blob: 8ab274e0f693f5878e192ee4ea434eaf5ded93a2 [file] [log] [blame]
//===- 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 "Config.h"
#include "Filesystem.h"
#include "LinkerScript.h"
#include "MapFile.h"
#include "Memory.h"
#include "OutputSections.h"
#include "Relocations.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Threads.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/FileOutputBuffer.h"
#include "llvm/Support/raw_ostream.h"
#include <climits>
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;
namespace {
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
typedef typename ELFT::Shdr Elf_Shdr;
typedef typename ELFT::Ehdr Elf_Ehdr;
typedef typename ELFT::Phdr Elf_Phdr;
void run();
void createSyntheticSections();
void copyLocalSymbols();
void addSectionSymbols();
void addReservedSymbols();
void createSections();
void forEachRelSec(std::function<void(InputSectionBase &)> Fn);
void sortSections();
void finalizeSections();
void addPredefinedSections();
std::vector<PhdrEntry> createPhdrs();
void removeEmptyPTLoad();
void addPtArmExid(std::vector<PhdrEntry> &Phdrs);
void assignFileOffsets();
void assignFileOffsetsBinary();
void setPhdrs();
void fixSectionAlignments();
void fixPredefinedSymbols();
void openFile();
void writeHeader();
void writeSections();
void writeSectionsBinary();
void writeBuildId();
std::unique_ptr<FileOutputBuffer> Buffer;
std::vector<OutputSection *> OutputSections;
OutputSectionFactory Factory{OutputSections};
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSection *Sec);
uint64_t getEntryAddr();
OutputSection *findSection(StringRef Name);
std::vector<PhdrEntry> Phdrs;
uint64_t FileSize;
uint64_t SectionHeaderOff;
} // anonymous namespace
StringRef elf::getOutputSectionName(StringRef Name) {
if (Config->Relocatable)
return Name;
// If -emit-relocs is given (which is rare), we need to copy
// relocation sections to the output. If input section .foo is
// output as .bar, we want to rename as well.
if (Config->EmitRelocs) {
for (StringRef V : {".rel.", ".rela."}) {
if (Name.startswith(V)) {
StringRef Inner = getOutputSectionName(Name.substr(V.size() - 1));
return + Inner);
for (StringRef V :
{".text.", ".rodata.", "", ".data.", "",
".bss.", ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
".gcc_except_table.", ".tdata.", ".ARM.exidx."}) {
StringRef Prefix = V.drop_back();
if (Name.startswith(V) || Name == Prefix)
return Prefix;
// CommonSection is identified as "COMMON" in linker scripts.
// By default, it should go to .bss section.
if (Name == "COMMON")
return ".bss";
// ".zdebug_" is a prefix for ZLIB-compressed sections.
// Because we decompressed input sections, we want to remove 'z'.
if (Name.startswith(".zdebug_"))
return"." + Name.substr(2));
return Name;
template <class ELFT> static bool needsInterpSection() {
return !Symtab<ELFT>::X->getSharedFiles().empty() &&
!Config->DynamicLinker.empty() && !Script->ignoreInterpSection();
template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
template <class ELFT> void Writer<ELFT>::removeEmptyPTLoad() {
auto I = std::remove_if(Phdrs.begin(), Phdrs.end(), [&](const PhdrEntry &P) {
if (P.p_type != PT_LOAD)
return false;
if (!P.First)
return true;
uint64_t Size = P.Last->Addr + P.Last->Size - P.First->Addr;
return Size == 0;
Phdrs.erase(I, Phdrs.end());
// This function scans over the input sections and creates mergeable
// synthetic sections. It removes MergeInputSections from array and
// adds new synthetic ones. Each synthetic section is added to the
// location of the first input section it replaces.
static void combineMergableSections() {
std::vector<MergeSyntheticSection *> MergeSections;
for (InputSectionBase *&S : InputSections) {
MergeInputSection *MS = dyn_cast<MergeInputSection>(S);
if (!MS)
// We do not want to handle sections that are not alive, so just remove
// them instead of trying to merge.
if (!MS->Live)
StringRef OutsecName = getOutputSectionName(MS->Name);
uint64_t Flags = MS->Flags & ~(uint64_t)(SHF_GROUP | SHF_COMPRESSED);
uint32_t Alignment = std::max<uint32_t>(MS->Alignment, MS->Entsize);
auto I = llvm::find_if(MergeSections, [=](MergeSyntheticSection *Sec) {
return Sec->Name == OutsecName && Sec->Flags == Flags &&
Sec->Alignment == Alignment;
if (I == MergeSections.end()) {
MergeSyntheticSection *Syn =
make<MergeSyntheticSection>(OutsecName, MS->Type, Flags, Alignment);
I = std::prev(MergeSections.end());
S = Syn;
} else {
S = nullptr;
std::vector<InputSectionBase *> &V = InputSections;
V.erase(std::remove(V.begin(), V.end(), nullptr), V.end());
template <class ELFT> static void combineEhFrameSections() {
for (InputSectionBase *&S : InputSections) {
EhInputSection *ES = dyn_cast<EhInputSection>(S);
if (!ES || !ES->Live)
S = nullptr;
std::vector<InputSectionBase *> &V = InputSections;
V.erase(std::remove(V.begin(), V.end(), nullptr), V.end());
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
// Create linker-synthesized sections such as .got or .plt.
// Such sections are of type input section.
if (!Config->Relocatable)
// We need to create some reserved symbols such as _end. Create them.
if (!Config->Relocatable)
// Create output sections.
Script->OutputSections = &OutputSections;
if (Script->Opt.HasSections) {
// If linker script contains SECTIONS commands, let it create sections.
// Linker scripts may have left some input sections unassigned.
// Assign such sections using the default rule.
} else {
// If linker script does not contain SECTIONS commands, create
// output sections by default rules. We still need to give the
// linker script a chance to run, because it might contain
// non-SECTIONS commands such as ASSERT.
if (Config->Discard != DiscardPolicy::All)
if (Config->CopyRelocs)
// Now that we have a complete set of output sections. This function
// completes section contents. For example, we need to add strings
// to the string table, and add entries to .got and .plt.
// finalizeSections does that.
if (ErrorCount)
if (Config->Relocatable) {
} else {
if (!Script->Opt.HasSections) {
// Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a
// 0 sized region. This has to be done late since only after assignAddresses
// we know the size of the sections.
if (!Config->OFormatBinary)
// It does not make sense try to open the file if we have error already.
if (ErrorCount)
// Write the result down to a file.
if (ErrorCount)
if (!Config->OFormatBinary) {
} else {
// Backfill section content. This is done at last
// because the content is usually a hash value of the entire output file.
if (ErrorCount)
// Handle -Map option.
if (ErrorCount)
if (auto EC = Buffer->commit())
error("failed to write to the output file: " + EC.message());
// Flush the output streams and exit immediately. A full shutdown
// is a good test that we are keeping track of all allocated memory,
// but actually freeing it is a waste of time in a regular linker run.
if (Config->ExitEarly)
// Initialize Out members.
template <class ELFT> void Writer<ELFT>::createSyntheticSections() {
// Initialize all pointers with NULL. This is needed because
// you can call lld::elf::main more than once as a library.
memset(&Out::First, 0, sizeof(Out));
auto Add = [](InputSectionBase *Sec) { InputSections.push_back(Sec); };
InX::DynStrTab = make<StringTableSection>(".dynstr", true);
InX::Dynamic = make<DynamicSection<ELFT>>();
In<ELFT>::RelaDyn = make<RelocationSection<ELFT>>(
Config->IsRela ? ".rela.dyn" : ".rel.dyn", Config->ZCombreloc);
InX::ShStrTab = make<StringTableSection>(".shstrtab", false);
Out::ElfHeader = make<OutputSection>("", 0, SHF_ALLOC);
Out::ElfHeader->Size = sizeof(Elf_Ehdr);
Out::ProgramHeaders = make<OutputSection>("", 0, SHF_ALLOC);
if (needsInterpSection<ELFT>()) {
InX::Interp = createInterpSection();
} else {
InX::Interp = nullptr;
if (!Config->Relocatable)
if (Config->Strip != StripPolicy::All) {
InX::StrTab = make<StringTableSection>(".strtab", false);
In<ELFT>::SymTab = make<SymbolTableSection<ELFT>>(*InX::StrTab);
if (Config->BuildId != BuildIdKind::None) {
InX::BuildId = make<BuildIdSection>();
InX::Common = createCommonSection<ELFT>();
if (InX::Common)
InX::Bss = make<BssSection>(".bss");
InX::BssRelRo = make<BssSection>("");
// Add MIPS-specific sections.
bool HasDynSymTab = !Symtab<ELFT>::X->getSharedFiles().empty() ||
Config->Pic || Config->ExportDynamic;
if (Config->EMachine == EM_MIPS) {
//if (!Config->Shared && HasDynSymTab) {
//InX::MipsRldMap = make<MipsRldMapSection>();
if (auto *Sec = MipsAbiFlagsSection<ELFT>::create())
if (auto *Sec = MipsOptionsSection<ELFT>::create())
if (auto *Sec = MipsReginfoSection<ELFT>::create())
//If ReadOnlyDynamic is set then we're going to need to emit a
//RldMapSection for DT_DEBUG_INDIRECT/DT_MIPS_RLD_MAP to point to.
if(!Config->Shared && HasDynSymTab && Config->ReadOnlyDynamic) {
In<ELFT>::RldMap = make<RldMapSection>();
if (HasDynSymTab) {
In<ELFT>::DynSymTab = make<SymbolTableSection<ELFT>>(*InX::DynStrTab);
In<ELFT>::VerSym = make<VersionTableSection<ELFT>>();
if (!Config->VersionDefinitions.empty()) {
In<ELFT>::VerDef = make<VersionDefinitionSection<ELFT>>();
In<ELFT>::VerNeed = make<VersionNeedSection<ELFT>>();
if (Config->GnuHash) {
In<ELFT>::GnuHashTab = make<GnuHashTableSection<ELFT>>();
if (Config->SysvHash) {
In<ELFT>::HashTab = make<HashTableSection<ELFT>>();
// Add .got. MIPS' .got is so different from the other archs,
// it has its own class.
if (Config->EMachine == EM_MIPS) {
InX::MipsGot = make<MipsGotSection>();
} else {
InX::Got = make<GotSection<ELFT>>();
InX::GotPlt = make<GotPltSection>();
InX::IgotPlt = make<IgotPltSection>();
if (Config->GdbIndex) {
InX::GdbIndex = make<GdbIndexSection>();
// We always need to add rel[a].plt to output if it has entries.
// Even for static linking it can contain R_[*]_IRELATIVE relocations.
In<ELFT>::RelaPlt = make<RelocationSection<ELFT>>(
Config->IsRela ? ".rela.plt" : ".rel.plt", false /*Sort*/);
// The RelaIplt immediately follows .rel.plt (.rel.dyn for ARM) to ensure
// that the IRelative relocations are processed last by the dynamic loader
In<ELFT>::RelaIplt = make<RelocationSection<ELFT>>(
(Config->EMachine == EM_ARM) ? ".rel.dyn" : In<ELFT>::RelaPlt->Name,
false /*Sort*/);
InX::Plt = make<PltSection>(Target->PltHeaderSize);
InX::Iplt = make<PltSection>(0);
if (!Config->Relocatable) {
if (Config->EhFrameHdr) {
In<ELFT>::EhFrameHdr = make<EhFrameHeader<ELFT>>();
In<ELFT>::EhFrame = make<EhFrameSection<ELFT>>();
if (In<ELFT>::SymTab)
if (InX::StrTab)
static bool shouldKeepInSymtab(SectionBase *Sec, StringRef SymName,
const SymbolBody &B) {
if (B.isFile() || B.isSection())
return false;
// If sym references a section in a discarded group, don't keep it.
if (Sec == &InputSection::Discarded)
return false;
if (Config->Discard == DiscardPolicy::None)
return true;
// In ELF assembly .L symbols are normally discarded by the assembler.
// If the assembler fails to do so, the linker discards them if
// * --discard-locals is used.
// * The symbol is in a SHF_MERGE section, which is normally the reason for
// the assembler keeping the .L symbol.
if (!SymName.startswith(".L") && !SymName.empty())
return true;
if (Config->Discard == DiscardPolicy::Locals)
return false;
return !Sec || !(Sec->Flags & SHF_MERGE);
static bool includeInSymtab(const SymbolBody &B) {
if (!B.isLocal() && !B.symbol()->IsUsedInRegularObj)
return false;
if (auto *D = dyn_cast<DefinedRegular>(&B)) {
// Always include absolute symbols.
SectionBase *Sec = D->Section;
if (!Sec)
return true;
if (auto *IS = dyn_cast<InputSectionBase>(Sec)) {
Sec = IS->Repl;
IS = cast<InputSectionBase>(Sec);
// Exclude symbols pointing to garbage-collected sections.
if (!IS->Live)
return false;
if (auto *S = dyn_cast<MergeInputSection>(Sec))
if (!S->getSectionPiece(D->Value)->Live)
return false;
return true;
// Local symbols are not in the linker's symbol table. This function scans
// each object file's symbol table to copy local symbols to the output.
template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
if (!In<ELFT>::SymTab)
for (elf::ObjectFile<ELFT> *F : Symtab<ELFT>::X->getObjectFiles()) {
for (SymbolBody *B : F->getLocalSymbols()) {
if (!B->IsLocal)
fatal(toString(F) +
": broken object: getLocalSymbols returns a non-local symbol");
auto *DR = dyn_cast<DefinedRegular>(B);
// No reason to keep local undefined symbol in symtab.
if (!DR)
if (!includeInSymtab(*B))
SectionBase *Sec = DR->Section;
if (!shouldKeepInSymtab(Sec, B->getName(), *B))
template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
// Create one STT_SECTION symbol for each output section we might
// have a relocation with.
for (OutputSection *Sec : OutputSections) {
if (Sec->Sections.empty())
InputSection *IS = Sec->Sections[0];
if (isa<SyntheticSection>(IS) || IS->Type == SHT_REL ||
IS->Type == SHT_RELA)
auto *Sym =
make<DefinedRegular>("", /*IsLocal=*/true, /*StOther=*/0, STT_SECTION,
/*Value=*/0, /*Size=*/0, IS, nullptr);
// Today's loaders have a feature to make segments read-only after
// processing dynamic relocations to enhance security. PT_GNU_RELRO
// is defined for that.
// This function returns true if a section needs to be put into a
// PT_GNU_RELRO segment.
bool elf::isRelroSection(const OutputSection *Sec) {
if (!Config->ZRelro)
return false;
uint64_t Flags = Sec->Flags;
// Non-allocatable or non-writable sections don't need RELRO because
// they are not writable or not even mapped to memory in the first place.
// RELRO is for sections that are essentially read-only but need to
// be writable only at process startup to allow dynamic linker to
// apply relocations.
if (!(Flags & SHF_ALLOC) || !(Flags & SHF_WRITE))
return false;
// Once initialized, TLS data segments are used as data templates
// for a thread-local storage. For each new thread, runtime
// allocates memory for a TLS and copy templates there. No thread
// are supposed to use templates directly. Thus, it can be in RELRO.
if (Flags & SHF_TLS)
return true;
// .init_array, .preinit_array and .fini_array contain pointers to
// functions that are executed on process startup or exit. These
// pointers are set by the static linker, and they are not expected
// to change at runtime. But if you are an attacker, you could do
// interesting things by manipulating pointers in .fini_array, for
// example. So they are put into RELRO.
uint32_t Type = Sec->Type;
if (Type == SHT_INIT_ARRAY || Type == SHT_FINI_ARRAY ||
return true;
// .got contains pointers to external symbols. They are resolved by
// the dynamic linker when a module is loaded into memory, and after
// that they are not expected to change. So, it can be in RELRO.
if (InX::Got && Sec == InX::Got->OutSec)
return true;
// .got.plt contains pointers to external function symbols. They are
// by default resolved lazily, so we usually cannot put it into RELRO.
// However, if "-z now" is given, the lazy symbol resolution is
// disabled, which enables us to put it into RELRO.
if (Sec == InX::GotPlt->OutSec)
return Config->ZNow;
// .dynamic section contains data for the dynamic linker, and
// there's no need to write to it at runtime, so it's better to put
// it into RELRO.
if (Sec == InX::Dynamic->OutSec)
return true;
// is used for copy relocations for read-only symbols.
// Since the dynamic linker needs to process copy relocations, the
// section cannot be read-only, but once initialized, they shouldn't
// change.
if (Sec == InX::BssRelRo->OutSec)
return true;
// Sections with some special names are put into RELRO. This is a
// bit unfortunate because section names shouldn't be significant in
// ELF in spirit. But in reality many linker features depend on
// magic section names.
StringRef S = Sec->Name;
return S == "" || S == ".ctors" || S == ".dtors" || S == ".jcr" ||
S == ".eh_frame" || S == ".openbsd.randomdata";
// We compute a rank for each section. The rank indicates where the
// section should be placed in the file. Instead of using simple
// numbers (0,1,2...), we use a series of flags. One for each decision
// point when placing the section.
// Using flags has two key properties:
// * It is easy to check if a give branch was taken.
// * It is easy two see how similar two ranks are (see getRankProximity).
enum RankFlags {
RF_NOT_ADDR_SET = 1 << 16,
RF_NOT_INTERP = 1 << 15,
RF_NOT_ALLOC = 1 << 14,
RF_WRITE = 1 << 13,
RF_EXEC = 1 << 12,
RF_NON_TLS_BSS = 1 << 11,
RF_NON_TLS_BSS_RO = 1 << 10,
RF_NOT_TLS = 1 << 9,
RF_BSS = 1 << 8,
RF_PPC_OPD = 1 << 6,
RF_PPC_TOCL = 1 << 5,
RF_PPC_TOC = 1 << 4,
RF_PPC_BRANCH_LT = 1 << 3,
RF_MIPS_GPREL = 1 << 2,
RF_MIPS_NOT_GOT = 1 << 1
static unsigned getSectionRank(const OutputSection *Sec) {
unsigned Rank = 0;
// We want to put section specified by -T option first, so we
// can start assigning VA starting from them later.
if (Config->SectionStartMap.count(Sec->Name))
return Rank;
// Put .interp first because some loaders want to see that section
// on the first page of the executable file when loaded into memory.
if (Sec->Name == ".interp")
return Rank;
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
if (!(Sec->Flags & SHF_ALLOC))
return Rank | RF_NOT_ALLOC;
// We want the read only sections first so that they go in the PT_LOAD
// covering the program headers at the start of the file.
if (Sec->Flags & SHF_WRITE)
Rank |= RF_WRITE;
if (Sec->Flags & SHF_EXECINSTR) {
// For a corresponding reason, put non exec sections first (the program
// header PT_LOAD is not executable).
// We only do that if we are not using linker scripts, since with linker
// scripts ro and rx sections are in the same PT_LOAD, so their relative
// order is not important. The same applies for -no-rosegment.
if ((Rank & RF_WRITE) || !Config->SingleRoRx)
Rank |= RF_EXEC;
// If we got here we know that both A and B are in the same PT_LOAD.
bool IsTls = Sec->Flags & SHF_TLS;
bool IsNoBits = Sec->Type == SHT_NOBITS;
// The first requirement we have is to put (non-TLS) nobits sections last. The
// reason is that the only thing the dynamic linker will see about them is a
// p_memsz that is larger than p_filesz. Seeing that it zeros the end of the
// PT_LOAD, so that has to correspond to the nobits sections.
bool IsNonTlsNoBits = IsNoBits && !IsTls;
if (IsNonTlsNoBits)
// We place nobits RelRo sections before plain r/w ones, and non-nobits RelRo
// sections after r/w ones, so that the RelRo sections are contiguous.
bool IsRelRo = isRelroSection(Sec);
if (IsNonTlsNoBits && !IsRelRo)
if (!IsNonTlsNoBits && IsRelRo)
// The TLS initialization block needs to be a single contiguous block in a R/W
// PT_LOAD, so stick TLS sections directly before the other RelRo R/W
// sections. The TLS NOBITS sections are placed here as they don't take up
// virtual address space in the PT_LOAD.
if (!IsTls)
Rank |= RF_NOT_TLS;
// Within the TLS initialization block, the non-nobits sections need to appear
// first.
if (IsNoBits)
Rank |= RF_BSS;
// // Some architectures have additional ordering restrictions for sections
// // within the same PT_LOAD.
if (Config->EMachine == EM_PPC64) {
// PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
// that we would like to make sure appear is a specific order to maximize
// their coverage by a single signed 16-bit offset from the TOC base
// pointer. Conversely, the special .tocbss section should be first among
// all SHT_NOBITS sections. This will put it next to the loaded special
// PPC64 sections (and, thus, within reach of the TOC base pointer).
StringRef Name = Sec->Name;
if (Name != ".tocbss")
if (Name == ".opd")
Rank |= RF_PPC_OPD;
if (Name == ".toc1")
Rank |= RF_PPC_TOCL;
if (Name == ".toc")
Rank |= RF_PPC_TOC;
if (Name == ".branch_lt")
if (Config->EMachine == EM_MIPS) {
// All sections with SHF_MIPS_GPREL flag should be grouped together
// because data in these sections is addressable with a gp relative address.
if (Sec->Flags & SHF_MIPS_GPREL)
if (Sec->Name != ".got")
return Rank;
static bool compareSectionsNonScript(const OutputSection *A,
const OutputSection *B) {
if (A->SortRank != B->SortRank)
return A->SortRank < B->SortRank;
if (!(A->SortRank & RF_NOT_ADDR_SET))
return Config->SectionStartMap.lookup(A->Name) <
return false;
// Output section ordering is determined by this function.
static bool compareSections(const OutputSection *A, const OutputSection *B) {
// For now, put sections mentioned in a linker script
// first. Sections not on linker script will have a SectionIndex of
int AIndex = A->SectionIndex;
int BIndex = B->SectionIndex;
if (AIndex != BIndex)
return AIndex < BIndex;
return compareSectionsNonScript(A, B);
// Program header entry
PhdrEntry::PhdrEntry(unsigned Type, unsigned Flags) {
p_type = Type;
p_flags = Flags;
void PhdrEntry::add(OutputSection *Sec) {
Last = Sec;
if (!First)
First = Sec;
p_align = std::max(p_align, Sec->Alignment);
if (p_type == PT_LOAD)
Sec->FirstInPtLoad = First;
template <class ELFT>
static Symbol *addRegular(StringRef Name, SectionBase *Sec, uint64_t Value,
uint8_t StOther = STV_HIDDEN,
uint8_t Binding = STB_WEAK) {
// The linker generated symbols are added as STB_WEAK to allow user defined
// ones to override them.
return Symtab<ELFT>::X->addRegular(Name, StOther, STT_NOTYPE, Value,
/*Size=*/0, Binding, Sec,
template <class ELFT>
static DefinedRegular *
addOptionalRegular(StringRef Name, SectionBase *Sec, uint64_t Val,
uint8_t StOther = STV_HIDDEN, uint8_t Binding = STB_GLOBAL) {
SymbolBody *S = Symtab<ELFT>::X->find(Name);
if (!S)
return nullptr;
if (S->isInCurrentDSO())
return nullptr;
return cast<DefinedRegular>(
addRegular<ELFT>(Name, Sec, Val, StOther, Binding)->body());
// The beginning and the ending of .rel[a].plt section are marked
// with __rel[a]_iplt_{start,end} symbols if it is a statically linked
// executable. The runtime needs these symbols in order to resolve
// all IRELATIVE relocs on startup. For dynamic executables, we don't
// need these symbols, since IRELATIVE relocs are resolved through GOT
// and PLT. For details, see
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
if (In<ELFT>::DynSymTab)
StringRef S = Config->IsRela ? "__rela_iplt_start" : "__rel_iplt_start";
addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, 0, STV_HIDDEN, STB_WEAK);
S = Config->IsRela ? "__rela_iplt_end" : "__rel_iplt_end";
addOptionalRegular<ELFT>(S, In<ELFT>::RelaIplt, -1, STV_HIDDEN, STB_WEAK);
// The linker is expected to define some symbols depending on
// the linking result. This function defines such symbols.
template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
if (Config->EMachine == EM_MIPS) {
// Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
// so that it points to an absolute address which by default is relative
// to GOT. Default offset is 0x7ff0.
// See "Global Data Symbols" in Chapter 6 in the following document:
ElfSym::MipsGp = Symtab<ELFT>::X->addAbsolute("_gp", STV_HIDDEN, STB_LOCAL);
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT.
if (Symtab<ELFT>::X->find("_gp_disp"))
ElfSym::MipsGpDisp =
Symtab<ELFT>::X->addAbsolute("_gp_disp", STV_HIDDEN, STB_LOCAL);
// The __gnu_local_gp is a magic symbol equal to the current value of 'gp'
// pointer. This symbol is used in the code generated by .cpload pseudo-op
// in case of using -mno-shared option.
if (Symtab<ELFT>::X->find("__gnu_local_gp"))
ElfSym::MipsLocalGp =
Symtab<ELFT>::X->addAbsolute("__gnu_local_gp", STV_HIDDEN, STB_LOCAL);
// In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol
// is magical and is used to produce a R_386_GOTPC relocation.
// The R_386_GOTPC relocation value doesn't actually depend on the
// symbol value, so it could use an index of STN_UNDEF which, according
// to the spec, means the symbol value is 0.
// Unfortunately both gas and MC keep the _GLOBAL_OFFSET_TABLE_ symbol in
// the object file.
// The situation is even stranger on x86_64 where the assembly doesn't
// need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
// an undefined symbol in the .o files.
// Given that the symbol is effectively unused, we just create a dummy
// hidden one to avoid the undefined symbol error.
// __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
// static linking the linker is required to optimize away any references to
// __tls_get_addr, so it's not defined anywhere. Create a hidden definition
// to avoid the undefined symbol error.
if (!In<ELFT>::DynSymTab)
// __ehdr_start is the location of ELF file headers. Note that we define
// this symbol unconditionally even when using a linker script, which
// differs from the behavior implemented by GNU linker which only define
// this symbol if ELF headers are in the memory mapped segment.
addOptionalRegular<ELFT>("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
// If linker script do layout we do not need to create any standart symbols.
if (Script->Opt.HasSections)
auto Add = [](StringRef S) {
return addOptionalRegular<ELFT>(S, Out::ElfHeader, 0, STV_DEFAULT);
ElfSym::Bss = Add("__bss_start");
ElfSym::End1 = Add("end");
ElfSym::End2 = Add("_end");
ElfSym::Etext1 = Add("etext");
ElfSym::Etext2 = Add("_etext");
ElfSym::Edata1 = Add("edata");
ElfSym::Edata2 = Add("_edata");
// Sort input sections by section name suffixes for
// __attribute__((init_priority(N))).
static void sortInitFini(OutputSection *S) {
if (S)
reinterpret_cast<OutputSection *>(S)->sortInitFini();
// Sort input sections by the special rule for .ctors and .dtors.
static void sortCtorsDtors(OutputSection *S) {
if (S)
reinterpret_cast<OutputSection *>(S)->sortCtorsDtors();
// Sort input sections using the list provided by --symbol-ordering-file.
template <class ELFT>
static void sortBySymbolsOrder(ArrayRef<OutputSection *> OutputSections) {
if (Config->SymbolOrderingFile.empty())
// Build a map from symbols to their priorities. Symbols that didn't
// appear in the symbol ordering file have the lowest priority 0.
// All explicitly mentioned symbols have negative (higher) priorities.
DenseMap<StringRef, int> SymbolOrder;
int Priority = -Config->SymbolOrderingFile.size();
for (StringRef S : Config->SymbolOrderingFile)
SymbolOrder.insert({S, Priority++});
// Build a map from sections to their priorities.
DenseMap<SectionBase *, int> SectionOrder;
for (elf::ObjectFile<ELFT> *File : Symtab<ELFT>::X->getObjectFiles()) {
for (SymbolBody *Body : File->getSymbols()) {
auto *D = dyn_cast<DefinedRegular>(Body);
if (!D || !D->Section)
int &Priority = SectionOrder[D->Section];
Priority = std::min(Priority, SymbolOrder.lookup(D->getName()));
// Sort sections by priority.
for (OutputSection *Base : OutputSections)
if (auto *Sec = dyn_cast<OutputSection>(Base))
Sec->sort([&](InputSectionBase *S) { return SectionOrder.lookup(S); });
template <class ELFT>
void Writer<ELFT>::forEachRelSec(std::function<void(InputSectionBase &)> Fn) {
for (InputSectionBase *IS : InputSections) {
if (!IS->Live)
// Scan all relocations. Each relocation goes through a series
// of tests to determine if it needs special treatment, such as
// creating GOT, PLT, copy relocations, etc.
// Note that relocations for non-alloc sections are directly
// processed by InputSection::relocateNonAlloc.
if (!(IS->Flags & SHF_ALLOC))
if (isa<InputSection>(IS) || isa<EhInputSection>(IS))
if (!Config->Relocatable) {
for (EhInputSection *ES : In<ELFT>::EhFrame->Sections)
template <class ELFT> void Writer<ELFT>::createSections() {
for (InputSectionBase *IS : InputSections)
if (IS)
Factory.addInputSec(IS, getOutputSectionName(IS->Name));
for (OutputSection *Sec : OutputSections)
// We want to find how similar two ranks are.
// The more branches in getSectionRank that match, the more similar they are.
// Since each branch corresponds to a bit flag, we can just use
// countLeadingZeros.
static unsigned getRankProximity(OutputSection *A, OutputSection *B) {
return countLeadingZeros(A->SortRank ^ B->SortRank);
// We want to place orphan sections so that they share as much
// characteristics with their neighbors as possible. For example, if
// both are rw, or both are tls.
template <typename ELFT>
static std::vector<OutputSection *>::iterator
findOrphanPos(std::vector<OutputSection *>::iterator B,
std::vector<OutputSection *>::iterator E) {
OutputSection *Sec = *E;
// Find the first element that has as close a rank as possible.
auto I = std::max_element(B, E, [=](OutputSection *A, OutputSection *B) {
return getRankProximity(Sec, A) < getRankProximity(Sec, B);
if (I == E)
return E;
// Consider all existing sections with the same proximity.
unsigned Proximity = getRankProximity(Sec, *I);
while (I != E && getRankProximity(Sec, *I) == Proximity &&
Sec->SortRank >= (*I)->SortRank)
return I;
template <class ELFT> void Writer<ELFT>::sortSections() {
// Don't sort if using -r. It is not necessary and we want to preserve the
// relative order for SHF_LINK_ORDER sections.
if (Config->Relocatable)
if (Script->Opt.HasSections)
for (OutputSection *Sec : OutputSections)
Sec->SortRank = getSectionRank(Sec);
if (!Script->Opt.HasSections) {
std::stable_sort(OutputSections.begin(), OutputSections.end(),
// The order of the sections in the script is arbitrary and may not agree with
// compareSectionsNonScript. This means that we cannot easily define a
// strict weak ordering. To see why, consider a comparison of a section in the
// script and one not in the script. We have a two simple options:
// * Make them equivalent (a is not less than b, and b is not less than a).
// The problem is then that equivalence has to be transitive and we can
// have sections a, b and c with only b in a script and a less than c
// which breaks this property.
// * Use compareSectionsNonScript. Given that the script order doesn't have
// to match, we can end up with sections a, b, c, d where b and c are in the
// script and c is compareSectionsNonScript less than b. In which case d
// can be equivalent to c, a to b and d < a. As a concrete example:
// .a (rx) # not in script
// .b (rx) # in script
// .c (ro) # in script
// .d (ro) # not in script
// The way we define an order then is:
// * First put script sections at the start and sort the script sections.
// * Move each non-script section to its preferred position. We try
// to put each section in the last position where it it can share
// a PT_LOAD.
std::stable_sort(OutputSections.begin(), OutputSections.end(),
auto I = OutputSections.begin();
auto E = OutputSections.end();
auto NonScriptI =
std::find_if(OutputSections.begin(), E,
[](OutputSection *S) { return S->SectionIndex == INT_MAX; });
while (NonScriptI != E) {
auto Pos = findOrphanPos<ELFT>(I, NonScriptI);
// As an optimization, find all sections with the same sort rank
// and insert them with one rotate.
unsigned Rank = (*NonScriptI)->SortRank;
auto End = std::find_if(NonScriptI + 1, E, [=](OutputSection *Sec) {
return Sec->SortRank != Rank;
std::rotate(Pos, NonScriptI, End);
NonScriptI = End;
static void applySynthetic(const std::vector<SyntheticSection *> &Sections,
std::function<void(SyntheticSection *)> Fn) {
for (SyntheticSection *SS : Sections)
if (SS && SS->OutSec && !SS->empty()) {
// We need to add input synthetic sections early in createSyntheticSections()
// to make them visible from linkescript side. But not all sections are always
// required to be in output. For example we don't need dynamic section content
// sometimes. This function filters out such unused sections from the output.
static void removeUnusedSyntheticSections(std::vector<OutputSection *> &V) {
// All input synthetic sections that can be empty are placed after
// all regular ones. We iterate over them all and exit at first
// non-synthetic.
for (InputSectionBase *S : llvm::reverse(InputSections)) {
SyntheticSection *SS = dyn_cast<SyntheticSection>(S);
if (!SS)
if (!SS->empty() || !SS->OutSec)
SS->OutSec->Sections.end(), SS));
SS->Live = false;
// If there are no other sections in the output section, remove it from the
// output.
if (SS->OutSec->Sections.empty())
V.erase(std::find(V.begin(), V.end(), SS->OutSec));
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::finalizeSections() {
Out::DebugInfo = findSection(".debug_info");
Out::PreinitArray = findSection(".preinit_array");
Out::InitArray = findSection(".init_array");
Out::FiniArray = findSection(".fini_array");
// The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
// symbols for sections, so that the runtime can get the start and end
// addresses of each section by section name. Add such symbols.
if (!Config->Relocatable) {
for (OutputSection *Sec : OutputSections)
// Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
// It should be okay as no one seems to care about the type.
// Even the author of gold doesn't remember why gold behaves that way.
if (In<ELFT>::DynSymTab)
addRegular<ELFT>("_DYNAMIC", InX::Dynamic, 0);
// Define __rel[a]_iplt_{start,end} symbols if needed.
// This responsible for splitting up .eh_frame section into
// pieces. The relocation scan uses those pieces, so this has to be
// earlier.
[](SyntheticSection *SS) { SS->finalizeContents(); });
// Scan relocations. This must be done after every symbol is declared so that
// we can correctly decide if a dynamic relocation is needed.
if (InX::Plt && !InX::Plt->empty())
if (InX::Iplt && !InX::Iplt->empty())
// Now that we have defined all possible global symbols including linker-
// synthesized ones. Visit all symbols to give the finishing touches.
for (Symbol *S : Symtab<ELFT>::X->getSymbols()) {
SymbolBody *Body = S->body();
if (!includeInSymtab(*Body))
if (In<ELFT>::SymTab)
if (In<ELFT>::DynSymTab && S->includeInDynsym()) {
if (auto *SS = dyn_cast<SharedSymbol>(Body))
if (cast<SharedFile<ELFT>>(SS->File)->isNeeded())
// Do not proceed if there was an undefined symbol.
if (ErrorCount)
// So far we have added sections from input object files.
// This function adds linker-created Out::* sections.
// This is a bit of a hack. A value of 0 means undef, so we set it
// to 1 t make __ehdr_start defined. The section number is not
// particularly relevant.
Out::ElfHeader->SectionIndex = 1;
unsigned I = 1;
for (OutputSection *Sec : OutputSections) {
Sec->SectionIndex = I++;
Sec->ShName = InX::ShStrTab->addString(Sec->Name);
// Binary and relocatable output does not have PHDRS.
// The headers have to be created before finalize as that can influence the
// image base and the dynamic section on mips includes the image base.
if (!Config->Relocatable && !Config->OFormatBinary) {
Phdrs = Script->hasPhdrsCommands() ? Script->createPhdrs() : createPhdrs();
Out::ProgramHeaders->Size = sizeof(Elf_Phdr) * Phdrs.size();
// Dynamic section must be the last one in this list and dynamic
// symbol table section (DynSymTab) must be the first one.
[](SyntheticSection *SS) { SS->finalizeContents(); });
// Some architectures use small displacements for jump instructions.
// It is linker's responsibility to create thunks containing long
// jump instructions if jump targets are too far. Create thunks.
if (Target->NeedsThunks) {
// FIXME: only ARM Interworking and Mips LA25 Thunks are implemented,
// these
// do not require address information. To support range extension Thunks
// we need to assign addresses so that we can tell if jump instructions
// are out of range. This will need to turn into a loop that converges
// when no more Thunks are added
ThunkCreator<ELFT> TC;
if (TC.createThunks(OutputSections))
[](SyntheticSection *SS) { SS->updateAllocSize(); });
// Fill other section headers. The dynamic table is finalized
// at the end because some tags like RELSZ depend on result
// of finalizing other sections.
for (OutputSection *Sec : OutputSections)
// If -compressed-debug-sections is specified, we need to compress
// .debug_* sections. Do it right now because it changes the size of
// output sections.
parallelForEach(OutputSections.begin(), OutputSections.end(),
[](OutputSection *S) { S->maybeCompress<ELFT>(); });
// createThunks may have added local symbols to the static symbol table
applySynthetic({In<ELFT>::SymTab, InX::ShStrTab, InX::StrTab},
[](SyntheticSection *SS) { SS->postThunkContents(); });
template <class ELFT> void Writer<ELFT>::addPredefinedSections() {
// ARM ABI requires .ARM.exidx to be terminated by some piece of data.
// We have the terminater synthetic section class. Add that at the end.
auto *OS = dyn_cast_or_null<OutputSection>(findSection(".ARM.exidx"));
if (OS && !OS->Sections.empty() && !Config->Relocatable)
// The linker is expected to define SECNAME_start and SECNAME_end
// symbols for a few sections. This function defines them.
template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
auto Define = [&](StringRef Start, StringRef End, OutputSection *OS) {
// These symbols resolve to the image base if the section does not exist.
// A special value -1 indicates end of the section.
if (OS) {
addOptionalRegular<ELFT>(Start, OS, 0);
addOptionalRegular<ELFT>(End, OS, -1);
} else {
if (Config->Pic)
OS = Out::ElfHeader;
addOptionalRegular<ELFT>(Start, OS, 0);
addOptionalRegular<ELFT>(End, OS, 0);
Define("__preinit_array_start", "__preinit_array_end", Out::PreinitArray);
Define("__init_array_start", "__init_array_end", Out::InitArray);
Define("__fini_array_start", "__fini_array_end", Out::FiniArray);
if (OutputSection *Sec = findSection(".ARM.exidx"))
Define("__exidx_start", "__exidx_end", Sec);
// If a section name is valid as a C identifier (which is rare because of
// the leading '.'), linkers are expected to define __start_<secname> and
// __stop_<secname> symbols. They are at beginning and end of the section,
// respectively. This is not requested by the ELF standard, but GNU ld and
// gold provide the feature, and used by many programs.
template <class ELFT>
void Writer<ELFT>::addStartStopSymbols(OutputSection *Sec) {
StringRef S = Sec->Name;
if (!isValidCIdentifier(S))
addOptionalRegular<ELFT>("__start_" + S), Sec, 0, STV_DEFAULT);
addOptionalRegular<ELFT>("__stop_" + S), Sec, -1, STV_DEFAULT);
template <class ELFT> OutputSection *Writer<ELFT>::findSection(StringRef Name) {
for (OutputSection *Sec : OutputSections)
if (Sec->Name == Name)
return Sec;
return nullptr;
static bool needsPtLoad(OutputSection *Sec) {
if (!(Sec->Flags & SHF_ALLOC))
return false;
// Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
// responsible for allocating space for them, not the PT_LOAD that
// contains the TLS initialization image.
if (Sec->Flags & SHF_TLS && Sec->Type == SHT_NOBITS)
return false;
return true;
// Linker scripts are responsible for aligning addresses. Unfortunately, most
// linker scripts are designed for creating two PT_LOADs only, one RX and one
// RW. This means that there is no alignment in the RO to RX transition and we
// cannot create a PT_LOAD there.
static uint64_t computeFlags(uint64_t Flags) {
if (Config->Omagic)
return PF_R | PF_W | PF_X;
if (Config->SingleRoRx && !(Flags & PF_W))
return Flags | PF_X;
return Flags;
// Decide which program headers to create and which sections to include in each
// one.
template <class ELFT> std::vector<PhdrEntry> Writer<ELFT>::createPhdrs() {
std::vector<PhdrEntry> Ret;
auto AddHdr = [&](unsigned Type, unsigned Flags) -> PhdrEntry * {
Ret.emplace_back(Type, Flags);
return &Ret.back();
// The first phdr entry is PT_PHDR which describes the program header itself.
AddHdr(PT_PHDR, PF_R)->add(Out::ProgramHeaders);
// PT_INTERP must be the second entry if exists.
if (OutputSection *Sec = findSection(".interp"))
AddHdr(PT_INTERP, Sec->getPhdrFlags())->add(Sec);
// Add the first PT_LOAD segment for regular output sections.
uint64_t Flags = computeFlags(PF_R);
PhdrEntry *Load = AddHdr(PT_LOAD, Flags);
// Add the headers. We will remove them if they don't fit.
for (OutputSection *Sec : OutputSections) {
if (!(Sec->Flags & SHF_ALLOC))
if (!needsPtLoad(Sec))
// Segments are contiguous memory regions that has the same attributes
// (e.g. executable or writable). There is one phdr for each segment.
// Therefore, we need to create a new phdr when the next section has
// different flags or is loaded at a discontiguous address using AT linker
// script command.
uint64_t NewFlags = computeFlags(Sec->getPhdrFlags());
if (Script->hasLMA(Sec) || Flags != NewFlags) {
Load = AddHdr(PT_LOAD, NewFlags);
Flags = NewFlags;
// Add a TLS segment if any.
PhdrEntry TlsHdr(PT_TLS, PF_R);
for (OutputSection *Sec : OutputSections)
if (Sec->Flags & SHF_TLS)
if (TlsHdr.First)
// Add an entry for .dynamic.
if (In<ELFT>::DynSymTab)
AddHdr(PT_DYNAMIC, InX::Dynamic->OutSec->getPhdrFlags())
// PT_GNU_RELRO includes all sections that should be marked as
// read-only by dynamic linker after proccessing relocations.
PhdrEntry RelRo(PT_GNU_RELRO, PF_R);
for (OutputSection *Sec : OutputSections)
if (needsPtLoad(Sec) && isRelroSection(Sec))
if (RelRo.First)
// PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
if (!In<ELFT>::EhFrame->empty() && In<ELFT>::EhFrameHdr &&
In<ELFT>::EhFrame->OutSec && In<ELFT>::EhFrameHdr->OutSec)
AddHdr(PT_GNU_EH_FRAME, In<ELFT>::EhFrameHdr->OutSec->getPhdrFlags())
// PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
// the dynamic linker fill the segment with random data.
if (OutputSection *Sec = findSection(".openbsd.randomdata"))
AddHdr(PT_OPENBSD_RANDOMIZE, Sec->getPhdrFlags())->add(Sec);
// PT_GNU_STACK is a special section to tell the loader to make the
// pages for the stack non-executable. If you really want an executable
// stack, you can pass -z execstack, but that's not recommended for
// security reasons.
unsigned Perm;
if (Config->ZExecstack)
Perm = PF_R | PF_W | PF_X;
Perm = PF_R | PF_W;
AddHdr(PT_GNU_STACK, Perm)->p_memsz = Config->ZStackSize;
// PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
// is expected to perform W^X violations, such as calling mprotect(2) or
// mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
// OpenBSD.
if (Config->ZWxneeded)
// Create one PT_NOTE per a group of contiguous .note sections.
PhdrEntry *Note = nullptr;
for (OutputSection *Sec : OutputSections) {
if (Sec->Type == SHT_NOTE) {
if (!Note || Script->hasLMA(Sec))
Note = AddHdr(PT_NOTE, PF_R);
} else {
Note = nullptr;
return Ret;
template <class ELFT>
void Writer<ELFT>::addPtArmExid(std::vector<PhdrEntry> &Phdrs) {
if (Config->EMachine != EM_ARM)
auto I = std::find_if(
OutputSections.begin(), OutputSections.end(),
[](OutputSection *Sec) { return Sec->Type == SHT_ARM_EXIDX; });
if (I == OutputSections.end())
// PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
PhdrEntry ARMExidx(PT_ARM_EXIDX, PF_R);
// The first section of each PT_LOAD, the first section in PT_GNU_RELRO and the
// first section after PT_GNU_RELRO have to be page aligned so that the dynamic
// linker can set the permissions.
template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
for (const PhdrEntry &P : Phdrs)
if (P.p_type == PT_LOAD && P.First)
P.First->PageAlign = true;
for (const PhdrEntry &P : Phdrs) {
if (P.p_type != PT_GNU_RELRO)
if (P.First)
P.First->PageAlign = true;
// Find the first section after PT_GNU_RELRO. If it is in a PT_LOAD we
// have to align it to a page.
auto End = OutputSections.end();
auto I = std::find(OutputSections.begin(), End, P.Last);
if (I == End || (I + 1) == End)
OutputSection *Sec = *(I + 1);
if (needsPtLoad(Sec))
Sec->PageAlign = true;
// Adjusts the file alignment for a given output section and returns
// its new file offset. The file offset must be the same with its
// virtual address (modulo the page size) so that the loader can load
// executables without any address adjustment.
static uint64_t getFileAlignment(uint64_t Off, OutputSection *Sec) {
OutputSection *First = Sec->FirstInPtLoad;
// If the section is not in a PT_LOAD, we just have to align it.
if (!First)
return alignTo(Off, Sec->Alignment);
// The first section in a PT_LOAD has to have congruent offset and address
// module the page size.
if (Sec == First)
return alignTo(Off, Config->MaxPageSize, Sec->Addr);
// If two sections share the same PT_LOAD the file offset is calculated
// using this formula: Off2 = Off1 + (VA2 - VA1).
return First->Offset + Sec->Addr - First->Addr;
static uint64_t setOffset(OutputSection *Sec, uint64_t Off) {
if (Sec->Type == SHT_NOBITS) {
Sec->Offset = Off;
return Off;
Off = getFileAlignment(Off, Sec);
Sec->Offset = Off;
return Off + Sec->Size;
template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
uint64_t Off = 0;
for (OutputSection *Sec : OutputSections)
if (Sec->Flags & SHF_ALLOC)
Off = setOffset(Sec, Off);
FileSize = alignTo(Off, Config->Wordsize);
// Assign file offsets to output sections.
template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
uint64_t Off = 0;
Off = setOffset(Out::ElfHeader, Off);
Off = setOffset(Out::ProgramHeaders, Off);
for (OutputSection *Sec : OutputSections)
Off = setOffset(Sec, Off);
SectionHeaderOff = alignTo(Off, Config->Wordsize);
FileSize = SectionHeaderOff + (OutputSections.size() + 1) * sizeof(Elf_Shdr);
// Finalize the program headers. We call this function after we assign
// file offsets and VAs to all sections.
template <class ELFT> void Writer<ELFT>::setPhdrs() {
for (PhdrEntry &P : Phdrs) {
OutputSection *First = P.First;
OutputSection *Last = P.Last;
if (First) {
P.p_filesz = Last->Offset - First->Offset;
if (Last->Type != SHT_NOBITS)
P.p_filesz += Last->Size;
P.p_memsz = Last->Addr + Last->Size - First->Addr;
P.p_offset = First->Offset;
P.p_vaddr = First->Addr;
if (!P.HasLMA)
P.p_paddr = First->getLMA();
if (P.p_type == PT_LOAD)
P.p_align = Config->MaxPageSize;
else if (P.p_type == PT_GNU_RELRO) {
P.p_align = 1;
// The glibc dynamic loader rounds the size down, so we need to round up
// to protect the last page. This is a no-op on FreeBSD which always
// rounds up.
P.p_memsz = alignTo(P.p_memsz, Target->PageSize);
// The TLS pointer goes after PT_TLS. At least glibc will align it,
// so round up the size to make sure the offsets are correct.
if (P.p_type == PT_TLS) {
Out::TlsPhdr = &P;
if (P.p_memsz)
P.p_memsz = alignTo(P.p_memsz, P.p_align);
// The entry point address is chosen in the following ways.
// 1. the '-e' entry command-line option;
// 2. the ENTRY(symbol) command in a linker control script;
// 3. the value of the symbol start, if present;
// 4. the address of the first byte of the .text section, if present;
// 5. the address 0.
template <class ELFT> uint64_t Writer<ELFT>::getEntryAddr() {
// Case 1, 2 or 3. As a special case, if the symbol is actually
// a number, we'll use that number as an address.
if (SymbolBody *B = Symtab<ELFT>::X->find(Config->Entry))
return B->getVA();
uint64_t Addr;
if (!Config->Entry.getAsInteger(0, Addr))
return Addr;
// Case 4
if (OutputSection *Sec = findSection(".text")) {
if (Config->WarnMissingEntry)
warn("cannot find entry symbol " + Config->Entry + "; defaulting to 0x" +
return Sec->Addr;
// Case 5
if (Config->WarnMissingEntry)
warn("cannot find entry symbol " + Config->Entry +
"; not setting start address");
return 0;
static uint16_t getELFType() {
if (Config->Pic)
return ET_DYN;
if (Config->Relocatable)
return ET_REL;
return ET_EXEC;
// This function is called after we have assigned address and size
// to each section. This function fixes some predefined
// symbol values that depend on section address and size.
template <class ELFT> void Writer<ELFT>::fixPredefinedSymbols() {
auto Set = [](DefinedRegular *S1, DefinedRegular *S2, OutputSection *Sec,
uint64_t Value) {
if (S1) {
S1->Section = Sec;
S1->Value = Value;
if (S2) {
S2->Section = Sec;
S2->Value = Value;
// _etext is the first location after the last read-only loadable segment.
// _edata is the first location after the last read-write loadable segment.
// _end is the first location after the uninitialized data region.
PhdrEntry *Last = nullptr;
PhdrEntry *LastRO = nullptr;
PhdrEntry *LastRW = nullptr;
for (PhdrEntry &P : Phdrs) {
if (P.p_type != PT_LOAD)
Last = &P;
if (P.p_flags & PF_W)
LastRW = &P;
LastRO = &P;
if (Last)
Set(ElfSym::End1, ElfSym::End2, Last->First, Last->p_memsz);
if (LastRO)
Set(ElfSym::Etext1, ElfSym::Etext2, LastRO->First, LastRO->p_filesz);
if (LastRW)
Set(ElfSym::Edata1, ElfSym::Edata2, LastRW->First, LastRW->p_filesz);
if (ElfSym::Bss)
ElfSym::Bss->Section = findSection(".bss");
// Setup MIPS _gp_disp/__gnu_local_gp symbols which should
// be equal to the _gp symbol's value.
if (Config->EMachine == EM_MIPS) {
if (!ElfSym::MipsGp->Value) {
// Find GP-relative section with the lowest address
// and use this address to calculate default _gp value.
uint64_t Gp = -1;
for (const OutputSection *OS : OutputSections)
if ((OS->Flags & SHF_MIPS_GPREL) && OS->Addr < Gp)
Gp = OS->Addr;
if (Gp != (uint64_t)-1)
ElfSym::MipsGp->Value = Gp + 0x7ff0;
template <class ELFT> void Writer<ELFT>::writeHeader() {
uint8_t *Buf = Buffer->getBufferStart();
memcpy(Buf, "\177ELF", 4);
// Write the ELF header.
auto *EHdr = reinterpret_cast<Elf_Ehdr *>(Buf);
EHdr->e_ident[EI_CLASS] = Config->Is64 ? ELFCLASS64 : ELFCLASS32;
EHdr->e_ident[EI_DATA] = Config->IsLE ? ELFDATA2LSB : ELFDATA2MSB;
EHdr->e_ident[EI_OSABI] = Config->OSABI;
EHdr->e_type = getELFType();
EHdr->e_machine = Config->EMachine;
EHdr->e_version = EV_CURRENT;
EHdr->e_entry = getEntryAddr();
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_ehsize = sizeof(Elf_Ehdr);
EHdr->e_phnum = Phdrs.size();
EHdr->e_shentsize = sizeof(Elf_Shdr);
EHdr->e_shnum = OutputSections.size() + 1;
EHdr->e_shstrndx = InX::ShStrTab->OutSec->SectionIndex;
if (Config->EMachine == EM_ARM)
// We don't currently use any features incompatible with EF_ARM_EABI_VER5,
// but we don't have any firm guarantees of conformance. Linux AArch64
// kernels (as of 2016) require an EABI version to be set.
EHdr->e_flags = EF_ARM_EABI_VER5;
else if (Config->EMachine == EM_MIPS)
EHdr->e_flags = getMipsEFlags<ELFT>();
if (!Config->Relocatable) {
EHdr->e_phoff = sizeof(Elf_Ehdr);
EHdr->e_phentsize = sizeof(Elf_Phdr);
// Write the program header table.
auto *HBuf = reinterpret_cast<Elf_Phdr *>(Buf + EHdr->e_phoff);
for (PhdrEntry &P : Phdrs) {
HBuf->p_type = P.p_type;
HBuf->p_flags = P.p_flags;
HBuf->p_offset = P.p_offset;
HBuf->p_vaddr = P.p_vaddr;
HBuf->p_paddr = P.p_paddr;
HBuf->p_filesz = P.p_filesz;
HBuf->p_memsz = P.p_memsz;
HBuf->p_align = P.p_align;
// Write the section header table. Note that the first table entry is null.
auto *SHdrs = reinterpret_cast<Elf_Shdr *>(Buf + EHdr->e_shoff);
for (OutputSection *Sec : OutputSections)
// Open a result file.
template <class ELFT> void Writer<ELFT>::openFile() {
if (!Config->Is64 && FileSize > UINT32_MAX) {
error("output file too large: " + Twine(FileSize) + " bytes");
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Config->OutputFile, FileSize,
if (auto EC = BufferOrErr.getError())
error("failed to open " + Config->OutputFile + ": " + EC.message());
Buffer = std::move(*BufferOrErr);
template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
uint8_t *Buf = Buffer->getBufferStart();
for (OutputSection *Sec : OutputSections)
if (Sec->Flags & SHF_ALLOC)
Sec->writeTo<ELFT>(Buf + Sec->Offset);
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
uint8_t *Buf = Buffer->getBufferStart();
// PPC64 needs to process relocations in the .opd section
// before processing relocations in code-containing sections.
Out::Opd = findSection(".opd");
if (Out::Opd) {
Out::OpdBuf = Buf + Out::Opd->Offset;
Out::Opd->template writeTo<ELFT>(Buf + Out::Opd->Offset);
OutputSection *EhFrameHdr =
In<ELFT>::EhFrameHdr ? In<ELFT>::EhFrameHdr->OutSec : nullptr;
// In -r or -emit-relocs mode, write the relocation sections first as in
// ELf_Rel targets we might find out that we need to modify the relocated
// section while doing it.
for (OutputSection *Sec : OutputSections)
if (Sec->Type == SHT_REL || Sec->Type == SHT_RELA)
Sec->writeTo<ELFT>(Buf + Sec->Offset);
for (OutputSection *Sec : OutputSections)
if (Sec != Out::Opd && Sec != EhFrameHdr && Sec->Type != SHT_REL &&
Sec->Type != SHT_RELA)
Sec->writeTo<ELFT>(Buf + Sec->Offset);
// The .eh_frame_hdr depends on .eh_frame section contents, therefore
// it should be written after .eh_frame is written.
if (EhFrameHdr && !EhFrameHdr->Sections.empty())
EhFrameHdr->writeTo<ELFT>(Buf + EhFrameHdr->Offset);
template <class ELFT> void Writer<ELFT>::writeBuildId() {
if (!InX::BuildId || !InX::BuildId->OutSec)
// Compute a hash of all sections of the output file.
uint8_t *Start = Buffer->getBufferStart();
uint8_t *End = Start + FileSize;
InX::BuildId->writeBuildId({Start, End});
template void elf::writeResult<ELF32LE>();
template void elf::writeResult<ELF32BE>();
template void elf::writeResult<ELF64LE>();
template void elf::writeResult<ELF64BE>();