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

601 lines
19 KiB
C++
Raw Blame History

//===- Writer.cpp ---------------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "Writer.h"
#include "Chunks.h"
#include "Config.h"
#include "Error.h"
#include "Symbols.h"
#include "SymbolTable.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace lld;
using namespace lld::elf2;
static const int PageSize = 4096;
namespace {
// OutputSection represents a section in an output file. It's a
// container of chunks. OutputSection and Chunk are 1:N relationship.
// Chunks cannot belong to more than one OutputSections. The writer
// creates multiple OutputSections and assign them unique,
// non-overlapping file offsets and VAs.
template <bool Is64Bits> class OutputSectionBase {
public:
typedef typename std::conditional<Is64Bits, uint64_t, uint32_t>::type uintX_t;
typedef
typename std::conditional<Is64Bits, Elf64_Shdr, Elf32_Shdr>::type HeaderT;
OutputSectionBase(StringRef Name, uint32_t sh_type, uintX_t sh_flags)
: Name(Name) {
memset(&Header, 0, sizeof(HeaderT));
Header.sh_type = sh_type;
Header.sh_flags = sh_flags;
}
void setVA(uintX_t VA) { Header.sh_addr = VA; }
uintX_t getVA() { return Header.sh_addr; }
void setFileOffset(uintX_t Off) { Header.sh_offset = Off; }
template <endianness E>
void writeHeaderTo(typename ELFFile<ELFType<E, Is64Bits>>::Elf_Shdr *SHdr);
StringRef getName() { return Name; }
void setNameOffset(uintX_t Offset) { Header.sh_name = Offset; }
unsigned getSectionIndex() const { return SectionIndex; }
void setSectionIndex(unsigned I) { SectionIndex = I; }
// Returns the size of the section in the output file.
uintX_t getSize() { return Header.sh_size; }
void setSize(uintX_t Val) { Header.sh_size = Val; }
uintX_t getFlags() { return Header.sh_flags; }
uintX_t getFileOff() { return Header.sh_offset; }
uintX_t getAlign() {
// The ELF spec states that a value of 0 means the section has no alignment
// constraits.
return std::max<uintX_t>(Header.sh_addralign, 1);
}
uint32_t getType() { return Header.sh_type; }
virtual void finalize() {}
virtual void writeTo(uint8_t *Buf) = 0;
protected:
StringRef Name;
HeaderT Header;
unsigned SectionIndex;
~OutputSectionBase() = default;
};
}
template <class ELFT>
class lld::elf2::OutputSection final
: public OutputSectionBase<ELFT::Is64Bits> {
public:
typedef typename OutputSectionBase<ELFT::Is64Bits>::uintX_t uintX_t;
typedef typename ELFFile<ELFT>::Elf_Shdr Elf_Shdr;
typedef typename ELFFile<ELFT>::Elf_Rela Elf_Rela;
OutputSection(StringRef Name, uint32_t sh_type, uintX_t sh_flags)
: OutputSectionBase<ELFT::Is64Bits>(Name, sh_type, sh_flags) {}
void addChunk(SectionChunk<ELFT> *C);
void writeTo(uint8_t *Buf) override;
private:
std::vector<SectionChunk<ELFT> *> Chunks;
};
namespace {
template <bool Is64Bits>
class StringTableSection final : public OutputSectionBase<Is64Bits> {
public:
llvm::StringTableBuilder StrTabBuilder;
typedef typename OutputSectionBase<Is64Bits>::uintX_t uintX_t;
StringTableSection() : OutputSectionBase<Is64Bits>(".strtab", SHT_STRTAB, 0) {
this->Header.sh_addralign = 1;
}
void add(StringRef S) { StrTabBuilder.add(S); }
size_t getFileOff(StringRef S) { return StrTabBuilder.getOffset(S); }
void writeTo(uint8_t *Buf) override;
void finalize() override {
StrTabBuilder.finalize(StringTableBuilder::ELF);
this->Header.sh_size = StrTabBuilder.data().size();
}
};
template <class ELFT>
class SymbolTableSection final : public OutputSectionBase<ELFT::Is64Bits> {
public:
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
typedef typename OutputSectionBase<ELFT::Is64Bits>::uintX_t uintX_t;
SymbolTableSection(SymbolTable &Table, llvm::StringTableBuilder &Builder)
: OutputSectionBase<ELFT::Is64Bits>(".symtab", SHT_SYMTAB, 0),
Table(Table), Builder(Builder) {
typedef OutputSectionBase<ELFT::Is64Bits> Base;
typename Base::HeaderT &Header = this->Header;
// For now the only local symbol is going to be the one at index 0
Header.sh_info = 1;
Header.sh_entsize = sizeof(Elf_Sym);
Header.sh_addralign = ELFT::Is64Bits ? 8 : 4;
}
void finalize() override {
this->Header.sh_size = (NumVisible + 1) * sizeof(Elf_Sym);
}
void setStringTableIndex(uint32_t Index) { this->Header.sh_link = Index; }
void writeTo(uint8_t *Buf) override;
const SymbolTable &getSymTable() { return Table; }
OutputSection<ELFT> *BSSSec = nullptr;
unsigned NumVisible = 0;
private:
SymbolTable &Table;
llvm::StringTableBuilder &Builder;
};
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
public:
typedef typename llvm::object::ELFFile<ELFT>::uintX_t uintX_t;
typedef typename llvm::object::ELFFile<ELFT>::Elf_Shdr Elf_Shdr;
typedef typename llvm::object::ELFFile<ELFT>::Elf_Sym Elf_Sym;
Writer(SymbolTable *T) : SymTable(*T, StringTable.StrTabBuilder) {}
void run();
private:
void createSections();
void assignAddresses();
void openFile(StringRef OutputPath);
void writeHeader();
void writeSections();
std::unique_ptr<llvm::FileOutputBuffer> Buffer;
llvm::SpecificBumpPtrAllocator<OutputSection<ELFT>> CAlloc;
std::vector<OutputSectionBase<ELFT::Is64Bits> *> OutputSections;
uintX_t FileSize;
uintX_t SizeOfHeaders;
uintX_t SectionHeaderOff;
unsigned StringTableIndex;
StringTableSection<ELFT::Is64Bits> StringTable;
SymbolTableSection<ELFT> SymTable;
unsigned NumSections;
void addOutputSection(OutputSectionBase<ELFT::Is64Bits> *Sec) {
OutputSections.push_back(Sec);
Sec->setSectionIndex(OutputSections.size());
}
};
} // anonymous namespace
namespace lld {
namespace elf2 {
template <class ELFT>
void writeResult(SymbolTable *Symtab) { Writer<ELFT>(Symtab).run(); }
template void writeResult<ELF32LE>(SymbolTable *);
template void writeResult<ELF32BE>(SymbolTable *);
template void writeResult<ELF64LE>(SymbolTable *);
template void writeResult<ELF64BE>(SymbolTable *);
} // namespace elf2
} // namespace lld
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
createSections();
assignAddresses();
openFile(Config->OutputFile);
writeHeader();
writeSections();
error(Buffer->commit());
}
template <class ELFT>
void OutputSection<ELFT>::addChunk(SectionChunk<ELFT> *C) {
Chunks.push_back(C);
C->setOutputSection(this);
uint32_t Align = C->getAlign();
if (Align > this->Header.sh_addralign)
this->Header.sh_addralign = Align;
uintX_t Off = this->Header.sh_size;
Off = RoundUpToAlignment(Off, Align);
C->setOutputSectionOff(Off);
Off += C->getSize();
this->Header.sh_size = Off;
}
template <class ELFT>
static typename llvm::object::ELFFile<ELFT>::uintX_t
getSymVA(DefinedRegular<ELFT> *DR) {
const SectionChunk<ELFT> *SC = &DR->Section;
OutputSection<ELFT> *OS = SC->getOutputSection();
return OS->getVA() + SC->getOutputSectionOff() + DR->Sym.st_value;
}
template <class ELFT> void OutputSection<ELFT>::writeTo(uint8_t *Buf) {
for (SectionChunk<ELFT> *C : Chunks) {
C->writeTo(Buf);
ObjectFile<ELFT> *File = C->getFile();
ELFFile<ELFT> *EObj = File->getObj();
uint8_t *Base = Buf + C->getOutputSectionOff();
// Iterate over all relocation sections that apply to this section.
for (const Elf_Shdr *RelSec : C->RelocSections) {
// Only support RELA for now.
if (RelSec->sh_type != SHT_RELA)
continue;
for (const Elf_Rela &RI : EObj->relas(RelSec)) {
uint32_t SymIndex = RI.getSymbol(EObj->isMips64EL());
SymbolBody *Body = File->getSymbolBody(SymIndex);
if (!Body)
continue;
// Skip unsupported for now.
if (!isa<DefinedRegular<ELFT>>(Body))
continue;
uintX_t Offset = RI.r_offset;
uint32_t Type = RI.getType(EObj->isMips64EL());
uintX_t P = this->getVA() + C->getOutputSectionOff();
uintX_t SymVA = getSymVA<ELFT>(cast<DefinedRegular<ELFT>>(Body));
uint8_t *Location = Base + Offset;
switch (Type) {
case llvm::ELF::R_X86_64_PC32:
support::endian::write32le(Location,
SymVA + (RI.r_addend - (P + Offset)));
break;
case llvm::ELF::R_X86_64_32:
case llvm::ELF::R_X86_64_32S:
support::endian::write32le(Location, SymVA + RI.r_addend);
break;
case llvm::ELF::R_X86_64_64:
support::endian::write64le(Location, SymVA + RI.r_addend);
break;
default:
llvm::errs() << Twine("unrecognized reloc ") + Twine(Type) << '\n';
break;
}
}
}
}
}
template <bool Is64Bits>
void StringTableSection<Is64Bits>::writeTo(uint8_t *Buf) {
StringRef Data = StrTabBuilder.data();
memcpy(Buf, Data.data(), Data.size());
}
template <class ELFT>
static int compareSym(const typename ELFFile<ELFT>::Elf_Sym *A,
const typename ELFFile<ELFT>::Elf_Sym *B) {
uint32_t AN = A->st_name;
uint32_t BN = B->st_name;
assert(AN != BN);
return AN - BN;
}
template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *Buf) {
uint8_t *BufStart = Buf;
Buf += sizeof(Elf_Sym);
for (auto &P : Table.getSymbols()) {
StringRef Name = P.first;
Symbol *Sym = P.second;
SymbolBody *Body = Sym->Body;
if (Body->isLazy())
continue;
const Elf_Sym &InputSym = cast<ELFSymbolBody<ELFT>>(Body)->Sym;
uint8_t V = Body->getMostConstrainingVisibility();
if (V != STV_DEFAULT && V != STV_PROTECTED)
continue;
auto *ESym = reinterpret_cast<Elf_Sym *>(Buf);
ESym->st_name = Builder.getOffset(Name);
const SectionChunk<ELFT> *Section = nullptr;
OutputSection<ELFT> *Out = nullptr;
switch (Body->kind()) {
case SymbolBody::DefinedRegularKind:
Section = &cast<DefinedRegular<ELFT>>(Body)->Section;
break;
case SymbolBody::DefinedCommonKind:
Out = BSSSec;
break;
case SymbolBody::UndefinedKind:
assert(Body->isWeak() && "Should be defined by now");
case SymbolBody::DefinedAbsoluteKind:
break;
case SymbolBody::LazyKind:
llvm_unreachable("Lazy symbol got to output symbol table!");
}
ESym->setBindingAndType(InputSym.getBinding(), InputSym.getType());
ESym->st_size = InputSym.st_size;
ESym->setVisibility(Body->getMostConstrainingVisibility());
if (InputSym.isAbsolute()) {
ESym->st_shndx = SHN_ABS;
ESym->st_value = InputSym.st_value;
}
if (Section)
Out = Section->getOutputSection();
if (Out) {
ESym->st_shndx = Out->getSectionIndex();
uintX_t VA = Out->getVA();
if (Section)
VA += Section->getOutputSectionOff();
if (auto *C = dyn_cast<DefinedCommon<ELFT>>(Body))
VA += C->OffsetInBSS;
else
VA += InputSym.st_value;
ESym->st_value = VA;
}
Buf += sizeof(Elf_Sym);
}
// The order the global symbols are in is not defined. We can use an arbitrary
// order, but it has to be reproducible. That is true even when cross linking.
// The default hashing of StringRef produces different results on 32 and 64
// bit systems so we sort by st_name. That is arbitrary but deterministic.
// FIXME: Experiment with passing in a custom hashing instead.
auto *Syms = reinterpret_cast<Elf_Sym *>(BufStart);
++Syms;
array_pod_sort(Syms, Syms + NumVisible, compareSym<ELFT>);
}
template <bool Is64Bits>
template <endianness E>
void OutputSectionBase<Is64Bits>::writeHeaderTo(
typename ELFFile<ELFType<E, Is64Bits>>::Elf_Shdr *SHdr) {
SHdr->sh_name = Header.sh_name;
SHdr->sh_type = Header.sh_type;
SHdr->sh_flags = Header.sh_flags;
SHdr->sh_addr = Header.sh_addr;
SHdr->sh_offset = Header.sh_offset;
SHdr->sh_size = Header.sh_size;
SHdr->sh_link = Header.sh_link;
SHdr->sh_info = Header.sh_info;
SHdr->sh_addralign = Header.sh_addralign;
SHdr->sh_entsize = Header.sh_entsize;
}
namespace {
template <bool Is64Bits> struct SectionKey {
typedef typename std::conditional<Is64Bits, uint64_t, uint32_t>::type uintX_t;
StringRef Name;
uint32_t sh_type;
uintX_t sh_flags;
};
}
namespace llvm {
template <bool Is64Bits> struct DenseMapInfo<SectionKey<Is64Bits>> {
static SectionKey<Is64Bits> getEmptyKey() {
return SectionKey<Is64Bits>{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
}
static SectionKey<Is64Bits> getTombstoneKey() {
return SectionKey<Is64Bits>{DenseMapInfo<StringRef>::getTombstoneKey(), 0,
0};
}
static unsigned getHashValue(const SectionKey<Is64Bits> &Val) {
return hash_combine(Val.Name, Val.sh_type, Val.sh_flags);
}
static bool isEqual(const SectionKey<Is64Bits> &LHS,
const SectionKey<Is64Bits> &RHS) {
return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
LHS.sh_type == RHS.sh_type && LHS.sh_flags == RHS.sh_flags;
}
};
}
template <class ELFT>
static bool cmpAlign(const DefinedCommon<ELFT> *A,
const DefinedCommon<ELFT> *B) {
return A->MaxAlignment > B->MaxAlignment;
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::createSections() {
SmallDenseMap<SectionKey<ELFT::Is64Bits>, OutputSection<ELFT> *> Map;
auto getSection = [&](StringRef Name, uint32_t sh_type,
uintX_t sh_flags) -> OutputSection<ELFT> * {
SectionKey<ELFT::Is64Bits> Key{Name, sh_type, sh_flags};
OutputSection<ELFT> *&Sec = Map[Key];
if (!Sec) {
Sec = new (CAlloc.Allocate())
OutputSection<ELFT>(Key.Name, Key.sh_type, Key.sh_flags);
addOutputSection(Sec);
}
return Sec;
};
const SymbolTable &Symtab = SymTable.getSymTable();
for (const std::unique_ptr<ObjectFileBase> &FileB : Symtab.getObjectFiles()) {
auto &File = cast<ObjectFile<ELFT>>(*FileB);
for (SectionChunk<ELFT> *C : File.getChunks()) {
if (!C)
continue;
const Elf_Shdr *H = C->getSectionHdr();
OutputSection<ELFT> *Sec =
getSection(C->getSectionName(), H->sh_type, H->sh_flags);
Sec->addChunk(C);
}
}
SymTable.BSSSec = getSection(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);
OutputSection<ELFT> *BSSSec = SymTable.BSSSec;
// FIXME: Try to avoid the extra walk over all global symbols.
unsigned &NumVisible = SymTable.NumVisible;
llvm::StringTableBuilder &Builder = StringTable.StrTabBuilder;
std::vector<DefinedCommon<ELFT> *> CommonSymbols;
for (auto &P : Symtab.getSymbols()) {
StringRef Name = P.first;
SymbolBody *Body = P.second->Body;
if (Body->isLazy())
continue;
if (auto *C = dyn_cast<DefinedCommon<ELFT>>(Body))
CommonSymbols.push_back(C);
uint8_t V = Body->getMostConstrainingVisibility();
if (V != STV_DEFAULT && V != STV_PROTECTED)
continue;
NumVisible++;
Builder.add(Name);
}
// Sort the common symbols by alignment as an heuristic to pack them better.
std::stable_sort(CommonSymbols.begin(), CommonSymbols.end(), cmpAlign<ELFT>);
uintX_t Off = BSSSec->getSize();
for (DefinedCommon<ELFT> *C : CommonSymbols) {
const Elf_Sym &Sym = C->Sym;
uintX_t Align = C->MaxAlignment;
Off = RoundUpToAlignment(Off, Align);
C->OffsetInBSS = Off;
Off += Sym.st_size;
}
BSSSec->setSize(Off);
}
template <bool Is64Bits>
static bool compSec(OutputSectionBase<Is64Bits> *A,
OutputSectionBase<Is64Bits> *B) {
// Place SHF_ALLOC sections first.
return (A->getFlags() & SHF_ALLOC) && !(B->getFlags() & SHF_ALLOC);
}
// Visits all sections to assign incremental, non-overlapping RVAs and
// file offsets.
template <class ELFT> void Writer<ELFT>::assignAddresses() {
SizeOfHeaders = RoundUpToAlignment(sizeof(Elf_Ehdr_Impl<ELFT>), PageSize);
uintX_t VA = 0x1000; // The first page is kept unmapped.
uintX_t FileOff = SizeOfHeaders;
std::stable_sort(OutputSections.begin(), OutputSections.end(),
compSec<ELFT::Is64Bits>);
addOutputSection(&SymTable);
addOutputSection(&StringTable);
StringTableIndex = OutputSections.size();
SymTable.setStringTableIndex(StringTableIndex);
for (OutputSectionBase<ELFT::Is64Bits> *Sec : OutputSections) {
StringTable.add(Sec->getName());
Sec->finalize();
uintX_t Align = Sec->getAlign();
uintX_t Size = Sec->getSize();
if (Sec->getFlags() & SHF_ALLOC) {
Sec->setVA(VA);
VA += RoundUpToAlignment(Size, Align);
}
Sec->setFileOffset(FileOff);
if (Sec->getType() != SHT_NOBITS)
FileOff += RoundUpToAlignment(Size, Align);
}
// Regular sections.
NumSections = OutputSections.size();
// First dummy section.
NumSections++;
FileOff += OffsetToAlignment(FileOff, ELFT::Is64Bits ? 8 : 4);
// Add space for section headers.
SectionHeaderOff = FileOff;
FileOff += NumSections * sizeof(Elf_Shdr_Impl<ELFT>);
FileSize = SizeOfHeaders + RoundUpToAlignment(FileOff - SizeOfHeaders, 8);
}
template <class ELFT> void Writer<ELFT>::writeHeader() {
uint8_t *Buf = Buffer->getBufferStart();
auto *EHdr = reinterpret_cast<Elf_Ehdr_Impl<ELFT> *>(Buf);
EHdr->e_ident[EI_MAG0] = 0x7F;
EHdr->e_ident[EI_MAG1] = 0x45;
EHdr->e_ident[EI_MAG2] = 0x4C;
EHdr->e_ident[EI_MAG3] = 0x46;
EHdr->e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
EHdr->e_ident[EI_DATA] = ELFT::TargetEndianness == llvm::support::little
? ELFDATA2LSB
: ELFDATA2MSB;
EHdr->e_ident[EI_VERSION] = EV_CURRENT;
EHdr->e_ident[EI_OSABI] = ELFOSABI_NONE;
EHdr->e_type = ET_EXEC;
const SymbolTable &Symtab = SymTable.getSymTable();
auto &FirstObj = cast<ObjectFile<ELFT>>(*Symtab.getFirstELF());
EHdr->e_machine = FirstObj.getEMachine();
EHdr->e_version = EV_CURRENT;
EHdr->e_entry = 0x401000;
EHdr->e_phoff = sizeof(Elf_Ehdr_Impl<ELFT>);
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_ehsize = sizeof(Elf_Ehdr_Impl<ELFT>);
EHdr->e_phentsize = sizeof(Elf_Phdr_Impl<ELFT>);
EHdr->e_phnum = 1;
EHdr->e_shentsize = sizeof(Elf_Shdr_Impl<ELFT>);
EHdr->e_shnum = NumSections;
EHdr->e_shstrndx = StringTableIndex;
auto PHdrs = reinterpret_cast<Elf_Phdr_Impl<ELFT> *>(Buf + EHdr->e_phoff);
PHdrs->p_type = PT_LOAD;
PHdrs->p_flags = PF_R | PF_X;
PHdrs->p_offset = 0x0000;
PHdrs->p_vaddr = 0x400000;
PHdrs->p_paddr = PHdrs->p_vaddr;
PHdrs->p_filesz = FileSize;
PHdrs->p_memsz = FileSize;
PHdrs->p_align = 0x4000;
auto SHdrs = reinterpret_cast<Elf_Shdr_Impl<ELFT> *>(Buf + EHdr->e_shoff);
// First entry is null.
++SHdrs;
for (OutputSectionBase<ELFT::Is64Bits> *Sec : OutputSections) {
Sec->setNameOffset(StringTable.getFileOff(Sec->getName()));
Sec->template writeHeaderTo<ELFT::TargetEndianness>(SHdrs++);
}
}
template <class ELFT> void Writer<ELFT>::openFile(StringRef Path) {
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Path, FileSize, FileOutputBuffer::F_executable);
error(BufferOrErr, Twine("failed to open ") + Path);
Buffer = std::move(*BufferOrErr);
}
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
uint8_t *Buf = Buffer->getBufferStart();
for (OutputSectionBase<ELFT::Is64Bits> *Sec : OutputSections)
Sec->writeTo(Buf + Sec->getFileOff());
}
Reference in New Issue View Git Blame Copy Permalink