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llvm-project/lld/MachO/Writer.cpp

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//===- 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 "Config.h"
#include "InputFiles.h"
#include "InputSection.h"
#include "MergedOutputSection.h"
#include "OutputSection.h"
#include "OutputSegment.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "UnwindInfoSection.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Memory.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/Path.h"
#include <algorithm>
using namespace llvm;
using namespace llvm::MachO;
using namespace lld;
using namespace lld::macho;
namespace {
class LCLinkEdit;
class LCDyldInfo;
class LCSymtab;
class Writer {
public:
Writer() : buffer(errorHandler().outputBuffer) {}
void scanRelocations();
void createOutputSections();
void createLoadCommands();
void assignAddresses(OutputSegment *);
void createSymtabContents();
void openFile();
void writeSections();
void run();
std::unique_ptr<FileOutputBuffer> &buffer;
uint64_t addr = 0;
uint64_t fileOff = 0;
MachHeaderSection *header = nullptr;
StringTableSection *stringTableSection = nullptr;
SymtabSection *symtabSection = nullptr;
IndirectSymtabSection *indirectSymtabSection = nullptr;
UnwindInfoSection *unwindInfoSection = nullptr;
};
// LC_DYLD_INFO_ONLY stores the offsets of symbol import/export information.
class LCDyldInfo : public LoadCommand {
public:
LCDyldInfo(RebaseSection *rebaseSection, BindingSection *bindingSection,
WeakBindingSection *weakBindingSection,
LazyBindingSection *lazyBindingSection,
ExportSection *exportSection)
: rebaseSection(rebaseSection), bindingSection(bindingSection),
weakBindingSection(weakBindingSection),
lazyBindingSection(lazyBindingSection), exportSection(exportSection) {}
uint32_t getSize() const override { return sizeof(dyld_info_command); }
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<dyld_info_command *>(buf);
c->cmd = LC_DYLD_INFO_ONLY;
c->cmdsize = getSize();
if (rebaseSection->isNeeded()) {
c->rebase_off = rebaseSection->fileOff;
c->rebase_size = rebaseSection->getFileSize();
}
if (bindingSection->isNeeded()) {
c->bind_off = bindingSection->fileOff;
c->bind_size = bindingSection->getFileSize();
}
if (weakBindingSection->isNeeded()) {
c->weak_bind_off = weakBindingSection->fileOff;
c->weak_bind_size = weakBindingSection->getFileSize();
}
if (lazyBindingSection->isNeeded()) {
c->lazy_bind_off = lazyBindingSection->fileOff;
c->lazy_bind_size = lazyBindingSection->getFileSize();
}
if (exportSection->isNeeded()) {
c->export_off = exportSection->fileOff;
c->export_size = exportSection->getFileSize();
}
}
RebaseSection *rebaseSection;
BindingSection *bindingSection;
WeakBindingSection *weakBindingSection;
LazyBindingSection *lazyBindingSection;
ExportSection *exportSection;
};
class LCDysymtab : public LoadCommand {
public:
LCDysymtab(IndirectSymtabSection *indirectSymtabSection)
: indirectSymtabSection(indirectSymtabSection) {}
uint32_t getSize() const override { return sizeof(dysymtab_command); }
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<dysymtab_command *>(buf);
c->cmd = LC_DYSYMTAB;
c->cmdsize = getSize();
c->indirectsymoff = indirectSymtabSection->fileOff;
c->nindirectsyms = indirectSymtabSection->getNumSymbols();
}
IndirectSymtabSection *indirectSymtabSection = nullptr;
};
class LCSegment : public LoadCommand {
public:
LCSegment(StringRef name, OutputSegment *seg) : name(name), seg(seg) {}
uint32_t getSize() const override {
return sizeof(segment_command_64) +
seg->numNonHiddenSections() * sizeof(section_64);
}
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<segment_command_64 *>(buf);
buf += sizeof(segment_command_64);
c->cmd = LC_SEGMENT_64;
c->cmdsize = getSize();
memcpy(c->segname, name.data(), name.size());
c->fileoff = seg->fileOff;
c->maxprot = seg->maxProt;
c->initprot = seg->initProt;
if (seg->getSections().empty())
return;
c->vmaddr = seg->firstSection()->addr;
c->vmsize =
seg->lastSection()->addr + seg->lastSection()->getSize() - c->vmaddr;
c->nsects = seg->numNonHiddenSections();
for (OutputSection *osec : seg->getSections()) {
if (!isZeroFill(osec->flags)) {
assert(osec->fileOff >= seg->fileOff);
c->filesize = std::max(
c->filesize, osec->fileOff + osec->getFileSize() - seg->fileOff);
}
if (osec->isHidden())
continue;
auto *sectHdr = reinterpret_cast<section_64 *>(buf);
buf += sizeof(section_64);
memcpy(sectHdr->sectname, osec->name.data(), osec->name.size());
memcpy(sectHdr->segname, name.data(), name.size());
sectHdr->addr = osec->addr;
sectHdr->offset = osec->fileOff;
sectHdr->align = Log2_32(osec->align);
sectHdr->flags = osec->flags;
sectHdr->size = osec->getSize();
sectHdr->reserved1 = osec->reserved1;
sectHdr->reserved2 = osec->reserved2;
}
}
private:
StringRef name;
OutputSegment *seg;
};
class LCMain : public LoadCommand {
uint32_t getSize() const override { return sizeof(entry_point_command); }
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<entry_point_command *>(buf);
c->cmd = LC_MAIN;
c->cmdsize = getSize();
if (config->entry->isInStubs())
c->entryoff =
in.stubs->fileOff + config->entry->stubsIndex * target->stubSize;
else
c->entryoff = config->entry->getFileOffset();
c->stacksize = 0;
}
};
class LCSymtab : public LoadCommand {
public:
LCSymtab(SymtabSection *symtabSection, StringTableSection *stringTableSection)
: symtabSection(symtabSection), stringTableSection(stringTableSection) {}
uint32_t getSize() const override { return sizeof(symtab_command); }
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<symtab_command *>(buf);
c->cmd = LC_SYMTAB;
c->cmdsize = getSize();
c->symoff = symtabSection->fileOff;
c->nsyms = symtabSection->getNumSymbols();
c->stroff = stringTableSection->fileOff;
c->strsize = stringTableSection->getFileSize();
}
SymtabSection *symtabSection = nullptr;
StringTableSection *stringTableSection = nullptr;
};
// There are several dylib load commands that share the same structure:
// * LC_LOAD_DYLIB
// * LC_ID_DYLIB
// * LC_REEXPORT_DYLIB
class LCDylib : public LoadCommand {
public:
LCDylib(LoadCommandType type, StringRef path) : type(type), path(path) {
instanceCount++;
}
uint32_t getSize() const override {
return alignTo(sizeof(dylib_command) + path.size() + 1, 8);
}
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<dylib_command *>(buf);
buf += sizeof(dylib_command);
c->cmd = type;
c->cmdsize = getSize();
c->dylib.name = sizeof(dylib_command);
memcpy(buf, path.data(), path.size());
buf[path.size()] = '\0';
}
static uint32_t getInstanceCount() { return instanceCount; }
private:
LoadCommandType type;
StringRef path;
static uint32_t instanceCount;
};
uint32_t LCDylib::instanceCount = 0;
class LCLoadDylinker : public LoadCommand {
public:
uint32_t getSize() const override {
return alignTo(sizeof(dylinker_command) + path.size() + 1, 8);
}
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<dylinker_command *>(buf);
buf += sizeof(dylinker_command);
c->cmd = LC_LOAD_DYLINKER;
c->cmdsize = getSize();
c->name = sizeof(dylinker_command);
memcpy(buf, path.data(), path.size());
buf[path.size()] = '\0';
}
private:
// Recent versions of Darwin won't run any binary that has dyld at a
// different location.
const StringRef path = "/usr/lib/dyld";
};
class LCRPath : public LoadCommand {
public:
LCRPath(StringRef path) : path(path) {}
uint32_t getSize() const override {
return alignTo(sizeof(rpath_command) + path.size() + 1, WordSize);
}
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<rpath_command *>(buf);
buf += sizeof(rpath_command);
c->cmd = LC_RPATH;
c->cmdsize = getSize();
c->path = sizeof(rpath_command);
memcpy(buf, path.data(), path.size());
buf[path.size()] = '\0';
}
private:
StringRef path;
};
class LCBuildVersion : public LoadCommand {
public:
LCBuildVersion(const PlatformInfo &platform) : platform(platform) {}
const int ntools = 1;
uint32_t getSize() const override {
return sizeof(build_version_command) + ntools * sizeof(build_tool_version);
}
void writeTo(uint8_t *buf) const override {
auto *c = reinterpret_cast<build_version_command *>(buf);
c->cmd = LC_BUILD_VERSION;
c->cmdsize = getSize();
c->platform = static_cast<uint32_t>(platform.kind);
c->minos = ((platform.minimum.getMajor() << 020) |
(platform.minimum.getMinor().getValueOr(0) << 010) |
platform.minimum.getSubminor().getValueOr(0));
c->sdk = ((platform.sdk.getMajor() << 020) |
(platform.sdk.getMinor().getValueOr(0) << 010) |
platform.sdk.getSubminor().getValueOr(0));
c->ntools = ntools;
auto *t = reinterpret_cast<build_tool_version *>(&c[1]);
t->tool = TOOL_LD;
t->version = (LLVM_VERSION_MAJOR << 020) | (LLVM_VERSION_MINOR << 010) |
LLVM_VERSION_PATCH;
}
const PlatformInfo &platform;
};
} // namespace
void Writer::scanRelocations() {
for (InputSection *isec : inputSections) {
// We do not wish to add rebase opcodes for __LD,__compact_unwind, because
// it doesn't actually end up in the final binary. TODO: filtering it out
// before Writer runs might be cleaner...
if (isec->segname == segment_names::ld)
continue;
for (Reloc &r : isec->relocs) {
if (auto *s = r.referent.dyn_cast<lld::macho::Symbol *>()) {
if (isa<Undefined>(s))
error("undefined symbol " + s->getName() + ", referenced from " +
sys::path::filename(isec->file->getName()));
else
target->prepareSymbolRelocation(s, isec, r);
} else {
assert(r.referent.is<InputSection *>());
if (!r.pcrel)
in.rebase->addEntry(isec, r.offset);
}
}
}
}
void Writer::createLoadCommands() {
in.header->addLoadCommand(make<LCDyldInfo>(
in.rebase, in.binding, in.weakBinding, in.lazyBinding, in.exports));
in.header->addLoadCommand(make<LCSymtab>(symtabSection, stringTableSection));
in.header->addLoadCommand(make<LCDysymtab>(indirectSymtabSection));
for (StringRef path : config->runtimePaths)
in.header->addLoadCommand(make<LCRPath>(path));
switch (config->outputType) {
case MH_EXECUTE:
in.header->addLoadCommand(make<LCMain>());
in.header->addLoadCommand(make<LCLoadDylinker>());
break;
case MH_DYLIB:
in.header->addLoadCommand(make<LCDylib>(LC_ID_DYLIB, config->installName));
break;
case MH_BUNDLE:
break;
default:
llvm_unreachable("unhandled output file type");
}
in.header->addLoadCommand(make<LCBuildVersion>(config->platform));
uint8_t segIndex = 0;
for (OutputSegment *seg : outputSegments) {
in.header->addLoadCommand(make<LCSegment>(seg->name, seg));
seg->index = segIndex++;
}
uint64_t dylibOrdinal = 1;
for (InputFile *file : inputFiles) {
if (auto *dylibFile = dyn_cast<DylibFile>(file)) {
// TODO: dylibs that are only referenced by weak refs should also be
// loaded via LC_LOAD_WEAK_DYLIB.
LoadCommandType lcType =
dylibFile->forceWeakImport ? LC_LOAD_WEAK_DYLIB : LC_LOAD_DYLIB;
in.header->addLoadCommand(make<LCDylib>(lcType, dylibFile->dylibName));
dylibFile->ordinal = dylibOrdinal++;
if (dylibFile->reexport)
in.header->addLoadCommand(
make<LCDylib>(LC_REEXPORT_DYLIB, dylibFile->dylibName));
}
}
const uint32_t MACOS_MAXPATHLEN = 1024;
config->headerPad = std::max(
config->headerPad, (config->headerPadMaxInstallNames
? LCDylib::getInstanceCount() * MACOS_MAXPATHLEN
: 0));
}
static size_t getSymbolPriority(const SymbolPriorityEntry &entry,
const InputFile &file) {
return std::max(entry.objectFiles.lookup(sys::path::filename(file.getName())),
entry.anyObjectFile);
}
// Each section gets assigned the priority of the highest-priority symbol it
// contains.
static DenseMap<const InputSection *, size_t> buildInputSectionPriorities() {
DenseMap<const InputSection *, size_t> sectionPriorities;
if (config->priorities.empty())
return sectionPriorities;
auto addSym = [&](Defined &sym) {
auto it = config->priorities.find(sym.getName());
if (it == config->priorities.end())
return;
SymbolPriorityEntry &entry = it->second;
size_t &priority = sectionPriorities[sym.isec];
priority = std::max(priority, getSymbolPriority(entry, *sym.isec->file));
};
// TODO: Make sure this handles weak symbols correctly.
for (InputFile *file : inputFiles)
if (isa<ObjFile>(file) || isa<ArchiveFile>(file))
for (lld::macho::Symbol *sym : file->symbols)
if (auto *d = dyn_cast<Defined>(sym))
addSym(*d);
return sectionPriorities;
}
static int segmentOrder(OutputSegment *seg) {
return StringSwitch<int>(seg->name)
.Case(segment_names::pageZero, -2)
.Case(segment_names::text, -1)
// Make sure __LINKEDIT is the last segment (i.e. all its hidden
// sections must be ordered after other sections).
.Case(segment_names::linkEdit, std::numeric_limits<int>::max())
.Default(0);
}
static int sectionOrder(OutputSection *osec) {
StringRef segname = osec->parent->name;
// Sections are uniquely identified by their segment + section name.
if (segname == segment_names::text) {
return StringSwitch<int>(osec->name)
.Case(section_names::header, -1)
.Case(section_names::unwindInfo, std::numeric_limits<int>::max() - 1)
.Case(section_names::ehFrame, std::numeric_limits<int>::max())
.Default(0);
} else if (segname == segment_names::linkEdit) {
return StringSwitch<int>(osec->name)
.Case(section_names::rebase, -8)
.Case(section_names::binding, -7)
.Case(section_names::weakBinding, -6)
.Case(section_names::lazyBinding, -5)
.Case(section_names::export_, -4)
.Case(section_names::symbolTable, -3)
.Case(section_names::indirectSymbolTable, -2)
.Case(section_names::stringTable, -1)
.Default(0);
}
// ZeroFill sections must always be the at the end of their segments,
// otherwise subsequent sections may get overwritten with zeroes at runtime.
if (isZeroFill(osec->flags))
return std::numeric_limits<int>::max();
return 0;
}
template <typename T, typename F>
static std::function<bool(T, T)> compareByOrder(F ord) {
return [=](T a, T b) { return ord(a) < ord(b); };
}
// Sorting only can happen once all outputs have been collected. Here we sort
// segments, output sections within each segment, and input sections within each
// output segment.
static void sortSegmentsAndSections() {
llvm::stable_sort(outputSegments,
compareByOrder<OutputSegment *>(segmentOrder));
DenseMap<const InputSection *, size_t> isecPriorities =
buildInputSectionPriorities();
uint32_t sectionIndex = 0;
for (OutputSegment *seg : outputSegments) {
seg->sortOutputSections(compareByOrder<OutputSection *>(sectionOrder));
for (auto *osec : seg->getSections()) {
// Now that the output sections are sorted, assign the final
// output section indices.
if (!osec->isHidden())
osec->index = ++sectionIndex;
if (!isecPriorities.empty()) {
if (auto *merged = dyn_cast<MergedOutputSection>(osec)) {
llvm::stable_sort(merged->inputs,
[&](InputSection *a, InputSection *b) {
return isecPriorities[a] > isecPriorities[b];
});
}
}
}
}
}
void Writer::createOutputSections() {
// First, create hidden sections
stringTableSection = make<StringTableSection>();
unwindInfoSection = make<UnwindInfoSection>(); // TODO(gkm): only when no -r
symtabSection = make<SymtabSection>(*stringTableSection);
indirectSymtabSection = make<IndirectSymtabSection>();
switch (config->outputType) {
case MH_EXECUTE:
make<PageZeroSection>();
break;
case MH_DYLIB:
case MH_BUNDLE:
break;
default:
llvm_unreachable("unhandled output file type");
}
// Then merge input sections into output sections.
MapVector<std::pair<StringRef, StringRef>, MergedOutputSection *>
mergedOutputSections;
for (InputSection *isec : inputSections) {
MergedOutputSection *&osec =
mergedOutputSections[{isec->segname, isec->name}];
if (osec == nullptr)
osec = make<MergedOutputSection>(isec->name);
osec->mergeInput(isec);
}
for (const auto &it : mergedOutputSections) {
StringRef segname = it.first.first;
MergedOutputSection *osec = it.second;
if (unwindInfoSection && segname == segment_names::ld) {
assert(osec->name == section_names::compactUnwind);
unwindInfoSection->setCompactUnwindSection(osec);
} else
getOrCreateOutputSegment(segname)->addOutputSection(osec);
}
for (SyntheticSection *ssec : syntheticSections) {
auto it = mergedOutputSections.find({ssec->segname, ssec->name});
if (it == mergedOutputSections.end()) {
if (ssec->isNeeded())
getOrCreateOutputSegment(ssec->segname)->addOutputSection(ssec);
} else {
error("section from " + it->second->firstSection()->file->getName() +
" conflicts with synthetic section " + ssec->segname + "," +
ssec->name);
}
}
}
void Writer::assignAddresses(OutputSegment *seg) {
addr = alignTo(addr, PageSize);
fileOff = alignTo(fileOff, PageSize);
seg->fileOff = fileOff;
for (auto *osec : seg->getSections()) {
if (!osec->isNeeded())
continue;
addr = alignTo(addr, osec->align);
fileOff = alignTo(fileOff, osec->align);
osec->addr = addr;
osec->fileOff = isZeroFill(osec->flags) ? 0 : fileOff;
osec->finalize();
addr += osec->getSize();
fileOff += osec->getFileSize();
}
}
void Writer::openFile() {
Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
FileOutputBuffer::create(config->outputFile, fileOff,
FileOutputBuffer::F_executable);
if (!bufferOrErr)
error("failed to open " + config->outputFile + ": " +
llvm::toString(bufferOrErr.takeError()));
else
buffer = std::move(*bufferOrErr);
}
void Writer::writeSections() {
uint8_t *buf = buffer->getBufferStart();
for (OutputSegment *seg : outputSegments)
for (OutputSection *osec : seg->getSections())
osec->writeTo(buf + osec->fileOff);
}
void Writer::run() {
// dyld requires __LINKEDIT segment to always exist (even if empty).
OutputSegment *linkEditSegment =
getOrCreateOutputSegment(segment_names::linkEdit);
prepareBranchTarget(config->entry);
scanRelocations();
if (in.stubHelper->isNeeded())
in.stubHelper->setup();
for (const macho::Symbol *sym : symtab->getSymbols())
if (const auto *defined = dyn_cast<Defined>(sym))
if (defined->overridesWeakDef)
in.weakBinding->addNonWeakDefinition(defined);
// Sort and assign sections to their respective segments. No more sections nor
// segments may be created after these methods run.
createOutputSections();
sortSegmentsAndSections();
createLoadCommands();
// Ensure that segments (and the sections they contain) are allocated
// addresses in ascending order, which dyld requires.
//
// Note that at this point, __LINKEDIT sections are empty, but we need to
// determine addresses of other segments/sections before generating its
// contents.
for (OutputSegment *seg : outputSegments)
if (seg != linkEditSegment)
assignAddresses(seg);
// Fill __LINKEDIT contents.
in.rebase->finalizeContents();
in.binding->finalizeContents();
in.weakBinding->finalizeContents();
in.lazyBinding->finalizeContents();
in.exports->finalizeContents();
symtabSection->finalizeContents();
indirectSymtabSection->finalizeContents();
// Now that __LINKEDIT is filled out, do a proper calculation of its
// addresses and offsets.
assignAddresses(linkEditSegment);
openFile();
if (errorCount())
return;
writeSections();
if (auto e = buffer->commit())
error("failed to write to the output file: " + toString(std::move(e)));
}
void macho::writeResult() { Writer().run(); }
void macho::createSyntheticSections() {
in.header = make<MachHeaderSection>();
in.rebase = make<RebaseSection>();
in.binding = make<BindingSection>();
in.weakBinding = make<WeakBindingSection>();
in.lazyBinding = make<LazyBindingSection>();
in.exports = make<ExportSection>();
in.got = make<GotSection>();
in.tlvPointers = make<TlvPointerSection>();
in.lazyPointers = make<LazyPointerSection>();
in.stubs = make<StubsSection>();
in.stubHelper = make<StubHelperSection>();
in.imageLoaderCache = make<ImageLoaderCacheSection>();
}
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