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Diffstat (limited to 'contrib/llvm-project/lld/ELF/Writer.cpp')
| -rw-r--r-- | contrib/llvm-project/lld/ELF/Writer.cpp | 2691 |
1 files changed, 2691 insertions, 0 deletions
diff --git a/contrib/llvm-project/lld/ELF/Writer.cpp b/contrib/llvm-project/lld/ELF/Writer.cpp new file mode 100644 index 000000000000..10b171e8c0d7 --- /dev/null +++ b/contrib/llvm-project/lld/ELF/Writer.cpp @@ -0,0 +1,2691 @@ +//===- 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 "CallGraphSort.h" +#include "Config.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/Filesystem.h" +#include "lld/Common/Memory.h" +#include "lld/Common/Strings.h" +#include "lld/Common/Threads.h" +#include "llvm/ADT/StringMap.h" +#include "llvm/ADT/StringSwitch.h" +#include "llvm/Support/RandomNumberGenerator.h" +#include "llvm/Support/SHA1.h" +#include "llvm/Support/xxhash.h" +#include <climits> + +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 ELFT> class Writer { +public: + Writer() : buffer(errorHandler().outputBuffer) {} + using Elf_Shdr = typename ELFT::Shdr; + using Elf_Ehdr = typename ELFT::Ehdr; + using Elf_Phdr = typename ELFT::Phdr; + + void run(); + +private: + void copyLocalSymbols(); + void addSectionSymbols(); + void forEachRelSec(llvm::function_ref<void(InputSectionBase &)> fn); + void sortSections(); + void resolveShfLinkOrder(); + void finalizeAddressDependentContent(); + void sortInputSections(); + void finalizeSections(); + void checkExecuteOnly(); + void setReservedSymbolSections(); + + std::vector<PhdrEntry *> createPhdrs(Partition &part); + void removeEmptyPTLoad(std::vector<PhdrEntry *> &phdrEntry); + 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<FileOutputBuffer> &buffer; + + void addRelIpltSymbols(); + void addStartEndSymbols(); + void addStartStopSymbols(OutputSection *sec); + + uint64_t fileSize; + uint64_t sectionHeaderOff; +}; +} // anonymous namespace + +static bool isSectionPrefix(StringRef prefix, StringRef name) { + return name.startswith(prefix) || name == prefix.drop_back(); +} + +StringRef elf::getOutputSectionName(const InputSectionBase *s) { + if (config->relocatable) + return s->name; + + // This is for --emit-relocs. If .text.foo is emitted as .text.bar, we want + // to emit .rela.text.foo as .rela.text.bar for consistency (this is not + // technically required, but not doing it is odd). This code guarantees that. + if (auto *isec = dyn_cast<InputSection>(s)) { + if (InputSectionBase *rel = isec->getRelocatedSection()) { + OutputSection *out = rel->getOutputSection(); + if (s->type == SHT_RELA) + return saver.save(".rela" + out->name); + return saver.save(".rel" + out->name); + } + } + + // This check is for -z keep-text-section-prefix. This option separates text + // sections with prefix ".text.hot", ".text.unlikely", ".text.startup" or + // ".text.exit". + // When enabled, this allows identifying the hot code region (.text.hot) in + // the final binary which can be selectively mapped to huge pages or mlocked, + // for instance. + if (config->zKeepTextSectionPrefix) + for (StringRef v : + {".text.hot.", ".text.unlikely.", ".text.startup.", ".text.exit."}) + if (isSectionPrefix(v, s->name)) + return v.drop_back(); + + for (StringRef v : + {".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.rel.ro.", + ".bss.", ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.", + ".gcc_except_table.", ".tdata.", ".ARM.exidx.", ".ARM.extab."}) + if (isSectionPrefix(v, s->name)) + return v.drop_back(); + + // CommonSection is identified as "COMMON" in linker scripts. + // By default, it should go to .bss section. + if (s->name == "COMMON") + return ".bss"; + + return s->name; +} + +static bool needsInterpSection() { + return !sharedFiles.empty() && !config->dynamicLinker.empty() && + script->needsInterpSection(); +} + +template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); } + +template <class ELFT> +void Writer<ELFT>::removeEmptyPTLoad(std::vector<PhdrEntry *> &phdrs) { + llvm::erase_if(phdrs, [&](const PhdrEntry *p) { + if (p->p_type != PT_LOAD) + return false; + if (!p->firstSec) + return true; + uint64_t size = p->lastSec->addr + p->lastSec->size - p->firstSec->addr; + return size == 0; + }); +} + +template <class ELFT> static void copySectionsIntoPartitions() { + std::vector<InputSectionBase *> newSections; + for (unsigned part = 2; part != partitions.size() + 1; ++part) { + for (InputSectionBase *s : inputSections) { + if (!(s->flags & SHF_ALLOC) || !s->isLive()) + continue; + InputSectionBase *copy; + if (s->type == SHT_NOTE) + copy = make<InputSection>(cast<InputSection>(*s)); + else if (auto *es = dyn_cast<EhInputSection>(s)) + copy = make<EhInputSection>(*es); + else + continue; + copy->partition = part; + newSections.push_back(copy); + } + } + + inputSections.insert(inputSections.end(), newSections.begin(), + newSections.end()); +} + +template <class ELFT> static void combineEhSections() { + for (InputSectionBase *&s : inputSections) { + // Ignore dead sections and the partition end marker (.part.end), + // whose partition number is out of bounds. + if (!s->isLive() || s->partition == 255) + continue; + + Partition &part = s->getPartition(); + if (auto *es = dyn_cast<EhInputSection>(s)) { + part.ehFrame->addSection<ELFT>(es); + s = nullptr; + } else if (s->kind() == SectionBase::Regular && part.armExidx && + part.armExidx->addSection(cast<InputSection>(s))) { + s = nullptr; + } + } + + std::vector<InputSectionBase *> &v = inputSections; + v.erase(std::remove(v.begin(), v.end(), nullptr), v.end()); +} + +static Defined *addOptionalRegular(StringRef name, SectionBase *sec, + uint64_t val, uint8_t stOther = STV_HIDDEN, + uint8_t binding = STB_GLOBAL) { + Symbol *s = symtab->find(name); + if (!s || s->isDefined()) + return nullptr; + + s->resolve(Defined{/*file=*/nullptr, name, binding, stOther, STT_NOTYPE, val, + /*size=*/0, sec}); + return cast<Defined>(s); +} + +static Defined *addAbsolute(StringRef name) { + Symbol *sym = symtab->addSymbol(Defined{nullptr, name, STB_GLOBAL, STV_HIDDEN, + STT_NOTYPE, 0, 0, nullptr}); + return cast<Defined>(sym); +} + +// 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) { + // 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); + } + + // 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{/*file=*/nullptr, gotSymName, STB_GLOBAL, STV_HIDDEN, + STT_NOTYPE, gotOff, /*size=*/0, Out::elfHeader}); + ElfSym::globalOffsetTable = cast<Defined>(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 standart 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 OutputSection *findSection(StringRef name, unsigned partition = 1) { + for (BaseCommand *base : script->sectionCommands) + if (auto *sec = dyn_cast<OutputSection>(base)) + if (sec->name == name && sec->partition == partition) + return sec; + return nullptr; +} + +// Initialize Out members. +template <class ELFT> static void createSyntheticSections() { + // Initialize all pointers with NULL. This is needed because + // you can call lld::elf::main more than once as a library. + memset(&Out::first, 0, sizeof(Out)); + + auto add = [](InputSectionBase *sec) { inputSections.push_back(sec); }; + + in.shStrTab = make<StringTableSection>(".shstrtab", false); + + Out::programHeaders = make<OutputSection>("", 0, SHF_ALLOC); + Out::programHeaders->alignment = config->wordsize; + + if (config->strip != StripPolicy::All) { + in.strTab = make<StringTableSection>(".strtab", false); + in.symTab = make<SymbolTableSection<ELFT>>(*in.strTab); + in.symTabShndx = make<SymtabShndxSection>(); + } + + in.bss = make<BssSection>(".bss", 0, 1); + add(in.bss); + + // If there is a SECTIONS command and a .data.rel.ro section name use name + // .data.rel.ro.bss so that we match in the .data.rel.ro output section. + // This makes sure our relro is contiguous. + bool hasDataRelRo = + script->hasSectionsCommand && findSection(".data.rel.ro", 0); + in.bssRelRo = + make<BssSection>(hasDataRelRo ? ".data.rel.ro.bss" : ".bss.rel.ro", 0, 1); + add(in.bssRelRo); + + // Add MIPS-specific sections. + if (config->emachine == EM_MIPS) { + if (!config->shared && config->hasDynSymTab) { + in.mipsRldMap = make<MipsRldMapSection>(); + add(in.mipsRldMap); + } + if (auto *sec = MipsAbiFlagsSection<ELFT>::create()) + add(sec); + if (auto *sec = MipsOptionsSection<ELFT>::create()) + add(sec); + if (auto *sec = MipsReginfoSection<ELFT>::create()) + add(sec); + } + + for (Partition &part : partitions) { + auto add = [&](InputSectionBase *sec) { + sec->partition = part.getNumber(); + inputSections.push_back(sec); + }; + + if (!part.name.empty()) { + part.elfHeader = make<PartitionElfHeaderSection<ELFT>>(); + part.elfHeader->name = part.name; + add(part.elfHeader); + + part.programHeaders = make<PartitionProgramHeadersSection<ELFT>>(); + add(part.programHeaders); + } + + if (config->buildId != BuildIdKind::None) { + part.buildId = make<BuildIdSection>(); + add(part.buildId); + } + + part.dynStrTab = make<StringTableSection>(".dynstr", true); + part.dynSymTab = make<SymbolTableSection<ELFT>>(*part.dynStrTab); + part.dynamic = make<DynamicSection<ELFT>>(); + if (config->androidPackDynRelocs) { + part.relaDyn = make<AndroidPackedRelocationSection<ELFT>>( + config->isRela ? ".rela.dyn" : ".rel.dyn"); + } else { + part.relaDyn = make<RelocationSection<ELFT>>( + config->isRela ? ".rela.dyn" : ".rel.dyn", config->zCombreloc); + } + + if (needsInterpSection()) + add(createInterpSection()); + + if (config->hasDynSymTab) { + part.dynSymTab = make<SymbolTableSection<ELFT>>(*part.dynStrTab); + add(part.dynSymTab); + + part.verSym = make<VersionTableSection>(); + add(part.verSym); + + if (!config->versionDefinitions.empty()) { + part.verDef = make<VersionDefinitionSection>(); + add(part.verDef); + } + + part.verNeed = make<VersionNeedSection<ELFT>>(); + add(part.verNeed); + + if (config->gnuHash) { + part.gnuHashTab = make<GnuHashTableSection>(); + add(part.gnuHashTab); + } + + if (config->sysvHash) { + part.hashTab = make<HashTableSection>(); + add(part.hashTab); + } + + add(part.dynamic); + add(part.dynStrTab); + add(part.relaDyn); + } + + if (config->relrPackDynRelocs) { + part.relrDyn = make<RelrSection<ELFT>>(); + add(part.relrDyn); + } + + if (!config->relocatable) { + if (config->ehFrameHdr) { + part.ehFrameHdr = make<EhFrameHeader>(); + add(part.ehFrameHdr); + } + part.ehFrame = make<EhFrameSection>(); + add(part.ehFrame); + } + + if (config->emachine == EM_ARM && !config->relocatable) { + // The ARMExidxsyntheticsection replaces all the individual .ARM.exidx + // InputSections. + part.armExidx = make<ARMExidxSyntheticSection>(); + add(part.armExidx); + } + } + + if (partitions.size() != 1) { + // Create the partition end marker. This needs to be in partition number 255 + // so that it is sorted after all other partitions. It also has other + // special handling (see createPhdrs() and combineEhSections()). + in.partEnd = make<BssSection>(".part.end", config->maxPageSize, 1); + in.partEnd->partition = 255; + add(in.partEnd); + + in.partIndex = make<PartitionIndexSection>(); + addOptionalRegular("__part_index_begin", in.partIndex, 0); + addOptionalRegular("__part_index_end", in.partIndex, + in.partIndex->getSize()); + add(in.partIndex); + } + + // Add .got. MIPS' .got is so different from the other archs, + // it has its own class. + if (config->emachine == EM_MIPS) { + in.mipsGot = make<MipsGotSection>(); + add(in.mipsGot); + } else { + in.got = make<GotSection>(); + add(in.got); + } + + if (config->emachine == EM_PPC) { + in.ppc32Got2 = make<PPC32Got2Section>(); + add(in.ppc32Got2); + } + + if (config->emachine == EM_PPC64) { + in.ppc64LongBranchTarget = make<PPC64LongBranchTargetSection>(); + add(in.ppc64LongBranchTarget); + } + + if (config->emachine == EM_RISCV) { + in.riscvSdata = make<RISCVSdataSection>(); + add(in.riscvSdata); + } + + in.gotPlt = make<GotPltSection>(); + add(in.gotPlt); + in.igotPlt = make<IgotPltSection>(); + add(in.igotPlt); + + // _GLOBAL_OFFSET_TABLE_ is defined relative to either .got.plt or .got. Treat + // it as a relocation and ensure the referenced section is created. + if (ElfSym::globalOffsetTable && config->emachine != EM_MIPS) { + if (target->gotBaseSymInGotPlt) + in.gotPlt->hasGotPltOffRel = true; + else + in.got->hasGotOffRel = true; + } + + if (config->gdbIndex) + add(GdbIndexSection::create<ELFT>()); + + // We always need to add rel[a].plt to output if it has entries. + // Even for static linking it can contain R_[*]_IRELATIVE relocations. + in.relaPlt = make<RelocationSection<ELFT>>( + config->isRela ? ".rela.plt" : ".rel.plt", /*sort=*/false); + add(in.relaPlt); + + // The relaIplt immediately follows .rel.plt (.rel.dyn for ARM) to ensure + // that the IRelative relocations are processed last by the dynamic loader. + // We cannot place the iplt section in .rel.dyn when Android relocation + // packing is enabled because that would cause a section type mismatch. + // However, because the Android dynamic loader reads .rel.plt after .rel.dyn, + // we can get the desired behaviour by placing the iplt section in .rel.plt. + in.relaIplt = make<RelocationSection<ELFT>>( + (config->emachine == EM_ARM && !config->androidPackDynRelocs) + ? ".rel.dyn" + : in.relaPlt->name, + /*sort=*/false); + add(in.relaIplt); + + in.plt = make<PltSection>(false); + add(in.plt); + in.iplt = make<PltSection>(true); + add(in.iplt); + + if (config->andFeatures) + add(make<GnuPropertySection>()); + + // .note.GNU-stack is always added when we are creating a re-linkable + // object file. Other linkers are using the presence of this marker + // section to control the executable-ness of the stack area, but that + // is irrelevant these days. Stack area should always be non-executable + // by default. So we emit this section unconditionally. + if (config->relocatable) + add(make<GnuStackSection>()); + + if (in.symTab) + add(in.symTab); + if (in.symTabShndx) + add(in.symTabShndx); + add(in.shStrTab); + if (in.strTab) + add(in.strTab); +} + +// The main function of the writer. +template <class ELFT> void Writer<ELFT>::run() { + // Make copies of any input sections that need to be copied into each + // partition. + copySectionsIntoPartitions<ELFT>(); + + // Create linker-synthesized sections such as .got or .plt. + // Such sections are of type input section. + createSyntheticSections<ELFT>(); + + // Some input sections that are used for exception handling need to be moved + // into synthetic sections. Do that now so that they aren't assigned to + // output sections in the usual way. + if (!config->relocatable) + combineEhSections<ELFT>(); + + // We want to process linker script commands. When SECTIONS command + // is given we let it create sections. + script->processSectionCommands(); + + // Linker scripts controls how input sections are assigned to output sections. + // Input sections that were not handled by scripts are called "orphans", and + // they are assigned to output sections by the default rule. Process that. + script->addOrphanSections(); + + if (config->discard != DiscardPolicy::All) + copyLocalSymbols(); + + if (config->copyRelocs) + addSectionSymbols(); + + // 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 (errorCount()) + return; + + script->assignAddresses(); + + // If -compressed-debug-sections is specified, we need to compress + // .debug_* sections. Do it right now because it changes the size of + // output sections. + for (OutputSection *sec : outputSections) + sec->maybeCompress<ELFT>(); + + 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); + + if (config->relocatable) + for (OutputSection *sec : outputSections) + sec->addr = 0; + + if (config->checkSections) + checkSections(); + + // It does not make sense try to open the file if we have error already. + if (errorCount()) + return; + // Write the result down to a file. + openFile(); + if (errorCount()) + return; + + if (!config->oFormatBinary) { + 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; + + // Handle -Map and -cref options. + writeMapFile(); + writeCrossReferenceTable(); + if (errorCount()) + return; + + if (auto e = buffer->commit()) + error("failed to write to the output file: " + toString(std::move(e))); +} + +static bool shouldKeepInSymtab(const Defined &sym) { + if (sym.isSection()) + return false; + + if (config->discard == DiscardPolicy::None) + return true; + + // If -emit-reloc is given, all symbols including local ones need to be + // copied because they may be referenced by relocations. + if (config->emitRelocs) + return true; + + // 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. + StringRef name = sym.getName(); + bool isLocal = name.startswith(".L") || name.empty(); + if (!isLocal) + return true; + + if (config->discard == DiscardPolicy::Locals) + return false; + + SectionBase *sec = sym.section; + return !sec || !(sec->flags & SHF_MERGE); +} + +static bool includeInSymtab(const Symbol &b) { + if (!b.isLocal() && !b.isUsedInRegularObj) + return false; + + if (auto *d = dyn_cast<Defined>(&b)) { + // Always include absolute symbols. + SectionBase *sec = d->section; + if (!sec) + return true; + sec = sec->repl; + + // Exclude symbols pointing to garbage-collected sections. + if (isa<InputSectionBase>(sec) && !sec->isLive()) + return false; + + if (auto *s = dyn_cast<MergeInputSection>(sec)) + if (!s->getSectionPiece(d->value)->live) + return false; + return true; + } + return b.used; +} + +// Local symbols are not in the linker's symbol table. This function scans +// each object file's symbol table to copy local symbols to the output. +template <class ELFT> void Writer<ELFT>::copyLocalSymbols() { + if (!in.symTab) + return; + for (InputFile *file : objectFiles) { + ObjFile<ELFT> *f = cast<ObjFile<ELFT>>(file); + for (Symbol *b : f->getLocalSymbols()) { + if (!b->isLocal()) + fatal(toString(f) + + ": broken object: getLocalSymbols returns a non-local symbol"); + auto *dr = dyn_cast<Defined>(b); + + // No reason to keep local undefined symbol in symtab. + if (!dr) + continue; + if (!includeInSymtab(*b)) + continue; + if (!shouldKeepInSymtab(*dr)) + continue; + 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 <class ELFT> void Writer<ELFT>::addSectionSymbols() { + for (BaseCommand *base : script->sectionCommands) { + auto *sec = dyn_cast<OutputSection>(base); + if (!sec) + continue; + auto i = llvm::find_if(sec->sectionCommands, [](BaseCommand *base) { + if (auto *isd = dyn_cast<InputSectionDescription>(base)) + return !isd->sections.empty(); + return false; + }); + if (i == sec->sectionCommands.end()) + continue; + InputSection *isec = cast<InputSectionDescription>(*i)->sections[0]; + + // Relocations are not using REL[A] section symbols. + if (isec->type == SHT_REL || isec->type == SHT_RELA) + 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 are given. + if (isa<SyntheticSection>(isec) && !(isec->flags & SHF_MERGE)) + continue; + + auto *sym = + make<Defined>(isec->file, "", STB_LOCAL, /*stOther=*/0, STT_SECTION, + /*value=*/0, /*size=*/0, isec); + in.symTab->addSymbol(sym); + } +} + +// 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; + + 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.equals(".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; + + // .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; + return s == ".data.rel.ro" || s == ".bss.rel.ro" || s == ".ctors" || + s == ".dtors" || s == ".jcr" || s == ".eh_frame" || + s == ".openbsd.randomdata"; +} + +// 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_NOT_PART_EHDR = 1 << 17, + RF_NOT_PART_PHDR = 1 << 16, + RF_NOT_INTERP = 1 << 15, + RF_NOT_NOTE = 1 << 14, + RF_WRITE = 1 << 13, + RF_EXEC_WRITE = 1 << 12, + RF_EXEC = 1 << 11, + RF_RODATA = 1 << 10, + RF_NOT_RELRO = 1 << 9, + RF_NOT_TLS = 1 << 8, + RF_BSS = 1 << 7, + RF_PPC_NOT_TOCBSS = 1 << 6, + RF_PPC_TOCL = 1 << 5, + RF_PPC_TOC = 1 << 4, + RF_PPC_GOT = 1 << 3, + RF_PPC_BRANCH_LT = 1 << 2, + RF_MIPS_GPREL = 1 << 1, + RF_MIPS_NOT_GOT = 1 << 0 +}; + +static unsigned getSectionRank(const OutputSection *sec) { + unsigned rank = sec->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(sec->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 (!(sec->flags & SHF_ALLOC)) + return rank | RF_NOT_ALLOC; + + if (sec->type == SHT_LLVM_PART_EHDR) + return rank; + rank |= RF_NOT_PART_EHDR; + + if (sec->type == SHT_LLVM_PART_PHDR) + return rank; + rank |= RF_NOT_PART_PHDR; + + // Put .interp first because some loaders want to see that section + // on the first page of the executable file when loaded into memory. + if (sec->name == ".interp") + return rank; + rank |= RF_NOT_INTERP; + + // Put .note sections (which make up one PT_NOTE) at the beginning so that + // they are likely to be included in a core file even if core file size is + // limited. In particular, we want a .note.gnu.build-id and a .note.tag to be + // included in a core to match core files with executables. + if (sec->type == SHT_NOTE) + return rank; + rank |= RF_NOT_NOTE; + + // Sort sections based on their access permission in the following + // order: R, RX, RWX, RW. This order is based on the following + // considerations: + // * Read-only sections come first such that they go in the + // PT_LOAD covering the program headers at the start of the file. + // * Read-only, executable sections come next. + // * Writable, executable sections follow such that .plt on + // architectures where it needs to be writable will be placed + // between .text and .data. + // * Writable sections come last, such that .bss lands at the very + // end of the last PT_LOAD. + bool isExec = sec->flags & SHF_EXECINSTR; + bool isWrite = sec->flags & SHF_WRITE; + + if (isExec) { + if (isWrite) + rank |= RF_EXEC_WRITE; + else + rank |= RF_EXEC; + } else if (isWrite) { + rank |= RF_WRITE; + } else if (sec->type == SHT_PROGBITS) { + // Make non-executable and non-writable PROGBITS sections (e.g .rodata + // .eh_frame) closer to .text. They likely contain PC or GOT relative + // relocations and there could be relocation overflow if other huge sections + // (.dynstr .dynsym) were placed in between. + rank |= RF_RODATA; + } + + // Place RelRo sections first. After considering SHT_NOBITS below, the + // ordering 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. + if (!isRelroSection(sec)) + rank |= RF_NOT_RELRO; + + // If we got here we know that both A and B are in the same PT_LOAD. + + // The TLS initialization block needs to be a single contiguous block in a R/W + // PT_LOAD, so stick TLS sections directly before the other RelRo R/W + // sections. Since p_filesz can be less than p_memsz, place NOBITS sections + // after PROGBITS. + if (!(sec->flags & SHF_TLS)) + rank |= RF_NOT_TLS; + + // Within TLS sections, or within other RelRo sections, or within non-RelRo + // sections, place non-NOBITS sections first. + if (sec->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. Conversely, the special .tocbss section should be first among + // all SHT_NOBITS sections. This will put it next to the loaded special + // PPC64 sections (and, thus, within reach of the TOC base pointer). + StringRef name = sec->name; + if (name != ".tocbss") + rank |= RF_PPC_NOT_TOCBSS; + + if (name == ".toc1") + rank |= RF_PPC_TOCL; + + if (name == ".toc") + rank |= RF_PPC_TOC; + + if (name == ".got") + rank |= RF_PPC_GOT; + + if (name == ".branch_lt") + rank |= RF_PPC_BRANCH_LT; + } + + if (config->emachine == EM_MIPS) { + // All sections with SHF_MIPS_GPREL flag should be grouped together + // because data in these sections is addressable with a gp relative address. + if (sec->flags & SHF_MIPS_GPREL) + rank |= RF_MIPS_GPREL; + + if (sec->name != ".got") + rank |= RF_MIPS_NOT_GOT; + } + + return rank; +} + +static bool compareSections(const BaseCommand *aCmd, const BaseCommand *bCmd) { + const OutputSection *a = cast<OutputSection>(aCmd); + const OutputSection *b = cast<OutputSection>(bCmd); + + 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->alignment); + if (p_type == PT_LOAD) + sec->ptLoad = this; +} + +// The beginning and the ending of .rel[a].plt section are marked +// with __rel[a]_iplt_{start,end} symbols if it is a statically linked +// executable. The runtime needs these symbols in order to resolve +// all IRELATIVE relocs on startup. For dynamic executables, we don't +// need these symbols, since IRELATIVE relocs are resolved through GOT +// and PLT. For details, see http://www.airs.com/blog/archives/403. +template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() { + if (config->relocatable || needsInterpSection()) + return; + + // By default, __rela_iplt_{start,end} belong to a dummy section 0 + // because .rela.plt might be empty and thus removed from output. + // We'll override Out::elfHeader with In.relaIplt later when we are + // sure that .rela.plt exists in output. + ElfSym::relaIpltStart = addOptionalRegular( + config->isRela ? "__rela_iplt_start" : "__rel_iplt_start", + Out::elfHeader, 0, STV_HIDDEN, STB_WEAK); + + ElfSym::relaIpltEnd = addOptionalRegular( + config->isRela ? "__rela_iplt_end" : "__rel_iplt_end", + Out::elfHeader, 0, STV_HIDDEN, STB_WEAK); +} + +template <class ELFT> +void Writer<ELFT>::forEachRelSec( + llvm::function_ref<void(InputSectionBase &)> fn) { + // Scan all relocations. Each relocation goes through a series + // of tests to determine if it needs special treatment, such as + // creating GOT, PLT, copy relocations, etc. + // Note that relocations for non-alloc sections are directly + // processed by InputSection::relocateNonAlloc. + for (InputSectionBase *isec : inputSections) + if (isec->isLive() && isa<InputSection>(isec) && (isec->flags & SHF_ALLOC)) + fn(*isec); + for (Partition &part : partitions) { + for (EhInputSection *es : part.ehFrame->sections) + fn(*es); + if (part.armExidx && part.armExidx->isLive()) + for (InputSection *ex : part.armExidx->exidxSections) + fn(*ex); + } +} + +// 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 <class ELFT> void Writer<ELFT>::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 *gotSection = in.gotPlt; + if (!target->gotBaseSymInGotPlt) + gotSection = in.mipsGot ? cast<InputSection>(in.mipsGot) + : cast<InputSection>(in.got); + ElfSym::globalOffsetTable->section = gotSection; + } + + // .rela_iplt_{start,end} mark the start and the end of .rela.plt section. + if (ElfSym::relaIpltStart && in.relaIplt->isNeeded()) { + ElfSym::relaIpltStart->section = in.relaIplt; + ElfSym::relaIpltEnd->section = in.relaIplt; + ElfSym::relaIpltEnd->value = in.relaIplt->getSize(); + } + + PhdrEntry *last = nullptr; + PhdrEntry *lastRO = nullptr; + + for (Partition &part : partitions) { + for (PhdrEntry *p : part.phdrs) { + if (p->p_type != PT_LOAD) + continue; + last = p; + if (!(p->p_flags & PF_W)) + lastRO = p; + } + } + + if (lastRO) { + // _etext is the first location after the last read-only loadable segment. + if (ElfSym::etext1) + ElfSym::etext1->section = lastRO->lastSec; + if (ElfSym::etext2) + ElfSym::etext2->section = lastRO->lastSec; + } + + if (last) { + // _edata points to the end of the last mapped initialized section. + OutputSection *edata = nullptr; + for (OutputSection *os : outputSections) { + if (os->type != SHT_NOBITS) + 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) + ElfSym::bss->section = 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 getRankProximityAux(OutputSection *a, OutputSection *b) { + return countLeadingZeros(a->sortRank ^ b->sortRank); +} + +static int getRankProximity(OutputSection *a, BaseCommand *b) { + auto *sec = dyn_cast<OutputSection>(b); + return (sec && sec->hasInputSections) ? getRankProximityAux(a, sec) : -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(BaseCommand *cmd) { + if (auto *assign = dyn_cast<SymbolAssignment>(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 std::vector<BaseCommand *>::iterator +findOrphanPos(std::vector<BaseCommand *>::iterator b, + std::vector<BaseCommand *>::iterator e) { + OutputSection *sec = cast<OutputSection>(*e); + + // Find the first element that has as close a rank as possible. + auto i = std::max_element(b, e, [=](BaseCommand *a, BaseCommand *b) { + return getRankProximity(sec, a) < getRankProximity(sec, b); + }); + if (i == e) + return e; + + // Consider all existing sections with the same proximity. + int proximity = getRankProximity(sec, *i); + for (; i != e; ++i) { + auto *curSec = dyn_cast<OutputSection>(*i); + if (!curSec || !curSec->hasInputSections) + continue; + if (getRankProximity(sec, curSec) != proximity || + sec->sortRank < curSec->sortRank) + break; + } + + auto isOutputSecWithInputSections = [](BaseCommand *cmd) { + auto *os = dyn_cast<OutputSection>(cmd); + return os && os->hasInputSections; + }; + auto j = std::find_if(llvm::make_reverse_iterator(i), + llvm::make_reverse_iterator(b), + isOutputSecWithInputSections); + i = j.base(); + + // 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). + auto nextSec = std::find_if(i, e, isOutputSecWithInputSections); + if (nextSec == e) + return e; + + while (i != e && shouldSkip(*i)) + ++i; + return i; +} + +// Builds section order for handling --symbol-ordering-file. +static DenseMap<const InputSectionBase *, int> buildSectionOrder() { + DenseMap<const InputSectionBase *, int> 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<StringRef, SymbolOrderEntry> symbolOrder; + int priority = -config->symbolOrderingFile.size(); + for (StringRef s : config->symbolOrderingFile) + symbolOrder.insert({s, {priority++, false}}); + + // Build a map from sections to their priorities. + auto addSym = [&](Symbol &sym) { + auto it = symbolOrder.find(sym.getName()); + if (it == symbolOrder.end()) + return; + SymbolOrderEntry &ent = it->second; + ent.present = true; + + maybeWarnUnorderableSymbol(&sym); + + if (auto *d = dyn_cast<Defined>(&sym)) { + if (auto *sec = dyn_cast_or_null<InputSectionBase>(d->section)) { + int &priority = sectionOrder[cast<InputSectionBase>(sec->repl)]; + 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. + symtab->forEachSymbol([&](Symbol *sym) { + if (!sym->isLazy()) + addSym(*sym); + }); + + for (InputFile *file : objectFiles) + for (Symbol *sym : file->getSymbols()) + if (sym->isLocal()) + addSym(*sym); + + if (config->warnSymbolOrdering) + for (auto orderEntry : symbolOrder) + if (!orderEntry.second.present) + warn("symbol ordering file: no such symbol: " + orderEntry.first); + + return sectionOrder; +} + +// Sorts the sections in ISD according to the provided section order. +static void +sortISDBySectionOrder(InputSectionDescription *isd, + const DenseMap<const InputSectionBase *, int> &order) { + std::vector<InputSection *> unorderedSections; + std::vector<std::pair<InputSection *, int>> orderedSections; + uint64_t unorderedSize = 0; + + for (InputSection *isec : isd->sections) { + 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, [&](std::pair<InputSection *, int> a, + std::pair<InputSection *, int> b) { + return a.second < b.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. + size_t insPt = 0; + if (target->getThunkSectionSpacing() && !orderedSections.empty()) { + uint64_t unorderedPos = 0; + for (; insPt != unorderedSections.size(); ++insPt) { + unorderedPos += unorderedSections[insPt]->getSize(); + if (unorderedPos > unorderedSize / 2) + break; + } + } + + isd->sections.clear(); + for (InputSection *isec : makeArrayRef(unorderedSections).slice(0, insPt)) + isd->sections.push_back(isec); + for (std::pair<InputSection *, int> p : orderedSections) + isd->sections.push_back(p.first); + for (InputSection *isec : makeArrayRef(unorderedSections).slice(insPt)) + isd->sections.push_back(isec); +} + +static void sortSection(OutputSection *sec, + const DenseMap<const InputSectionBase *, int> &order) { + StringRef name = sec->name; + + // Sort input sections by section name suffixes for + // __attribute__((init_priority(N))). + if (name == ".init_array" || name == ".fini_array") { + if (!script->hasSectionsCommand) + sec->sortInitFini(); + return; + } + + // Sort input sections by the special rule for .ctors and .dtors. + if (name == ".ctors" || name == ".dtors") { + if (!script->hasSectionsCommand) + sec->sortCtorsDtors(); + return; + } + + // Never sort these. + if (name == ".init" || name == ".fini") + return; + + // .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 + if (config->emachine == EM_PPC64 && name == ".toc") { + if (script->hasSectionsCommand) + return; + assert(sec->sectionCommands.size() == 1); + auto *isd = cast<InputSectionDescription>(sec->sectionCommands[0]); + llvm::stable_sort(isd->sections, + [](const InputSection *a, const InputSection *b) -> bool { + return a->file->ppc64SmallCodeModelTocRelocs && + !b->file->ppc64SmallCodeModelTocRelocs; + }); + return; + } + + // Sort input sections by priority using the list provided + // by --symbol-ordering-file. + if (!order.empty()) + for (BaseCommand *b : sec->sectionCommands) + if (auto *isd = dyn_cast<InputSectionDescription>(b)) + sortISDBySectionOrder(isd, order); +} + +// 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 <class ELFT> void Writer<ELFT>::sortInputSections() { + // Build the order once since it is expensive. + DenseMap<const InputSectionBase *, int> order = buildSectionOrder(); + for (BaseCommand *base : script->sectionCommands) + if (auto *sec = dyn_cast<OutputSection>(base)) + sortSection(sec, order); +} + +template <class ELFT> void Writer<ELFT>::sortSections() { + script->adjustSectionsBeforeSorting(); + + // 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) + return; + + sortInputSections(); + + for (BaseCommand *base : script->sectionCommands) { + auto *os = dyn_cast<OutputSection>(base); + if (!os) + continue; + os->sortRank = getSectionRank(os); + + // We want to assign rude approximation values to outSecOff fields + // to know the relative order of the input sections. We use it for + // sorting SHF_LINK_ORDER sections. See resolveShfLinkOrder(). + uint64_t i = 0; + for (InputSection *sec : getInputSections(os)) + sec->outSecOff = i++; + } + + if (!script->hasSectionsCommand) { + // We know that all the OutputSections are contiguous in this case. + auto isSection = [](BaseCommand *base) { return isa<OutputSection>(base); }; + std::stable_sort( + llvm::find_if(script->sectionCommands, isSection), + llvm::find_if(llvm::reverse(script->sectionCommands), isSection).base(), + compareSections); + return; + } + + // 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, [](BaseCommand *base) { + if (auto *sec = dyn_cast<OutputSection>(base)) + return sec->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, [](BaseCommand *cmd) { return !shouldSkip(cmd); }); + if (firstSectionOrDotAssignment != e && + isa<SymbolAssignment>(**firstSectionOrDotAssignment)) + ++firstSectionOrDotAssignment; + i = firstSectionOrDotAssignment; + + while (nonScriptI != e) { + auto pos = findOrphanPos(i, nonScriptI); + OutputSection *orphan = cast<OutputSection>(*nonScriptI); + + // 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, [=](BaseCommand *cmd) { + return cast<OutputSection>(cmd)->sortRank != rank; + }); + std::rotate(pos, nonScriptI, end); + nonScriptI = end; + } + + script->adjustSectionsAfterSorting(); +} + +static bool compareByFilePosition(InputSection *a, InputSection *b) { + InputSection *la = a->getLinkOrderDep(); + InputSection *lb = b->getLinkOrderDep(); + OutputSection *aOut = la->getParent(); + OutputSection *bOut = lb->getParent(); + + if (aOut != bOut) + return aOut->sectionIndex < bOut->sectionIndex; + return la->outSecOff < lb->outSecOff; +} + +template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() { + for (OutputSection *sec : outputSections) { + if (!(sec->flags & SHF_LINK_ORDER)) + continue; + + // Link order may be distributed across several InputSectionDescriptions + // but sort must consider them all at once. + std::vector<InputSection **> scriptSections; + std::vector<InputSection *> sections; + for (BaseCommand *base : sec->sectionCommands) { + if (auto *isd = dyn_cast<InputSectionDescription>(base)) { + for (InputSection *&isec : isd->sections) { + scriptSections.push_back(&isec); + sections.push_back(isec); + } + } + } + + // 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; + + llvm::stable_sort(sections, compareByFilePosition); + + for (int i = 0, n = sections.size(); i < n; ++i) + *scriptSections[i] = sections[i]; + } +} + +// 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<ELFT>::finalizeSections(). +template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() { + ThunkCreator tc; + AArch64Err843419Patcher a64p; + + // For some targets, like x86, this loop iterates only once. + for (;;) { + bool changed = false; + + script->assignAddresses(); + + if (target->needsThunks) + changed |= tc.createThunks(outputSections); + + if (config->fixCortexA53Errata843419) { + if (changed) + script->assignAddresses(); + changed |= a64p.createFixes(); + } + + if (in.mipsGot) + in.mipsGot->updateAllocSize(); + + for (Partition &part : partitions) { + changed |= part.relaDyn->updateAllocSize(); + if (part.relrDyn) + changed |= part.relrDyn->updateAllocSize(); + } + + if (!changed) + return; + } +} + +static void finalizeSynthetic(SyntheticSection *sec) { + if (sec && sec->isNeeded() && sec->getParent()) + sec->finalizeContents(); +} + +// 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. We iterate over them all and exit at first + // non-synthetic. + for (InputSectionBase *s : llvm::reverse(inputSections)) { + SyntheticSection *ss = dyn_cast<SyntheticSection>(s); + if (!ss) + return; + OutputSection *os = ss->getParent(); + if (!os || ss->isNeeded()) + continue; + + // If we reach here, then SS is an unused synthetic section and we want to + // remove it from corresponding input section description of output section. + for (BaseCommand *b : os->sectionCommands) + if (auto *isd = dyn_cast<InputSectionDescription>(b)) + llvm::erase_if(isd->sections, + [=](InputSection *isec) { return isec == ss; }); + } +} + +// Returns true if a symbol can be replaced at load-time by a symbol +// with the same name defined in other ELF executable or DSO. +static bool computeIsPreemptible(const Symbol &b) { + assert(!b.isLocal()); + + // Only symbols that appear in dynsym can be preempted. + if (!b.includeInDynsym()) + return false; + + // Only default visibility symbols can be preempted. + if (b.visibility != STV_DEFAULT) + return false; + + // At this point copy relocations have not been created yet, so any + // symbol that is not defined locally is preemptible. + if (!b.isDefined()) + return true; + + // If we have a dynamic list it specifies which local symbols are preemptible. + if (config->hasDynamicList) + return false; + + if (!config->shared) + return false; + + // -Bsymbolic means that definitions are not preempted. + if (config->bsymbolic || (config->bsymbolicFunctions && b.isFunc())) + return false; + return true; +} + +// Create output section objects and add them to OutputSections. +template <class ELFT> void Writer<ELFT>::finalizeSections() { + 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. + if (!config->relocatable) { + addStartEndSymbols(); + for (BaseCommand *base : script->sectionCommands) + if (auto *sec = dyn_cast<OutputSection>(base)) + addStartStopSymbols(sec); + } + + // 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) + symtab->addSymbol(Defined{/*file=*/nullptr, "_DYNAMIC", STB_WEAK, + STV_HIDDEN, STT_NOTYPE, + /*value=*/0, /*size=*/0, mainPart->dynamic}); + + // Define __rel[a]_iplt_{start,end} symbols if needed. + addRelIpltSymbols(); + + // RISC-V's gp can address +/- 2 KiB, set it to .sdata + 0x800 if not defined. + // This symbol should only be defined in an executable. + if (config->emachine == EM_RISCV && !config->shared) + ElfSym::riscvGlobalPointer = + addOptionalRegular("__global_pointer$", findSection(".sdata"), 0x800, + STV_DEFAULT, STB_GLOBAL); + + if (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{/*file=*/nullptr, s->getName(), STB_GLOBAL, STV_HIDDEN, + STT_TLS, /*value=*/0, 0, + /*section=*/nullptr}); + ElfSym::tlsModuleBase = cast<Defined>(s); + } + } + + // This responsible for splitting up .eh_frame section into + // pieces. The relocation scan uses those pieces, so this has to be + // earlier. + for (Partition &part : partitions) + finalizeSynthetic(part.ehFrame); + + symtab->forEachSymbol([](Symbol *s) { + if (!s->isPreemptible) + s->isPreemptible = computeIsPreemptible(*s); + }); + + // Scan relocations. This must be done after every symbol is declared so that + // we can correctly decide if a dynamic relocation is needed. + if (!config->relocatable) { + forEachRelSec(scanRelocations<ELFT>); + reportUndefinedSymbols<ELFT>(); + } + + addIRelativeRelocs(); + + if (in.plt && in.plt->isNeeded()) + in.plt->addSymbols(); + if (in.iplt && in.iplt->isNeeded()) + in.iplt->addSymbols(); + + if (!config->allowShlibUndefined) { + // Error on undefined symbols in a shared object, if all of its DT_NEEDED + // entires 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. + for (SharedFile *file : sharedFiles) + file->allNeededIsKnown = + llvm::all_of(file->dtNeeded, [&](StringRef needed) { + return symtab->soNames.count(needed); + }); + + symtab->forEachSymbol([](Symbol *sym) { + if (sym->isUndefined() && !sym->isWeak()) + if (auto *f = dyn_cast_or_null<SharedFile>(sym->file)) + if (f->allNeededIsKnown) + error(toString(f) + ": undefined reference to " + toString(*sym)); + }); + } + + // Now that we have defined all possible global symbols including linker- + // synthesized ones. Visit all symbols to give the finishing touches. + symtab->forEachSymbol([](Symbol *sym) { + if (!includeInSymtab(*sym)) + return; + if (in.symTab) + in.symTab->addSymbol(sym); + + if (sym->includeInDynsym()) { + partitions[sym->partition - 1].dynSymTab->addSymbol(sym); + if (auto *file = dyn_cast_or_null<SharedFile>(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<Partition>(partitions).slice(1)) { + DenseSet<Symbol *> syms; + for (const SymbolTableEntry &e : part.dynSymTab->getSymbols()) + syms.insert(e.sym); + for (DynamicReloc &reloc : part.relaDyn->relocs) + if (reloc.sym && !reloc.useSymVA && syms.insert(reloc.sym).second) + part.dynSymTab->addSymbol(reloc.sym); + } + + // Do not proceed if there was an undefined symbol. + if (errorCount()) + return; + + if (in.mipsGot) + in.mipsGot->build(); + + removeUnusedSyntheticSections(); + + sortSections(); + + // Now that we have the final list, create a list of all the + // OutputSections for convenience. + for (BaseCommand *base : script->sectionCommands) + if (auto *sec = dyn_cast<OutputSection>(base)) + outputSections.push_back(sec); + + // 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; }; + } + + // 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; + + for (size_t i = 0, e = outputSections.size(); i != e; ++i) { + OutputSection *sec = outputSections[i]; + sec->sectionIndex = i + 1; + sec->shName = in.shStrTab->addString(sec->name); + } + + // 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); + } + } + 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(); + + finalizeSynthetic(in.bss); + finalizeSynthetic(in.bssRelRo); + finalizeSynthetic(in.symTabShndx); + finalizeSynthetic(in.shStrTab); + finalizeSynthetic(in.strTab); + finalizeSynthetic(in.got); + finalizeSynthetic(in.mipsGot); + finalizeSynthetic(in.igotPlt); + finalizeSynthetic(in.gotPlt); + finalizeSynthetic(in.relaIplt); + finalizeSynthetic(in.relaPlt); + finalizeSynthetic(in.plt); + finalizeSynthetic(in.iplt); + finalizeSynthetic(in.ppc32Got2); + finalizeSynthetic(in.riscvSdata); + finalizeSynthetic(in.partIndex); + + // 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) { + finalizeSynthetic(part.armExidx); + finalizeSynthetic(part.dynSymTab); + finalizeSynthetic(part.gnuHashTab); + finalizeSynthetic(part.hashTab); + finalizeSynthetic(part.verDef); + finalizeSynthetic(part.relaDyn); + finalizeSynthetic(part.relrDyn); + finalizeSynthetic(part.ehFrameHdr); + finalizeSynthetic(part.verSym); + finalizeSynthetic(part.verNeed); + finalizeSynthetic(part.dynamic); + } + + if (!script->hasSectionsCommand && !config->relocatable) + fixSectionAlignments(); + + // SHFLinkOrder processing must be processed after relative section placements are + // known but before addresses are allocated. + resolveShfLinkOrder(); + + // 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(); + + // finalizeAddressDependentContent may have added local symbols to the static symbol table. + finalizeSynthetic(in.symTab); + finalizeSynthetic(in.ppc64LongBranchTarget); + + // 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(); +} + +// Ensure data sections are not mixed with executable sections when +// -execute-only is used. -execute-only is a feature to make pages executable +// but not readable, and the feature is currently supported only on AArch64. +template <class ELFT> void Writer<ELFT>::checkExecuteOnly() { + if (!config->executeOnly) + return; + + for (OutputSection *os : outputSections) + if (os->flags & SHF_EXECINSTR) + for (InputSection *isec : getInputSections(os)) + if (!(isec->flags & SHF_EXECINSTR)) + error("cannot place " + toString(isec) + " into " + toString(os->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 <class ELFT> void Writer<ELFT>::addStartEndSymbols() { + // If a section does not exist, there's ambiguity as to how we + // define _start and _end symbols for an init/fini section. Since + // the loader assume that the symbols are always defined, we need to + // always define them. But what value? The loader iterates over all + // pointers between _start and _end to run global ctors/dtors, so if + // the section is empty, their symbol values don't actually matter + // as long as _start and _end point to the same location. + // + // That said, we don't want to set the symbols to 0 (which is + // probably the simplest value) because that could cause some + // program to fail to link due to relocation overflow, if their + // program text is above 2 GiB. We use the address of the .text + // section instead to prevent that failure. + // + // In a rare sitaution, .text section may not exist. If that's the + // case, use the image base address as a last resort. + OutputSection *Default = findSection(".text"); + if (!Default) + Default = Out::elfHeader; + + auto define = [=](StringRef start, StringRef end, OutputSection *os) { + if (os) { + addOptionalRegular(start, os, 0); + addOptionalRegular(end, os, -1); + } else { + addOptionalRegular(start, Default, 0); + addOptionalRegular(end, Default, 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); + + 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_<secname> and +// __stop_<secname> 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 <class ELFT> +void Writer<ELFT>::addStartStopSymbols(OutputSection *sec) { + StringRef s = sec->name; + if (!isValidCIdentifier(s)) + return; + addOptionalRegular(saver.save("__start_" + s), sec, 0, STV_PROTECTED); + addOptionalRegular(saver.save("__stop_" + s), sec, -1, STV_PROTECTED); +} + +static bool needsPtLoad(OutputSection *sec) { + if (!(sec->flags & SHF_ALLOC) || sec->noload) + 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; +} + +// Linker scripts are responsible for aligning addresses. Unfortunately, most +// linker scripts are designed for creating two PT_LOADs only, one RX and one +// RW. This means that there is no alignment in the RO to RX transition and we +// cannot create a PT_LOAD there. +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; + if (config->singleRoRx && !(flags & PF_W)) + return flags | PF_X; + return flags; +} + +// Decide which program headers to create and which sections to include in each +// one. +template <class ELFT> +std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs(Partition &part) { + std::vector<PhdrEntry *> ret; + auto addHdr = [&](unsigned type, unsigned flags) -> PhdrEntry * { + ret.push_back(make<PhdrEntry>(type, flags)); + return ret.back(); + }; + + unsigned partNo = part.getNumber(); + bool isMain = partNo == 1; + + // 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 first PT_LOAD segment for regular output sections. + uint64_t flags = computeFlags(PF_R); + PhdrEntry *load = nullptr; + + // 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 proccessing 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<PhdrEntry>(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; + } + } + + for (OutputSection *sec : outputSections) { + if (!(sec->flags & SHF_ALLOC)) + break; + 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 + // different flags or is loaded at a discontiguous address or memory + // region using AT or AT> linker script command, respectively. At the same + // time, we don't want to create a separate load segment for the headers, + // even if the first output section has an AT or AT> attribute. + uint64_t newFlags = computeFlags(sec->getPhdrFlags()); + if (!load || + ((sec->lmaExpr || + (sec->lmaRegion && (sec->lmaRegion != load->firstSec->lmaRegion))) && + load->lastSec != Out::programHeaders) || + sec->memRegion != load->firstSec->memRegion || flags != newFlags || + sec == relroEnd) { + load = addHdr(PT_LOAD, newFlags); + flags = newFlags; + } + + load->add(sec); + } + + // Add a TLS segment if any. + PhdrEntry *tlsHdr = make<PhdrEntry>(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()); + + // PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That 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_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->zExecstack) + 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); + + // 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->alignment != sec->alignment) + note = addHdr(PT_NOTE, PF_R); + note->add(sec); + } else { + note = nullptr; + } + } + return ret; +} + +template <class ELFT> +void Writer<ELFT>::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<PhdrEntry>(pType, pFlags); + entry->add(*i); + part.phdrs.push_back(entry); +} + +// The first section of each PT_LOAD, the first section in PT_GNU_RELRO and the +// first section after PT_GNU_RELRO have to be page aligned so that the dynamic +// linker can set the permissions. +template <class ELFT> void Writer<ELFT>::fixSectionAlignments() { + auto pageAlign = [](OutputSection *cmd) { + if (cmd && !cmd->addrExpr) + cmd->addrExpr = [=] { + return alignTo(script->getDot(), config->maxPageSize); + }; + }; + + for (Partition &part : partitions) { + for (const PhdrEntry *p : part.phdrs) + if (p->p_type == PT_LOAD && p->firstSec) + pageAlign(p->firstSec); + } +} + +// 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 + // module the page size. + if (os->ptLoad && os->ptLoad->firstSec == os) { + uint64_t alignment = + std::max<uint64_t>(os->ptLoad->p_align, config->maxPageSize); + return alignTo(off, alignment, os->addr); + } + + // File offsets are not significant for .bss sections other than the first one + // in a PT_LOAD. By convention, we keep section offsets monotonically + // increasing rather than setting to zero. + if (os->type == SHT_NOBITS) + return off; + + // If the section is not in a PT_LOAD, we just have to align it. + if (!os->ptLoad) + return alignTo(off, os->alignment); + + // 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; +} + +// Set an in-file position to a given section and returns the end position of +// the section. +static uint64_t setFileOffset(OutputSection *os, uint64_t off) { + off = computeFileOffset(os, off); + os->offset = off; + + if (os->type == SHT_NOBITS) + return off; + return off + os->size; +} + +template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() { + uint64_t off = 0; + for (OutputSection *sec : outputSections) + if (sec->flags & SHF_ALLOC) + off = setFileOffset(sec, off); + fileSize = alignTo(off, config->wordsize); +} + +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 <class ELFT> void Writer<ELFT>::assignFileOffsets() { + uint64_t off = 0; + off = setFileOffset(Out::elfHeader, off); + off = setFileOffset(Out::programHeaders, off); + + 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; + + for (OutputSection *sec : outputSections) { + off = setFileOffset(sec, off); + if (script->hasSectionsCommand) + continue; + + // 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 (lastRX && lastRX->lastSec == sec) + off = alignTo(off, config->commonPageSize); + } + + sectionHeaderOff = alignTo(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 <class ELFT> void Writer<ELFT>::setPhdrs(Partition &part) { + for (PhdrEntry *p : part.phdrs) { + OutputSection *first = p->firstSec; + OutputSection *last = p->lastSec; + + 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(); + } + + if (p->p_type == PT_LOAD) { + p->p_align = std::max<uint64_t>(p->p_align, config->maxPageSize); + } else if (p->p_type == PT_GNU_RELRO) { + p->p_align = 1; + // The glibc dynamic loader rounds the size down, so we need to round up + // to protect the last page. This is a no-op on FreeBSD which always + // rounds up. + p->p_memsz = alignTo(p->p_memsz, config->commonPageSize); + } + } +} + +// 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 adresses). +static void checkOverlap(StringRef name, std::vector<SectionOffset> §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 <class ELFT> void Writer<ELFT>::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 > UINT32_MAX)) + 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<SectionOffset> 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<SectionOffset> 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<SectionOffset> 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 of the first byte of the .text section, if present; +// 6. 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 (OutputSection *sec = findSection(".text")) { + if (config->warnMissingEntry) + warn("cannot find entry symbol " + config->entry + "; defaulting to 0x" + + utohexstr(sec->addr)); + return sec->addr; + } + + // Case 6 + 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 <class ELFT> void Writer<ELFT>::writeHeader() { + writeEhdr<ELFT>(Out::bufferStart, *mainPart); + writePhdrs<ELFT>(Out::bufferStart + sizeof(Elf_Ehdr), *mainPart); + + auto *eHdr = reinterpret_cast<Elf_Ehdr *>(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<Elf_Shdr *>(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<ELFT>(++sHdrs); +} + +// Open a result file. +template <class ELFT> void Writer<ELFT>::openFile() { + uint64_t maxSize = config->is64 ? INT64_MAX : UINT32_MAX; + if (fileSize != size_t(fileSize) || maxSize < fileSize) { + error("output file too large: " + Twine(fileSize) + " bytes"); + return; + } + + unlinkAsync(config->outputFile); + unsigned flags = 0; + if (!config->relocatable) + flags = FileOutputBuffer::F_executable; + Expected<std::unique_ptr<FileOutputBuffer>> 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 <class ELFT> void Writer<ELFT>::writeSectionsBinary() { + for (OutputSection *sec : outputSections) + if (sec->flags & SHF_ALLOC) + sec->writeTo<ELFT>(Out::bufferStart + sec->offset); +} + +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 <class ELFT> void Writer<ELFT>::writeTrapInstr() { + if (script->hasSectionsCommand) + return; + + 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, + config->commonPageSize), + Out::bufferStart + alignTo(p->firstSec->offset + p->p_filesz, + config->commonPageSize)); + + // 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 = + alignTo(last->p_filesz, config->commonPageSize); + } +} + +// Write section contents to a mmap'ed file. +template <class ELFT> void Writer<ELFT>::writeSections() { + // 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. + for (OutputSection *sec : outputSections) + if (sec->type == SHT_REL || sec->type == SHT_RELA) + sec->writeTo<ELFT>(Out::bufferStart + sec->offset); + + for (OutputSection *sec : outputSections) + if (sec->type != SHT_REL && sec->type != SHT_RELA) + sec->writeTo<ELFT>(Out::bufferStart + sec->offset); +} + +// Split one uint8 array into small pieces of uint8 arrays. +static std::vector<ArrayRef<uint8_t>> split(ArrayRef<uint8_t> arr, + size_t chunkSize) { + std::vector<ArrayRef<uint8_t>> ret; + while (arr.size() > chunkSize) { + ret.push_back(arr.take_front(chunkSize)); + arr = arr.drop_front(chunkSize); + } + if (!arr.empty()) + ret.push_back(arr); + return ret; +} + +// 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<uint8_t> hashBuf, + llvm::ArrayRef<uint8_t> data, + std::function<void(uint8_t *dest, ArrayRef<uint8_t> arr)> hashFn) { + std::vector<ArrayRef<uint8_t>> chunks = split(data, 1024 * 1024); + std::vector<uint8_t> hashes(chunks.size() * hashBuf.size()); + + // Compute hash values. + parallelForEachN(0, chunks.size(), [&](size_t i) { + hashFn(hashes.data() + i * hashBuf.size(), chunks[i]); + }); + + // Write to the final output buffer. + hashFn(hashBuf.data(), hashes); +} + +template <class ELFT> void Writer<ELFT>::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::vector<uint8_t> buildId(hashSize); + llvm::ArrayRef<uint8_t> buf{Out::bufferStart, size_t(fileSize)}; + + switch (config->buildId) { + case BuildIdKind::Fast: + computeHash(buildId, buf, [](uint8_t *dest, ArrayRef<uint8_t> arr) { + write64le(dest, xxHash64(arr)); + }); + break; + case BuildIdKind::Md5: + computeHash(buildId, buf, [&](uint8_t *dest, ArrayRef<uint8_t> arr) { + memcpy(dest, MD5::hash(arr).data(), hashSize); + }); + break; + case BuildIdKind::Sha1: + computeHash(buildId, buf, [&](uint8_t *dest, ArrayRef<uint8_t> arr) { + memcpy(dest, SHA1::hash(arr).data(), hashSize); + }); + break; + case BuildIdKind::Uuid: + if (auto ec = llvm::getRandomBytes(buildId.data(), hashSize)) + error("entropy source failure: " + ec.message()); + break; + default: + llvm_unreachable("unknown BuildIdKind"); + } + for (Partition &part : partitions) + part.buildId->writeBuildId(buildId); +} + +template void elf::writeResult<ELF32LE>(); +template void elf::writeResult<ELF32BE>(); +template void elf::writeResult<ELF64LE>(); +template void elf::writeResult<ELF64BE>(); |