//===- LinkerScript.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 // //===----------------------------------------------------------------------===// // // This file contains the parser/evaluator of the linker script. // //===----------------------------------------------------------------------===// #include "LinkerScript.h" #include "Config.h" #include "InputFiles.h" #include "InputSection.h" #include "OutputSections.h" #include "SymbolTable.h" #include "Symbols.h" #include "SyntheticSections.h" #include "Target.h" #include "Writer.h" #include "lld/Common/CommonLinkerContext.h" #include "lld/Common/Strings.h" #include "llvm/ADT/STLExtras.h" #include "llvm/ADT/StringRef.h" #include "llvm/BinaryFormat/ELF.h" #include "llvm/Support/Casting.h" #include "llvm/Support/Endian.h" #include "llvm/Support/ErrorHandling.h" #include "llvm/Support/TimeProfiler.h" #include #include #include #include #include #include #include using namespace llvm; using namespace llvm::ELF; using namespace llvm::object; using namespace llvm::support::endian; using namespace lld; using namespace lld::elf; std::unique_ptr elf::script; static bool isSectionPrefix(StringRef prefix, StringRef name) { return name.consume_front(prefix) && (name.empty() || name[0] == '.'); } static StringRef getOutputSectionName(const InputSectionBase *s) { // This is for --emit-relocs and -r. 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(s)) { if (InputSectionBase *rel = isec->getRelocatedSection()) { OutputSection *out = rel->getOutputSection(); if (!out) { assert(config->relocatable && (rel->flags & SHF_LINK_ORDER)); return s->name; } if (s->type == SHT_RELA) return saver().save(".rela" + out->name); return saver().save(".rel" + out->name); } } if (config->relocatable) return s->name; // A BssSection created for a common symbol is identified as "COMMON" in // linker scripts. It should go to .bss section. if (s->name == "COMMON") return ".bss"; if (script->hasSectionsCommand) return s->name; // When no SECTIONS is specified, emulate GNU ld's internal linker scripts // by grouping sections with certain prefixes. // GNU ld places text sections with prefix ".text.hot.", ".text.unknown.", // ".text.unlikely.", ".text.startup." or ".text.exit." before others. // We provide an option -z keep-text-section-prefix to group such sections // into separate output sections. This is more flexible. See also // sortISDBySectionOrder(). // ".text.unknown" means the hotness of the section is unknown. When // SampleFDO is used, if a function doesn't have sample, it could be very // cold or it could be a new function never being sampled. Those functions // will be kept in the ".text.unknown" section. // ".text.split." holds symbols which are split out from functions in other // input sections. For example, with -fsplit-machine-functions, placing the // cold parts in .text.split instead of .text.unlikely mitigates against poor // profile inaccuracy. Techniques such as hugepage remapping can make // conservative decisions at the section granularity. if (isSectionPrefix(".text", s->name)) { if (config->zKeepTextSectionPrefix) for (StringRef v : {".text.hot", ".text.unknown", ".text.unlikely", ".text.startup", ".text.exit", ".text.split"}) if (isSectionPrefix(v.substr(5), s->name.substr(5))) return v; return ".text"; } for (StringRef v : {".data.rel.ro", ".data", ".rodata", ".bss.rel.ro", ".bss", ".ldata", ".lrodata", ".lbss", ".gcc_except_table", ".init_array", ".fini_array", ".tbss", ".tdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"}) if (isSectionPrefix(v, s->name)) return v; return s->name; } uint64_t ExprValue::getValue() const { if (sec) return alignToPowerOf2(sec->getOutputSection()->addr + sec->getOffset(val), alignment); return alignToPowerOf2(val, alignment); } uint64_t ExprValue::getSecAddr() const { return sec ? sec->getOutputSection()->addr + sec->getOffset(0) : 0; } uint64_t ExprValue::getSectionOffset() const { return getValue() - getSecAddr(); } OutputDesc *LinkerScript::createOutputSection(StringRef name, StringRef location) { OutputDesc *&secRef = nameToOutputSection[CachedHashStringRef(name)]; OutputDesc *sec; if (secRef && secRef->osec.location.empty()) { // There was a forward reference. sec = secRef; } else { sec = make(name, SHT_PROGBITS, 0); if (!secRef) secRef = sec; } sec->osec.location = std::string(location); return sec; } OutputDesc *LinkerScript::getOrCreateOutputSection(StringRef name) { OutputDesc *&cmdRef = nameToOutputSection[CachedHashStringRef(name)]; if (!cmdRef) cmdRef = make(name, SHT_PROGBITS, 0); return cmdRef; } // Expands the memory region by the specified size. static void expandMemoryRegion(MemoryRegion *memRegion, uint64_t size, StringRef secName) { memRegion->curPos += size; } void LinkerScript::expandMemoryRegions(uint64_t size) { if (state->memRegion) expandMemoryRegion(state->memRegion, size, state->outSec->name); // Only expand the LMARegion if it is different from memRegion. if (state->lmaRegion && state->memRegion != state->lmaRegion) expandMemoryRegion(state->lmaRegion, size, state->outSec->name); } void LinkerScript::expandOutputSection(uint64_t size) { state->outSec->size += size; expandMemoryRegions(size); } void LinkerScript::setDot(Expr e, const Twine &loc, bool inSec) { uint64_t val = e().getValue(); // If val is smaller and we are in an output section, record the error and // report it if this is the last assignAddresses iteration. dot may be smaller // if there is another assignAddresses iteration. if (val < dot && inSec) { backwardDotErr = (loc + ": unable to move location counter (0x" + Twine::utohexstr(dot) + ") backward to 0x" + Twine::utohexstr(val) + " for section '" + state->outSec->name + "'") .str(); } // Update to location counter means update to section size. if (inSec) expandOutputSection(val - dot); dot = val; } // Used for handling linker symbol assignments, for both finalizing // their values and doing early declarations. Returns true if symbol // should be defined from linker script. static bool shouldDefineSym(SymbolAssignment *cmd) { if (cmd->name == ".") return false; if (!cmd->provide) return true; // If a symbol was in PROVIDE(), we need to define it only // when it is a referenced undefined symbol. Symbol *b = symtab.find(cmd->name); if (b && !b->isDefined() && !b->isCommon()) return true; return false; } // Called by processSymbolAssignments() to assign definitions to // linker-script-defined symbols. void LinkerScript::addSymbol(SymbolAssignment *cmd) { if (!shouldDefineSym(cmd)) return; // Define a symbol. ExprValue value = cmd->expression(); SectionBase *sec = value.isAbsolute() ? nullptr : value.sec; uint8_t visibility = cmd->hidden ? STV_HIDDEN : STV_DEFAULT; // When this function is called, section addresses have not been // fixed yet. So, we may or may not know the value of the RHS // expression. // // For example, if an expression is `x = 42`, we know x is always 42. // However, if an expression is `x = .`, there's no way to know its // value at the moment. // // We want to set symbol values early if we can. This allows us to // use symbols as variables in linker scripts. Doing so allows us to // write expressions like this: `alignment = 16; . = ALIGN(., alignment)`. uint64_t symValue = value.sec ? 0 : value.getValue(); Defined newSym(createInternalFile(cmd->location), cmd->name, STB_GLOBAL, visibility, value.type, symValue, 0, sec); Symbol *sym = symtab.insert(cmd->name); sym->mergeProperties(newSym); newSym.overwrite(*sym); sym->isUsedInRegularObj = true; cmd->sym = cast(sym); } // This function is called from LinkerScript::declareSymbols. // It creates a placeholder symbol if needed. static void declareSymbol(SymbolAssignment *cmd) { if (!shouldDefineSym(cmd)) return; uint8_t visibility = cmd->hidden ? STV_HIDDEN : STV_DEFAULT; Defined newSym(ctx.internalFile, cmd->name, STB_GLOBAL, visibility, STT_NOTYPE, 0, 0, nullptr); // If the symbol is already defined, its order is 0 (with absence indicating // 0); otherwise it's assigned the order of the SymbolAssignment. Symbol *sym = symtab.insert(cmd->name); if (!sym->isDefined()) ctx.scriptSymOrder.insert({sym, cmd->symOrder}); // We can't calculate final value right now. sym->mergeProperties(newSym); newSym.overwrite(*sym); cmd->sym = cast(sym); cmd->provide = false; sym->isUsedInRegularObj = true; sym->scriptDefined = true; } using SymbolAssignmentMap = DenseMap>; // Collect section/value pairs of linker-script-defined symbols. This is used to // check whether symbol values converge. static SymbolAssignmentMap getSymbolAssignmentValues(ArrayRef sectionCommands) { SymbolAssignmentMap ret; for (SectionCommand *cmd : sectionCommands) { if (auto *assign = dyn_cast(cmd)) { if (assign->sym) // sym is nullptr for dot. ret.try_emplace(assign->sym, std::make_pair(assign->sym->section, assign->sym->value)); continue; } for (SectionCommand *subCmd : cast(cmd)->osec.commands) if (auto *assign = dyn_cast(subCmd)) if (assign->sym) ret.try_emplace(assign->sym, std::make_pair(assign->sym->section, assign->sym->value)); } return ret; } // Returns the lexicographical smallest (for determinism) Defined whose // section/value has changed. static const Defined * getChangedSymbolAssignment(const SymbolAssignmentMap &oldValues) { const Defined *changed = nullptr; for (auto &it : oldValues) { const Defined *sym = it.first; if (std::make_pair(sym->section, sym->value) != it.second && (!changed || sym->getName() < changed->getName())) changed = sym; } return changed; } // Process INSERT [AFTER|BEFORE] commands. For each command, we move the // specified output section to the designated place. void LinkerScript::processInsertCommands() { SmallVector moves; for (const InsertCommand &cmd : insertCommands) { for (StringRef name : cmd.names) { // If base is empty, it may have been discarded by // adjustOutputSections(). We do not handle such output sections. auto from = llvm::find_if(sectionCommands, [&](SectionCommand *subCmd) { return isa(subCmd) && cast(subCmd)->osec.name == name; }); if (from == sectionCommands.end()) continue; moves.push_back(cast(*from)); sectionCommands.erase(from); } auto insertPos = llvm::find_if(sectionCommands, [&cmd](SectionCommand *subCmd) { auto *to = dyn_cast(subCmd); return to != nullptr && to->osec.name == cmd.where; }); if (insertPos == sectionCommands.end()) { error("unable to insert " + cmd.names[0] + (cmd.isAfter ? " after " : " before ") + cmd.where); } else { if (cmd.isAfter) ++insertPos; sectionCommands.insert(insertPos, moves.begin(), moves.end()); } moves.clear(); } } // Symbols defined in script should not be inlined by LTO. At the same time // we don't know their final values until late stages of link. Here we scan // over symbol assignment commands and create placeholder symbols if needed. void LinkerScript::declareSymbols() { assert(!state); for (SectionCommand *cmd : sectionCommands) { if (auto *assign = dyn_cast(cmd)) { declareSymbol(assign); continue; } // If the output section directive has constraints, // we can't say for sure if it is going to be included or not. // Skip such sections for now. Improve the checks if we ever // need symbols from that sections to be declared early. const OutputSection &sec = cast(cmd)->osec; if (sec.constraint != ConstraintKind::NoConstraint) continue; for (SectionCommand *cmd : sec.commands) if (auto *assign = dyn_cast(cmd)) declareSymbol(assign); } } // This function is called from assignAddresses, while we are // fixing the output section addresses. This function is supposed // to set the final value for a given symbol assignment. void LinkerScript::assignSymbol(SymbolAssignment *cmd, bool inSec) { if (cmd->name == ".") { setDot(cmd->expression, cmd->location, inSec); return; } if (!cmd->sym) return; ExprValue v = cmd->expression(); if (v.isAbsolute()) { cmd->sym->section = nullptr; cmd->sym->value = v.getValue(); } else { cmd->sym->section = v.sec; cmd->sym->value = v.getSectionOffset(); } cmd->sym->type = v.type; } static inline StringRef getFilename(const InputFile *file) { return file ? file->getNameForScript() : StringRef(); } bool InputSectionDescription::matchesFile(const InputFile *file) const { if (filePat.isTrivialMatchAll()) return true; if (!matchesFileCache || matchesFileCache->first != file) matchesFileCache.emplace(file, filePat.match(getFilename(file))); return matchesFileCache->second; } bool SectionPattern::excludesFile(const InputFile *file) const { if (excludedFilePat.empty()) return false; if (!excludesFileCache || excludesFileCache->first != file) excludesFileCache.emplace(file, excludedFilePat.match(getFilename(file))); return excludesFileCache->second; } bool LinkerScript::shouldKeep(InputSectionBase *s) { for (InputSectionDescription *id : keptSections) if (id->matchesFile(s->file)) for (SectionPattern &p : id->sectionPatterns) if (p.sectionPat.match(s->name) && (s->flags & id->withFlags) == id->withFlags && (s->flags & id->withoutFlags) == 0) return true; return false; } // A helper function for the SORT() command. static bool matchConstraints(ArrayRef sections, ConstraintKind kind) { if (kind == ConstraintKind::NoConstraint) return true; bool isRW = llvm::any_of( sections, [](InputSectionBase *sec) { return sec->flags & SHF_WRITE; }); return (isRW && kind == ConstraintKind::ReadWrite) || (!isRW && kind == ConstraintKind::ReadOnly); } static void sortSections(MutableArrayRef vec, SortSectionPolicy k) { auto alignmentComparator = [](InputSectionBase *a, InputSectionBase *b) { // ">" is not a mistake. Sections with larger alignments are placed // before sections with smaller alignments in order to reduce the // amount of padding necessary. This is compatible with GNU. return a->addralign > b->addralign; }; auto nameComparator = [](InputSectionBase *a, InputSectionBase *b) { return a->name < b->name; }; auto priorityComparator = [](InputSectionBase *a, InputSectionBase *b) { return getPriority(a->name) < getPriority(b->name); }; switch (k) { case SortSectionPolicy::Default: case SortSectionPolicy::None: return; case SortSectionPolicy::Alignment: return llvm::stable_sort(vec, alignmentComparator); case SortSectionPolicy::Name: return llvm::stable_sort(vec, nameComparator); case SortSectionPolicy::Priority: return llvm::stable_sort(vec, priorityComparator); case SortSectionPolicy::Reverse: return std::reverse(vec.begin(), vec.end()); } } // Sort sections as instructed by SORT-family commands and --sort-section // option. Because SORT-family commands can be nested at most two depth // (e.g. SORT_BY_NAME(SORT_BY_ALIGNMENT(.text.*))) and because the command // line option is respected even if a SORT command is given, the exact // behavior we have here is a bit complicated. Here are the rules. // // 1. If two SORT commands are given, --sort-section is ignored. // 2. If one SORT command is given, and if it is not SORT_NONE, // --sort-section is handled as an inner SORT command. // 3. If one SORT command is given, and if it is SORT_NONE, don't sort. // 4. If no SORT command is given, sort according to --sort-section. static void sortInputSections(MutableArrayRef vec, SortSectionPolicy outer, SortSectionPolicy inner) { if (outer == SortSectionPolicy::None) return; if (inner == SortSectionPolicy::Default) sortSections(vec, config->sortSection); else sortSections(vec, inner); sortSections(vec, outer); } // Compute and remember which sections the InputSectionDescription matches. SmallVector LinkerScript::computeInputSections(const InputSectionDescription *cmd, ArrayRef sections) { SmallVector ret; SmallVector indexes; DenseSet seen; auto sortByPositionThenCommandLine = [&](size_t begin, size_t end) { llvm::sort(MutableArrayRef(indexes).slice(begin, end - begin)); for (size_t i = begin; i != end; ++i) ret[i] = sections[indexes[i]]; sortInputSections( MutableArrayRef(ret).slice(begin, end - begin), config->sortSection, SortSectionPolicy::None); }; // Collects all sections that satisfy constraints of Cmd. size_t sizeAfterPrevSort = 0; for (const SectionPattern &pat : cmd->sectionPatterns) { size_t sizeBeforeCurrPat = ret.size(); for (size_t i = 0, e = sections.size(); i != e; ++i) { // Skip if the section is dead or has been matched by a previous input // section description or a previous pattern. InputSectionBase *sec = sections[i]; if (!sec->isLive() || sec->parent || seen.contains(i)) continue; // For --emit-relocs we have to ignore entries like // .rela.dyn : { *(.rela.data) } // which are common because they are in the default bfd script. // We do not ignore SHT_REL[A] linker-synthesized sections here because // want to support scripts that do custom layout for them. if (isa(sec) && cast(sec)->getRelocatedSection()) continue; // Check the name early to improve performance in the common case. if (!pat.sectionPat.match(sec->name)) continue; if (!cmd->matchesFile(sec->file) || pat.excludesFile(sec->file) || (sec->flags & cmd->withFlags) != cmd->withFlags || (sec->flags & cmd->withoutFlags) != 0) continue; ret.push_back(sec); indexes.push_back(i); seen.insert(i); } if (pat.sortOuter == SortSectionPolicy::Default) continue; // Matched sections are ordered by radix sort with the keys being (SORT*, // --sort-section, input order), where SORT* (if present) is most // significant. // // Matched sections between the previous SORT* and this SORT* are sorted by // (--sort-alignment, input order). sortByPositionThenCommandLine(sizeAfterPrevSort, sizeBeforeCurrPat); // Matched sections by this SORT* pattern are sorted using all 3 keys. // ret[sizeBeforeCurrPat,ret.size()) are already in the input order, so we // just sort by sortOuter and sortInner. sortInputSections( MutableArrayRef(ret).slice(sizeBeforeCurrPat), pat.sortOuter, pat.sortInner); sizeAfterPrevSort = ret.size(); } // Matched sections after the last SORT* are sorted by (--sort-alignment, // input order). sortByPositionThenCommandLine(sizeAfterPrevSort, ret.size()); return ret; } void LinkerScript::discard(InputSectionBase &s) { if (&s == in.shStrTab.get()) error("discarding " + s.name + " section is not allowed"); s.markDead(); s.parent = nullptr; for (InputSection *sec : s.dependentSections) discard(*sec); } void LinkerScript::discardSynthetic(OutputSection &outCmd) { for (Partition &part : partitions) { if (!part.armExidx || !part.armExidx->isLive()) continue; SmallVector secs( part.armExidx->exidxSections.begin(), part.armExidx->exidxSections.end()); for (SectionCommand *cmd : outCmd.commands) if (auto *isd = dyn_cast(cmd)) for (InputSectionBase *s : computeInputSections(isd, secs)) discard(*s); } } SmallVector LinkerScript::createInputSectionList(OutputSection &outCmd) { SmallVector ret; for (SectionCommand *cmd : outCmd.commands) { if (auto *isd = dyn_cast(cmd)) { isd->sectionBases = computeInputSections(isd, ctx.inputSections); for (InputSectionBase *s : isd->sectionBases) s->parent = &outCmd; ret.insert(ret.end(), isd->sectionBases.begin(), isd->sectionBases.end()); } } return ret; } // Create output sections described by SECTIONS commands. void LinkerScript::processSectionCommands() { auto process = [this](OutputSection *osec) { SmallVector v = createInputSectionList(*osec); // The output section name `/DISCARD/' is special. // Any input section assigned to it is discarded. if (osec->name == "/DISCARD/") { for (InputSectionBase *s : v) discard(*s); discardSynthetic(*osec); osec->commands.clear(); return false; } // This is for ONLY_IF_RO and ONLY_IF_RW. An output section directive // ".foo : ONLY_IF_R[OW] { ... }" is handled only if all member input // sections satisfy a given constraint. If not, a directive is handled // as if it wasn't present from the beginning. // // Because we'll iterate over SectionCommands many more times, the easy // way to "make it as if it wasn't present" is to make it empty. if (!matchConstraints(v, osec->constraint)) { for (InputSectionBase *s : v) s->parent = nullptr; osec->commands.clear(); return false; } // Handle subalign (e.g. ".foo : SUBALIGN(32) { ... }"). If subalign // is given, input sections are aligned to that value, whether the // given value is larger or smaller than the original section alignment. if (osec->subalignExpr) { uint32_t subalign = osec->subalignExpr().getValue(); for (InputSectionBase *s : v) s->addralign = subalign; } // Set the partition field the same way OutputSection::recordSection() // does. Partitions cannot be used with the SECTIONS command, so this is // always 1. osec->partition = 1; return true; }; // Process OVERWRITE_SECTIONS first so that it can overwrite the main script // or orphans. DenseMap map; size_t i = 0; for (OutputDesc *osd : overwriteSections) { OutputSection *osec = &osd->osec; if (process(osec) && !map.try_emplace(CachedHashStringRef(osec->name), osd).second) warn("OVERWRITE_SECTIONS specifies duplicate " + osec->name); } for (SectionCommand *&base : sectionCommands) if (auto *osd = dyn_cast(base)) { OutputSection *osec = &osd->osec; if (OutputDesc *overwrite = map.lookup(CachedHashStringRef(osec->name))) { log(overwrite->osec.location + " overwrites " + osec->name); overwrite->osec.sectionIndex = i++; base = overwrite; } else if (process(osec)) { osec->sectionIndex = i++; } } // If an OVERWRITE_SECTIONS specified output section is not in // sectionCommands, append it to the end. The section will be inserted by // orphan placement. for (OutputDesc *osd : overwriteSections) if (osd->osec.partition == 1 && osd->osec.sectionIndex == UINT32_MAX) sectionCommands.push_back(osd); } void LinkerScript::processSymbolAssignments() { // Dot outside an output section still represents a relative address, whose // sh_shndx should not be SHN_UNDEF or SHN_ABS. Create a dummy aether section // that fills the void outside a section. It has an index of one, which is // indistinguishable from any other regular section index. aether = make("", 0, SHF_ALLOC); aether->sectionIndex = 1; // `st` captures the local AddressState and makes it accessible deliberately. // This is needed as there are some cases where we cannot just thread the // current state through to a lambda function created by the script parser. AddressState st; state = &st; st.outSec = aether; for (SectionCommand *cmd : sectionCommands) { if (auto *assign = dyn_cast(cmd)) addSymbol(assign); else for (SectionCommand *subCmd : cast(cmd)->osec.commands) if (auto *assign = dyn_cast(subCmd)) addSymbol(assign); } state = nullptr; } static OutputSection *findByName(ArrayRef vec, StringRef name) { for (SectionCommand *cmd : vec) if (auto *osd = dyn_cast(cmd)) if (osd->osec.name == name) return &osd->osec; return nullptr; } static OutputDesc *createSection(InputSectionBase *isec, StringRef outsecName) { OutputDesc *osd = script->createOutputSection(outsecName, ""); osd->osec.recordSection(isec); return osd; } static OutputDesc *addInputSec(StringMap> &map, InputSectionBase *isec, StringRef outsecName) { // Sections with SHT_GROUP or SHF_GROUP attributes reach here only when the -r // option is given. A section with SHT_GROUP defines a "section group", and // its members have SHF_GROUP attribute. Usually these flags have already been // stripped by InputFiles.cpp as section groups are processed and uniquified. // However, for the -r option, we want to pass through all section groups // as-is because adding/removing members or merging them with other groups // change their semantics. if (isec->type == SHT_GROUP || (isec->flags & SHF_GROUP)) return createSection(isec, outsecName); // Imagine .zed : { *(.foo) *(.bar) } script. Both foo and bar may have // relocation sections .rela.foo and .rela.bar for example. Most tools do // not allow multiple REL[A] sections for output section. Hence we // should combine these relocation sections into single output. // We skip synthetic sections because it can be .rela.dyn/.rela.plt or any // other REL[A] sections created by linker itself. if (!isa(isec) && (isec->type == SHT_REL || isec->type == SHT_RELA)) { auto *sec = cast(isec); OutputSection *out = sec->getRelocatedSection()->getOutputSection(); if (out->relocationSection) { out->relocationSection->recordSection(sec); return nullptr; } OutputDesc *osd = createSection(isec, outsecName); out->relocationSection = &osd->osec; return osd; } // The ELF spec just says // ---------------------------------------------------------------- // In the first phase, input sections that match in name, type and // attribute flags should be concatenated into single sections. // ---------------------------------------------------------------- // // However, it is clear that at least some flags have to be ignored for // section merging. At the very least SHF_GROUP and SHF_COMPRESSED have to be // ignored. We should not have two output .text sections just because one was // in a group and another was not for example. // // It also seems that wording was a late addition and didn't get the // necessary scrutiny. // // Merging sections with different flags is expected by some users. One // reason is that if one file has // // int *const bar __attribute__((section(".foo"))) = (int *)0; // // gcc with -fPIC will produce a read only .foo section. But if another // file has // // int zed; // int *const bar __attribute__((section(".foo"))) = (int *)&zed; // // gcc with -fPIC will produce a read write section. // // Last but not least, when using linker script the merge rules are forced by // the script. Unfortunately, linker scripts are name based. This means that // expressions like *(.foo*) can refer to multiple input sections with // different flags. We cannot put them in different output sections or we // would produce wrong results for // // start = .; *(.foo.*) end = .; *(.bar) // // and a mapping of .foo1 and .bar1 to one section and .foo2 and .bar2 to // another. The problem is that there is no way to layout those output // sections such that the .foo sections are the only thing between the start // and end symbols. // // Given the above issues, we instead merge sections by name and error on // incompatible types and flags. TinyPtrVector &v = map[outsecName]; for (OutputSection *sec : v) { if (sec->partition != isec->partition) continue; if (config->relocatable && (isec->flags & SHF_LINK_ORDER)) { // Merging two SHF_LINK_ORDER sections with different sh_link fields will // change their semantics, so we only merge them in -r links if they will // end up being linked to the same output section. The casts are fine // because everything in the map was created by the orphan placement code. auto *firstIsec = cast( cast(sec->commands[0])->sectionBases[0]); OutputSection *firstIsecOut = firstIsec->flags & SHF_LINK_ORDER ? firstIsec->getLinkOrderDep()->getOutputSection() : nullptr; if (firstIsecOut != isec->getLinkOrderDep()->getOutputSection()) continue; } sec->recordSection(isec); return nullptr; } OutputDesc *osd = createSection(isec, outsecName); v.push_back(&osd->osec); return osd; } // Add sections that didn't match any sections command. void LinkerScript::addOrphanSections() { StringMap> map; SmallVector v; auto add = [&](InputSectionBase *s) { if (s->isLive() && !s->parent) { orphanSections.push_back(s); StringRef name = getOutputSectionName(s); if (config->unique) { v.push_back(createSection(s, name)); } else if (OutputSection *sec = findByName(sectionCommands, name)) { sec->recordSection(s); } else { if (OutputDesc *osd = addInputSec(map, s, name)) v.push_back(osd); assert(isa(s) || s->getOutputSection()->sectionIndex == UINT32_MAX); } } }; // For further --emit-reloc handling code we need target output section // to be created before we create relocation output section, so we want // to create target sections first. We do not want priority handling // for synthetic sections because them are special. size_t n = 0; for (InputSectionBase *isec : ctx.inputSections) { // Process InputSection and MergeInputSection. if (LLVM_LIKELY(isa(isec))) ctx.inputSections[n++] = isec; // In -r links, SHF_LINK_ORDER sections are added while adding their parent // sections because we need to know the parent's output section before we // can select an output section for the SHF_LINK_ORDER section. if (config->relocatable && (isec->flags & SHF_LINK_ORDER)) continue; if (auto *sec = dyn_cast(isec)) if (InputSectionBase *rel = sec->getRelocatedSection()) if (auto *relIS = dyn_cast_or_null(rel->parent)) add(relIS); add(isec); if (config->relocatable) for (InputSectionBase *depSec : isec->dependentSections) if (depSec->flags & SHF_LINK_ORDER) add(depSec); } // Keep just InputSection. ctx.inputSections.resize(n); // If no SECTIONS command was given, we should insert sections commands // before others, so that we can handle scripts which refers them, // for example: "foo = ABSOLUTE(ADDR(.text)));". // When SECTIONS command is present we just add all orphans to the end. if (hasSectionsCommand) sectionCommands.insert(sectionCommands.end(), v.begin(), v.end()); else sectionCommands.insert(sectionCommands.begin(), v.begin(), v.end()); } void LinkerScript::diagnoseOrphanHandling() const { llvm::TimeTraceScope timeScope("Diagnose orphan sections"); if (config->orphanHandling == OrphanHandlingPolicy::Place) return; for (const InputSectionBase *sec : orphanSections) { // .relro_padding is inserted before DATA_SEGMENT_RELRO_END, if present, // automatically. The section is not supposed to be specified by scripts. if (sec == in.relroPadding.get()) continue; // Input SHT_REL[A] retained by --emit-relocs are ignored by // computeInputSections(). Don't warn/error. if (isa(sec) && cast(sec)->getRelocatedSection()) continue; StringRef name = getOutputSectionName(sec); if (config->orphanHandling == OrphanHandlingPolicy::Error) error(toString(sec) + " is being placed in '" + name + "'"); else warn(toString(sec) + " is being placed in '" + name + "'"); } } void LinkerScript::diagnoseMissingSGSectionAddress() const { if (!config->cmseImplib || !in.armCmseSGSection->isNeeded()) return; OutputSection *sec = findByName(sectionCommands, ".gnu.sgstubs"); if (sec && !sec->addrExpr && !config->sectionStartMap.count(".gnu.sgstubs")) error("no address assigned to the veneers output section " + sec->name); } // This function searches for a memory region to place the given output // section in. If found, a pointer to the appropriate memory region is // returned in the first member of the pair. Otherwise, a nullptr is returned. // The second member of the pair is a hint that should be passed to the // subsequent call of this method. std::pair LinkerScript::findMemoryRegion(OutputSection *sec, MemoryRegion *hint) { // Non-allocatable sections are not part of the process image. if (!(sec->flags & SHF_ALLOC)) { bool hasInputOrByteCommand = sec->hasInputSections || llvm::any_of(sec->commands, [](SectionCommand *comm) { return ByteCommand::classof(comm); }); if (!sec->memoryRegionName.empty() && hasInputOrByteCommand) warn("ignoring memory region assignment for non-allocatable section '" + sec->name + "'"); return {nullptr, nullptr}; } // If a memory region name was specified in the output section command, // then try to find that region first. if (!sec->memoryRegionName.empty()) { if (MemoryRegion *m = memoryRegions.lookup(sec->memoryRegionName)) return {m, m}; error("memory region '" + sec->memoryRegionName + "' not declared"); return {nullptr, nullptr}; } // If at least one memory region is defined, all sections must // belong to some memory region. Otherwise, we don't need to do // anything for memory regions. if (memoryRegions.empty()) return {nullptr, nullptr}; // An orphan section should continue the previous memory region. if (sec->sectionIndex == UINT32_MAX && hint) return {hint, hint}; // See if a region can be found by matching section flags. for (auto &pair : memoryRegions) { MemoryRegion *m = pair.second; if (m->compatibleWith(sec->flags)) return {m, nullptr}; } // Otherwise, no suitable region was found. error("no memory region specified for section '" + sec->name + "'"); return {nullptr, nullptr}; } static OutputSection *findFirstSection(PhdrEntry *load) { for (OutputSection *sec : outputSections) if (sec->ptLoad == load) return sec; return nullptr; } // This function assigns offsets to input sections and an output section // for a single sections command (e.g. ".text { *(.text); }"). void LinkerScript::assignOffsets(OutputSection *sec) { const bool isTbss = (sec->flags & SHF_TLS) && sec->type == SHT_NOBITS; const bool sameMemRegion = state->memRegion == sec->memRegion; const bool prevLMARegionIsDefault = state->lmaRegion == nullptr; const uint64_t savedDot = dot; state->memRegion = sec->memRegion; state->lmaRegion = sec->lmaRegion; if (!(sec->flags & SHF_ALLOC)) { // Non-SHF_ALLOC sections have zero addresses. dot = 0; } else if (isTbss) { // Allow consecutive SHF_TLS SHT_NOBITS output sections. The address range // starts from the end address of the previous tbss section. if (state->tbssAddr == 0) state->tbssAddr = dot; else dot = state->tbssAddr; } else { if (state->memRegion) dot = state->memRegion->curPos; if (sec->addrExpr) setDot(sec->addrExpr, sec->location, false); // If the address of the section has been moved forward by an explicit // expression so that it now starts past the current curPos of the enclosing // region, we need to expand the current region to account for the space // between the previous section, if any, and the start of this section. if (state->memRegion && state->memRegion->curPos < dot) expandMemoryRegion(state->memRegion, dot - state->memRegion->curPos, sec->name); } state->outSec = sec; if (sec->addrExpr && script->hasSectionsCommand) { // The alignment is ignored. sec->addr = dot; } else { // sec->alignment is the max of ALIGN and the maximum of input // section alignments. const uint64_t pos = dot; dot = alignToPowerOf2(dot, sec->addralign); sec->addr = dot; expandMemoryRegions(dot - pos); } // state->lmaOffset is LMA minus VMA. If LMA is explicitly specified via AT() // or AT>, recompute state->lmaOffset; otherwise, if both previous/current LMA // region is the default, and the two sections are in the same memory region, // reuse previous lmaOffset; otherwise, reset lmaOffset to 0. This emulates // heuristics described in // https://sourceware.org/binutils/docs/ld/Output-Section-LMA.html if (sec->lmaExpr) { state->lmaOffset = sec->lmaExpr().getValue() - dot; } else if (MemoryRegion *mr = sec->lmaRegion) { uint64_t lmaStart = alignToPowerOf2(mr->curPos, sec->addralign); if (mr->curPos < lmaStart) expandMemoryRegion(mr, lmaStart - mr->curPos, sec->name); state->lmaOffset = lmaStart - dot; } else if (!sameMemRegion || !prevLMARegionIsDefault) { state->lmaOffset = 0; } // Propagate state->lmaOffset to the first "non-header" section. if (PhdrEntry *l = sec->ptLoad) if (sec == findFirstSection(l)) l->lmaOffset = state->lmaOffset; // We can call this method multiple times during the creation of // thunks and want to start over calculation each time. sec->size = 0; // We visited SectionsCommands from processSectionCommands to // layout sections. Now, we visit SectionsCommands again to fix // section offsets. for (SectionCommand *cmd : sec->commands) { // This handles the assignments to symbol or to the dot. if (auto *assign = dyn_cast(cmd)) { assign->addr = dot; assignSymbol(assign, true); assign->size = dot - assign->addr; continue; } // Handle BYTE(), SHORT(), LONG(), or QUAD(). if (auto *data = dyn_cast(cmd)) { data->offset = dot - sec->addr; dot += data->size; expandOutputSection(data->size); continue; } // Handle a single input section description command. // It calculates and assigns the offsets for each section and also // updates the output section size. for (InputSection *isec : cast(cmd)->sections) { assert(isec->getParent() == sec); const uint64_t pos = dot; dot = alignToPowerOf2(dot, isec->addralign); isec->outSecOff = dot - sec->addr; dot += isec->getSize(); // Update output section size after adding each section. This is so that // SIZEOF works correctly in the case below: // .foo { *(.aaa) a = SIZEOF(.foo); *(.bbb) } expandOutputSection(dot - pos); } } // If .relro_padding is present, round up the end to a common-page-size // boundary to protect the last page. if (in.relroPadding && sec == in.relroPadding->getParent()) expandOutputSection(alignToPowerOf2(dot, config->commonPageSize) - dot); // Non-SHF_ALLOC sections do not affect the addresses of other OutputSections // as they are not part of the process image. if (!(sec->flags & SHF_ALLOC)) { dot = savedDot; } else if (isTbss) { // NOBITS TLS sections are similar. Additionally save the end address. state->tbssAddr = dot; dot = savedDot; } } static bool isDiscardable(const OutputSection &sec) { if (sec.name == "/DISCARD/") return true; // We do not want to remove OutputSections with expressions that reference // symbols even if the OutputSection is empty. We want to ensure that the // expressions can be evaluated and report an error if they cannot. if (sec.expressionsUseSymbols) return false; // OutputSections may be referenced by name in ADDR and LOADADDR expressions, // as an empty Section can has a valid VMA and LMA we keep the OutputSection // to maintain the integrity of the other Expression. if (sec.usedInExpression) return false; for (SectionCommand *cmd : sec.commands) { if (auto assign = dyn_cast(cmd)) // Don't create empty output sections just for unreferenced PROVIDE // symbols. if (assign->name != "." && !assign->sym) continue; if (!isa(*cmd)) return false; } return true; } bool LinkerScript::isDiscarded(const OutputSection *sec) const { return hasSectionsCommand && (getFirstInputSection(sec) == nullptr) && isDiscardable(*sec); } static void maybePropagatePhdrs(OutputSection &sec, SmallVector &phdrs) { if (sec.phdrs.empty()) { // To match the bfd linker script behaviour, only propagate program // headers to sections that are allocated. if (sec.flags & SHF_ALLOC) sec.phdrs = phdrs; } else { phdrs = sec.phdrs; } } void LinkerScript::adjustOutputSections() { // If the output section contains only symbol assignments, create a // corresponding output section. The issue is what to do with linker script // like ".foo : { symbol = 42; }". One option would be to convert it to // "symbol = 42;". That is, move the symbol out of the empty section // description. That seems to be what bfd does for this simple case. The // problem is that this is not completely general. bfd will give up and // create a dummy section too if there is a ". = . + 1" inside the section // for example. // Given that we want to create the section, we have to worry what impact // it will have on the link. For example, if we just create a section with // 0 for flags, it would change which PT_LOADs are created. // We could remember that particular section is dummy and ignore it in // other parts of the linker, but unfortunately there are quite a few places // that would need to change: // * The program header creation. // * The orphan section placement. // * The address assignment. // The other option is to pick flags that minimize the impact the section // will have on the rest of the linker. That is why we copy the flags from // the previous sections. We copy just SHF_ALLOC and SHF_WRITE to keep the // impact low. We do not propagate SHF_EXECINSTR as in some cases this can // lead to executable writeable section. uint64_t flags = SHF_ALLOC; SmallVector defPhdrs; bool seenRelro = false; for (SectionCommand *&cmd : sectionCommands) { if (!isa(cmd)) continue; auto *sec = &cast(cmd)->osec; // Handle align (e.g. ".foo : ALIGN(16) { ... }"). if (sec->alignExpr) sec->addralign = std::max(sec->addralign, sec->alignExpr().getValue()); bool isEmpty = (getFirstInputSection(sec) == nullptr); bool discardable = isEmpty && isDiscardable(*sec); // If sec has at least one input section and not discarded, remember its // flags to be inherited by subsequent output sections. (sec may contain // just one empty synthetic section.) if (sec->hasInputSections && !discardable) flags = sec->flags; // We do not want to keep any special flags for output section // in case it is empty. if (isEmpty) sec->flags = flags & ((sec->nonAlloc ? 0 : (uint64_t)SHF_ALLOC) | SHF_WRITE); // The code below may remove empty output sections. We should save the // specified program headers (if exist) and propagate them to subsequent // sections which do not specify program headers. // An example of such a linker script is: // SECTIONS { .empty : { *(.empty) } :rw // .foo : { *(.foo) } } // Note: at this point the order of output sections has not been finalized, // because orphans have not been inserted into their expected positions. We // will handle them in adjustSectionsAfterSorting(). if (sec->sectionIndex != UINT32_MAX) maybePropagatePhdrs(*sec, defPhdrs); // Discard .relro_padding if we have not seen one RELRO section. Note: when // .tbss is the only RELRO section, there is no associated PT_LOAD segment // (needsPtLoad), so we don't append .relro_padding in the case. if (in.relroPadding && in.relroPadding->getParent() == sec && !seenRelro) discardable = true; if (discardable) { sec->markDead(); cmd = nullptr; } else { seenRelro |= sec->relro && !(sec->type == SHT_NOBITS && (sec->flags & SHF_TLS)); } } // It is common practice to use very generic linker scripts. So for any // given run some of the output sections in the script will be empty. // We could create corresponding empty output sections, but that would // clutter the output. // We instead remove trivially empty sections. The bfd linker seems even // more aggressive at removing them. llvm::erase_if(sectionCommands, [&](SectionCommand *cmd) { return !cmd; }); } void LinkerScript::adjustSectionsAfterSorting() { // Try and find an appropriate memory region to assign offsets in. MemoryRegion *hint = nullptr; for (SectionCommand *cmd : sectionCommands) { if (auto *osd = dyn_cast(cmd)) { OutputSection *sec = &osd->osec; if (!sec->lmaRegionName.empty()) { if (MemoryRegion *m = memoryRegions.lookup(sec->lmaRegionName)) sec->lmaRegion = m; else error("memory region '" + sec->lmaRegionName + "' not declared"); } std::tie(sec->memRegion, hint) = findMemoryRegion(sec, hint); } } // If output section command doesn't specify any segments, // and we haven't previously assigned any section to segment, // then we simply assign section to the very first load segment. // Below is an example of such linker script: // PHDRS { seg PT_LOAD; } // SECTIONS { .aaa : { *(.aaa) } } SmallVector defPhdrs; auto firstPtLoad = llvm::find_if(phdrsCommands, [](const PhdrsCommand &cmd) { return cmd.type == PT_LOAD; }); if (firstPtLoad != phdrsCommands.end()) defPhdrs.push_back(firstPtLoad->name); // Walk the commands and propagate the program headers to commands that don't // explicitly specify them. for (SectionCommand *cmd : sectionCommands) if (auto *osd = dyn_cast(cmd)) maybePropagatePhdrs(osd->osec, defPhdrs); } static uint64_t computeBase(uint64_t min, bool allocateHeaders) { // If there is no SECTIONS or if the linkerscript is explicit about program // headers, do our best to allocate them. if (!script->hasSectionsCommand || allocateHeaders) return 0; // Otherwise only allocate program headers if that would not add a page. return alignDown(min, config->maxPageSize); } // When the SECTIONS command is used, try to find an address for the file and // program headers output sections, which can be added to the first PT_LOAD // segment when program headers are created. // // We check if the headers fit below the first allocated section. If there isn't // enough space for these sections, we'll remove them from the PT_LOAD segment, // and we'll also remove the PT_PHDR segment. void LinkerScript::allocateHeaders(SmallVector &phdrs) { uint64_t min = std::numeric_limits::max(); for (OutputSection *sec : outputSections) if (sec->flags & SHF_ALLOC) min = std::min(min, sec->addr); auto it = llvm::find_if( phdrs, [](const PhdrEntry *e) { return e->p_type == PT_LOAD; }); if (it == phdrs.end()) return; PhdrEntry *firstPTLoad = *it; bool hasExplicitHeaders = llvm::any_of(phdrsCommands, [](const PhdrsCommand &cmd) { return cmd.hasPhdrs || cmd.hasFilehdr; }); bool paged = !config->omagic && !config->nmagic; uint64_t headerSize = getHeaderSize(); if ((paged || hasExplicitHeaders) && headerSize <= min - computeBase(min, hasExplicitHeaders)) { min = alignDown(min - headerSize, config->maxPageSize); Out::elfHeader->addr = min; Out::programHeaders->addr = min + Out::elfHeader->size; return; } // Error if we were explicitly asked to allocate headers. if (hasExplicitHeaders) error("could not allocate headers"); Out::elfHeader->ptLoad = nullptr; Out::programHeaders->ptLoad = nullptr; firstPTLoad->firstSec = findFirstSection(firstPTLoad); llvm::erase_if(phdrs, [](const PhdrEntry *e) { return e->p_type == PT_PHDR; }); } LinkerScript::AddressState::AddressState() { for (auto &mri : script->memoryRegions) { MemoryRegion *mr = mri.second; mr->curPos = (mr->origin)().getValue(); } } // Here we assign addresses as instructed by linker script SECTIONS // sub-commands. Doing that allows us to use final VA values, so here // we also handle rest commands like symbol assignments and ASSERTs. // Returns a symbol that has changed its section or value, or nullptr if no // symbol has changed. const Defined *LinkerScript::assignAddresses() { if (script->hasSectionsCommand) { // With a linker script, assignment of addresses to headers is covered by // allocateHeaders(). dot = config->imageBase.value_or(0); } else { // Assign addresses to headers right now. dot = target->getImageBase(); Out::elfHeader->addr = dot; Out::programHeaders->addr = dot + Out::elfHeader->size; dot += getHeaderSize(); } AddressState st; state = &st; errorOnMissingSection = true; st.outSec = aether; backwardDotErr.clear(); SymbolAssignmentMap oldValues = getSymbolAssignmentValues(sectionCommands); for (SectionCommand *cmd : sectionCommands) { if (auto *assign = dyn_cast(cmd)) { assign->addr = dot; assignSymbol(assign, false); assign->size = dot - assign->addr; continue; } assignOffsets(&cast(cmd)->osec); } state = nullptr; return getChangedSymbolAssignment(oldValues); } // Creates program headers as instructed by PHDRS linker script command. SmallVector LinkerScript::createPhdrs() { SmallVector ret; // Process PHDRS and FILEHDR keywords because they are not // real output sections and cannot be added in the following loop. for (const PhdrsCommand &cmd : phdrsCommands) { PhdrEntry *phdr = make(cmd.type, cmd.flags.value_or(PF_R)); if (cmd.hasFilehdr) phdr->add(Out::elfHeader); if (cmd.hasPhdrs) phdr->add(Out::programHeaders); if (cmd.lmaExpr) { phdr->p_paddr = cmd.lmaExpr().getValue(); phdr->hasLMA = true; } ret.push_back(phdr); } // Add output sections to program headers. for (OutputSection *sec : outputSections) { // Assign headers specified by linker script for (size_t id : getPhdrIndices(sec)) { ret[id]->add(sec); if (!phdrsCommands[id].flags) ret[id]->p_flags |= sec->getPhdrFlags(); } } return ret; } // Returns true if we should emit an .interp section. // // We usually do. But if PHDRS commands are given, and // no PT_INTERP is there, there's no place to emit an // .interp, so we don't do that in that case. bool LinkerScript::needsInterpSection() { if (phdrsCommands.empty()) return true; for (PhdrsCommand &cmd : phdrsCommands) if (cmd.type == PT_INTERP) return true; return false; } ExprValue LinkerScript::getSymbolValue(StringRef name, const Twine &loc) { if (name == ".") { if (state) return {state->outSec, false, dot - state->outSec->addr, loc}; error(loc + ": unable to get location counter value"); return 0; } if (Symbol *sym = symtab.find(name)) { if (auto *ds = dyn_cast(sym)) { ExprValue v{ds->section, false, ds->value, loc}; // Retain the original st_type, so that the alias will get the same // behavior in relocation processing. Any operation will reset st_type to // STT_NOTYPE. v.type = ds->type; return v; } if (isa(sym)) if (!errorOnMissingSection) return {nullptr, false, 0, loc}; } error(loc + ": symbol not found: " + name); return 0; } // Returns the index of the segment named Name. static std::optional getPhdrIndex(ArrayRef vec, StringRef name) { for (size_t i = 0; i < vec.size(); ++i) if (vec[i].name == name) return i; return std::nullopt; } // Returns indices of ELF headers containing specific section. Each index is a // zero based number of ELF header listed within PHDRS {} script block. SmallVector LinkerScript::getPhdrIndices(OutputSection *cmd) { SmallVector ret; for (StringRef s : cmd->phdrs) { if (std::optional idx = getPhdrIndex(phdrsCommands, s)) ret.push_back(*idx); else if (s != "NONE") error(cmd->location + ": program header '" + s + "' is not listed in PHDRS"); } return ret; } void LinkerScript::printMemoryUsage(raw_ostream& os) { auto printSize = [&](uint64_t size) { if ((size & 0x3fffffff) == 0) os << format_decimal(size >> 30, 10) << " GB"; else if ((size & 0xfffff) == 0) os << format_decimal(size >> 20, 10) << " MB"; else if ((size & 0x3ff) == 0) os << format_decimal(size >> 10, 10) << " KB"; else os << " " << format_decimal(size, 10) << " B"; }; os << "Memory region Used Size Region Size %age Used\n"; for (auto &pair : memoryRegions) { MemoryRegion *m = pair.second; uint64_t usedLength = m->curPos - m->getOrigin(); os << right_justify(m->name, 16) << ": "; printSize(usedLength); uint64_t length = m->getLength(); if (length != 0) { printSize(length); double percent = usedLength * 100.0 / length; os << " " << format("%6.2f%%", percent); } os << '\n'; } } static void checkMemoryRegion(const MemoryRegion *region, const OutputSection *osec, uint64_t addr) { uint64_t osecEnd = addr + osec->size; uint64_t regionEnd = region->getOrigin() + region->getLength(); if (osecEnd > regionEnd) { error("section '" + osec->name + "' will not fit in region '" + region->name + "': overflowed by " + Twine(osecEnd - regionEnd) + " bytes"); } } void LinkerScript::checkFinalScriptConditions() const { if (backwardDotErr.size()) errorOrWarn(backwardDotErr); for (const OutputSection *sec : outputSections) { if (const MemoryRegion *memoryRegion = sec->memRegion) checkMemoryRegion(memoryRegion, sec, sec->addr); if (const MemoryRegion *lmaRegion = sec->lmaRegion) checkMemoryRegion(lmaRegion, sec, sec->getLMA()); } }