group of contiguous SHT_NOTE sections with the // same alignment. PhdrEntry *note = nullptr; for (OutputSection *sec : outputSections) { if (sec->partition != partNo) continue; if (sec->type == SHT_NOTE && (sec->flags & SHF_ALLOC)) { if (!note || sec->lmaExpr || note->lastSec->addralign != sec->addralign) note = addHdr(PT_NOTE, PF_R); note->add(sec); } else { note = nullptr; } } return ret; } template void Writer::addPhdrForSection(Partition &part, unsigned shType, unsigned pType, unsigned pFlags) { unsigned partNo = part.getNumber(); auto i = llvm::find_if(outputSections, [=](OutputSection *cmd) { return cmd->partition == partNo && cmd->type == shType; }); if (i == outputSections.end()) return; PhdrEntry *entry = make(pType, pFlags); entry->add(*i); part.phdrs.push_back(entry); } // Place the first section of each PT_LOAD to a different page (of maxPageSize). // This is achieved by assigning an alignment expression to addrExpr of each // such section. template void Writer::fixSectionAlignments() { const PhdrEntry *prev; auto pageAlign = [&](const PhdrEntry *p) { OutputSection *cmd = p->firstSec; if (!cmd) return; cmd->alignExpr = [align = cmd->addralign]() { return align; }; if (!cmd->addrExpr) { // Prefer advancing to align(dot, maxPageSize) + dot%maxPageSize to avoid // padding in the file contents. // // When -z separate-code is used we must not have any overlap in pages // between an executable segment and a non-executable segment. We align to // the next maximum page size boundary on transitions between executable // and non-executable segments. // // SHT_LLVM_PART_EHDR marks the start of a partition. The partition // sections will be extracted to a separate file. Align to the next // maximum page size boundary so that we can find the ELF header at the // start. We cannot benefit from overlapping p_offset ranges with the // previous segment anyway. if (config->zSeparate == SeparateSegmentKind::Loadable || (config->zSeparate == SeparateSegmentKind::Code && prev && (prev->p_flags & PF_X) != (p->p_flags & PF_X)) || cmd->type == SHT_LLVM_PART_EHDR) cmd->addrExpr = [] { return alignToPowerOf2(script->getDot(), config->maxPageSize); }; // PT_TLS is at the start of the first RW PT_LOAD. If `p` includes PT_TLS, // it must be the RW. Align to p_align(PT_TLS) to make sure // p_vaddr(PT_LOAD)%p_align(PT_LOAD) = 0. Otherwise, if // sh_addralign(.tdata) < sh_addralign(.tbss), we will set p_align(PT_TLS) // to sh_addralign(.tbss), while p_vaddr(PT_TLS)=p_vaddr(PT_LOAD) may not // be congruent to 0 modulo p_align(PT_TLS). // // Technically this is not required, but as of 2019, some dynamic loaders // don't handle p_vaddr%p_align != 0 correctly, e.g. glibc (i386 and // x86-64) doesn't make runtime address congruent to p_vaddr modulo // p_align for dynamic TLS blocks (PR/24606), FreeBSD rtld has the same // bug, musl (TLS Variant 1 architectures) before 1.1.23 handled TLS // blocks correctly. We need to keep the workaround for a while. else if (Out::tlsPhdr && Out::tlsPhdr->firstSec == p->firstSec) cmd->addrExpr = [] { return alignToPowerOf2(script->getDot(), config->maxPageSize) + alignToPowerOf2(script->getDot() % config->maxPageSize, Out::tlsPhdr->p_align); }; else cmd->addrExpr = [] { return alignToPowerOf2(script->getDot(), config->maxPageSize) + script->getDot() % config->maxPageSize; }; } }; for (Partition &part : partitions) { prev = nullptr; for (const PhdrEntry *p : part.phdrs) if (p->p_type == PT_LOAD && p->firstSec) { pageAlign(p); prev = p; } } } // Compute an in-file position for a given section. The file offset must be the // same with its virtual address modulo the page size, so that the loader can // load executables without any address adjustment. static uint64_t computeFileOffset(OutputSection *os, uint64_t off) { // The first section in a PT_LOAD has to have congruent offset and address // modulo the maximum page size. if (os->ptLoad && os->ptLoad->firstSec == os) return alignTo(off, os->ptLoad->p_align, os->addr); // File offsets are not significant for .bss sections other than the first one // in a PT_LOAD/PT_TLS. By convention, we keep section offsets monotonically // increasing rather than setting to zero. if (os->type == SHT_NOBITS && (!Out::tlsPhdr || Out::tlsPhdr->firstSec != os)) return off; // If the section is not in a PT_LOAD, we just have to align it. if (!os->ptLoad) return alignToPowerOf2(off, os->addralign); // If two sections share the same PT_LOAD the file offset is calculated // using this formula: Off2 = Off1 + (VA2 - VA1). OutputSection *first = os->ptLoad->firstSec; return first->offset + os->addr - first->addr; } template void Writer::assignFileOffsetsBinary() { // Compute the minimum LMA of all non-empty non-NOBITS sections as minAddr. auto needsOffset = [](OutputSection &sec) { return sec.type != SHT_NOBITS && (sec.flags & SHF_ALLOC) && sec.size > 0; }; uint64_t minAddr = UINT64_MAX; for (OutputSection *sec : outputSections) if (needsOffset(*sec)) { sec->offset = sec->getLMA(); minAddr = std::min(minAddr, sec->offset); } // Sections are laid out at LMA minus minAddr. fileSize = 0; for (OutputSection *sec : outputSections) if (needsOffset(*sec)) { sec->offset -= minAddr; fileSize = std::max(fileSize, sec->offset + sec->size); } } static std::string rangeToString(uint64_t addr, uint64_t len) { return "[0x" + utohexstr(addr) + ", 0x" + utohexstr(addr + len - 1) + "]"; } // Assign file offsets to output sections. template void Writer::assignFileOffsets() { Out::programHeaders->offset = Out::elfHeader->size; uint64_t off = Out::elfHeader->size + Out::programHeaders->size; PhdrEntry *lastRX = nullptr; for (Partition &part : partitions) for (PhdrEntry *p : part.phdrs) if (p->p_type == PT_LOAD && (p->p_flags & PF_X)) lastRX = p; // Layout SHF_ALLOC sections before non-SHF_ALLOC sections. A non-SHF_ALLOC // will not occupy file offsets contained by a PT_LOAD. for (OutputSection *sec : outputSections) { if (!(sec->flags & SHF_ALLOC)) continue; off = computeFileOffset(sec, off); sec->offset = off; if (sec->type != SHT_NOBITS) off += sec->size; // If this is a last section of the last executable segment and that // segment is the last loadable segment, align the offset of the // following section to avoid loading non-segments parts of the file. if (config->zSeparate != SeparateSegmentKind::None && lastRX && lastRX->lastSec == sec) off = alignToPowerOf2(off, config->maxPageSize); } for (OutputSection *osec : outputSections) { if (osec->flags & SHF_ALLOC) continue; osec->offset = alignToPowerOf2(off, osec->addralign); off = osec->offset + osec->size; } sectionHeaderOff = alignToPowerOf2(off, config->wordsize); fileSize = sectionHeaderOff + (outputSections.size() + 1) * sizeof(Elf_Shdr); // Our logic assumes that sections have rising VA within the same segment. // With use of linker scripts it is possible to violate this rule and get file // offset overlaps or overflows. That should never happen with a valid script // which does not move the location counter backwards and usually scripts do // not do that. Unfortunately, there are apps in the wild, for example, Linux // kernel, which control segment distribution explicitly and move the counter // backwards, so we have to allow doing that to support linking them. We // perform non-critical checks for overlaps in checkSectionOverlap(), but here // we want to prevent file size overflows because it would crash the linker. for (OutputSection *sec : outputSections) { if (sec->type == SHT_NOBITS) continue; if ((sec->offset > fileSize) || (sec->offset + sec->size > fileSize)) error("unable to place section " + sec->name + " at file offset " + rangeToString(sec->offset, sec->size) + "; check your linker script for overflows"); } } // Finalize the program headers. We call this function after we assign // file offsets and VAs to all sections. template void Writer::setPhdrs(Partition &part) { for (PhdrEntry *p : part.phdrs) { OutputSection *first = p->firstSec; OutputSection *last = p->lastSec; // .ARM.exidx sections may not be within a single .ARM.exidx // output section. We always want to describe just the // SyntheticSection. if (part.armExidx && p->p_type == PT_ARM_EXIDX) { p->p_filesz = part.armExidx->getSize(); p->p_memsz = part.armExidx->getSize(); p->p_offset = first->offset + part.armExidx->outSecOff; p->p_vaddr = first->addr + part.armExidx->outSecOff; p->p_align = part.armExidx->addralign; if (part.elfHeader) p->p_offset -= part.elfHeader->getParent()->offset; if (!p->hasLMA) p->p_paddr = first->getLMA() + part.armExidx->outSecOff; return; } if (first) { p->p_filesz = last->offset - first->offset; if (last->type != SHT_NOBITS) p->p_filesz += last->size; p->p_memsz = last->addr + last->size - first->addr; p->p_offset = first->offset; p->p_vaddr = first->addr; // File offsets in partitions other than the main partition are relative // to the offset of the ELF headers. Perform that adjustment now. if (part.elfHeader) p->p_offset -= part.elfHeader->getParent()->offset; if (!p->hasLMA) p->p_paddr = first->getLMA(); } } } // A helper struct for checkSectionOverlap. namespace { struct SectionOffset { OutputSection *sec; uint64_t offset; }; } // namespace // Check whether sections overlap for a specific address range (file offsets, // load and virtual addresses). static void checkOverlap(StringRef name, std::vector §ions, bool isVirtualAddr) { llvm::sort(sections, [=](const SectionOffset &a, const SectionOffset &b) { return a.offset < b.offset; }); // Finding overlap is easy given a vector is sorted by start position. // If an element starts before the end of the previous element, they overlap. for (size_t i = 1, end = sections.size(); i < end; ++i) { SectionOffset a = sections[i - 1]; SectionOffset b = sections[i]; if (b.offset >= a.offset + a.sec->size) continue; // If both sections are in OVERLAY we allow the overlapping of virtual // addresses, because it is what OVERLAY was designed for. if (isVirtualAddr && a.sec->inOverlay && b.sec->inOverlay) continue; errorOrWarn("section " + a.sec->name + " " + name + " range overlaps with " + b.sec->name + "\n>>> " + a.sec->name + " range is " + rangeToString(a.offset, a.sec->size) + "\n>>> " + b.sec->name + " range is " + rangeToString(b.offset, b.sec->size)); } } // Check for overlapping sections and address overflows. // // In this function we check that none of the output sections have overlapping // file offsets. For SHF_ALLOC sections we also check that the load address // ranges and the virtual address ranges don't overlap template void Writer::checkSections() { // First, check that section's VAs fit in available address space for target. for (OutputSection *os : outputSections) if ((os->addr + os->size < os->addr) || (!ELFT::Is64Bits && os->addr + os->size > uint64_t(UINT32_MAX) + 1)) errorOrWarn("section " + os->name + " at 0x" + utohexstr(os->addr) + " of size 0x" + utohexstr(os->size) + " exceeds available address space"); // Check for overlapping file offsets. In this case we need to skip any // section marked as SHT_NOBITS. These sections don't actually occupy space in // the file so Sec->Offset + Sec->Size can overlap with others. If --oformat // binary is specified only add SHF_ALLOC sections are added to the output // file so we skip any non-allocated sections in that case. std::vector fileOffs; for (OutputSection *sec : outputSections) if (sec->size > 0 && sec->type != SHT_NOBITS && (!config->oFormatBinary || (sec->flags & SHF_ALLOC))) fileOffs.push_back({sec, sec->offset}); checkOverlap("file", fileOffs, false); // When linking with -r there is no need to check for overlapping virtual/load // addresses since those addresses will only be assigned when the final // executable/shared object is created. if (config->relocatable) return; // Checking for overlapping virtual and load addresses only needs to take // into account SHF_ALLOC sections since others will not be loaded. // Furthermore, we also need to skip SHF_TLS sections since these will be // mapped to other addresses at runtime and can therefore have overlapping // ranges in the file. std::vector vmas; for (OutputSection *sec : outputSections) if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS)) vmas.push_back({sec, sec->addr}); checkOverlap("virtual address", vmas, true); // Finally, check that the load addresses don't overlap. This will usually be // the same as the virtual addresses but can be different when using a linker // script with AT(). std::vector lmas; for (OutputSection *sec : outputSections) if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS)) lmas.push_back({sec, sec->getLMA()}); checkOverlap("load address", lmas, false); } // The entry point address is chosen in the following ways. // // 1. the '-e' entry command-line option; // 2. the ENTRY(symbol) command in a linker control script; // 3. the value of the symbol _start, if present; // 4. the number represented by the entry symbol, if it is a number; // 5. the address 0. static uint64_t getEntryAddr() { // Case 1, 2 or 3 if (Symbol *b = symtab.find(config->entry)) return b->getVA(); // Case 4 uint64_t addr; if (to_integer(config->entry, addr)) return addr; // Case 5 if (config->warnMissingEntry) warn("cannot find entry symbol " + config->entry + "; not setting start address"); return 0; } static uint16_t getELFType() { if (config->isPic) return ET_DYN; if (config->relocatable) return ET_REL; return ET_EXEC; } template void Writer::writeHeader() { writeEhdr(Out::bufferStart, *mainPart); writePhdrs(Out::bufferStart + sizeof(Elf_Ehdr), *mainPart); auto *eHdr = reinterpret_cast(Out::bufferStart); eHdr->e_type = getELFType(); eHdr->e_entry = getEntryAddr(); eHdr->e_shoff = sectionHeaderOff; // Write the section header table. // // The ELF header can only store numbers up to SHN_LORESERVE in the e_shnum // and e_shstrndx fields. When the value of one of these fields exceeds // SHN_LORESERVE ELF requires us to put sentinel values in the ELF header and // use fields in the section header at index 0 to store // the value. The sentinel values and fields are: // e_shnum = 0, SHdrs[0].sh_size = number of sections. // e_shstrndx = SHN_XINDEX, SHdrs[0].sh_link = .shstrtab section index. auto *sHdrs = reinterpret_cast(Out::bufferStart + eHdr->e_shoff); size_t num = outputSections.size() + 1; if (num >= SHN_LORESERVE) sHdrs->sh_size = num; else eHdr->e_shnum = num; uint32_t strTabIndex = in.shStrTab->getParent()->sectionIndex; if (strTabIndex >= SHN_LORESERVE) { sHdrs->sh_link = strTabIndex; eHdr->e_shstrndx = SHN_XINDEX; } else { eHdr->e_shstrndx = strTabIndex; } for (OutputSection *sec : outputSections) sec->writeHeaderTo(++sHdrs); } // Open a result file. template void Writer::openFile() { uint64_t maxSize = config->is64 ? INT64_MAX : UINT32_MAX; if (fileSize != size_t(fileSize) || maxSize < fileSize) { std::string msg; raw_string_ostream s(msg); s << "output file too large: " << Twine(fileSize) << " bytes\n" << "section sizes:\n"; for (OutputSection *os : outputSections) s << os->name << ' ' << os->size << "\n"; error(s.str()); return; } unlinkAsync(config->outputFile); unsigned flags = 0; if (!config->relocatable) flags |= FileOutputBuffer::F_executable; if (!config->mmapOutputFile) flags |= FileOutputBuffer::F_no_mmap; Expected> bufferOrErr = FileOutputBuffer::create(config->outputFile, fileSize, flags); if (!bufferOrErr) { error("failed to open " + config->outputFile + ": " + llvm::toString(bufferOrErr.takeError())); return; } buffer = std::move(*bufferOrErr); Out::bufferStart = buffer->getBufferStart(); } template void Writer::writeSectionsBinary() { parallel::TaskGroup tg; for (OutputSection *sec : outputSections) if (sec->flags & SHF_ALLOC) sec->writeTo(Out::bufferStart + sec->offset, tg); } static void fillTrap(uint8_t *i, uint8_t *end) { for (; i + 4 <= end; i += 4) memcpy(i, &target->trapInstr, 4); } // Fill the last page of executable segments with trap instructions // instead of leaving them as zero. Even though it is not required by any // standard, it is in general a good thing to do for security reasons. // // We'll leave other pages in segments as-is because the rest will be // overwritten by output sections. template void Writer::writeTrapInstr() { for (Partition &part : partitions) { // Fill the last page. for (PhdrEntry *p : part.phdrs) if (p->p_type == PT_LOAD && (p->p_flags & PF_X)) fillTrap(Out::bufferStart + alignDown(p->firstSec->offset + p->p_filesz, 4), Out::bufferStart + alignToPowerOf2(p->firstSec->offset + p->p_filesz, config->maxPageSize)); // Round up the file size of the last segment to the page boundary iff it is // an executable segment to ensure that other tools don't accidentally // trim the instruction padding (e.g. when stripping the file). PhdrEntry *last = nullptr; for (PhdrEntry *p : part.phdrs) if (p->p_type == PT_LOAD) last = p; if (last && (last->p_flags & PF_X)) last->p_memsz = last->p_filesz = alignToPowerOf2(last->p_filesz, config->maxPageSize); } } // Write section contents to a mmap'ed file. template void Writer::writeSections() { llvm::TimeTraceScope timeScope("Write sections"); { // In -r or --emit-relocs mode, write the relocation sections first as in // ELf_Rel targets we might find out that we need to modify the relocated // section while doing it. parallel::TaskGroup tg; for (OutputSection *sec : outputSections) if (isStaticRelSecType(sec->type)) sec->writeTo(Out::bufferStart + sec->offset, tg); } { parallel::TaskGroup tg; for (OutputSection *sec : outputSections) if (!isStaticRelSecType(sec->type)) sec->writeTo(Out::bufferStart + sec->offset, tg); } // Finally, check that all dynamic relocation addends were written correctly. if (config->checkDynamicRelocs && config->writeAddends) { for (OutputSection *sec : outputSections) if (isStaticRelSecType(sec->type)) sec->checkDynRelAddends(Out::bufferStart); } } // Computes a hash value of Data using a given hash function. // In order to utilize multiple cores, we first split data into 1MB // chunks, compute a hash for each chunk, and then compute a hash value // of the hash values. static void computeHash(llvm::MutableArrayRef hashBuf, llvm::ArrayRef data, std::function arr)> hashFn) { std::vector> chunks = split(data, 1024 * 1024); const size_t hashesSize = chunks.size() * hashBuf.size(); std::unique_ptr hashes(new uint8_t[hashesSize]); // Compute hash values. parallelFor(0, chunks.size(), [&](size_t i) { hashFn(hashes.get() + i * hashBuf.size(), chunks[i]); }); // Write to the final output buffer. hashFn(hashBuf.data(), ArrayRef(hashes.get(), hashesSize)); } template void Writer::writeBuildId() { if (!mainPart->buildId || !mainPart->buildId->getParent()) return; if (config->buildId == BuildIdKind::Hexstring) { for (Partition &part : partitions) part.buildId->writeBuildId(config->buildIdVector); return; } // Compute a hash of all sections of the output file. size_t hashSize = mainPart->buildId->hashSize; std::unique_ptr buildId(new uint8_t[hashSize]); MutableArrayRef output(buildId.get(), hashSize); llvm::ArrayRef input{Out::bufferStart, size_t(fileSize)}; // Fedora introduced build ID as "approximation of true uniqueness across all // binaries that might be used by overlapping sets of people". It does not // need some security goals that some hash algorithms strive to provide, e.g. // (second-)preimage and collision resistance. In practice people use 'md5' // and 'sha1' just for different lengths. Implement them with the more // efficient BLAKE3. switch (config->buildId) { case BuildIdKind::Fast: computeHash(output, input, [](uint8_t *dest, ArrayRef arr) { write64le(dest, xxh3_64bits(arr)); }); break; case BuildIdKind::Md5: computeHash(output, input, [&](uint8_t *dest, ArrayRef arr) { memcpy(dest, BLAKE3::hash<16>(arr).data(), hashSize); }); break; case BuildIdKind::Sha1: computeHash(output, input, [&](uint8_t *dest, ArrayRef arr) { memcpy(dest, BLAKE3::hash<20>(arr).data(), hashSize); }); break; case BuildIdKind::Uuid: if (auto ec = llvm::getRandomBytes(buildId.get(), hashSize)) error("entropy source failure: " + ec.message()); break; default: llvm_unreachable("unknown BuildIdKind"); } for (Partition &part : partitions) part.buildId->writeBuildId(output); } template void elf::writeResult(); template void elf::writeResult(); template void elf::writeResult(); template void elf::writeResult();