maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity
llvm-project/lld/MachO/InputFiles.cpp

434 lines
17 KiB
C++
Raw Blame History

//===- InputFiles.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 functions to parse Mach-O object files. In this comment,
// we describe the Mach-O file structure and how we parse it.
//
// Mach-O is not very different from ELF or COFF. The notion of symbols,
// sections and relocations exists in Mach-O as it does in ELF and COFF.
//
// Perhaps the notion that is new to those who know ELF/COFF is "subsections".
// In ELF/COFF, sections are an atomic unit of data copied from input files to
// output files. When we merge or garbage-collect sections, we treat each
// section as an atomic unit. In Mach-O, that's not the case. Sections can
// consist of multiple subsections, and subsections are a unit of merging and
// garbage-collecting. Therefore, Mach-O's subsections are more similar to
// ELF/COFF's sections than Mach-O's sections are.
//
// A section can have multiple symbols. A symbol that does not have the
// N_ALT_ENTRY attribute indicates a beginning of a subsection. Therefore, by
// definition, a symbol is always present at the beginning of each subsection. A
// symbol with N_ALT_ENTRY attribute does not start a new subsection and can
// point to a middle of a subsection.
//
// The notion of subsections also affects how relocations are represented in
// Mach-O. All references within a section need to be explicitly represented as
// relocations if they refer to different subsections, because we obviously need
// to fix up addresses if subsections are laid out in an output file differently
// than they were in object files. To represent that, Mach-O relocations can
// refer to an unnamed location via its address. Scattered relocations (those
// with the R_SCATTERED bit set) always refer to unnamed locations.
// Non-scattered relocations refer to an unnamed location if r_extern is not set
// and r_symbolnum is zero.
//
// Without the above differences, I think you can use your knowledge about ELF
// and COFF for Mach-O.
//
//===----------------------------------------------------------------------===//
#include "InputFiles.h"
#include "Config.h"
#include "ExportTrie.h"
#include "InputSection.h"
#include "MachOStructs.h"
#include "OutputSection.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Target.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Memory.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Path.h"
using namespace llvm;
using namespace llvm::MachO;
using namespace llvm::support::endian;
using namespace llvm::sys;
using namespace lld;
using namespace lld::macho;
std::vector<InputFile *> macho::inputFiles;
// Open a given file path and return it as a memory-mapped file.
Optional<MemoryBufferRef> macho::readFile(StringRef path) {
// Open a file.
auto mbOrErr = MemoryBuffer::getFile(path);
if (auto ec = mbOrErr.getError()) {
error("cannot open " + path + ": " + ec.message());
return None;
}
std::unique_ptr<MemoryBuffer> &mb = *mbOrErr;
MemoryBufferRef mbref = mb->getMemBufferRef();
make<std::unique_ptr<MemoryBuffer>>(std::move(mb)); // take mb ownership
// If this is a regular non-fat file, return it.
const char *buf = mbref.getBufferStart();
auto *hdr = reinterpret_cast<const MachO::fat_header *>(buf);
if (read32be(&hdr->magic) != MachO::FAT_MAGIC)
return mbref;
// Object files and archive files may be fat files, which contains
// multiple real files for different CPU ISAs. Here, we search for a
// file that matches with the current link target and returns it as
// a MemoryBufferRef.
auto *arch = reinterpret_cast<const MachO::fat_arch *>(buf + sizeof(*hdr));
for (uint32_t i = 0, n = read32be(&hdr->nfat_arch); i < n; ++i) {
if (reinterpret_cast<const char *>(arch + i + 1) >
buf + mbref.getBufferSize()) {
error(path + ": fat_arch struct extends beyond end of file");
return None;
}
if (read32be(&arch[i].cputype) != target->cpuType ||
read32be(&arch[i].cpusubtype) != target->cpuSubtype)
continue;
uint32_t offset = read32be(&arch[i].offset);
uint32_t size = read32be(&arch[i].size);
if (offset + size > mbref.getBufferSize())
error(path + ": slice extends beyond end of file");
return MemoryBufferRef(StringRef(buf + offset, size), path.copy(bAlloc));
}
error("unable to find matching architecture in " + path);
return None;
}
static const load_command *findCommand(const mach_header_64 *hdr,
uint32_t type) {
const uint8_t *p =
reinterpret_cast<const uint8_t *>(hdr) + sizeof(mach_header_64);
for (uint32_t i = 0, n = hdr->ncmds; i < n; ++i) {
auto *cmd = reinterpret_cast<const load_command *>(p);
if (cmd->cmd == type)
return cmd;
p += cmd->cmdsize;
}
return nullptr;
}
void InputFile::parseSections(ArrayRef<section_64> sections) {
subsections.reserve(sections.size());
auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
for (const section_64 &sec : sections) {
InputSection *isec = make<InputSection>();
isec->file = this;
isec->name = StringRef(sec.sectname, strnlen(sec.sectname, 16));
isec->segname = StringRef(sec.segname, strnlen(sec.segname, 16));
isec->data = {isZeroFill(sec.flags) ? nullptr : buf + sec.offset,
static_cast<size_t>(sec.size)};
if (sec.align >= 32)
error("alignment " + std::to_string(sec.align) + " of section " +
isec->name + " is too large");
else
isec->align = 1 << sec.align;
isec->flags = sec.flags;
subsections.push_back({{0, isec}});
}
}
// Find the subsection corresponding to the greatest section offset that is <=
// that of the given offset.
//
// offset: an offset relative to the start of the original InputSection (before
// any subsection splitting has occurred). It will be updated to represent the
// same location as an offset relative to the start of the containing
// subsection.
static InputSection *findContainingSubsection(SubsectionMap &map,
uint32_t *offset) {
auto it = std::prev(map.upper_bound(*offset));
*offset -= it->first;
return it->second;
}
void InputFile::parseRelocations(const section_64 &sec,
SubsectionMap &subsecMap) {
auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
ArrayRef<any_relocation_info> relInfos(
reinterpret_cast<const any_relocation_info *>(buf + sec.reloff),
sec.nreloc);
for (const any_relocation_info &anyRel : relInfos) {
if (anyRel.r_word0 & R_SCATTERED)
fatal("TODO: Scattered relocations not supported");
auto rel = reinterpret_cast<const relocation_info &>(anyRel);
Reloc r;
r.type = rel.r_type;
r.pcrel = rel.r_pcrel;
r.length = rel.r_length;
uint64_t rawAddend = target->getImplicitAddend(mb, sec, rel);
if (rel.r_extern) {
r.target = symbols[rel.r_symbolnum];
r.addend = rawAddend;
} else {
if (rel.r_symbolnum == 0 || rel.r_symbolnum > subsections.size())
fatal("invalid section index in relocation for offset " +
std::to_string(r.offset) + " in section " + sec.sectname +
" of " + getName());
SubsectionMap &targetSubsecMap = subsections[rel.r_symbolnum - 1];
const section_64 &targetSec = sectionHeaders[rel.r_symbolnum - 1];
uint32_t targetOffset;
if (rel.r_pcrel) {
// The implicit addend for pcrel section relocations is the pcrel offset
// in terms of the addresses in the input file. Here we adjust it so
// that it describes the offset from the start of the target section.
// TODO: The offset of 4 is probably not right for ARM64, nor for
// relocations with r_length != 2.
targetOffset =
sec.addr + rel.r_address + 4 + rawAddend - targetSec.addr;
} else {
// The addend for a non-pcrel relocation is its absolute address.
targetOffset = rawAddend - targetSec.addr;
}
r.target = findContainingSubsection(targetSubsecMap, &targetOffset);
r.addend = targetOffset;
}
r.offset = rel.r_address;
InputSection *subsec = findContainingSubsection(subsecMap, &r.offset);
subsec->relocs.push_back(r);
}
}
void InputFile::parseSymbols(ArrayRef<structs::nlist_64> nList,
const char *strtab, bool subsectionsViaSymbols) {
// resize(), not reserve(), because we are going to create N_ALT_ENTRY symbols
// out-of-sequence.
symbols.resize(nList.size());
std::vector<size_t> altEntrySymIdxs;
auto createDefined = [&](const structs::nlist_64 &sym, InputSection *isec,
uint32_t value) -> Symbol * {
StringRef name = strtab + sym.n_strx;
if (sym.n_type & N_EXT)
// Global defined symbol
return symtab->addDefined(name, isec, value);
else
// Local defined symbol
return make<Defined>(name, isec, value);
};
for (size_t i = 0, n = nList.size(); i < n; ++i) {
const structs::nlist_64 &sym = nList[i];
// Undefined symbol
if (!sym.n_sect) {
StringRef name = strtab + sym.n_strx;
symbols[i] = symtab->addUndefined(name);
continue;
}
const section_64 &sec = sectionHeaders[sym.n_sect - 1];
SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
uint64_t offset = sym.n_value - sec.addr;
// If the input file does not use subsections-via-symbols, all symbols can
// use the same subsection. Otherwise, we must split the sections along
// symbol boundaries.
if (!subsectionsViaSymbols) {
symbols[i] = createDefined(sym, subsecMap[0], offset);
continue;
}
// nList entries aren't necessarily arranged in address order. Therefore,
// we can't create alt-entry symbols at this point because a later symbol
// may split its section, which may affect which subsection the alt-entry
// symbol is assigned to. So we need to handle them in a second pass below.
if (sym.n_desc & N_ALT_ENTRY) {
altEntrySymIdxs.push_back(i);
continue;
}
// Find the subsection corresponding to the greatest section offset that is
// <= that of the current symbol. The subsection that we find either needs
// to be used directly or split in two.
uint32_t firstSize = offset;
InputSection *firstIsec = findContainingSubsection(subsecMap, &firstSize);
if (firstSize == 0) {
// Alias of an existing symbol, or the first symbol in the section. These
// are handled by reusing the existing section.
symbols[i] = createDefined(sym, firstIsec, 0);
continue;
}
// We saw a symbol definition at a new offset. Split the section into two
// subsections. The new symbol uses the second subsection.
auto *secondIsec = make<InputSection>(*firstIsec);
secondIsec->data = firstIsec->data.slice(firstSize);
firstIsec->data = firstIsec->data.slice(0, firstSize);
// TODO: ld64 appears to preserve the original alignment as well as each
// subsection's offset from the last aligned address. We should consider
// emulating that behavior.
secondIsec->align = MinAlign(firstIsec->align, offset);
subsecMap[offset] = secondIsec;
// By construction, the symbol will be at offset zero in the new section.
symbols[i] = createDefined(sym, secondIsec, 0);
}
for (size_t idx : altEntrySymIdxs) {
const structs::nlist_64 &sym = nList[idx];
SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
uint32_t off = sym.n_value - sectionHeaders[sym.n_sect - 1].addr;
InputSection *subsec = findContainingSubsection(subsecMap, &off);
symbols[idx] = createDefined(sym, subsec, off);
}
}
ObjFile::ObjFile(MemoryBufferRef mb) : InputFile(ObjKind, mb) {
auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
auto *hdr = reinterpret_cast<const mach_header_64 *>(mb.getBufferStart());
if (const load_command *cmd = findCommand(hdr, LC_SEGMENT_64)) {
auto *c = reinterpret_cast<const segment_command_64 *>(cmd);
sectionHeaders = ArrayRef<section_64>{
reinterpret_cast<const section_64 *>(c + 1), c->nsects};
parseSections(sectionHeaders);
}
// TODO: Error on missing LC_SYMTAB?
if (const load_command *cmd = findCommand(hdr, LC_SYMTAB)) {
auto *c = reinterpret_cast<const symtab_command *>(cmd);
ArrayRef<structs::nlist_64> nList(
reinterpret_cast<const structs::nlist_64 *>(buf + c->symoff), c->nsyms);
const char *strtab = reinterpret_cast<const char *>(buf) + c->stroff;
bool subsectionsViaSymbols = hdr->flags & MH_SUBSECTIONS_VIA_SYMBOLS;
parseSymbols(nList, strtab, subsectionsViaSymbols);
}
// The relocations may refer to the symbols, so we parse them after we have
// parsed all the symbols.
for (size_t i = 0, n = subsections.size(); i < n; ++i)
parseRelocations(sectionHeaders[i], subsections[i]);
}
DylibFile::DylibFile(MemoryBufferRef mb, DylibFile *umbrella)
: InputFile(DylibKind, mb) {
if (umbrella == nullptr)
umbrella = this;
auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
auto *hdr = reinterpret_cast<const mach_header_64 *>(mb.getBufferStart());
// Initialize dylibName.
if (const load_command *cmd = findCommand(hdr, LC_ID_DYLIB)) {
auto *c = reinterpret_cast<const dylib_command *>(cmd);
dylibName = reinterpret_cast<const char *>(cmd) + read32le(&c->dylib.name);
} else {
error("dylib " + getName() + " missing LC_ID_DYLIB load command");
return;
}
// Initialize symbols.
if (const load_command *cmd = findCommand(hdr, LC_DYLD_INFO_ONLY)) {
auto *c = reinterpret_cast<const dyld_info_command *>(cmd);
parseTrie(buf + c->export_off, c->export_size,
[&](const Twine &name, uint64_t flags) {
symbols.push_back(symtab->addDylib(saver.save(name), umbrella));
});
} else {
error("LC_DYLD_INFO_ONLY not found in " + getName());
return;
}
if (hdr->flags & MH_NO_REEXPORTED_DYLIBS)
return;
const uint8_t *p =
reinterpret_cast<const uint8_t *>(hdr) + sizeof(mach_header_64);
for (uint32_t i = 0, n = hdr->ncmds; i < n; ++i) {
auto *cmd = reinterpret_cast<const load_command *>(p);
p += cmd->cmdsize;
if (cmd->cmd != LC_REEXPORT_DYLIB)
continue;
auto *c = reinterpret_cast<const dylib_command *>(cmd);
StringRef reexportPath =
reinterpret_cast<const char *>(c) + read32le(&c->dylib.name);
// TODO: Expand @loader_path, @executable_path etc in reexportPath
Optional<MemoryBufferRef> buffer = readFile(reexportPath);
if (!buffer) {
error("unable to read re-exported dylib at " + reexportPath);
return;
}
reexported.push_back(make<DylibFile>(*buffer, umbrella));
}
}
DylibFile::DylibFile(std::shared_ptr<llvm::MachO::InterfaceFile> interface,
DylibFile *umbrella)
: InputFile(DylibKind, MemoryBufferRef()) {
if (umbrella == nullptr)
umbrella = this;
dylibName = saver.save(interface->getInstallName());
// TODO(compnerd) filter out symbols based on the target platform
for (const auto symbol : interface->symbols())
if (symbol->getArchitectures().has(config->arch))
symbols.push_back(
symtab->addDylib(saver.save(symbol->getName()), umbrella));
// TODO(compnerd) properly represent the hierarchy of the documents as it is
// in theory possible to have re-exported dylibs from re-exported dylibs which
// should be parent'ed to the child.
for (auto document : interface->documents())
reexported.push_back(make<DylibFile>(document, umbrella));
}
ArchiveFile::ArchiveFile(std::unique_ptr<llvm::object::Archive> &&f)
: InputFile(ArchiveKind, f->getMemoryBufferRef()), file(std::move(f)) {
for (const object::Archive::Symbol &sym : file->symbols())
symtab->addLazy(sym.getName(), this, sym);
}
void ArchiveFile::fetch(const object::Archive::Symbol &sym) {
object::Archive::Child c =
CHECK(sym.getMember(), toString(this) +
": could not get the member for symbol " +
sym.getName());
if (!seen.insert(c.getChildOffset()).second)
return;
MemoryBufferRef mb =
CHECK(c.getMemoryBufferRef(),
toString(this) +
": could not get the buffer for the member defining symbol " +
sym.getName());
auto file = make<ObjFile>(mb);
symbols.insert(symbols.end(), file->symbols.begin(), file->symbols.end());
subsections.insert(subsections.end(), file->subsections.begin(),
file->subsections.end());
}
// Returns "<internal>" or "baz.o".
std::string lld::toString(const InputFile *file) {
return file ? std::string(file->getName()) : "<internal>";
}
Reference in New Issue View Git Blame Copy Permalink