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//===- 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 "AArch64ErrataFix.h"
#include "CallGraphSort.h"
#include "Config.h"
#include "Filesystem.h"
#include "LinkerScript.h"
#include "MapFile.h"
#include "OutputSections.h"
#include "Relocations.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "lld/Common/Threads.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringSwitch.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 {
Writer() : Buffer(errorHandler().OutputBuffer) {}
typedef typename ELFT::Shdr Elf_Shdr;
typedef typename ELFT::Ehdr Elf_Ehdr;
typedef typename ELFT::Phdr Elf_Phdr;
void run();
void copyLocalSymbols();
void addSectionSymbols();
void forEachRelSec(llvm::function_ref<void(InputSectionBase &)> Fn);
void sortSections();
void resolveShfLinkOrder();
void maybeAddThunks();
void sortInputSections();
void finalizeSections();
void checkExecuteOnly();
void setReservedSymbolSections();
std::vector<PhdrEntry *> createPhdrs();
void removeEmptyPTLoad();
void addPtArmExid(std::vector<PhdrEntry *> &Phdrs);
void assignFileOffsets();
void assignFileOffsetsBinary();
void setPhdrs();
void checkSections();
void fixSectionAlignments();
void openFile();
void writeTrapInstr();
void writeHeader();
void writeSections();
void writeSectionsBinary();
void writeBuildId();
std::unique_ptr<FileOutputBuffer> &Buffer;
void addRelIpltSymbols();
void addStartEndSymbols();
void addStartStopSymbols(OutputSection *Sec);
std::vector<PhdrEntry *> Phdrs;
uint64_t FileSize;
uint64_t SectionHeaderOff;
} // anonymous namespace
static bool isSectionPrefix(StringRef Prefix, StringRef Name) {
return Name.startswith(Prefix) || Name == Prefix.drop_back();
StringRef elf::getOutputSectionName(const InputSectionBase *S) {
if (Config->Relocatable)
return S->Name;
// This is for --emit-relocs. If is emitted as, we want
// to emit as for consistency (this is not
// technically required, but not doing it is odd). This code guarantees that.
if (auto *IS = dyn_cast<InputSection>(S)) {
if (InputSectionBase *Rel = IS->getRelocatedSection()) {
OutputSection *Out = Rel->getOutputSection();
if (S->Type == SHT_RELA)
return".rela" + Out->Name);
return".rel" + Out->Name);
// This check is for -z keep-text-section-prefix. This option separates text
// sections with prefix "", ".text.unlikely", ".text.startup" or
// ".text.exit".
// When enabled, this allows identifying the hot code region ( in
// the final binary which can be selectively mapped to huge pages or mlocked,
// for instance.
if (Config->ZKeepTextSectionPrefix)
for (StringRef V :
{"", ".text.unlikely.", ".text.startup.", ".text.exit."})
if (isSectionPrefix(V, S->Name))
return V.drop_back();
for (StringRef V :
{".text.", ".rodata.", "", ".data.", "",
".bss.", ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
".gcc_except_table.", ".tdata.", ".ARM.exidx.", ".ARM.extab."})
if (isSectionPrefix(V, S->Name))
return V.drop_back();
// CommonSection is identified as "COMMON" in linker scripts.
// By default, it should go to .bss section.
if (S->Name == "COMMON")
return ".bss";
return S->Name;
static bool needsInterpSection() {
return !SharedFiles.empty() && !Config->DynamicLinker.empty() &&
template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
template <class ELFT> void Writer<ELFT>::removeEmptyPTLoad() {
llvm::erase_if(Phdrs, [&](const PhdrEntry *P) {
if (P->p_type != PT_LOAD)
return false;
if (!P->FirstSec)
return true;
uint64_t Size = P->LastSec->Addr + P->LastSec->Size - P->FirstSec->Addr;
return Size == 0;
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());
static Defined *addOptionalRegular(StringRef Name, SectionBase *Sec,
uint64_t Val, uint8_t StOther = STV_HIDDEN,
uint8_t Binding = STB_GLOBAL) {
Symbol *S = Symtab->find(Name);
if (!S || S->isDefined())
return nullptr;
Symbol *Sym = Symtab->addDefined(Name, StOther, STT_NOTYPE, Val,
/*Size=*/0, Binding, Sec,
return cast<Defined>(Sym);
static Defined *addAbsolute(StringRef Name) {
return cast<Defined>(Symtab->addDefined(Name, STV_HIDDEN, STT_NOTYPE, 0, 0,
STB_GLOBAL, nullptr, nullptr));
// The linker is expected to define some symbols depending on
// the linking result. This function defines such symbols.
void elf::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 = addAbsolute("_gp");
// On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
// start of function and 'gp' pointer into GOT.
if (Symtab->find("_gp_disp"))
ElfSym::MipsGpDisp = addAbsolute("_gp_disp");
// 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->find("__gnu_local_gp"))
ElfSym::MipsLocalGp = addAbsolute("__gnu_local_gp");
// The Power Architecture 64-bit v2 ABI defines a TableOfContents (TOC) which
// combines the typical ELF GOT with the small data sections. It commonly
// includes .got .toc .sdata .sbss. The .TOC. symbol replaces both
// _GLOBAL_OFFSET_TABLE_ and _SDA_BASE_ from the 32-bit ABI. It is used to
// represent the TOC base which is offset by 0x8000 bytes from the start of
// the .got section.
ElfSym::GlobalOffsetTable = addOptionalRegular(
(Config->EMachine == EM_PPC64) ? ".TOC." : "_GLOBAL_OFFSET_TABLE_",
Out::ElfHeader, Target->GotBaseSymOff);
// __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("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
// __executable_start is not documented, but the expectation of at
// least the Android libc is that it points to the ELF header.
addOptionalRegular("__executable_start", Out::ElfHeader, 0, STV_HIDDEN);
// __dso_handle symbol is passed to cxa_finalize as a marker to identify
// each DSO. The address of the symbol doesn't matter as long as they are
// different in different DSOs, so we chose the start address of the DSO.
addOptionalRegular("__dso_handle", Out::ElfHeader, 0, STV_HIDDEN);
// If linker script do layout we do not need to create any standart symbols.
if (Script->HasSectionsCommand)
auto Add = [](StringRef S, int64_t Pos) {
return addOptionalRegular(S, Out::ElfHeader, Pos, STV_DEFAULT);
ElfSym::Bss = Add("__bss_start", 0);
ElfSym::End1 = Add("end", -1);
ElfSym::End2 = Add("_end", -1);
ElfSym::Etext1 = Add("etext", -1);
ElfSym::Etext2 = Add("_etext", -1);
ElfSym::Edata1 = Add("edata", -1);
ElfSym::Edata2 = Add("_edata", -1);
static OutputSection *findSection(StringRef Name) {
for (BaseCommand *Base : Script->SectionCommands)
if (auto *Sec = dyn_cast<OutputSection>(Base))
if (Sec->Name == Name)
return Sec;
return nullptr;
// Initialize Out members.
template <class ELFT> static void 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); };
In.DynStrTab = make<StringTableSection>(".dynstr", true);
In.Dynamic = make<DynamicSection<ELFT>>();
if (Config->AndroidPackDynRelocs) {
In.RelaDyn = make<AndroidPackedRelocationSection<ELFT>>(
Config->IsRela ? ".rela.dyn" : ".rel.dyn");
} else {
In.RelaDyn = make<RelocationSection<ELFT>>(
Config->IsRela ? ".rela.dyn" : ".rel.dyn", Config->ZCombreloc);
In.ShStrTab = make<StringTableSection>(".shstrtab", false);
Out::ProgramHeaders = make<OutputSection>("", 0, SHF_ALLOC);
Out::ProgramHeaders->Alignment = Config->Wordsize;
if (needsInterpSection()) {
In.Interp = createInterpSection();
if (Config->Strip != StripPolicy::All) {
In.StrTab = make<StringTableSection>(".strtab", false);
In.SymTab = make<SymbolTableSection<ELFT>>(*In.StrTab);
In.SymTabShndx = make<SymtabShndxSection>();
if (Config->BuildId != BuildIdKind::None) {
In.BuildId = make<BuildIdSection>();
In.Bss = make<BssSection>(".bss", 0, 1);
// If there is a SECTIONS command and a section name use name
// so that we match in the output section.
// This makes sure our relro is contiguous.
bool HasDataRelRo = Script->HasSectionsCommand && findSection("");
In.BssRelRo =
make<BssSection>(HasDataRelRo ? "" : "", 0, 1);
// Add MIPS-specific sections.
if (Config->EMachine == EM_MIPS) {
if (!Config->Shared && Config->HasDynSymTab) {
In.MipsRldMap = make<MipsRldMapSection>();
if (auto *Sec = MipsAbiFlagsSection<ELFT>::create())
if (auto *Sec = MipsOptionsSection<ELFT>::create())
if (auto *Sec = MipsReginfoSection<ELFT>::create())
if (Config->HasDynSymTab) {
In.DynSymTab = make<SymbolTableSection<ELFT>>(*In.DynStrTab);
InX<ELFT>::VerSym = make<VersionTableSection<ELFT>>();
if (!Config->VersionDefinitions.empty()) {
In.VerDef = make<VersionDefinitionSection>();
InX<ELFT>::VerNeed = make<VersionNeedSection<ELFT>>();
if (Config->GnuHash) {
In.GnuHashTab = make<GnuHashTableSection>();
if (Config->SysvHash) {
In.HashTab = make<HashTableSection>();
if (Config->RelrPackDynRelocs) {
In.RelrDyn = make<RelrSection<ELFT>>();
// Add .got. MIPS' .got is so different from the other archs,
// it has its own class.
if (Config->EMachine == EM_MIPS) {
In.MipsGot = make<MipsGotSection>();
} else {
In.Got = make<GotSection>();
if (Config->EMachine == EM_PPC64) {
In.PPC64LongBranchTarget = make<PPC64LongBranchTargetSection>();
In.GotPlt = make<GotPltSection>();
In.IgotPlt = make<IgotPltSection>();
if (Config->GdbIndex) {
In.GdbIndex = GdbIndexSection::create<ELFT>();
// 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.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.
// We cannot place the iplt section in .rel.dyn when Android relocation
// packing is enabled because that would cause a section type mismatch.
// However, because the Android dynamic loader reads .rel.plt after .rel.dyn,
// we can get the desired behaviour by placing the iplt section in .rel.plt.
In.RelaIplt = make<RelocationSection<ELFT>>(
(Config->EMachine == EM_ARM && !Config->AndroidPackDynRelocs)
? ".rel.dyn"
: In.RelaPlt->Name,
false /*Sort*/);
In.Plt = make<PltSection>(false);
In.Iplt = make<PltSection>(true);
// .note.GNU-stack is always added when we are creating a re-linkable
// object file. Other linkers are using the presence of this marker
// section to control the executable-ness of the stack area, but that
// is irrelevant these days. Stack area should always be non-executable
// by default. So we emit this section unconditionally.
if (Config->Relocatable)
if (!Config->Relocatable) {
if (Config->EhFrameHdr) {
In.EhFrameHdr = make<EhFrameHeader>();
In.EhFrame = make<EhFrameSection>();
if (In.SymTab)
if (In.SymTabShndx)
if (In.StrTab)
if (Config->EMachine == EM_ARM && !Config->Relocatable)
// Add a sentinel to terminate .ARM.exidx. It helps an unwinder
// to find the exact address range of the last entry.
// 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 want to process linker script commands. When SECTIONS command
// is given we let it create sections.
// Linker scripts controls how input sections are assigned to output sections.
// Input sections that were not handled by scripts are called "orphans", and
// they are assigned to output sections by the default rule. Process that.
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 -compressed-debug-sections is specified, we need to compress
// .debug_* sections. Do it right now because it changes the size of
// output sections.
for (OutputSection *Sec : OutputSections)
// 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)
if (Config->Relocatable)
for (OutputSection *Sec : OutputSections)
Sec->Addr = 0;
if (Config->CheckSections)
// 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 and -cref options.
if (errorCount())
if (auto E = Buffer->commit())
error("failed to write to the output file: " + toString(std::move(E)));
static bool shouldKeepInSymtab(SectionBase *Sec, StringRef SymName,
const Symbol &B) {
if (B.isSection())
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 Symbol &B) {
if (!B.isLocal() && !B.IsUsedInRegularObj)
return false;
if (auto *D = dyn_cast<Defined>(&B)) {
// Always include absolute symbols.
SectionBase *Sec = D->Section;
if (!Sec)
return true;
Sec = Sec->Repl;
// Exclude symbols pointing to garbage-collected sections.
if (isa<InputSectionBase>(Sec) && !Sec->Live)
return false;
if (auto *S = dyn_cast<MergeInputSection>(Sec))
if (!S->getSectionPiece(D->Value)->Live)
return false;
return true;
return B.Used;
// 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.SymTab)
for (InputFile *File : ObjectFiles) {
ObjFile<ELFT> *F = cast<ObjFile<ELFT>>(File);
for (Symbol *B : F->getLocalSymbols()) {
if (!B->isLocal())
fatal(toString(F) +
": broken object: getLocalSymbols returns a non-local symbol");
auto *DR = dyn_cast<Defined>(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))
// Create a section symbol for each output section so that we can represent
// relocations that point to the section. If we know that no relocation is
// referring to a section (that happens if the section is a synthetic one), we
// don't create a section symbol for that section.
template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
for (BaseCommand *Base : Script->SectionCommands) {
auto *Sec = dyn_cast<OutputSection>(Base);
if (!Sec)
auto I = llvm::find_if(Sec->SectionCommands, [](BaseCommand *Base) {
if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
return !ISD->Sections.empty();
return false;
if (I == Sec->SectionCommands.end())
InputSection *IS = cast<InputSectionDescription>(*I)->Sections[0];
// Relocations are not using REL[A] section symbols.
if (IS->Type == SHT_REL || IS->Type == SHT_RELA)
// Unlike other synthetic sections, mergeable output sections contain data
// copied from input sections, and there may be a relocation pointing to its
// contents if -r or -emit-reloc are given.
if (isa<SyntheticSection>(IS) && !(IS->Flags & SHF_MERGE))
auto *Sym =
make<Defined>(IS->File, "", STB_LOCAL, /*StOther=*/0, STT_SECTION,
/*Value=*/0, /*Size=*/0, IS);
// 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.
static bool 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 (In.Got && Sec == In.Got->getParent())
return true;
// .toc is a GOT-ish section for PowerPC64. Their contents are accessed
// through r2 register, which is reserved for that purpose. Since r2 is used
// for accessing .got as well, .got and .toc need to be close enough in the
// virtual address space. Usually, .toc comes just after .got. Since we place
// .got into RELRO, .toc needs to be placed into RELRO too.
if (Sec->Name.equals(".toc"))
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 == In.GotPlt->getParent())
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 == In.Dynamic->getParent())
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 == "" || 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 << 18,
RF_NOT_INTERP = 1 << 17,
RF_NOT_ALLOC = 1 << 16,
RF_WRITE = 1 << 15,
RF_EXEC_WRITE = 1 << 14,
RF_EXEC = 1 << 13,
RF_RODATA = 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_NOTE = 1 << 7,
RF_PPC_TOCL = 1 << 5,
RF_PPC_TOC = 1 << 4,
RF_PPC_GOT = 1 << 3,
RF_PPC_BRANCH_LT = 1 << 2,
RF_MIPS_GPREL = 1 << 1,
RF_MIPS_NOT_GOT = 1 << 0
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;
// Sort sections based on their access permission in the following
// order: R, RX, RWX, RW. This order is based on the following
// considerations:
// * Read-only sections come first such that they go in the
// PT_LOAD covering the program headers at the start of the file.
// * Read-only, executable sections come next.
// * Writable, executable sections follow such that .plt on
// architectures where it needs to be writable will be placed
// between .text and .data.
// * Writable sections come last, such that .bss lands at the very
// end of the last PT_LOAD.
bool IsExec = Sec->Flags & SHF_EXECINSTR;
bool IsWrite = Sec->Flags & SHF_WRITE;
if (IsExec) {
if (IsWrite)
Rank |= RF_EXEC;
} else if (IsWrite) {
Rank |= RF_WRITE;
} else if (Sec->Type == SHT_PROGBITS) {
// Make non-executable and non-writable PROGBITS sections (e.g .rodata
// .eh_frame) closer to .text. They likely contain PC or GOT relative
// relocations and there could be relocation overflow if other huge sections
// (.dynstr .dynsym) were placed in between.
Rank |= RF_RODATA;
// 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;
// We create a NOTE segment for contiguous .note sections, so make
// them contigous if there are more than one .note section with the
// same attributes.
if (Sec->Type == SHT_NOTE)
Rank |= RF_NOTE;
// 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 == ".toc1")
Rank |= RF_PPC_TOCL;
if (Name == ".toc")
Rank |= RF_PPC_TOC;
if (Name == ".got")
Rank |= RF_PPC_GOT;
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 compareSections(const BaseCommand *ACmd, const BaseCommand *BCmd) {
const OutputSection *A = cast<OutputSection>(ACmd);
const OutputSection *B = cast<OutputSection>(BCmd);
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;
void PhdrEntry::add(OutputSection *Sec) {
LastSec = Sec;
if (!FirstSec)
FirstSec = Sec;
p_align = std::max(p_align, Sec->Alignment);
if (p_type == PT_LOAD)
Sec->PtLoad = this;
// 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 (Config->Relocatable || needsInterpSection())
StringRef S = Config->IsRela ? "__rela_iplt_start" : "__rel_iplt_start";
addOptionalRegular(S, In.RelaIplt, 0, STV_HIDDEN, STB_WEAK);
S = Config->IsRela ? "__rela_iplt_end" : "__rel_iplt_end";
ElfSym::RelaIpltEnd =
addOptionalRegular(S, In.RelaIplt, 0, STV_HIDDEN, STB_WEAK);
template <class ELFT>
void Writer<ELFT>::forEachRelSec(
llvm::function_ref<void(InputSectionBase &)> Fn) {
// 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.
for (InputSectionBase *IS : InputSections)
if (IS->Live && isa<InputSection>(IS) && (IS->Flags & SHF_ALLOC))
for (EhInputSection *ES : In.EhFrame->Sections)
// This function generates assignments for predefined symbols (e.g. _end or
// _etext) and inserts them into the commands sequence to be processed at the
// appropriate time. This ensures that the value is going to be correct by the
// time any references to these symbols are processed and is equivalent to
// defining these symbols explicitly in the linker script.
template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
if (ElfSym::GlobalOffsetTable) {
// The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually
// to the start of the .got or .got.plt section.
InputSection *GotSection = In.GotPlt;
if (!Target->GotBaseSymInGotPlt)
GotSection = In.MipsGot ? cast<InputSection>(In.MipsGot)
: cast<InputSection>(In.Got);
ElfSym::GlobalOffsetTable->Section = GotSection;
if (ElfSym::RelaIpltEnd)
ElfSym::RelaIpltEnd->Value = In.RelaIplt->getSize();
PhdrEntry *Last = nullptr;
PhdrEntry *LastRO = nullptr;
for (PhdrEntry *P : Phdrs) {
if (P->p_type != PT_LOAD)
Last = P;
if (!(P->p_flags & PF_W))
LastRO = P;
if (LastRO) {
// _etext is the first location after the last read-only loadable segment.
if (ElfSym::Etext1)
ElfSym::Etext1->Section = LastRO->LastSec;
if (ElfSym::Etext2)
ElfSym::Etext2->Section = LastRO->LastSec;
if (Last) {
// _edata points to the end of the last mapped initialized section.
OutputSection *Edata = nullptr;
for (OutputSection *OS : OutputSections) {
if (OS->Type != SHT_NOBITS)
Edata = OS;
if (OS == Last->LastSec)
if (ElfSym::Edata1)
ElfSym::Edata1->Section = Edata;
if (ElfSym::Edata2)
ElfSym::Edata2->Section = Edata;
// _end is the first location after the uninitialized data region.
if (ElfSym::End1)
ElfSym::End1->Section = Last->LastSec;
if (ElfSym::End2)
ElfSym::End2->Section = Last->LastSec;
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 (ElfSym::MipsGp) {
// Find GP-relative section with the lowest address
// and use this address to calculate default _gp value.
for (OutputSection *OS : OutputSections) {
if (OS->Flags & SHF_MIPS_GPREL) {
ElfSym::MipsGp->Section = OS;
ElfSym::MipsGp->Value = 0x7ff0;
// 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 int getRankProximityAux(OutputSection *A, OutputSection *B) {
return countLeadingZeros(A->SortRank ^ B->SortRank);
static int getRankProximity(OutputSection *A, BaseCommand *B) {
if (auto *Sec = dyn_cast<OutputSection>(B))
return getRankProximityAux(A, Sec);
return -1;
// When placing orphan sections, we want to place them after symbol assignments
// so that an orphan after
// begin_foo = .;
// foo : { *(foo) }
// end_foo = .;
// doesn't break the intended meaning of the begin/end symbols.
// We don't want to go over sections since findOrphanPos is the
// one in charge of deciding the order of the sections.
// We don't want to go over changes to '.', since doing so in
// rx_sec : { *(rx_sec) }
// . = ALIGN(0x1000);
// /* The RW PT_LOAD starts here*/
// rw_sec : { *(rw_sec) }
// would mean that the RW PT_LOAD would become unaligned.
static bool shouldSkip(BaseCommand *Cmd) {
if (auto *Assign = dyn_cast<SymbolAssignment>(Cmd))
return Assign->Name != ".";
return false;
// 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<BaseCommand *>::iterator
findOrphanPos(std::vector<BaseCommand *>::iterator B,
std::vector<BaseCommand *>::iterator E) {
OutputSection *Sec = cast<OutputSection>(*E);
// Find the first element that has as close a rank as possible.
auto I = std::max_element(B, E, [=](BaseCommand *A, BaseCommand *B) {
return getRankProximity(Sec, A) < getRankProximity(Sec, B);
if (I == E)
return E;
// Consider all existing sections with the same proximity.
int Proximity = getRankProximity(Sec, *I);
for (; I != E; ++I) {
auto *CurSec = dyn_cast<OutputSection>(*I);
if (!CurSec)
if (getRankProximity(Sec, CurSec) != Proximity ||
Sec->SortRank < CurSec->SortRank)
auto IsOutputSec = [](BaseCommand *Cmd) { return isa<OutputSection>(Cmd); };
auto J = std::find_if(llvm::make_reverse_iterator(I),
llvm::make_reverse_iterator(B), IsOutputSec);
I = J.base();
// As a special case, if the orphan section is the last section, put
// it at the very end, past any other commands.
// This matches bfd's behavior and is convenient when the linker script fully
// specifies the start of the file, but doesn't care about the end (the non
// alloc sections for example).
auto NextSec = std::find_if(I, E, IsOutputSec);
if (NextSec == E)
return E;
while (I != E && shouldSkip(*I))
return I;
// Builds section order for handling --symbol-ordering-file.
static DenseMap<const InputSectionBase *, int> buildSectionOrder() {
DenseMap<const InputSectionBase *, int> SectionOrder;
// Use the rarely used option -call-graph-ordering-file to sort sections.
if (!Config->CallGraphProfile.empty())
return computeCallGraphProfileOrder();
if (Config->SymbolOrderingFile.empty())
return SectionOrder;
struct SymbolOrderEntry {
int Priority;
bool Present;
// 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, SymbolOrderEntry> SymbolOrder;
int Priority = -Config->SymbolOrderingFile.size();
for (StringRef S : Config->SymbolOrderingFile)
SymbolOrder.insert({S, {Priority++, false}});
// Build a map from sections to their priorities.
auto AddSym = [&](Symbol &Sym) {
auto It = SymbolOrder.find(Sym.getName());
if (It == SymbolOrder.end())
SymbolOrderEntry &Ent = It->second;
Ent.Present = true;
if (auto *D = dyn_cast<Defined>(&Sym)) {
if (auto *Sec = dyn_cast_or_null<InputSectionBase>(D->Section)) {
int &Priority = SectionOrder[cast<InputSectionBase>(Sec->Repl)];
Priority = std::min(Priority, Ent.Priority);
// We want both global and local symbols. We get the global ones from the
// symbol table and iterate the object files for the local ones.
for (Symbol *Sym : Symtab->getSymbols())
if (!Sym->isLazy())
for (InputFile *File : ObjectFiles)
for (Symbol *Sym : File->getSymbols())
if (Sym->isLocal())
if (Config->WarnSymbolOrdering)
for (auto OrderEntry : SymbolOrder)
if (!OrderEntry.second.Present)
warn("symbol ordering file: no such symbol: " + OrderEntry.first);
return SectionOrder;
// Sorts the sections in ISD according to the provided section order.
static void
sortISDBySectionOrder(InputSectionDescription *ISD,
const DenseMap<const InputSectionBase *, int> &Order) {
std::vector<InputSection *> UnorderedSections;
std::vector<std::pair<InputSection *, int>> OrderedSections;
uint64_t UnorderedSize = 0;
for (InputSection *IS : ISD->Sections) {
auto I = Order.find(IS);
if (I == Order.end()) {
UnorderedSize += IS->getSize();
OrderedSections.push_back({IS, I->second});
llvm::sort(OrderedSections, [&](std::pair<InputSection *, int> A,
std::pair<InputSection *, int> B) {
return A.second < B.second;
// Find an insertion point for the ordered section list in the unordered
// section list. On targets with limited-range branches, this is the mid-point
// of the unordered section list. This decreases the likelihood that a range
// extension thunk will be needed to enter or exit the ordered region. If the
// ordered section list is a list of hot functions, we can generally expect
// the ordered functions to be called more often than the unordered functions,
// making it more likely that any particular call will be within range, and
// therefore reducing the number of thunks required.
// For example, imagine that you have 8MB of hot code and 32MB of cold code.
// If the layout is:
// 8MB hot
// 32MB cold
// only the first 8-16MB of the cold code (depending on which hot function it
// is actually calling) can call the hot code without a range extension thunk.
// However, if we use this layout:
// 16MB cold
// 8MB hot
// 16MB cold
// both the last 8-16MB of the first block of cold code and the first 8-16MB
// of the second block of cold code can call the hot code without a thunk. So
// we effectively double the amount of code that could potentially call into
// the hot code without a thunk.
size_t InsPt = 0;
if (Target->getThunkSectionSpacing() && !OrderedSections.empty()) {
uint64_t UnorderedPos = 0;
for (; InsPt != UnorderedSections.size(); ++InsPt) {
UnorderedPos += UnorderedSections[InsPt]->getSize();
if (UnorderedPos > UnorderedSize / 2)
for (InputSection *IS : makeArrayRef(UnorderedSections).slice(0, InsPt))
for (std::pair<InputSection *, int> P : OrderedSections)
for (InputSection *IS : makeArrayRef(UnorderedSections).slice(InsPt))
static void sortSection(OutputSection *Sec,
const DenseMap<const InputSectionBase *, int> &Order) {
StringRef Name = Sec->Name;
// Sort input sections by section name suffixes for
// __attribute__((init_priority(N))).
if (Name == ".init_array" || Name == ".fini_array") {
if (!Script->HasSectionsCommand)
// Sort input sections by the special rule for .ctors and .dtors.
if (Name == ".ctors" || Name == ".dtors") {
if (!Script->HasSectionsCommand)
// Never sort these.
if (Name == ".init" || Name == ".fini")
// Sort input sections by priority using the list provided
// by --symbol-ordering-file.
if (!Order.empty())
for (BaseCommand *B : Sec->SectionCommands)
if (auto *ISD = dyn_cast<InputSectionDescription>(B))
sortISDBySectionOrder(ISD, Order);
// If no layout was provided by linker script, we want to apply default
// sorting for special input sections. This also handles --symbol-ordering-file.
template <class ELFT> void Writer<ELFT>::sortInputSections() {
// Build the order once since it is expensive.
DenseMap<const InputSectionBase *, int> Order = buildSectionOrder();
for (BaseCommand *Base : Script->SectionCommands)
if (auto *Sec = dyn_cast<OutputSection>(Base))
sortSection(Sec, Order);
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)
for (BaseCommand *Base : Script->SectionCommands) {
auto *OS = dyn_cast<OutputSection>(Base);
if (!OS)
OS->SortRank = getSectionRank(OS);
// We want to assign rude approximation values to OutSecOff fields
// to know the relative order of the input sections. We use it for
// sorting SHF_LINK_ORDER sections. See resolveShfLinkOrder().
uint64_t I = 0;
for (InputSection *Sec : getInputSections(OS))
Sec->OutSecOff = I++;
if (!Script->HasSectionsCommand) {
// We know that all the OutputSections are contiguous in this case.
auto IsSection = [](BaseCommand *Base) { return isa<OutputSection>(Base); };
llvm::find_if(Script->SectionCommands, IsSection),
llvm::find_if(llvm::reverse(Script->SectionCommands), IsSection).base(),
// Orphan sections are sections present in the input files which are
// not explicitly placed into the output file by the linker script.
// The sections in the linker script are already in the correct
// order. We have to figuere out where to insert the orphan
// sections.
// The order of the sections in the script is arbitrary and may not agree with
// compareSections. 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:
// * Sort only the orphan sections. They are in the end right now.
// * Move each orphan section to its preferred position. We try
// to put each section in the last position where it can share
// a PT_LOAD.
// There is some ambiguity as to where exactly a new entry should be
// inserted, because Commands contains not only output section
// commands but also other types of commands such as symbol assignment
// expressions. There's no correct answer here due to the lack of the
// formal specification of the linker script. We use heuristics to
// determine whether a new output command should be added before or
// after another commands. For the details, look at shouldSkip
// function.
auto I = Script->SectionCommands.begin();
auto E = Script->SectionCommands.end();
auto NonScriptI = std::find_if(I, E, [](BaseCommand *Base) {
if (auto *Sec = dyn_cast<OutputSection>(Base))
return Sec->SectionIndex == UINT32_MAX;
return false;
// Sort the orphan sections.
std::stable_sort(NonScriptI, E, compareSections);
// As a horrible special case, skip the first . assignment if it is before any
// section. We do this because it is common to set a load address by starting
// the script with ". = 0xabcd" and the expectation is that every section is
// after that.
auto FirstSectionOrDotAssignment =
std::find_if(I, E, [](BaseCommand *Cmd) { return !shouldSkip(Cmd); });
if (FirstSectionOrDotAssignment != E &&
I = FirstSectionOrDotAssignment;
while (NonScriptI != E) {
auto Pos = findOrphanPos<ELFT>(I, NonScriptI);
OutputSection *Orphan = cast<OutputSection>(*NonScriptI);
// As an optimization, find all sections with the same sort rank
// and insert them with one rotate.
unsigned Rank = Orphan->SortRank;
auto End = std::find_if(NonScriptI + 1, E, [=](BaseCommand *Cmd) {
return cast<OutputSection>(Cmd)->SortRank != Rank;
std::rotate(Pos, NonScriptI, End);
NonScriptI = End;
static bool compareByFilePosition(InputSection *A, InputSection *B) {
// Synthetic, i. e. a sentinel section, should go last.
if (A->kind() == InputSectionBase::Synthetic ||
B->kind() == InputSectionBase::Synthetic)
return A->kind() != InputSectionBase::Synthetic;
InputSection *LA = A->getLinkOrderDep();
InputSection *LB = B->getLinkOrderDep();
OutputSection *AOut = LA->getParent();
OutputSection *BOut = LB->getParent();
if (AOut != BOut)
return AOut->SectionIndex < BOut->SectionIndex;
return LA->OutSecOff < LB->OutSecOff;
// This function is used by the --merge-exidx-entries to detect duplicate
// .ARM.exidx sections. It is Arm only.
// The .ARM.exidx section is of the form:
// | PREL31 offset to function | Unwind instructions for function |
// where the unwind instructions are either a small number of unwind
// instructions inlined into the table entry, the special CANT_UNWIND value of
// 0x1 or a PREL31 offset into a .ARM.extab Section that contains unwind
// instructions.
// We return true if all the unwind instructions in the .ARM.exidx entries of
// Cur can be merged into the last entry of Prev.
static bool isDuplicateArmExidxSec(InputSection *Prev, InputSection *Cur) {
// References to .ARM.Extab Sections have bit 31 clear and are not the
// special EXIDX_CANTUNWIND bit-pattern.
auto IsExtabRef = [](uint32_t Unwind) {
return (Unwind & 0x80000000) == 0 && Unwind != 0x1;
struct ExidxEntry {
ulittle32_t Fn;
ulittle32_t Unwind;
// Get the last table Entry from the previous .ARM.exidx section.
const ExidxEntry &PrevEntry = Prev->getDataAs<ExidxEntry>().back();
if (IsExtabRef(PrevEntry.Unwind))
return false;
// We consider the unwind instructions of an .ARM.exidx table entry
// a duplicate if the previous unwind instructions if:
// - Both are the special EXIDX_CANTUNWIND.
// - Both are the same inline unwind instructions.
// We do not attempt to follow and check links into .ARM.extab tables as
// consecutive identical entries are rare and the effort to check that they
// are identical is high.
for (const ExidxEntry Entry : Cur->getDataAs<ExidxEntry>())
if (IsExtabRef(Entry.Unwind) || Entry.Unwind != PrevEntry.Unwind)
return false;
// All table entries in this .ARM.exidx Section can be merged into the
// previous Section.
return true;
template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
for (OutputSection *Sec : OutputSections) {
if (!(Sec->Flags & SHF_LINK_ORDER))
// Link order may be distributed across several InputSectionDescriptions
// but sort must consider them all at once.
std::vector<InputSection **> ScriptSections;
std::vector<InputSection *> Sections;
for (BaseCommand *Base : Sec->SectionCommands) {
if (auto *ISD = dyn_cast<InputSectionDescription>(Base)) {
for (InputSection *&IS : ISD->Sections) {
std::stable_sort(Sections.begin(), Sections.end(), compareByFilePosition);
if (!Config->Relocatable && Config->EMachine == EM_ARM &&
Sec->Type == SHT_ARM_EXIDX) {
if (auto *Sentinel = dyn_cast<ARMExidxSentinelSection>(Sections.back())) {
assert(Sections.size() >= 2 &&
"We should create a sentinel section only if there are "
"alive regular exidx sections.");
// The last executable section is required to fill the sentinel.
// Remember it here so that we don't have to find it again.
Sentinel->Highest = Sections[Sections.size() - 2]->getLinkOrderDep();
// The EHABI for the Arm Architecture permits consecutive identical
// table entries to be merged. We use a simple implementation that
// removes a .ARM.exidx Input Section if it can be merged into the
// previous one. This does not require any rewriting of InputSection
// contents but misses opportunities for fine grained deduplication
// where only a subset of the InputSection contents can be merged.
if (Config->MergeArmExidx) {
size_t Prev = 0;
// The last one is a sentinel entry which should not be removed.
for (size_t I = 1; I < Sections.size() - 1; ++I) {
if (isDuplicateArmExidxSec(Sections[Prev], Sections[I]))
Sections[I] = nullptr;
Prev = I;
for (int I = 0, N = Sections.size(); I < N; ++I)
*ScriptSections[I] = Sections[I];
// Remove the Sections we marked as duplicate earlier.
for (BaseCommand *Base : Sec->SectionCommands)
if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
llvm::erase_if(ISD->Sections, [](InputSection *IS) { return !IS; });
// For most RISC ISAs, we need to generate content that depends on the address
// of InputSections. For example some architectures such as AArch64 use small
// displacements for jump instructions that is the linker's responsibility for
// creating range extension thunks for. As the generation of the content may
// also alter InputSection addresses we must converge to a fixed point.
template <class ELFT> void Writer<ELFT>::maybeAddThunks() {
if (!Target->NeedsThunks && !Config->AndroidPackDynRelocs &&
ThunkCreator TC;
AArch64Err843419Patcher A64P;
for (;;) {
bool Changed = false;
if (Target->NeedsThunks)
Changed |= TC.createThunks(OutputSections);
if (Config->FixCortexA53Errata843419) {
if (Changed)
Changed |= A64P.createFixes();
if (In.MipsGot)
Changed |= In.RelaDyn->updateAllocSize();
if (In.RelrDyn)
Changed |= In.RelrDyn->updateAllocSize();
if (!Changed)
static void finalizeSynthetic(SyntheticSection *Sec) {
if (Sec && !Sec->empty() && Sec->getParent())
// In order to allow users to manipulate linker-synthesized sections,
// we had to add synthetic sections to the input section list early,
// even before we make decisions whether they are needed. This allows
// users to write scripts like this: ".mygot : { .got }".
// Doing it has an unintended side effects. If it turns out that we
// don't need a .got (for example) at all because there's no
// relocation that needs a .got, we don't want to emit .got.
// To deal with the above problem, this function is called after
// scanRelocations is called to remove synthetic sections that turn
// out to be empty.
static void removeUnusedSyntheticSections() {
// 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)
OutputSection *OS = SS->getParent();
if (!OS || !SS->empty())
// If we reach here, then SS is an unused synthetic section and we want to
// remove it from corresponding input section description of output section.
for (BaseCommand *B : OS->SectionCommands)
if (auto *ISD = dyn_cast<InputSectionDescription>(B))
[=](InputSection *IS) { return IS == SS; });
// Returns true if a symbol can be replaced at load-time by a symbol
// with the same name defined in other ELF executable or DSO.
static bool computeIsPreemptible(const Symbol &B) {
// Only symbols that appear in dynsym can be preempted.
if (!B.includeInDynsym())
return false;
// Only default visibility symbols can be preempted.
if (B.Visibility != STV_DEFAULT)
return false;
// At this point copy relocations have not been created yet, so any
// symbol that is not defined locally is preemptible.
if (!B.isDefined())
return true;
// If we have a dynamic list it specifies which local symbols are preemptible.
if (Config->HasDynamicList)
return false;
if (!Config->Shared)
return false;
// -Bsymbolic means that definitions are not preempted.
if (Config->Bsymbolic || (Config->BsymbolicFunctions && B.isFunc()))
return false;
return true;
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::finalizeSections() {
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 (BaseCommand *Base : Script->SectionCommands)
if (auto *Sec = dyn_cast<OutputSection>(Base))
// 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.DynSymTab)
Symtab->addDefined("_DYNAMIC", STV_HIDDEN, STT_NOTYPE, 0 /*Value*/,
/*Size=*/0, STB_WEAK, In.Dynamic,
// Define __rel[a]_iplt_{start,end} symbols if needed.
// RISC-V's gp can address +/- 2 KiB, set it to .sdata + 0x800 if not defined.
if (Config->EMachine == EM_RISCV)
if (!dyn_cast_or_null<Defined>(Symtab->find("__global_pointer$")))
addOptionalRegular("__global_pointer$", findSection(".sdata"), 0x800);
// This responsible for splitting up .eh_frame section into
// pieces. The relocation scan uses those pieces, so this has to be
// earlier.
for (Symbol *S : Symtab->getSymbols())
if (!S->IsPreemptible)
S->IsPreemptible = computeIsPreemptible(*S);
// Scan relocations. This must be done after every symbol is declared so that
// we can correctly decide if a dynamic relocation is needed.
if (!Config->Relocatable)
if (In.Plt && !In.Plt->empty())
if (In.Iplt && !In.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 *Sym : Symtab->getSymbols()) {
if (!includeInSymtab(*Sym))
if (In.SymTab)
if (In.DynSymTab && Sym->includeInDynsym()) {
if (auto *File = dyn_cast_or_null<SharedFile<ELFT>>(Sym->File))
if (File->IsNeeded && !Sym->isUndefined())
// Do not proceed if there was an undefined symbol.
if (errorCount())
if (In.MipsGot)
// Now that we have the final list, create a list of all the
// OutputSections for convenience.
for (BaseCommand *Base : Script->SectionCommands)
if (auto *Sec = dyn_cast<OutputSection>(Base))
// Prefer command line supplied address over other constraints.
for (OutputSection *Sec : OutputSections) {
auto I = Config->SectionStartMap.find(Sec->Name);
if (I != Config->SectionStartMap.end())
Sec->AddrExpr = [=] { return I->second; };
// This is a bit of a hack. A value of 0 means undef, so we set it
// to 1 to make __ehdr_start defined. The section number is not
// particularly relevant.
Out::ElfHeader->SectionIndex = 1;
for (size_t I = 0, E = OutputSections.size(); I != E; ++I) {
OutputSection *Sec = OutputSections[I];
Sec->SectionIndex = I + 1;
Sec->ShName = In.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();
// Find the TLS segment. This happens before the section layout loop so that
// Android relocation packing can look up TLS symbol addresses.
for (PhdrEntry *P : Phdrs)
if (P->p_type == PT_TLS)
Out::TlsPhdr = P;
// Some symbols are defined in term of program headers. Now that we
// have the headers, we can find out which sections they point to.
// Dynamic section must be the last one in this list and dynamic
// symbol table section (DynSymTab) must be the first one.
if (!Script->HasSectionsCommand && !Config->Relocatable)
// After link order processing .ARM.exidx sections can be deduplicated, which
// needs to be resolved before any other address dependent operation.
// Jump instructions in many ISAs have small displacements, and therefore they
// cannot jump to arbitrary addresses in memory. For example, RISC-V JAL
// instruction can target only +-1 MiB from PC. It is a linker's
// responsibility to create and insert small pieces of code between sections
// to extend the ranges if jump targets are out of range. Such code pieces are
// called "thunks".
// We add thunks at this stage. We couldn't do this before this point because
// this is the earliest point where we know sizes of sections and their
// layouts (that are needed to determine if jump targets are in range).
// maybeAddThunks may have added local symbols to the static symbol table.
// 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)
// Ensure data sections are not mixed with executable sections when
// -execute-only is used. -execute-only is a feature to make pages executable
// but not readable, and the feature is currently supported only on AArch64.
template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
if (!Config->ExecuteOnly)
for (OutputSection *OS : OutputSections)
if (OS->Flags & SHF_EXECINSTR)
for (InputSection *IS : getInputSections(OS))
if (!(IS->Flags & SHF_EXECINSTR))
error("cannot place " + toString(IS) + " into " + toString(OS->Name) +
": -execute-only does not support intermingling data and code");
// 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() {
// If a section does not exist, there's ambiguity as to how we
// define _start and _end symbols for an init/fini section. Since
// the loader assume that the symbols are always defined, we need to
// always define them. But what value? The loader iterates over all
// pointers between _start and _end to run global ctors/dtors, so if
// the section is empty, their symbol values don't actually matter
// as long as _start and _end point to the same location.
// That said, we don't want to set the symbols to 0 (which is
// probably the simplest value) because that could cause some
// program to fail to link due to relocation overflow, if their
// program text is above 2 GiB. We use the address of the .text
// section instead to prevent that failure.
// In a rare sitaution, .text section may not exist. If that's the
// case, use the image base address as a last resort.
OutputSection *Default = findSection(".text");
if (!Default)
Default = Out::ElfHeader;
auto Define = [=](StringRef Start, StringRef End, OutputSection *OS) {
if (OS) {
addOptionalRegular(Start, OS, 0);
addOptionalRegular(End, OS, -1);
} else {
addOptionalRegular(Start, Default, 0);
addOptionalRegular(End, Default, 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("__start_" + S), Sec, 0, STV_PROTECTED);
addOptionalRegular("__stop_" + S), Sec, -1, STV_PROTECTED);
static bool needsPtLoad(OutputSection *Sec) {
if (!(Sec->Flags & SHF_ALLOC) || Sec->Noload)
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->ExecuteOnly && (Flags & PF_X))
return Flags & ~PF_R;
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.push_back(make<PhdrEntry>(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 *Cmd = findSection(".interp"))
AddHdr(PT_INTERP, Cmd->getPhdrFlags())->add(Cmd);
// 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 or memory
// region using AT or AT> linker script command, respectively. At the same
// time, we don't want to create a separate load segment for the headers,
// even if the first output section has an AT or AT> attribute.
uint64_t NewFlags = computeFlags(Sec->getPhdrFlags());
if (((Sec->LMAExpr ||
(Sec->LMARegion && (Sec->LMARegion != Load->FirstSec->LMARegion))) &&
Load->LastSec != Out::ProgramHeaders) ||
Sec->MemRegion != Load->FirstSec->MemRegion || Flags != NewFlags) {
Load = AddHdr(PT_LOAD, NewFlags);
Flags = NewFlags;
// Add a TLS segment if any.
PhdrEntry *TlsHdr = make<PhdrEntry>(PT_TLS, PF_R);
for (OutputSection *Sec : OutputSections)
if (Sec->Flags & SHF_TLS)
if (TlsHdr->FirstSec)
// Add an entry for .dynamic.
if (In.DynSymTab)
AddHdr(PT_DYNAMIC, In.Dynamic->getParent()->getPhdrFlags())
// PT_GNU_RELRO includes all sections that should be marked as
// read-only by dynamic linker after proccessing relocations.
// Current dynamic loaders only support one PT_GNU_RELRO PHDR, give
// an error message if more than one PT_GNU_RELRO PHDR is required.
PhdrEntry *RelRo = make<PhdrEntry>(PT_GNU_RELRO, PF_R);
bool InRelroPhdr = false;
bool IsRelroFinished = false;
for (OutputSection *Sec : OutputSections) {
if (!needsPtLoad(Sec))
if (isRelroSection(Sec)) {
InRelroPhdr = true;
if (!IsRelroFinished)
error("section: " + Sec->Name + " is not contiguous with other relro" +
" sections");
} else if (InRelroPhdr) {
InRelroPhdr = false;
IsRelroFinished = true;
if (RelRo->FirstSec)
// PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
if (!In.EhFrame->empty() && In.EhFrameHdr && In.EhFrame->getParent() &&
AddHdr(PT_GNU_EH_FRAME, In.EhFrameHdr->getParent()->getPhdrFlags())
// PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
// the dynamic linker fill the segment with random data.
if (OutputSection *Cmd = findSection(".openbsd.randomdata"))
AddHdr(PT_OPENBSD_RANDOMIZE, Cmd->getPhdrFlags())->add(Cmd);
// 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 = PF_R | PF_W;
if (Config->ZExecstack)
Perm |= PF_X;
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 && (Sec->Flags & SHF_ALLOC)) {
if (!Note || Sec->LMAExpr)
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 = llvm::find_if(OutputSections, [](OutputSection *Cmd) {
return Cmd->Type == SHT_ARM_EXIDX;
if (I == OutputSections.end())
// PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
PhdrEntry *ARMExidx = make<PhdrEntry>(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() {
auto PageAlign = [](OutputSection *Cmd) {
if (Cmd && !Cmd->AddrExpr)
Cmd->AddrExpr = [=] {
return alignTo(Script->getDot(), Config->MaxPageSize);
for (const PhdrEntry *P : Phdrs)
if (P->p_type == PT_LOAD && P->FirstSec)
for (const PhdrEntry *P : Phdrs) {
if (P->p_type != PT_GNU_RELRO)
if (P->FirstSec)
// 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->LastSec);
if (I == End || (I + 1) == End)
OutputSection *Cmd = (*(I + 1));
if (needsPtLoad(Cmd))
// Compute an in-file position for a given section. 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 computeFileOffset(OutputSection *OS, uint64_t Off) {
// File offsets are not significant for .bss sections. By convention, we keep
// section offsets monotonically increasing rather than setting to zero.
if (OS->Type == SHT_NOBITS)
return Off;
// If the section is not in a PT_LOAD, we just have to align it.
if (!OS->PtLoad)
return alignTo(Off, OS->Alignment);
// The first section in a PT_LOAD has to have congruent offset and address
// module the page size.
OutputSection *First = OS->PtLoad->FirstSec;
if (OS == First) {
uint64_t Alignment = std::max<uint64_t>(OS->Alignment, Config->MaxPageSize);
return alignTo(Off, Alignment, OS->Addr);
// If two sections share the same PT_LOAD the file offset is calculated
// using this formula: Off2 = Off1 + (VA2 - VA1).
return First->Offset + OS->Addr - First->Addr;
// Set an in-file position to a given section and returns the end position of
// the section.
static uint64_t setFileOffset(OutputSection *OS, uint64_t Off) {
Off = computeFileOffset(OS, Off);
OS->Offset = Off;
if (OS->Type == SHT_NOBITS)
return Off;
return Off + OS->Size;
template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
uint64_t Off = 0;
for (OutputSection *Sec : OutputSections)
if (Sec->Flags & SHF_ALLOC)
Off = setFileOffset(Sec, Off);
FileSize = alignTo(Off, Config->Wordsize);
static std::string rangeToString(uint64_t Addr, uint64_t Len) {
return "[0x" + utohexstr(Addr) + ", 0x" + utohexstr(Addr + Len - 1) + "]";
// Assign file offsets to output sections.
template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
uint64_t Off = 0;
Off = setFileOffset(Out::ElfHeader, Off);
Off = setFileOffset(Out::ProgramHeaders, Off);
PhdrEntry *LastRX = nullptr;
for (PhdrEntry *P : Phdrs)
if (P->p_type == PT_LOAD && (P->p_flags & PF_X))
LastRX = P;
for (OutputSection *Sec : OutputSections) {
Off = setFileOffset(Sec, Off);
if (Script->HasSectionsCommand)
// If this is a last section of the last executable segment and that
// segment is the last loadable segment, align the offset of the
// following section to avoid loading non-segments parts of the file.
if (LastRX && LastRX->LastSec == Sec)
Off = alignTo(Off, Target->PageSize);
SectionHeaderOff = alignTo(Off, Config->Wordsize);
FileSize = SectionHeaderOff + (OutputSections.size() + 1) * sizeof(Elf_Shdr);
// Our logic assumes that sections have rising VA within the same segment.
// With use of linker scripts it is possible to violate this rule and get file
// offset overlaps or overflows. That should never happen with a valid script
// which does not move the location counter backwards and usually scripts do
// not do that. Unfortunately, there are apps in the wild, for example, Linux
// kernel, which control segment distribution explicitly and move the counter
// backwards, so we have to allow doing that to support linking them. We
// perform non-critical checks for overlaps in checkSectionOverlap(), but here
// we want to prevent file size overflows because it would crash the linker.
for (OutputSection *Sec : OutputSections) {
if (Sec->Type == SHT_NOBITS)
if ((Sec->Offset > FileSize) || (Sec->Offset + Sec->Size > FileSize))
error("unable to place section " + Sec->Name + " at file offset " +
rangeToString(Sec->Offset, Sec->Size) +
"; check your linker script for overflows");
// 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->FirstSec;
OutputSection *Last = P->LastSec;
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 = std::max<uint64_t>(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 for variant 2 targets. At least glibc
// will align it, so round up the size to make sure the offsets are
// correct.
if (P->p_type == PT_TLS && P->p_memsz)
P->p_memsz = alignTo(P->p_memsz, P->p_align);
// A helper struct for checkSectionOverlap.
namespace {
struct SectionOffset {
OutputSection *Sec;
uint64_t Offset;
} // namespace
// Check whether sections overlap for a specific address range (file offsets,
// load and virtual adresses).
static void checkOverlap(StringRef Name, std::vector<SectionOffset> &Sections,
bool IsVirtualAddr) {
llvm::sort(Sections, [=](const SectionOffset &A, const SectionOffset &B) {
return A.Offset < B.Offset;
// Finding overlap is easy given a vector is sorted by start position.
// If an element starts before the end of the previous element, they overlap.
for (size_t I = 1, End = Sections.size(); I < End; ++I) {
SectionOffset A = Sections[I - 1];
SectionOffset B = Sections[I];
if (B.Offset >= A.Offset + A.Sec->Size)
// If both sections are in OVERLAY we allow the overlapping of virtual
// addresses, because it is what OVERLAY was designed for.
if (IsVirtualAddr && A.Sec->InOverlay && B.Sec->InOverlay)
errorOrWarn("section " + A.Sec->Name + " " + Name +
" range overlaps with " + B.Sec->Name + "\n>>> " + A.Sec->Name +
" range is " + rangeToString(A.Offset, A.Sec->Size) + "\n>>> " +
B.Sec->Name + " range is " +
rangeToString(B.Offset, B.Sec->Size));
// Check for overlapping sections and address overflows.
// In this function we check that none of the output sections have overlapping
// file offsets. For SHF_ALLOC sections we also check that the load address
// ranges and the virtual address ranges don't overlap
template <class ELFT> void Writer<ELFT>::checkSections() {
// First, check that section's VAs fit in available address space for target.
for (OutputSection *OS : OutputSections)
if ((OS->Addr + OS->Size < OS->Addr) ||
(!ELFT::Is64Bits && OS->Addr + OS->Size > UINT32_MAX))
errorOrWarn("section " + OS->Name + " at 0x" + utohexstr(OS->Addr) +
" of size 0x" + utohexstr(OS->Size) +
" exceeds available address space");
// Check for overlapping file offsets. In this case we need to skip any
// section marked as SHT_NOBITS. These sections don't actually occupy space in
// the file so Sec->Offset + Sec->Size can overlap with others. If --oformat
// binary is specified only add SHF_ALLOC sections are added to the output
// file so we skip any non-allocated sections in that case.
std::vector<SectionOffset> FileOffs;
for (OutputSection *Sec : OutputSections)
if (Sec->Size > 0 && Sec->Type != SHT_NOBITS &&
(!Config->OFormatBinary || (Sec->Flags & SHF_ALLOC)))
FileOffs.push_back({Sec, Sec->Offset});
checkOverlap("file", FileOffs, false);
// When linking with -r there is no need to check for overlapping virtual/load
// addresses since those addresses will only be assigned when the final
// executable/shared object is created.
if (Config->Relocatable)
// Checking for overlapping virtual and load addresses only needs to take
// into account SHF_ALLOC sections since others will not be loaded.
// Furthermore, we also need to skip SHF_TLS sections since these will be
// mapped to other addresses at runtime and can therefore have overlapping
// ranges in the file.
std::vector<SectionOffset> VMAs;
for (OutputSection *Sec : OutputSections)
if (Sec->Size > 0 && (Sec->Flags & SHF_ALLOC) && !(Sec->Flags & SHF_TLS))
VMAs.push_back({Sec, Sec->Addr});
checkOverlap("virtual address", VMAs, true);
// Finally, check that the load addresses don't overlap. This will usually be
// the same as the virtual addresses but can be different when using a linker
// script with AT().
std::vector<SectionOffset> LMAs;
for (OutputSection *Sec : OutputSections)
if (Sec->Size > 0 && (Sec->Flags & SHF_ALLOC) && !(Sec->Flags & SHF_TLS))
LMAs.push_back({Sec, Sec->getLMA()});
checkOverlap("load address", LMAs, false);
// 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 number represented by the entry symbol, if it is a number;
// 5. the address of the first byte of the .text section, if present;
// 6. the address 0.
static uint64_t getEntryAddr() {
// Case 1, 2 or 3
if (Symbol *B = Symtab->find(Config->Entry))
return B->getVA();
// Case 4
uint64_t Addr;
if (to_integer(Config->Entry, Addr))
return Addr;
// Case 5
if (OutputSection *Sec = findSection(".text")) {
if (Config->WarnMissingEntry)
warn("cannot find entry symbol " + Config->Entry + "; defaulting to 0x" +
return Sec->Addr;
// Case 6
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;
static uint8_t getAbiVersion() {
// MIPS non-PIC executable gets ABI version 1.
if (Config->EMachine == EM_MIPS && getELFType() == ET_EXEC &&
(Config->EFlags & (EF_MIPS_PIC | EF_MIPS_CPIC)) == EF_MIPS_CPIC)
return 1;
return 0;
template <class ELFT> void Writer<ELFT>::writeHeader() {
uint8_t *Buf = Buffer->getBufferStart();
// For executable segments, the trap instructions are written before writing
// the header. Setting Elf header bytes to zero ensures that any unused bytes
// in header are zero-cleared, instead of having trap instructions.
memset(Buf, 0, sizeof(Elf_Ehdr));
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_ident[EI_ABIVERSION] = getAbiVersion();
EHdr->e_type = getELFType();
EHdr->e_machine = Config->EMachine;
EHdr->e_version = EV_CURRENT;
EHdr->e_entry = getEntryAddr();
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_flags = Config->EFlags;
EHdr->e_ehsize = sizeof(Elf_Ehdr);
EHdr->e_phnum = Phdrs.size();
EHdr->e_shentsize = sizeof(Elf_Shdr);
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.
// The ELF header can only store numbers up to SHN_LORESERVE in the e_shnum
// and e_shstrndx fields. When the value of one of these fields exceeds
// SHN_LORESERVE ELF requires us to put sentinel values in the ELF header and
// use fields in the section header at index 0 to store
// the value. The sentinel values and fields are:
// e_shnum = 0, SHdrs[0].sh_size = number of sections.
// e_shstrndx = SHN_XINDEX, SHdrs[0].sh_link = .shstrtab section index.
auto *SHdrs = reinterpret_cast<Elf_Shdr *>(Buf + EHdr->e_shoff);
size_t Num = OutputSections.size() + 1;
SHdrs->sh_size = Num;
EHdr->e_shnum = Num;
uint32_t StrTabIndex = In.ShStrTab->getParent()->SectionIndex;
if (StrTabIndex >= SHN_LORESERVE) {
SHdrs->sh_link = StrTabIndex;
EHdr->e_shstrndx = SHN_XINDEX;
} else {
EHdr->e_shstrndx = StrTabIndex;
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");
unsigned Flags = 0;
if (!Config->Relocatable)
Flags = FileOutputBuffer::F_executable;
Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Config->OutputFile, FileSize, Flags);
if (!BufferOrErr)
error("failed to open " + Config->OutputFile + ": " +
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);
static void fillTrap(uint8_t *I, uint8_t *End) {
for (; I + 4 <= End; I += 4)
memcpy(I, &Target->TrapInstr, 4);
// Fill the last page of executable segments with trap instructions
// instead of leaving them as zero. Even though it is not required by any
// standard, it is in general a good thing to do for security reasons.
// We'll leave other pages in segments as-is because the rest will be
// overwritten by output sections.
template <class ELFT> void Writer<ELFT>::writeTrapInstr() {
if (Script->HasSectionsCommand)
// Fill the last page.
uint8_t *Buf = Buffer->getBufferStart();
for (PhdrEntry *P : Phdrs)
if (P->p_type == PT_LOAD && (P->p_flags & PF_X))
fillTrap(Buf + alignDown(P->p_offset + P->p_filesz, Target->PageSize),
Buf + alignTo(P->p_offset + P->p_filesz, Target->PageSize));
// Round up the file size of the last segment to the page boundary iff it is
// an executable segment to ensure that other tools don't accidentally
// trim the instruction padding (e.g. when stripping the file).
PhdrEntry *Last = nullptr;
for (PhdrEntry *P : Phdrs)
if (P->p_type == PT_LOAD)
Last = P;
if (Last && (Last->p_flags & PF_X))
Last->p_memsz = Last->p_filesz = alignTo(Last->p_filesz, Target->PageSize);
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
uint8_t *Buf = Buffer->getBufferStart();
OutputSection *EhFrameHdr = nullptr;
if (In.EhFrameHdr && !In.EhFrameHdr->empty())
EhFrameHdr = In.EhFrameHdr->getParent();
// 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 != 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->writeTo<ELFT>(Buf + EhFrameHdr->Offset);
template <class ELFT> void Writer<ELFT>::writeBuildId() {
if (!In.BuildId || !In.BuildId->getParent())
// Compute a hash of all sections of the output file.
uint8_t *Start = Buffer->getBufferStart();
uint8_t *End = Start + FileSize;
In.BuildId->writeBuildId({Start, End});
template void elf::writeResult<ELF32LE>();
template void elf::writeResult<ELF32BE>();
template void elf::writeResult<ELF64LE>();
template void elf::writeResult<ELF64BE>();