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llvm-project/lld/ELF/SyntheticSections.cpp

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//===- SyntheticSections.cpp ----------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains linker-synthesized sections. Currently,
// synthetic sections are created either output sections or input sections,
// but we are rewriting code so that all synthetic sections are created as
// input sections.
//
//===----------------------------------------------------------------------===//
#include "SyntheticSections.h"
#include "Bits.h"
#include "Config.h"
#include "InputFiles.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "Target.h"
#include "Writer.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "lld/Common/Threads.h"
#include "lld/Common/Version.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
#include "llvm/Object/ELFObjectFile.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/MD5.h"
#include "llvm/Support/RandomNumberGenerator.h"
#include "llvm/Support/SHA1.h"
#include "llvm/Support/xxhash.h"
#include <cstdlib>
#include <thread>
using namespace llvm;
using namespace llvm::dwarf;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support;
using namespace lld;
using namespace lld::elf;
using llvm::support::endian::read32le;
using llvm::support::endian::write32le;
using llvm::support::endian::write64le;
constexpr size_t MergeNoTailSection::NumShards;
// Returns an LLD version string.
static ArrayRef<uint8_t> getVersion() {
// Check LLD_VERSION first for ease of testing.
// You can get consistent output by using the environment variable.
// This is only for testing.
StringRef S = getenv("LLD_VERSION");
if (S.empty())
S = Saver.save(Twine("Linker: ") + getLLDVersion());
// +1 to include the terminating '\0'.
return {(const uint8_t *)S.data(), S.size() + 1};
}
// Creates a .comment section containing LLD version info.
// With this feature, you can identify LLD-generated binaries easily
// by "readelf --string-dump .comment <file>".
// The returned object is a mergeable string section.
MergeInputSection *elf::createCommentSection() {
return make<MergeInputSection>(SHF_MERGE | SHF_STRINGS, SHT_PROGBITS, 1,
getVersion(), ".comment");
}
// .MIPS.abiflags section.
template <class ELFT>
MipsAbiFlagsSection<ELFT>::MipsAbiFlagsSection(Elf_Mips_ABIFlags Flags)
: SyntheticSection(SHF_ALLOC, SHT_MIPS_ABIFLAGS, 8, ".MIPS.abiflags"),
Flags(Flags) {
this->Entsize = sizeof(Elf_Mips_ABIFlags);
}
template <class ELFT> void MipsAbiFlagsSection<ELFT>::writeTo(uint8_t *Buf) {
memcpy(Buf, &Flags, sizeof(Flags));
}
template <class ELFT>
MipsAbiFlagsSection<ELFT> *MipsAbiFlagsSection<ELFT>::create() {
Elf_Mips_ABIFlags Flags = {};
bool Create = false;
for (InputSectionBase *Sec : InputSections) {
if (Sec->Type != SHT_MIPS_ABIFLAGS)
continue;
Sec->Live = false;
Create = true;
std::string Filename = toString(Sec->File);
const size_t Size = Sec->data().size();
// Older version of BFD (such as the default FreeBSD linker) concatenate
// .MIPS.abiflags instead of merging. To allow for this case (or potential
// zero padding) we ignore everything after the first Elf_Mips_ABIFlags
if (Size < sizeof(Elf_Mips_ABIFlags)) {
error(Filename + ": invalid size of .MIPS.abiflags section: got " +
Twine(Size) + " instead of " + Twine(sizeof(Elf_Mips_ABIFlags)));
return nullptr;
}
auto *S = reinterpret_cast<const Elf_Mips_ABIFlags *>(Sec->data().data());
if (S->version != 0) {
error(Filename + ": unexpected .MIPS.abiflags version " +
Twine(S->version));
return nullptr;
}
// LLD checks ISA compatibility in calcMipsEFlags(). Here we just
// select the highest number of ISA/Rev/Ext.
Flags.isa_level = std::max(Flags.isa_level, S->isa_level);
Flags.isa_rev = std::max(Flags.isa_rev, S->isa_rev);
Flags.isa_ext = std::max(Flags.isa_ext, S->isa_ext);
Flags.gpr_size = std::max(Flags.gpr_size, S->gpr_size);
Flags.cpr1_size = std::max(Flags.cpr1_size, S->cpr1_size);
Flags.cpr2_size = std::max(Flags.cpr2_size, S->cpr2_size);
Flags.ases |= S->ases;
Flags.flags1 |= S->flags1;
Flags.flags2 |= S->flags2;
Flags.fp_abi = elf::getMipsFpAbiFlag(Flags.fp_abi, S->fp_abi, Filename);
};
if (Create)
return make<MipsAbiFlagsSection<ELFT>>(Flags);
return nullptr;
}
// .MIPS.options section.
template <class ELFT>
MipsOptionsSection<ELFT>::MipsOptionsSection(Elf_Mips_RegInfo Reginfo)
: SyntheticSection(SHF_ALLOC, SHT_MIPS_OPTIONS, 8, ".MIPS.options"),
Reginfo(Reginfo) {
this->Entsize = sizeof(Elf_Mips_Options) + sizeof(Elf_Mips_RegInfo);
}
template <class ELFT> void MipsOptionsSection<ELFT>::writeTo(uint8_t *Buf) {
auto *Options = reinterpret_cast<Elf_Mips_Options *>(Buf);
Options->kind = ODK_REGINFO;
Options->size = getSize();
if (!Config->Relocatable)
Reginfo.ri_gp_value = In.MipsGot->getGp();
memcpy(Buf + sizeof(Elf_Mips_Options), &Reginfo, sizeof(Reginfo));
}
template <class ELFT>
MipsOptionsSection<ELFT> *MipsOptionsSection<ELFT>::create() {
// N64 ABI only.
if (!ELFT::Is64Bits)
return nullptr;
std::vector<InputSectionBase *> Sections;
for (InputSectionBase *Sec : InputSections)
if (Sec->Type == SHT_MIPS_OPTIONS)
Sections.push_back(Sec);
if (Sections.empty())
return nullptr;
Elf_Mips_RegInfo Reginfo = {};
for (InputSectionBase *Sec : Sections) {
Sec->Live = false;
std::string Filename = toString(Sec->File);
ArrayRef<uint8_t> D = Sec->data();
while (!D.empty()) {
if (D.size() < sizeof(Elf_Mips_Options)) {
error(Filename + ": invalid size of .MIPS.options section");
break;
}
auto *Opt = reinterpret_cast<const Elf_Mips_Options *>(D.data());
if (Opt->kind == ODK_REGINFO) {
Reginfo.ri_gprmask |= Opt->getRegInfo().ri_gprmask;
Sec->getFile<ELFT>()->MipsGp0 = Opt->getRegInfo().ri_gp_value;
break;
}
if (!Opt->size)
fatal(Filename + ": zero option descriptor size");
D = D.slice(Opt->size);
}
};
return make<MipsOptionsSection<ELFT>>(Reginfo);
}
// MIPS .reginfo section.
template <class ELFT>
MipsReginfoSection<ELFT>::MipsReginfoSection(Elf_Mips_RegInfo Reginfo)
: SyntheticSection(SHF_ALLOC, SHT_MIPS_REGINFO, 4, ".reginfo"),
Reginfo(Reginfo) {
this->Entsize = sizeof(Elf_Mips_RegInfo);
}
template <class ELFT> void MipsReginfoSection<ELFT>::writeTo(uint8_t *Buf) {
if (!Config->Relocatable)
Reginfo.ri_gp_value = In.MipsGot->getGp();
memcpy(Buf, &Reginfo, sizeof(Reginfo));
}
template <class ELFT>
MipsReginfoSection<ELFT> *MipsReginfoSection<ELFT>::create() {
// Section should be alive for O32 and N32 ABIs only.
if (ELFT::Is64Bits)
return nullptr;
std::vector<InputSectionBase *> Sections;
for (InputSectionBase *Sec : InputSections)
if (Sec->Type == SHT_MIPS_REGINFO)
Sections.push_back(Sec);
if (Sections.empty())
return nullptr;
Elf_Mips_RegInfo Reginfo = {};
for (InputSectionBase *Sec : Sections) {
Sec->Live = false;
if (Sec->data().size() != sizeof(Elf_Mips_RegInfo)) {
error(toString(Sec->File) + ": invalid size of .reginfo section");
return nullptr;
}
auto *R = reinterpret_cast<const Elf_Mips_RegInfo *>(Sec->data().data());
Reginfo.ri_gprmask |= R->ri_gprmask;
Sec->getFile<ELFT>()->MipsGp0 = R->ri_gp_value;
};
return make<MipsReginfoSection<ELFT>>(Reginfo);
}
InputSection *elf::createInterpSection() {
// StringSaver guarantees that the returned string ends with '\0'.
StringRef S = Saver.save(Config->DynamicLinker);
ArrayRef<uint8_t> Contents = {(const uint8_t *)S.data(), S.size() + 1};
auto *Sec = make<InputSection>(nullptr, SHF_ALLOC, SHT_PROGBITS, 1, Contents,
".interp");
Sec->Live = true;
return Sec;
}
Defined *elf::addSyntheticLocal(StringRef Name, uint8_t Type, uint64_t Value,
uint64_t Size, InputSectionBase &Section) {
auto *S = make<Defined>(Section.File, Name, STB_LOCAL, STV_DEFAULT, Type,
Value, Size, &Section);
if (In.SymTab)
In.SymTab->addSymbol(S);
return S;
}
static size_t getHashSize() {
switch (Config->BuildId) {
case BuildIdKind::Fast:
return 8;
case BuildIdKind::Md5:
case BuildIdKind::Uuid:
return 16;
case BuildIdKind::Sha1:
return 20;
case BuildIdKind::Hexstring:
return Config->BuildIdVector.size();
default:
llvm_unreachable("unknown BuildIdKind");
}
}
BuildIdSection::BuildIdSection()
: SyntheticSection(SHF_ALLOC, SHT_NOTE, 4, ".note.gnu.build-id"),
HashSize(getHashSize()) {}
void BuildIdSection::writeTo(uint8_t *Buf) {
write32(Buf, 4); // Name size
write32(Buf + 4, HashSize); // Content size
write32(Buf + 8, NT_GNU_BUILD_ID); // Type
memcpy(Buf + 12, "GNU", 4); // Name string
HashBuf = Buf + 16;
}
// Split one uint8 array into small pieces of uint8 arrays.
static std::vector<ArrayRef<uint8_t>> split(ArrayRef<uint8_t> Arr,
size_t ChunkSize) {
std::vector<ArrayRef<uint8_t>> Ret;
while (Arr.size() > ChunkSize) {
Ret.push_back(Arr.take_front(ChunkSize));
Arr = Arr.drop_front(ChunkSize);
}
if (!Arr.empty())
Ret.push_back(Arr);
return Ret;
}
// Computes a hash value of Data using a given hash function.
// In order to utilize multiple cores, we first split data into 1MB
// chunks, compute a hash for each chunk, and then compute a hash value
// of the hash values.
void BuildIdSection::computeHash(
llvm::ArrayRef<uint8_t> Data,
std::function<void(uint8_t *Dest, ArrayRef<uint8_t> Arr)> HashFn) {
std::vector<ArrayRef<uint8_t>> Chunks = split(Data, 1024 * 1024);
std::vector<uint8_t> Hashes(Chunks.size() * HashSize);
// Compute hash values.
parallelForEachN(0, Chunks.size(), [&](size_t I) {
HashFn(Hashes.data() + I * HashSize, Chunks[I]);
});
// Write to the final output buffer.
HashFn(HashBuf, Hashes);
}
BssSection::BssSection(StringRef Name, uint64_t Size, uint32_t Alignment)
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_NOBITS, Alignment, Name) {
this->Bss = true;
this->Size = Size;
}
void BuildIdSection::writeBuildId(ArrayRef<uint8_t> Buf) {
switch (Config->BuildId) {
case BuildIdKind::Fast:
computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
write64le(Dest, xxHash64(Arr));
});
break;
case BuildIdKind::Md5:
computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
memcpy(Dest, MD5::hash(Arr).data(), 16);
});
break;
case BuildIdKind::Sha1:
computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
memcpy(Dest, SHA1::hash(Arr).data(), 20);
});
break;
case BuildIdKind::Uuid:
if (auto EC = getRandomBytes(HashBuf, HashSize))
error("entropy source failure: " + EC.message());
break;
case BuildIdKind::Hexstring:
memcpy(HashBuf, Config->BuildIdVector.data(), Config->BuildIdVector.size());
break;
default:
llvm_unreachable("unknown BuildIdKind");
}
}
EhFrameSection::EhFrameSection()
: SyntheticSection(SHF_ALLOC, SHT_PROGBITS, 1, ".eh_frame") {}
// Search for an existing CIE record or create a new one.
// CIE records from input object files are uniquified by their contents
// and where their relocations point to.
template <class ELFT, class RelTy>
CieRecord *EhFrameSection::addCie(EhSectionPiece &Cie, ArrayRef<RelTy> Rels) {
Symbol *Personality = nullptr;
unsigned FirstRelI = Cie.FirstRelocation;
if (FirstRelI != (unsigned)-1)
Personality =
&Cie.Sec->template getFile<ELFT>()->getRelocTargetSym(Rels[FirstRelI]);
// Search for an existing CIE by CIE contents/relocation target pair.
CieRecord *&Rec = CieMap[{Cie.data(), Personality}];
// If not found, create a new one.
if (!Rec) {
Rec = make<CieRecord>();
Rec->Cie = &Cie;
CieRecords.push_back(Rec);
}
return Rec;
}
// There is one FDE per function. Returns true if a given FDE
// points to a live function.
template <class ELFT, class RelTy>
bool EhFrameSection::isFdeLive(EhSectionPiece &Fde, ArrayRef<RelTy> Rels) {
auto *Sec = cast<EhInputSection>(Fde.Sec);
unsigned FirstRelI = Fde.FirstRelocation;
// An FDE should point to some function because FDEs are to describe
// functions. That's however not always the case due to an issue of
// ld.gold with -r. ld.gold may discard only functions and leave their
// corresponding FDEs, which results in creating bad .eh_frame sections.
// To deal with that, we ignore such FDEs.
if (FirstRelI == (unsigned)-1)
return false;
const RelTy &Rel = Rels[FirstRelI];
Symbol &B = Sec->template getFile<ELFT>()->getRelocTargetSym(Rel);
// FDEs for garbage-collected or merged-by-ICF sections are dead.
if (auto *D = dyn_cast<Defined>(&B))
if (SectionBase *Sec = D->Section)
return Sec->Live;
return false;
}
// .eh_frame is a sequence of CIE or FDE records. In general, there
// is one CIE record per input object file which is followed by
// a list of FDEs. This function searches an existing CIE or create a new
// one and associates FDEs to the CIE.
template <class ELFT, class RelTy>
void EhFrameSection::addSectionAux(EhInputSection *Sec, ArrayRef<RelTy> Rels) {
OffsetToCie.clear();
for (EhSectionPiece &Piece : Sec->Pieces) {
// The empty record is the end marker.
if (Piece.Size == 4)
return;
size_t Offset = Piece.InputOff;
uint32_t ID = read32(Piece.data().data() + 4);
if (ID == 0) {
OffsetToCie[Offset] = addCie<ELFT>(Piece, Rels);
continue;
}
uint32_t CieOffset = Offset + 4 - ID;
CieRecord *Rec = OffsetToCie[CieOffset];
if (!Rec)
fatal(toString(Sec) + ": invalid CIE reference");
if (!isFdeLive<ELFT>(Piece, Rels))
continue;
Rec->Fdes.push_back(&Piece);
NumFdes++;
}
}
template <class ELFT> void EhFrameSection::addSection(InputSectionBase *C) {
auto *Sec = cast<EhInputSection>(C);
Sec->Parent = this;
Alignment = std::max(Alignment, Sec->Alignment);
Sections.push_back(Sec);
for (auto *DS : Sec->DependentSections)
DependentSections.push_back(DS);
if (Sec->Pieces.empty())
return;
if (Sec->AreRelocsRela)
addSectionAux<ELFT>(Sec, Sec->template relas<ELFT>());
else
addSectionAux<ELFT>(Sec, Sec->template rels<ELFT>());
}
static void writeCieFde(uint8_t *Buf, ArrayRef<uint8_t> D) {
memcpy(Buf, D.data(), D.size());
size_t Aligned = alignTo(D.size(), Config->Wordsize);
// Zero-clear trailing padding if it exists.
memset(Buf + D.size(), 0, Aligned - D.size());
// Fix the size field. -4 since size does not include the size field itself.
write32(Buf, Aligned - 4);
}
void EhFrameSection::finalizeContents() {
assert(!this->Size); // Not finalized.
size_t Off = 0;
for (CieRecord *Rec : CieRecords) {
Rec->Cie->OutputOff = Off;
Off += alignTo(Rec->Cie->Size, Config->Wordsize);
for (EhSectionPiece *Fde : Rec->Fdes) {
Fde->OutputOff = Off;
Off += alignTo(Fde->Size, Config->Wordsize);
}
}
// The LSB standard does not allow a .eh_frame section with zero
// Call Frame Information records. glibc unwind-dw2-fde.c
// classify_object_over_fdes expects there is a CIE record length 0 as a
// terminator. Thus we add one unconditionally.
Off += 4;
this->Size = Off;
}
// Returns data for .eh_frame_hdr. .eh_frame_hdr is a binary search table
// to get an FDE from an address to which FDE is applied. This function
// returns a list of such pairs.
std::vector<EhFrameSection::FdeData> EhFrameSection::getFdeData() const {
uint8_t *Buf = getParent()->Loc + OutSecOff;
std::vector<FdeData> Ret;
uint64_t VA = In.EhFrameHdr->getVA();
for (CieRecord *Rec : CieRecords) {
uint8_t Enc = getFdeEncoding(Rec->Cie);
for (EhSectionPiece *Fde : Rec->Fdes) {
uint64_t Pc = getFdePc(Buf, Fde->OutputOff, Enc);
uint64_t FdeVA = getParent()->Addr + Fde->OutputOff;
if (!isInt<32>(Pc - VA))
fatal(toString(Fde->Sec) + ": PC offset is too large: 0x" +
Twine::utohexstr(Pc - VA));
Ret.push_back({uint32_t(Pc - VA), uint32_t(FdeVA - VA)});
}
}
// Sort the FDE list by their PC and uniqueify. Usually there is only
// one FDE for a PC (i.e. function), but if ICF merges two functions
// into one, there can be more than one FDEs pointing to the address.
auto Less = [](const FdeData &A, const FdeData &B) {
return A.PcRel < B.PcRel;
};
std::stable_sort(Ret.begin(), Ret.end(), Less);
auto Eq = [](const FdeData &A, const FdeData &B) {
return A.PcRel == B.PcRel;
};
Ret.erase(std::unique(Ret.begin(), Ret.end(), Eq), Ret.end());
return Ret;
}
static uint64_t readFdeAddr(uint8_t *Buf, int Size) {
switch (Size) {
case DW_EH_PE_udata2:
return read16(Buf);
case DW_EH_PE_sdata2:
return (int16_t)read16(Buf);
case DW_EH_PE_udata4:
return read32(Buf);
case DW_EH_PE_sdata4:
return (int32_t)read32(Buf);
case DW_EH_PE_udata8:
case DW_EH_PE_sdata8:
return read64(Buf);
case DW_EH_PE_absptr:
return readUint(Buf);
}
fatal("unknown FDE size encoding");
}
// Returns the VA to which a given FDE (on a mmap'ed buffer) is applied to.
// We need it to create .eh_frame_hdr section.
uint64_t EhFrameSection::getFdePc(uint8_t *Buf, size_t FdeOff,
uint8_t Enc) const {
// The starting address to which this FDE applies is
// stored at FDE + 8 byte.
size_t Off = FdeOff + 8;
uint64_t Addr = readFdeAddr(Buf + Off, Enc & 0xf);
if ((Enc & 0x70) == DW_EH_PE_absptr)
return Addr;
if ((Enc & 0x70) == DW_EH_PE_pcrel)
return Addr + getParent()->Addr + Off;
fatal("unknown FDE size relative encoding");
}
void EhFrameSection::writeTo(uint8_t *Buf) {
// Write CIE and FDE records.
for (CieRecord *Rec : CieRecords) {
size_t CieOffset = Rec->Cie->OutputOff;
writeCieFde(Buf + CieOffset, Rec->Cie->data());
for (EhSectionPiece *Fde : Rec->Fdes) {
size_t Off = Fde->OutputOff;
writeCieFde(Buf + Off, Fde->data());
// FDE's second word should have the offset to an associated CIE.
// Write it.
write32(Buf + Off + 4, Off + 4 - CieOffset);
}
}
// Apply relocations. .eh_frame section contents are not contiguous
// in the output buffer, but relocateAlloc() still works because
// getOffset() takes care of discontiguous section pieces.
for (EhInputSection *S : Sections)
S->relocateAlloc(Buf, nullptr);
}
GotSection::GotSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS,
Target->GotEntrySize, ".got") {
// PPC64 saves the ElfSym::GlobalOffsetTable .TOC. as the first entry in the
// .got. If there are no references to .TOC. in the symbol table,
// ElfSym::GlobalOffsetTable will not be defined and we won't need to save
// .TOC. in the .got. When it is defined, we increase NumEntries by the number
// of entries used to emit ElfSym::GlobalOffsetTable.
if (ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt)
NumEntries += Target->GotHeaderEntriesNum;
}
void GotSection::addEntry(Symbol &Sym) {
Sym.GotIndex = NumEntries;
++NumEntries;
}
bool GotSection::addDynTlsEntry(Symbol &Sym) {
if (Sym.GlobalDynIndex != -1U)
return false;
Sym.GlobalDynIndex = NumEntries;
// Global Dynamic TLS entries take two GOT slots.
NumEntries += 2;
return true;
}
// Reserves TLS entries for a TLS module ID and a TLS block offset.
// In total it takes two GOT slots.
bool GotSection::addTlsIndex() {
if (TlsIndexOff != uint32_t(-1))
return false;
TlsIndexOff = NumEntries * Config->Wordsize;
NumEntries += 2;
return true;
}
uint64_t GotSection::getGlobalDynAddr(const Symbol &B) const {
return this->getVA() + B.GlobalDynIndex * Config->Wordsize;
}
uint64_t GotSection::getGlobalDynOffset(const Symbol &B) const {
return B.GlobalDynIndex * Config->Wordsize;
}
void GotSection::finalizeContents() {
Size = NumEntries * Config->Wordsize;
}
bool GotSection::empty() const {
// We need to emit a GOT even if it's empty if there's a relocation that is
// relative to GOT(such as GOTOFFREL) or there's a symbol that points to a GOT
// (i.e. _GLOBAL_OFFSET_TABLE_) that the target defines relative to the .got.
return NumEntries == 0 && !HasGotOffRel &&
!(ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt);
}
void GotSection::writeTo(uint8_t *Buf) {
// Buf points to the start of this section's buffer,
// whereas InputSectionBase::relocateAlloc() expects its argument
// to point to the start of the output section.
Target->writeGotHeader(Buf);
relocateAlloc(Buf - OutSecOff, Buf - OutSecOff + Size);
}
static uint64_t getMipsPageAddr(uint64_t Addr) {
return (Addr + 0x8000) & ~0xffff;
}
static uint64_t getMipsPageCount(uint64_t Size) {
return (Size + 0xfffe) / 0xffff + 1;
}
MipsGotSection::MipsGotSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE | SHF_MIPS_GPREL, SHT_PROGBITS, 16,
".got") {}
void MipsGotSection::addEntry(InputFile &File, Symbol &Sym, int64_t Addend,
RelExpr Expr) {
FileGot &G = getGot(File);
if (Expr == R_MIPS_GOT_LOCAL_PAGE) {
if (const OutputSection *OS = Sym.getOutputSection())
G.PagesMap.insert({OS, {}});
else
G.Local16.insert({{nullptr, getMipsPageAddr(Sym.getVA(Addend))}, 0});
} else if (Sym.isTls())
G.Tls.insert({&Sym, 0});
else if (Sym.IsPreemptible && Expr == R_ABS)
G.Relocs.insert({&Sym, 0});
else if (Sym.IsPreemptible)
G.Global.insert({&Sym, 0});
else if (Expr == R_MIPS_GOT_OFF32)
G.Local32.insert({{&Sym, Addend}, 0});
else
G.Local16.insert({{&Sym, Addend}, 0});
}
void MipsGotSection::addDynTlsEntry(InputFile &File, Symbol &Sym) {
getGot(File).DynTlsSymbols.insert({&Sym, 0});
}
void MipsGotSection::addTlsIndex(InputFile &File) {
getGot(File).DynTlsSymbols.insert({nullptr, 0});
}
size_t MipsGotSection::FileGot::getEntriesNum() const {
return getPageEntriesNum() + Local16.size() + Global.size() + Relocs.size() +
Tls.size() + DynTlsSymbols.size() * 2;
}
size_t MipsGotSection::FileGot::getPageEntriesNum() const {
size_t Num = 0;
for (const std::pair<const OutputSection *, FileGot::PageBlock> &P : PagesMap)
Num += P.second.Count;
return Num;
}
size_t MipsGotSection::FileGot::getIndexedEntriesNum() const {
size_t Count = getPageEntriesNum() + Local16.size() + Global.size();
// If there are relocation-only entries in the GOT, TLS entries
// are allocated after them. TLS entries should be addressable
// by 16-bit index so count both reloc-only and TLS entries.
if (!Tls.empty() || !DynTlsSymbols.empty())
Count += Relocs.size() + Tls.size() + DynTlsSymbols.size() * 2;
return Count;
}
MipsGotSection::FileGot &MipsGotSection::getGot(InputFile &F) {
if (!F.MipsGotIndex.hasValue()) {
Gots.emplace_back();
Gots.back().File = &F;
F.MipsGotIndex = Gots.size() - 1;
}
return Gots[*F.MipsGotIndex];
}
uint64_t MipsGotSection::getPageEntryOffset(const InputFile *F,
const Symbol &Sym,
int64_t Addend) const {
const FileGot &G = Gots[*F->MipsGotIndex];
uint64_t Index = 0;
if (const OutputSection *OutSec = Sym.getOutputSection()) {
uint64_t SecAddr = getMipsPageAddr(OutSec->Addr);
uint64_t SymAddr = getMipsPageAddr(Sym.getVA(Addend));
Index = G.PagesMap.lookup(OutSec).FirstIndex + (SymAddr - SecAddr) / 0xffff;
} else {
Index = G.Local16.lookup({nullptr, getMipsPageAddr(Sym.getVA(Addend))});
}
return Index * Config->Wordsize;
}
uint64_t MipsGotSection::getSymEntryOffset(const InputFile *F, const Symbol &S,
int64_t Addend) const {
const FileGot &G = Gots[*F->MipsGotIndex];
Symbol *Sym = const_cast<Symbol *>(&S);
if (Sym->isTls())
return G.Tls.lookup(Sym) * Config->Wordsize;
if (Sym->IsPreemptible)
return G.Global.lookup(Sym) * Config->Wordsize;
return G.Local16.lookup({Sym, Addend}) * Config->Wordsize;
}
uint64_t MipsGotSection::getTlsIndexOffset(const InputFile *F) const {
const FileGot &G = Gots[*F->MipsGotIndex];
return G.DynTlsSymbols.lookup(nullptr) * Config->Wordsize;
}
uint64_t MipsGotSection::getGlobalDynOffset(const InputFile *F,
const Symbol &S) const {
const FileGot &G = Gots[*F->MipsGotIndex];
Symbol *Sym = const_cast<Symbol *>(&S);
return G.DynTlsSymbols.lookup(Sym) * Config->Wordsize;
}
const Symbol *MipsGotSection::getFirstGlobalEntry() const {
if (Gots.empty())
return nullptr;
const FileGot &PrimGot = Gots.front();
if (!PrimGot.Global.empty())
return PrimGot.Global.front().first;
if (!PrimGot.Relocs.empty())
return PrimGot.Relocs.front().first;
return nullptr;
}
unsigned MipsGotSection::getLocalEntriesNum() const {
if (Gots.empty())
return HeaderEntriesNum;
return HeaderEntriesNum + Gots.front().getPageEntriesNum() +
Gots.front().Local16.size();
}
bool MipsGotSection::tryMergeGots(FileGot &Dst, FileGot &Src, bool IsPrimary) {
FileGot Tmp = Dst;
set_union(Tmp.PagesMap, Src.PagesMap);
set_union(Tmp.Local16, Src.Local16);
set_union(Tmp.Global, Src.Global);
set_union(Tmp.Relocs, Src.Relocs);
set_union(Tmp.Tls, Src.Tls);
set_union(Tmp.DynTlsSymbols, Src.DynTlsSymbols);
size_t Count = IsPrimary ? HeaderEntriesNum : 0;
Count += Tmp.getIndexedEntriesNum();
if (Count * Config->Wordsize > Config->MipsGotSize)
return false;
std::swap(Tmp, Dst);
return true;
}
void MipsGotSection::finalizeContents() { updateAllocSize(); }
bool MipsGotSection::updateAllocSize() {
Size = HeaderEntriesNum * Config->Wordsize;
for (const FileGot &G : Gots)
Size += G.getEntriesNum() * Config->Wordsize;
return false;
}
template <class ELFT> void MipsGotSection::build() {
if (Gots.empty())
return;
std::vector<FileGot> MergedGots(1);
// For each GOT move non-preemptible symbols from the `Global`
// to `Local16` list. Preemptible symbol might become non-preemptible
// one if, for example, it gets a related copy relocation.
for (FileGot &Got : Gots) {
for (auto &P: Got.Global)
if (!P.first->IsPreemptible)
Got.Local16.insert({{P.first, 0}, 0});
Got.Global.remove_if([&](const std::pair<Symbol *, size_t> &P) {
return !P.first->IsPreemptible;
});
}
// For each GOT remove "reloc-only" entry if there is "global"
// entry for the same symbol. And add local entries which indexed
// using 32-bit value at the end of 16-bit entries.
for (FileGot &Got : Gots) {
Got.Relocs.remove_if([&](const std::pair<Symbol *, size_t> &P) {
return Got.Global.count(P.first);
});
set_union(Got.Local16, Got.Local32);
Got.Local32.clear();
}
// Evaluate number of "reloc-only" entries in the resulting GOT.
// To do that put all unique "reloc-only" and "global" entries
// from all GOTs to the future primary GOT.
FileGot *PrimGot = &MergedGots.front();
for (FileGot &Got : Gots) {
set_union(PrimGot->Relocs, Got.Global);
set_union(PrimGot->Relocs, Got.Relocs);
Got.Relocs.clear();
}
// Evaluate number of "page" entries in each GOT.
for (FileGot &Got : Gots) {
for (std::pair<const OutputSection *, FileGot::PageBlock> &P :
Got.PagesMap) {
const OutputSection *OS = P.first;
uint64_t SecSize = 0;
for (BaseCommand *Cmd : OS->SectionCommands) {
if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
for (InputSection *IS : ISD->Sections) {
uint64_t Off = alignTo(SecSize, IS->Alignment);
SecSize = Off + IS->getSize();
}
}
P.second.Count = getMipsPageCount(SecSize);
}
}
// Merge GOTs. Try to join as much as possible GOTs but do not exceed
// maximum GOT size. At first, try to fill the primary GOT because
// the primary GOT can be accessed in the most effective way. If it
// is not possible, try to fill the last GOT in the list, and finally
// create a new GOT if both attempts failed.
for (FileGot &SrcGot : Gots) {
InputFile *File = SrcGot.File;
if (tryMergeGots(MergedGots.front(), SrcGot, true)) {
File->MipsGotIndex = 0;
} else {
// If this is the first time we failed to merge with the primary GOT,
// MergedGots.back() will also be the primary GOT. We must make sure not
// to try to merge again with IsPrimary=false, as otherwise, if the
// inputs are just right, we could allow the primary GOT to become 1 or 2
// words too big due to ignoring the header size.
if (MergedGots.size() == 1 ||
!tryMergeGots(MergedGots.back(), SrcGot, false)) {
MergedGots.emplace_back();
std::swap(MergedGots.back(), SrcGot);
}
File->MipsGotIndex = MergedGots.size() - 1;
}
}
std::swap(Gots, MergedGots);
// Reduce number of "reloc-only" entries in the primary GOT
// by substracting "global" entries exist in the primary GOT.
PrimGot = &Gots.front();
PrimGot->Relocs.remove_if([&](const std::pair<Symbol *, size_t> &P) {
return PrimGot->Global.count(P.first);
});
// Calculate indexes for each GOT entry.
size_t Index = HeaderEntriesNum;
for (FileGot &Got : Gots) {
Got.StartIndex = &Got == PrimGot ? 0 : Index;
for (std::pair<const OutputSection *, FileGot::PageBlock> &P :
Got.PagesMap) {
// For each output section referenced by GOT page relocations calculate
// and save into PagesMap an upper bound of MIPS GOT entries required
// to store page addresses of local symbols. We assume the worst case -
// each 64kb page of the output section has at least one GOT relocation
// against it. And take in account the case when the section intersects
// page boundaries.
P.second.FirstIndex = Index;
Index += P.second.Count;
}
for (auto &P: Got.Local16)
P.second = Index++;
for (auto &P: Got.Global)
P.second = Index++;
for (auto &P: Got.Relocs)
P.second = Index++;
for (auto &P: Got.Tls)
P.second = Index++;
for (auto &P: Got.DynTlsSymbols) {
P.second = Index;
Index += 2;
}
}
// Update Symbol::GotIndex field to use this
// value later in the `sortMipsSymbols` function.
for (auto &P : PrimGot->Global)
P.first->GotIndex = P.second;
for (auto &P : PrimGot->Relocs)
P.first->GotIndex = P.second;
// Create dynamic relocations.
for (FileGot &Got : Gots) {
// Create dynamic relocations for TLS entries.
for (std::pair<Symbol *, size_t> &P : Got.Tls) {
Symbol *S = P.first;
uint64_t Offset = P.second * Config->Wordsize;
if (S->IsPreemptible)
In.RelaDyn->addReloc(Target->TlsGotRel, this, Offset, S);
}
for (std::pair<Symbol *, size_t> &P : Got.DynTlsSymbols) {
Symbol *S = P.first;
uint64_t Offset = P.second * Config->Wordsize;
if (S == nullptr) {
if (!Config->Pic)
continue;
In.RelaDyn->addReloc(Target->TlsModuleIndexRel, this, Offset, S);
} else {
// When building a shared library we still need a dynamic relocation
// for the module index. Therefore only checking for
// S->IsPreemptible is not sufficient (this happens e.g. for
// thread-locals that have been marked as local through a linker script)
if (!S->IsPreemptible && !Config->Pic)
continue;
In.RelaDyn->addReloc(Target->TlsModuleIndexRel, this, Offset, S);
// However, we can skip writing the TLS offset reloc for non-preemptible
// symbols since it is known even in shared libraries
if (!S->IsPreemptible)
continue;
Offset += Config->Wordsize;
In.RelaDyn->addReloc(Target->TlsOffsetRel, this, Offset, S);
}
}
// Do not create dynamic relocations for non-TLS
// entries in the primary GOT.
if (&Got == PrimGot)
continue;
// Dynamic relocations for "global" entries.
for (const std::pair<Symbol *, size_t> &P : Got.Global) {
uint64_t Offset = P.second * Config->Wordsize;
In.RelaDyn->addReloc(Target->RelativeRel, this, Offset, P.first);
}
if (!Config->Pic)
continue;
// Dynamic relocations for "local" entries in case of PIC.
for (const std::pair<const OutputSection *, FileGot::PageBlock> &L :
Got.PagesMap) {
size_t PageCount = L.second.Count;
for (size_t PI = 0; PI < PageCount; ++PI) {
uint64_t Offset = (L.second.FirstIndex + PI) * Config->Wordsize;
In.RelaDyn->addReloc({Target->RelativeRel, this, Offset, L.first,
int64_t(PI * 0x10000)});
}
}
for (const std::pair<GotEntry, size_t> &P : Got.Local16) {
uint64_t Offset = P.second * Config->Wordsize;
In.RelaDyn->addReloc({Target->RelativeRel, this, Offset, true,
P.first.first, P.first.second});
}
}
}
bool MipsGotSection::empty() const {
// We add the .got section to the result for dynamic MIPS target because
// its address and properties are mentioned in the .dynamic section.
return Config->Relocatable;
}
uint64_t MipsGotSection::getGp(const InputFile *F) const {
// For files without related GOT or files refer a primary GOT
// returns "common" _gp value. For secondary GOTs calculate
// individual _gp values.
if (!F || !F->MipsGotIndex.hasValue() || *F->MipsGotIndex == 0)
return ElfSym::MipsGp->getVA(0);
return getVA() + Gots[*F->MipsGotIndex].StartIndex * Config->Wordsize +
0x7ff0;
}
void MipsGotSection::writeTo(uint8_t *Buf) {
// Set the MSB of the second GOT slot. This is not required by any
// MIPS ABI documentation, though.
//
// There is a comment in glibc saying that "The MSB of got[1] of a
// gnu object is set to identify gnu objects," and in GNU gold it
// says "the second entry will be used by some runtime loaders".
// But how this field is being used is unclear.
//
// We are not really willing to mimic other linkers behaviors
// without understanding why they do that, but because all files
// generated by GNU tools have this special GOT value, and because
// we've been doing this for years, it is probably a safe bet to
// keep doing this for now. We really need to revisit this to see
// if we had to do this.
writeUint(Buf + Config->Wordsize, (uint64_t)1 << (Config->Wordsize * 8 - 1));
for (const FileGot &G : Gots) {
auto Write = [&](size_t I, const Symbol *S, int64_t A) {
uint64_t VA = A;
if (S) {
VA = S->getVA(A);
if (S->StOther & STO_MIPS_MICROMIPS)
VA |= 1;
}
writeUint(Buf + I * Config->Wordsize, VA);
};
// Write 'page address' entries to the local part of the GOT.
for (const std::pair<const OutputSection *, FileGot::PageBlock> &L :
G.PagesMap) {
size_t PageCount = L.second.Count;
uint64_t FirstPageAddr = getMipsPageAddr(L.first->Addr);
for (size_t PI = 0; PI < PageCount; ++PI)
Write(L.second.FirstIndex + PI, nullptr, FirstPageAddr + PI * 0x10000);
}
// Local, global, TLS, reloc-only entries.
// If TLS entry has a corresponding dynamic relocations, leave it
// initialized by zero. Write down adjusted TLS symbol's values otherwise.
// To calculate the adjustments use offsets for thread-local storage.
// https://www.linux-mips.org/wiki/NPTL
for (const std::pair<GotEntry, size_t> &P : G.Local16)
Write(P.second, P.first.first, P.first.second);
// Write VA to the primary GOT only. For secondary GOTs that
// will be done by REL32 dynamic relocations.
if (&G == &Gots.front())
for (const std::pair<const Symbol *, size_t> &P : G.Global)
Write(P.second, P.first, 0);
for (const std::pair<Symbol *, size_t> &P : G.Relocs)
Write(P.second, P.first, 0);
for (const std::pair<Symbol *, size_t> &P : G.Tls)
Write(P.second, P.first, P.first->IsPreemptible ? 0 : -0x7000);
for (const std::pair<Symbol *, size_t> &P : G.DynTlsSymbols) {
if (P.first == nullptr && !Config->Pic)
Write(P.second, nullptr, 1);
else if (P.first && !P.first->IsPreemptible) {
// If we are emitting PIC code with relocations we mustn't write
// anything to the GOT here. When using Elf_Rel relocations the value
// one will be treated as an addend and will cause crashes at runtime
if (!Config->Pic)
Write(P.second, nullptr, 1);
Write(P.second + 1, P.first, -0x8000);
}
}
}
}
// On PowerPC the .plt section is used to hold the table of function addresses
// instead of the .got.plt, and the type is SHT_NOBITS similar to a .bss
// section. I don't know why we have a BSS style type for the section but it is
// consitent across both 64-bit PowerPC ABIs as well as the 32-bit PowerPC ABI.
GotPltSection::GotPltSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE,
Config->EMachine == EM_PPC64 ? SHT_NOBITS : SHT_PROGBITS,
Target->GotPltEntrySize,
Config->EMachine == EM_PPC64 ? ".plt" : ".got.plt") {}
void GotPltSection::addEntry(Symbol &Sym) {
assert(Sym.PltIndex == Entries.size());
Entries.push_back(&Sym);
}
size_t GotPltSection::getSize() const {
return (Target->GotPltHeaderEntriesNum + Entries.size()) *
Target->GotPltEntrySize;
}
void GotPltSection::writeTo(uint8_t *Buf) {
Target->writeGotPltHeader(Buf);
Buf += Target->GotPltHeaderEntriesNum * Target->GotPltEntrySize;
for (const Symbol *B : Entries) {
Target->writeGotPlt(Buf, *B);
Buf += Config->Wordsize;
}
}
bool GotPltSection::empty() const {
// We need to emit a GOT.PLT even if it's empty if there's a symbol that
// references the _GLOBAL_OFFSET_TABLE_ and the Target defines the symbol
// relative to the .got.plt section.
return Entries.empty() &&
!(ElfSym::GlobalOffsetTable && Target->GotBaseSymInGotPlt);
}
static StringRef getIgotPltName() {
// On ARM the IgotPltSection is part of the GotSection.
if (Config->EMachine == EM_ARM)
return ".got";
// On PowerPC64 the GotPltSection is renamed to '.plt' so the IgotPltSection
// needs to be named the same.
if (Config->EMachine == EM_PPC64)
return ".plt";
return ".got.plt";
}
// On PowerPC64 the GotPltSection type is SHT_NOBITS so we have to follow suit
// with the IgotPltSection.
IgotPltSection::IgotPltSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE,
Config->EMachine == EM_PPC64 ? SHT_NOBITS : SHT_PROGBITS,
Target->GotPltEntrySize, getIgotPltName()) {}
void IgotPltSection::addEntry(Symbol &Sym) {
Sym.IsInIgot = true;
assert(Sym.PltIndex == Entries.size());
Entries.push_back(&Sym);
}
size_t IgotPltSection::getSize() const {
return Entries.size() * Target->GotPltEntrySize;
}
void IgotPltSection::writeTo(uint8_t *Buf) {
for (const Symbol *B : Entries) {
Target->writeIgotPlt(Buf, *B);
Buf += Config->Wordsize;
}
}
StringTableSection::StringTableSection(StringRef Name, bool Dynamic)
: SyntheticSection(Dynamic ? (uint64_t)SHF_ALLOC : 0, SHT_STRTAB, 1, Name),
Dynamic(Dynamic) {
// ELF string tables start with a NUL byte.
addString("");
}
// Adds a string to the string table. If HashIt is true we hash and check for
// duplicates. It is optional because the name of global symbols are already
// uniqued and hashing them again has a big cost for a small value: uniquing
// them with some other string that happens to be the same.
unsigned StringTableSection::addString(StringRef S, bool HashIt) {
if (HashIt) {
auto R = StringMap.insert(std::make_pair(S, this->Size));
if (!R.second)
return R.first->second;
}
unsigned Ret = this->Size;
this->Size = this->Size + S.size() + 1;
Strings.push_back(S);
return Ret;
}
void StringTableSection::writeTo(uint8_t *Buf) {
for (StringRef S : Strings) {
memcpy(Buf, S.data(), S.size());
Buf[S.size()] = '\0';
Buf += S.size() + 1;
}
}
// Returns the number of version definition entries. Because the first entry
// is for the version definition itself, it is the number of versioned symbols
// plus one. Note that we don't support multiple versions yet.
static unsigned getVerDefNum() { return Config->VersionDefinitions.size() + 1; }
template <class ELFT>
DynamicSection<ELFT>::DynamicSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_DYNAMIC, Config->Wordsize,
".dynamic") {
this->Entsize = ELFT::Is64Bits ? 16 : 8;
// .dynamic section is not writable on MIPS and on Fuchsia OS
// which passes -z rodynamic.
// See "Special Section" in Chapter 4 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
if (Config->EMachine == EM_MIPS || Config->ZRodynamic)
this->Flags = SHF_ALLOC;
// Add strings to .dynstr early so that .dynstr's size will be
// fixed early.
for (StringRef S : Config->FilterList)
addInt(DT_FILTER, In.DynStrTab->addString(S));
for (StringRef S : Config->AuxiliaryList)
addInt(DT_AUXILIARY, In.DynStrTab->addString(S));
if (!Config->Rpath.empty())
addInt(Config->EnableNewDtags ? DT_RUNPATH : DT_RPATH,
In.DynStrTab->addString(Config->Rpath));
for (InputFile *File : SharedFiles) {
SharedFile<ELFT> *F = cast<SharedFile<ELFT>>(File);
if (F->IsNeeded)
addInt(DT_NEEDED, In.DynStrTab->addString(F->SoName));
}
if (!Config->SoName.empty())
addInt(DT_SONAME, In.DynStrTab->addString(Config->SoName));
}
template <class ELFT>
void DynamicSection<ELFT>::add(int32_t Tag, std::function<uint64_t()> Fn) {
Entries.push_back({Tag, Fn});
}
template <class ELFT>
void DynamicSection<ELFT>::addInt(int32_t Tag, uint64_t Val) {
Entries.push_back({Tag, [=] { return Val; }});
}
template <class ELFT>
void DynamicSection<ELFT>::addInSec(int32_t Tag, InputSection *Sec) {
Entries.push_back({Tag, [=] { return Sec->getVA(0); }});
}
template <class ELFT>
void DynamicSection<ELFT>::addInSecRelative(int32_t Tag, InputSection *Sec) {
size_t TagOffset = Entries.size() * Entsize;
Entries.push_back(
{Tag, [=] { return Sec->getVA(0) - (getVA() + TagOffset); }});
}
template <class ELFT>
void DynamicSection<ELFT>::addOutSec(int32_t Tag, OutputSection *Sec) {
Entries.push_back({Tag, [=] { return Sec->Addr; }});
}
template <class ELFT>
void DynamicSection<ELFT>::addSize(int32_t Tag, OutputSection *Sec) {
Entries.push_back({Tag, [=] { return Sec->Size; }});
}
template <class ELFT>
void DynamicSection<ELFT>::addSym(int32_t Tag, Symbol *Sym) {
Entries.push_back({Tag, [=] { return Sym->getVA(); }});
}
// Add remaining entries to complete .dynamic contents.
template <class ELFT> void DynamicSection<ELFT>::finalizeContents() {
if (this->Size)
return; // Already finalized.
// Set DT_FLAGS and DT_FLAGS_1.
uint32_t DtFlags = 0;
uint32_t DtFlags1 = 0;
if (Config->Bsymbolic)
DtFlags |= DF_SYMBOLIC;
if (Config->ZGlobal)
DtFlags1 |= DF_1_GLOBAL;
if (Config->ZInitfirst)
DtFlags1 |= DF_1_INITFIRST;
if (Config->ZInterpose)
DtFlags1 |= DF_1_INTERPOSE;
if (Config->ZNodelete)
DtFlags1 |= DF_1_NODELETE;
if (Config->ZNodlopen)
DtFlags1 |= DF_1_NOOPEN;
if (Config->ZNow) {
DtFlags |= DF_BIND_NOW;
DtFlags1 |= DF_1_NOW;
}
if (Config->ZOrigin) {
DtFlags |= DF_ORIGIN;
DtFlags1 |= DF_1_ORIGIN;
}
if (!Config->ZText)
DtFlags |= DF_TEXTREL;
if (DtFlags)
addInt(DT_FLAGS, DtFlags);
if (DtFlags1)
addInt(DT_FLAGS_1, DtFlags1);
// DT_DEBUG is a pointer to debug informaion used by debuggers at runtime. We
// need it for each process, so we don't write it for DSOs. The loader writes
// the pointer into this entry.
//
// DT_DEBUG is the only .dynamic entry that needs to be written to. Some
// systems (currently only Fuchsia OS) provide other means to give the
// debugger this information. Such systems may choose make .dynamic read-only.
// If the target is such a system (used -z rodynamic) don't write DT_DEBUG.
if (!Config->Shared && !Config->Relocatable && !Config->ZRodynamic)
addInt(DT_DEBUG, 0);
this->Link = In.DynStrTab->getParent()->SectionIndex;
if (!In.RelaDyn->empty()) {
addInSec(In.RelaDyn->DynamicTag, In.RelaDyn);
addSize(In.RelaDyn->SizeDynamicTag, In.RelaDyn->getParent());
bool IsRela = Config->IsRela;
addInt(IsRela ? DT_RELAENT : DT_RELENT,
IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel));
// MIPS dynamic loader does not support RELCOUNT tag.
// The problem is in the tight relation between dynamic
// relocations and GOT. So do not emit this tag on MIPS.
if (Config->EMachine != EM_MIPS) {
size_t NumRelativeRels = In.RelaDyn->getRelativeRelocCount();
if (Config->ZCombreloc && NumRelativeRels)
addInt(IsRela ? DT_RELACOUNT : DT_RELCOUNT, NumRelativeRels);
}
}
if (In.RelrDyn && !In.RelrDyn->Relocs.empty()) {
addInSec(Config->UseAndroidRelrTags ? DT_ANDROID_RELR : DT_RELR,
In.RelrDyn);
addSize(Config->UseAndroidRelrTags ? DT_ANDROID_RELRSZ : DT_RELRSZ,
In.RelrDyn->getParent());
addInt(Config->UseAndroidRelrTags ? DT_ANDROID_RELRENT : DT_RELRENT,
sizeof(Elf_Relr));
}
// .rel[a].plt section usually consists of two parts, containing plt and
// iplt relocations. It is possible to have only iplt relocations in the
// output. In that case RelaPlt is empty and have zero offset, the same offset
// as RelaIplt have. And we still want to emit proper dynamic tags for that
// case, so here we always use RelaPlt as marker for the begining of
// .rel[a].plt section.
if (In.RelaPlt->getParent()->Live) {
addInSec(DT_JMPREL, In.RelaPlt);
addSize(DT_PLTRELSZ, In.RelaPlt->getParent());
switch (Config->EMachine) {
case EM_MIPS:
addInSec(DT_MIPS_PLTGOT, In.GotPlt);
break;
case EM_SPARCV9:
addInSec(DT_PLTGOT, In.Plt);
break;
default:
addInSec(DT_PLTGOT, In.GotPlt);
break;
}
addInt(DT_PLTREL, Config->IsRela ? DT_RELA : DT_REL);
}
addInSec(DT_SYMTAB, In.DynSymTab);
addInt(DT_SYMENT, sizeof(Elf_Sym));
addInSec(DT_STRTAB, In.DynStrTab);
addInt(DT_STRSZ, In.DynStrTab->getSize());
if (!Config->ZText)
addInt(DT_TEXTREL, 0);
if (In.GnuHashTab)
addInSec(DT_GNU_HASH, In.GnuHashTab);
if (In.HashTab)
addInSec(DT_HASH, In.HashTab);
if (Out::PreinitArray) {
addOutSec(DT_PREINIT_ARRAY, Out::PreinitArray);
addSize(DT_PREINIT_ARRAYSZ, Out::PreinitArray);
}
if (Out::InitArray) {
addOutSec(DT_INIT_ARRAY, Out::InitArray);
addSize(DT_INIT_ARRAYSZ, Out::InitArray);
}
if (Out::FiniArray) {
addOutSec(DT_FINI_ARRAY, Out::FiniArray);
addSize(DT_FINI_ARRAYSZ, Out::FiniArray);
}
if (Symbol *B = Symtab->find(Config->Init))
if (B->isDefined())
addSym(DT_INIT, B);
if (Symbol *B = Symtab->find(Config->Fini))
if (B->isDefined())
addSym(DT_FINI, B);
bool HasVerNeed = InX<ELFT>::VerNeed->getNeedNum() != 0;
if (HasVerNeed || In.VerDef)
addInSec(DT_VERSYM, InX<ELFT>::VerSym);
if (In.VerDef) {
addInSec(DT_VERDEF, In.VerDef);
addInt(DT_VERDEFNUM, getVerDefNum());
}
if (HasVerNeed) {
addInSec(DT_VERNEED, InX<ELFT>::VerNeed);
addInt(DT_VERNEEDNUM, InX<ELFT>::VerNeed->getNeedNum());
}
if (Config->EMachine == EM_MIPS) {
addInt(DT_MIPS_RLD_VERSION, 1);
addInt(DT_MIPS_FLAGS, RHF_NOTPOT);
addInt(DT_MIPS_BASE_ADDRESS, Target->getImageBase());
addInt(DT_MIPS_SYMTABNO, In.DynSymTab->getNumSymbols());
add(DT_MIPS_LOCAL_GOTNO, [] { return In.MipsGot->getLocalEntriesNum(); });
if (const Symbol *B = In.MipsGot->getFirstGlobalEntry())
addInt(DT_MIPS_GOTSYM, B->DynsymIndex);
else
addInt(DT_MIPS_GOTSYM, In.DynSymTab->getNumSymbols());
addInSec(DT_PLTGOT, In.MipsGot);
if (In.MipsRldMap) {
if (!Config->Pie)
addInSec(DT_MIPS_RLD_MAP, In.MipsRldMap);
// Store the offset to the .rld_map section
// relative to the address of the tag.
addInSecRelative(DT_MIPS_RLD_MAP_REL, In.MipsRldMap);
}
}
// Glink dynamic tag is required by the V2 abi if the plt section isn't empty.
if (Config->EMachine == EM_PPC64 && !In.Plt->empty()) {
// The Glink tag points to 32 bytes before the first lazy symbol resolution
// stub, which starts directly after the header.
Entries.push_back({DT_PPC64_GLINK, [=] {
unsigned Offset = Target->PltHeaderSize - 32;
return In.Plt->getVA(0) + Offset;
}});
}
addInt(DT_NULL, 0);
getParent()->Link = this->Link;
this->Size = Entries.size() * this->Entsize;
}
template <class ELFT> void DynamicSection<ELFT>::writeTo(uint8_t *Buf) {
auto *P = reinterpret_cast<Elf_Dyn *>(Buf);
for (std::pair<int32_t, std::function<uint64_t()>> &KV : Entries) {
P->d_tag = KV.first;
P->d_un.d_val = KV.second();
++P;
}
}
uint64_t DynamicReloc::getOffset() const {
return InputSec->getVA(OffsetInSec);
}
int64_t DynamicReloc::computeAddend() const {
if (UseSymVA)
return Sym->getVA(Addend);
if (!OutputSec)
return Addend;
// See the comment in the DynamicReloc ctor.
return getMipsPageAddr(OutputSec->Addr) + Addend;
}
uint32_t DynamicReloc::getSymIndex() const {
if (Sym && !UseSymVA)
return Sym->DynsymIndex;
return 0;
}
RelocationBaseSection::RelocationBaseSection(StringRef Name, uint32_t Type,
int32_t DynamicTag,
int32_t SizeDynamicTag)
: SyntheticSection(SHF_ALLOC, Type, Config->Wordsize, Name),
DynamicTag(DynamicTag), SizeDynamicTag(SizeDynamicTag) {}
void RelocationBaseSection::addReloc(RelType DynType, InputSectionBase *IS,
uint64_t OffsetInSec, Symbol *Sym) {
addReloc({DynType, IS, OffsetInSec, false, Sym, 0});
}
void RelocationBaseSection::addReloc(RelType DynType,
InputSectionBase *InputSec,
uint64_t OffsetInSec, Symbol *Sym,
int64_t Addend, RelExpr Expr,
RelType Type) {
// Write the addends to the relocated address if required. We skip
// it if the written value would be zero.
if (Config->WriteAddends && (Expr != R_ADDEND || Addend != 0))
InputSec->Relocations.push_back({Expr, Type, OffsetInSec, Addend, Sym});
addReloc({DynType, InputSec, OffsetInSec, Expr != R_ADDEND, Sym, Addend});
}
void RelocationBaseSection::addReloc(const DynamicReloc &Reloc) {
if (Reloc.Type == Target->RelativeRel)
++NumRelativeRelocs;
Relocs.push_back(Reloc);
}
void RelocationBaseSection::finalizeContents() {
// If all relocations are R_*_RELATIVE they don't refer to any
// dynamic symbol and we don't need a dynamic symbol table. If that
// is the case, just use 0 as the link.
getParent()->Link =
In.DynSymTab ? In.DynSymTab->getParent()->SectionIndex : 0;
}
RelrBaseSection::RelrBaseSection()
: SyntheticSection(SHF_ALLOC,
Config->UseAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR,
Config->Wordsize, ".relr.dyn") {}
template <class ELFT>
static void encodeDynamicReloc(typename ELFT::Rela *P,
const DynamicReloc &Rel) {
if (Config->IsRela)
P->r_addend = Rel.computeAddend();
P->r_offset = Rel.getOffset();
P->setSymbolAndType(Rel.getSymIndex(), Rel.Type, Config->IsMips64EL);
}
template <class ELFT>
RelocationSection<ELFT>::RelocationSection(StringRef Name, bool Sort)
: RelocationBaseSection(Name, Config->IsRela ? SHT_RELA : SHT_REL,
Config->IsRela ? DT_RELA : DT_REL,
Config->IsRela ? DT_RELASZ : DT_RELSZ),
Sort(Sort) {
this->Entsize = Config->IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
}
static bool compRelocations(const DynamicReloc &A, const DynamicReloc &B) {
bool AIsRel = A.Type == Target->RelativeRel;
bool BIsRel = B.Type == Target->RelativeRel;
if (AIsRel != BIsRel)
return AIsRel;
return A.getSymIndex() < B.getSymIndex();
}
template <class ELFT> void RelocationSection<ELFT>::writeTo(uint8_t *Buf) {
if (Sort)
std::stable_sort(Relocs.begin(), Relocs.end(), compRelocations);
for (const DynamicReloc &Rel : Relocs) {
encodeDynamicReloc<ELFT>(reinterpret_cast<Elf_Rela *>(Buf), Rel);
Buf += Config->IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
}
}
template <class ELFT> unsigned RelocationSection<ELFT>::getRelocOffset() {
return this->Entsize * Relocs.size();
}
template <class ELFT>
AndroidPackedRelocationSection<ELFT>::AndroidPackedRelocationSection(
StringRef Name)
: RelocationBaseSection(
Name, Config->IsRela ? SHT_ANDROID_RELA : SHT_ANDROID_REL,
Config->IsRela ? DT_ANDROID_RELA : DT_ANDROID_REL,
Config->IsRela ? DT_ANDROID_RELASZ : DT_ANDROID_RELSZ) {
this->Entsize = 1;
}
template <class ELFT>
bool AndroidPackedRelocationSection<ELFT>::updateAllocSize() {
// This function computes the contents of an Android-format packed relocation
// section.
//
// This format compresses relocations by using relocation groups to factor out
// fields that are common between relocations and storing deltas from previous
// relocations in SLEB128 format (which has a short representation for small
// numbers). A good example of a relocation type with common fields is
// R_*_RELATIVE, which is normally used to represent function pointers in
// vtables. In the REL format, each relative relocation has the same r_info
// field, and is only different from other relative relocations in terms of
// the r_offset field. By sorting relocations by offset, grouping them by
// r_info and representing each relocation with only the delta from the
// previous offset, each 8-byte relocation can be compressed to as little as 1
// byte (or less with run-length encoding). This relocation packer was able to
// reduce the size of the relocation section in an Android Chromium DSO from
// 2,911,184 bytes to 174,693 bytes, or 6% of the original size.
//
// A relocation section consists of a header containing the literal bytes
// 'APS2' followed by a sequence of SLEB128-encoded integers. The first two
// elements are the total number of relocations in the section and an initial
// r_offset value. The remaining elements define a sequence of relocation
// groups. Each relocation group starts with a header consisting of the
// following elements:
//
// - the number of relocations in the relocation group
// - flags for the relocation group
// - (if RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG is set) the r_offset delta
// for each relocation in the group.
// - (if RELOCATION_GROUPED_BY_INFO_FLAG is set) the value of the r_info
// field for each relocation in the group.
// - (if RELOCATION_GROUP_HAS_ADDEND_FLAG and
// RELOCATION_GROUPED_BY_ADDEND_FLAG are set) the r_addend delta for
// each relocation in the group.
//
// Following the relocation group header are descriptions of each of the
// relocations in the group. They consist of the following elements:
//
// - (if RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG is not set) the r_offset
// delta for this relocation.
// - (if RELOCATION_GROUPED_BY_INFO_FLAG is not set) the value of the r_info
// field for this relocation.
// - (if RELOCATION_GROUP_HAS_ADDEND_FLAG is set and
// RELOCATION_GROUPED_BY_ADDEND_FLAG is not set) the r_addend delta for
// this relocation.
size_t OldSize = RelocData.size();
RelocData = {'A', 'P', 'S', '2'};
raw_svector_ostream OS(RelocData);
auto Add = [&](int64_t V) { encodeSLEB128(V, OS); };
// The format header includes the number of relocations and the initial
// offset (we set this to zero because the first relocation group will
// perform the initial adjustment).
Add(Relocs.size());
Add(0);
std::vector<Elf_Rela> Relatives, NonRelatives;
for (const DynamicReloc &Rel : Relocs) {
Elf_Rela R;
encodeDynamicReloc<ELFT>(&R, Rel);
if (R.getType(Config->IsMips64EL) == Target->RelativeRel)
Relatives.push_back(R);
else
NonRelatives.push_back(R);
}
llvm::sort(Relatives, [](const Elf_Rel &A, const Elf_Rel &B) {
return A.r_offset < B.r_offset;
});
// Try to find groups of relative relocations which are spaced one word
// apart from one another. These generally correspond to vtable entries. The
// format allows these groups to be encoded using a sort of run-length
// encoding, but each group will cost 7 bytes in addition to the offset from
// the previous group, so it is only profitable to do this for groups of
// size 8 or larger.
std::vector<Elf_Rela> UngroupedRelatives;
std::vector<std::vector<Elf_Rela>> RelativeGroups;
for (auto I = Relatives.begin(), E = Relatives.end(); I != E;) {
std::vector<Elf_Rela> Group;
do {
Group.push_back(*I++);
} while (I != E && (I - 1)->r_offset + Config->Wordsize == I->r_offset);
if (Group.size() < 8)
UngroupedRelatives.insert(UngroupedRelatives.end(), Group.begin(),
Group.end());
else
RelativeGroups.emplace_back(std::move(Group));
}
unsigned HasAddendIfRela =
Config->IsRela ? RELOCATION_GROUP_HAS_ADDEND_FLAG : 0;
uint64_t Offset = 0;
uint64_t Addend = 0;
// Emit the run-length encoding for the groups of adjacent relative
// relocations. Each group is represented using two groups in the packed
// format. The first is used to set the current offset to the start of the
// group (and also encodes the first relocation), and the second encodes the
// remaining relocations.
for (std::vector<Elf_Rela> &G : RelativeGroups) {
// The first relocation in the group.
Add(1);
Add(RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG |
RELOCATION_GROUPED_BY_INFO_FLAG | HasAddendIfRela);
Add(G[0].r_offset - Offset);
Add(Target->RelativeRel);
if (Config->IsRela) {
Add(G[0].r_addend - Addend);
Addend = G[0].r_addend;
}
// The remaining relocations.
Add(G.size() - 1);
Add(RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG |
RELOCATION_GROUPED_BY_INFO_FLAG | HasAddendIfRela);
Add(Config->Wordsize);
Add(Target->RelativeRel);
if (Config->IsRela) {
for (auto I = G.begin() + 1, E = G.end(); I != E; ++I) {
Add(I->r_addend - Addend);
Addend = I->r_addend;
}
}
Offset = G.back().r_offset;
}
// Now the ungrouped relatives.
if (!UngroupedRelatives.empty()) {
Add(UngroupedRelatives.size());
Add(RELOCATION_GROUPED_BY_INFO_FLAG | HasAddendIfRela);
Add(Target->RelativeRel);
for (Elf_Rela &R : UngroupedRelatives) {
Add(R.r_offset - Offset);
Offset = R.r_offset;
if (Config->IsRela) {
Add(R.r_addend - Addend);
Addend = R.r_addend;
}
}
}
// Finally the non-relative relocations.
llvm::sort(NonRelatives, [](const Elf_Rela &A, const Elf_Rela &B) {
return A.r_offset < B.r_offset;
});
if (!NonRelatives.empty()) {
Add(NonRelatives.size());
Add(HasAddendIfRela);
for (Elf_Rela &R : NonRelatives) {
Add(R.r_offset - Offset);
Offset = R.r_offset;
Add(R.r_info);
if (Config->IsRela) {
Add(R.r_addend - Addend);
Addend = R.r_addend;
}
}
}
// Returns whether the section size changed. We need to keep recomputing both
// section layout and the contents of this section until the size converges
// because changing this section's size can affect section layout, which in
// turn can affect the sizes of the LEB-encoded integers stored in this
// section.
return RelocData.size() != OldSize;
}
template <class ELFT> RelrSection<ELFT>::RelrSection() {
this->Entsize = Config->Wordsize;
}
template <class ELFT> bool RelrSection<ELFT>::updateAllocSize() {
// This function computes the contents of an SHT_RELR packed relocation
// section.
//
// Proposal for adding SHT_RELR sections to generic-abi is here:
// https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg
//
// The encoded sequence of Elf64_Relr entries in a SHT_RELR section looks
// like [ AAAAAAAA BBBBBBB1 BBBBBBB1 ... AAAAAAAA BBBBBB1 ... ]
//
// i.e. start with an address, followed by any number of bitmaps. The address
// entry encodes 1 relocation. The subsequent bitmap entries encode up to 63
// relocations each, at subsequent offsets following the last address entry.
//
// The bitmap entries must have 1 in the least significant bit. The assumption
// here is that an address cannot have 1 in lsb. Odd addresses are not
// supported.
//
// Excluding the least significant bit in the bitmap, each non-zero bit in
// the bitmap represents a relocation to be applied to a corresponding machine
// word that follows the base address word. The second least significant bit
// represents the machine word immediately following the initial address, and
// each bit that follows represents the next word, in linear order. As such,
// a single bitmap can encode up to 31 relocations in a 32-bit object, and
// 63 relocations in a 64-bit object.
//
// This encoding has a couple of interesting properties:
// 1. Looking at any entry, it is clear whether it's an address or a bitmap:
// even means address, odd means bitmap.
// 2. Just a simple list of addresses is a valid encoding.
size_t OldSize = RelrRelocs.size();
RelrRelocs.clear();
// Same as Config->Wordsize but faster because this is a compile-time
// constant.
const size_t Wordsize = sizeof(typename ELFT::uint);
// Number of bits to use for the relocation offsets bitmap.
// Must be either 63 or 31.
const size_t NBits = Wordsize * 8 - 1;
// Get offsets for all relative relocations and sort them.
std::vector<uint64_t> Offsets;
for (const RelativeReloc &Rel : Relocs)
Offsets.push_back(Rel.getOffset());
llvm::sort(Offsets.begin(), Offsets.end());
// For each leading relocation, find following ones that can be folded
// as a bitmap and fold them.
for (size_t I = 0, E = Offsets.size(); I < E;) {
// Add a leading relocation.
RelrRelocs.push_back(Elf_Relr(Offsets[I]));
uint64_t Base = Offsets[I] + Wordsize;
++I;
// Find foldable relocations to construct bitmaps.
while (I < E) {
uint64_t Bitmap = 0;
while (I < E) {
uint64_t Delta = Offsets[I] - Base;
// If it is too far, it cannot be folded.
if (Delta >= NBits * Wordsize)
break;
// If it is not a multiple of wordsize away, it cannot be folded.
if (Delta % Wordsize)
break;
// Fold it.
Bitmap |= 1ULL << (Delta / Wordsize);
++I;
}
if (!Bitmap)
break;
RelrRelocs.push_back(Elf_Relr((Bitmap << 1) | 1));
Base += NBits * Wordsize;
}
}
return RelrRelocs.size() != OldSize;
}
SymbolTableBaseSection::SymbolTableBaseSection(StringTableSection &StrTabSec)
: SyntheticSection(StrTabSec.isDynamic() ? (uint64_t)SHF_ALLOC : 0,
StrTabSec.isDynamic() ? SHT_DYNSYM : SHT_SYMTAB,
Config->Wordsize,
StrTabSec.isDynamic() ? ".dynsym" : ".symtab"),
StrTabSec(StrTabSec) {}
// Orders symbols according to their positions in the GOT,
// in compliance with MIPS ABI rules.
// See "Global Offset Table" in Chapter 5 in the following document
// for detailed description:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
static bool sortMipsSymbols(const SymbolTableEntry &L,
const SymbolTableEntry &R) {
// Sort entries related to non-local preemptible symbols by GOT indexes.
// All other entries go to the beginning of a dynsym in arbitrary order.
if (L.Sym->isInGot() && R.Sym->isInGot())
return L.Sym->GotIndex < R.Sym->GotIndex;
if (!L.Sym->isInGot() && !R.Sym->isInGot())
return false;
return !L.Sym->isInGot();
}
void SymbolTableBaseSection::finalizeContents() {
getParent()->Link = StrTabSec.getParent()->SectionIndex;
if (this->Type != SHT_DYNSYM) {
sortSymTabSymbols();
return;
}
// If it is a .dynsym, there should be no local symbols, but we need
// to do a few things for the dynamic linker.
// Section's Info field has the index of the first non-local symbol.
// Because the first symbol entry is a null entry, 1 is the first.
getParent()->Info = 1;
if (In.GnuHashTab) {
// NB: It also sorts Symbols to meet the GNU hash table requirements.
In.GnuHashTab->addSymbols(Symbols);
} else if (Config->EMachine == EM_MIPS) {
std::stable_sort(Symbols.begin(), Symbols.end(), sortMipsSymbols);
}
size_t I = 0;
for (const SymbolTableEntry &S : Symbols)
S.Sym->DynsymIndex = ++I;
}
// The ELF spec requires that all local symbols precede global symbols, so we
// sort symbol entries in this function. (For .dynsym, we don't do that because
// symbols for dynamic linking are inherently all globals.)
//
// Aside from above, we put local symbols in groups starting with the STT_FILE
// symbol. That is convenient for purpose of identifying where are local symbols
// coming from.
void SymbolTableBaseSection::sortSymTabSymbols() {
// Move all local symbols before global symbols.
auto E = std::stable_partition(
Symbols.begin(), Symbols.end(), [](const SymbolTableEntry &S) {
return S.Sym->isLocal() || S.Sym->computeBinding() == STB_LOCAL;
});
size_t NumLocals = E - Symbols.begin();
getParent()->Info = NumLocals + 1;
// We want to group the local symbols by file. For that we rebuild the local
// part of the symbols vector. We do not need to care about the STT_FILE
// symbols, they are already naturally placed first in each group. That
// happens because STT_FILE is always the first symbol in the object and hence
// precede all other local symbols we add for a file.
MapVector<InputFile *, std::vector<SymbolTableEntry>> Arr;
for (const SymbolTableEntry &S : llvm::make_range(Symbols.begin(), E))
Arr[S.Sym->File].push_back(S);
auto I = Symbols.begin();
for (std::pair<InputFile *, std::vector<SymbolTableEntry>> &P : Arr)
for (SymbolTableEntry &Entry : P.second)
*I++ = Entry;
}
void SymbolTableBaseSection::addSymbol(Symbol *B) {
// Adding a local symbol to a .dynsym is a bug.
assert(this->Type != SHT_DYNSYM || !B->isLocal());
bool HashIt = B->isLocal();
Symbols.push_back({B, StrTabSec.addString(B->getName(), HashIt)});
}
size_t SymbolTableBaseSection::getSymbolIndex(Symbol *Sym) {
// Initializes symbol lookup tables lazily. This is used only
// for -r or -emit-relocs.
llvm::call_once(OnceFlag, [&] {
SymbolIndexMap.reserve(Symbols.size());
size_t I = 0;
for (const SymbolTableEntry &E : Symbols) {
if (E.Sym->Type == STT_SECTION)
SectionIndexMap[E.Sym->getOutputSection()] = ++I;
else
SymbolIndexMap[E.Sym] = ++I;
}
});
// Section symbols are mapped based on their output sections
// to maintain their semantics.
if (Sym->Type == STT_SECTION)
return SectionIndexMap.lookup(Sym->getOutputSection());
return SymbolIndexMap.lookup(Sym);
}
template <class ELFT>
SymbolTableSection<ELFT>::SymbolTableSection(StringTableSection &StrTabSec)
: SymbolTableBaseSection(StrTabSec) {
this->Entsize = sizeof(Elf_Sym);
}
static BssSection *getCommonSec(Symbol *Sym) {
if (!Config->DefineCommon)
if (auto *D = dyn_cast<Defined>(Sym))
return dyn_cast_or_null<BssSection>(D->Section);
return nullptr;
}
static uint32_t getSymSectionIndex(Symbol *Sym) {
if (getCommonSec(Sym))
return SHN_COMMON;
if (!isa<Defined>(Sym) || Sym->NeedsPltAddr)
return SHN_UNDEF;
if (const OutputSection *OS = Sym->getOutputSection())
return OS->SectionIndex >= SHN_LORESERVE ? (uint32_t)SHN_XINDEX
: OS->SectionIndex;
return SHN_ABS;
}
// Write the internal symbol table contents to the output symbol table.
template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *Buf) {
// The first entry is a null entry as per the ELF spec.
memset(Buf, 0, sizeof(Elf_Sym));
Buf += sizeof(Elf_Sym);
auto *ESym = reinterpret_cast<Elf_Sym *>(Buf);
for (SymbolTableEntry &Ent : Symbols) {
Symbol *Sym = Ent.Sym;
// Set st_info and st_other.
ESym->st_other = 0;
if (Sym->isLocal()) {
ESym->setBindingAndType(STB_LOCAL, Sym->Type);
} else {
ESym->setBindingAndType(Sym->computeBinding(), Sym->Type);
ESym->setVisibility(Sym->Visibility);
}
ESym->st_name = Ent.StrTabOffset;
ESym->st_shndx = getSymSectionIndex(Ent.Sym);
// Copy symbol size if it is a defined symbol. st_size is not significant
// for undefined symbols, so whether copying it or not is up to us if that's
// the case. We'll leave it as zero because by not setting a value, we can
// get the exact same outputs for two sets of input files that differ only
// in undefined symbol size in DSOs.
if (ESym->st_shndx == SHN_UNDEF)
ESym->st_size = 0;
else
ESym->st_size = Sym->getSize();
// st_value is usually an address of a symbol, but that has a
// special meaining for uninstantiated common symbols (this can
// occur if -r is given).
if (BssSection *CommonSec = getCommonSec(Ent.Sym))
ESym->st_value = CommonSec->Alignment;
else
ESym->st_value = Sym->getVA();
++ESym;
}
// On MIPS we need to mark symbol which has a PLT entry and requires
// pointer equality by STO_MIPS_PLT flag. That is necessary to help
// dynamic linker distinguish such symbols and MIPS lazy-binding stubs.
// https://sourceware.org/ml/binutils/2008-07/txt00000.txt
if (Config->EMachine == EM_MIPS) {
auto *ESym = reinterpret_cast<Elf_Sym *>(Buf);
for (SymbolTableEntry &Ent : Symbols) {
Symbol *Sym = Ent.Sym;
if (Sym->isInPlt() && Sym->NeedsPltAddr)
ESym->st_other |= STO_MIPS_PLT;
if (isMicroMips()) {
// Set STO_MIPS_MICROMIPS flag and less-significant bit for
// a defined microMIPS symbol and symbol should point to its
// PLT entry (in case of microMIPS, PLT entries always contain
// microMIPS code).
if (Sym->isDefined() &&
((Sym->StOther & STO_MIPS_MICROMIPS) || Sym->NeedsPltAddr)) {
if (StrTabSec.isDynamic())
ESym->st_value |= 1;
ESym->st_other |= STO_MIPS_MICROMIPS;
}
}
if (Config->Relocatable)
if (auto *D = dyn_cast<Defined>(Sym))
if (isMipsPIC<ELFT>(D))
ESym->st_other |= STO_MIPS_PIC;
++ESym;
}
}
}
SymtabShndxSection::SymtabShndxSection()
: SyntheticSection(0, SHT_SYMTAB_SHNDX, 4, ".symtab_shndxr") {
this->Entsize = 4;
}
void SymtabShndxSection::writeTo(uint8_t *Buf) {
// We write an array of 32 bit values, where each value has 1:1 association
// with an entry in .symtab. If the corresponding entry contains SHN_XINDEX,
// we need to write actual index, otherwise, we must write SHN_UNDEF(0).
Buf += 4; // Ignore .symtab[0] entry.
for (const SymbolTableEntry &Entry : In.SymTab->getSymbols()) {
if (getSymSectionIndex(Entry.Sym) == SHN_XINDEX)
write32(Buf, Entry.Sym->getOutputSection()->SectionIndex);
Buf += 4;
}
}
bool SymtabShndxSection::empty() const {
// SHT_SYMTAB can hold symbols with section indices values up to
// SHN_LORESERVE. If we need more, we want to use extension SHT_SYMTAB_SHNDX
// section. Problem is that we reveal the final section indices a bit too
// late, and we do not know them here. For simplicity, we just always create
// a .symtab_shndxr section when the amount of output sections is huge.
size_t Size = 0;
for (BaseCommand *Base : Script->SectionCommands)
if (isa<OutputSection>(Base))
++Size;
return Size < SHN_LORESERVE;
}
void SymtabShndxSection::finalizeContents() {
getParent()->Link = In.SymTab->getParent()->SectionIndex;
}
size_t SymtabShndxSection::getSize() const {
return In.SymTab->getNumSymbols() * 4;
}
// .hash and .gnu.hash sections contain on-disk hash tables that map
// symbol names to their dynamic symbol table indices. Their purpose
// is to help the dynamic linker resolve symbols quickly. If ELF files
// don't have them, the dynamic linker has to do linear search on all
// dynamic symbols, which makes programs slower. Therefore, a .hash
// section is added to a DSO by default. A .gnu.hash is added if you
// give the -hash-style=gnu or -hash-style=both option.
//
// The Unix semantics of resolving dynamic symbols is somewhat expensive.
// Each ELF file has a list of DSOs that the ELF file depends on and a
// list of dynamic symbols that need to be resolved from any of the
// DSOs. That means resolving all dynamic symbols takes O(m)*O(n)
// where m is the number of DSOs and n is the number of dynamic
// symbols. For modern large programs, both m and n are large. So
// making each step faster by using hash tables substiantially
// improves time to load programs.
//
// (Note that this is not the only way to design the shared library.
// For instance, the Windows DLL takes a different approach. On
// Windows, each dynamic symbol has a name of DLL from which the symbol
// has to be resolved. That makes the cost of symbol resolution O(n).
// This disables some hacky techniques you can use on Unix such as
// LD_PRELOAD, but this is arguably better semantics than the Unix ones.)
//
// Due to historical reasons, we have two different hash tables, .hash
// and .gnu.hash. They are for the same purpose, and .gnu.hash is a new
// and better version of .hash. .hash is just an on-disk hash table, but
// .gnu.hash has a bloom filter in addition to a hash table to skip
// DSOs very quickly. If you are sure that your dynamic linker knows
// about .gnu.hash, you want to specify -hash-style=gnu. Otherwise, a
// safe bet is to specify -hash-style=both for backward compatibilty.
GnuHashTableSection::GnuHashTableSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_HASH, Config->Wordsize, ".gnu.hash") {
}
void GnuHashTableSection::finalizeContents() {
getParent()->Link = In.DynSymTab->getParent()->SectionIndex;
// Computes bloom filter size in word size. We want to allocate 12
// bits for each symbol. It must be a power of two.
if (Symbols.empty()) {
MaskWords = 1;
} else {
uint64_t NumBits = Symbols.size() * 12;
MaskWords = NextPowerOf2(NumBits / (Config->Wordsize * 8));
}
Size = 16; // Header
Size += Config->Wordsize * MaskWords; // Bloom filter
Size += NBuckets * 4; // Hash buckets
Size += Symbols.size() * 4; // Hash values
}
void GnuHashTableSection::writeTo(uint8_t *Buf) {
// The output buffer is not guaranteed to be zero-cleared because we pre-
// fill executable sections with trap instructions. This is a precaution
// for that case, which happens only when -no-rosegment is given.
memset(Buf, 0, Size);
// Write a header.
write32(Buf, NBuckets);
write32(Buf + 4, In.DynSymTab->getNumSymbols() - Symbols.size());
write32(Buf + 8, MaskWords);
write32(Buf + 12, Shift2);
Buf += 16;
// Write a bloom filter and a hash table.
writeBloomFilter(Buf);
Buf += Config->Wordsize * MaskWords;
writeHashTable(Buf);
}
// This function writes a 2-bit bloom filter. This bloom filter alone
// usually filters out 80% or more of all symbol lookups [1].
// The dynamic linker uses the hash table only when a symbol is not
// filtered out by a bloom filter.
//
// [1] Ulrich Drepper (2011), "How To Write Shared Libraries" (Ver. 4.1.2),
// p.9, https://www.akkadia.org/drepper/dsohowto.pdf
void GnuHashTableSection::writeBloomFilter(uint8_t *Buf) {
unsigned C = Config->Is64 ? 64 : 32;
for (const Entry &Sym : Symbols) {
size_t I = (Sym.Hash / C) & (MaskWords - 1);
uint64_t Val = readUint(Buf + I * Config->Wordsize);
Val |= uint64_t(1) << (Sym.Hash % C);
Val |= uint64_t(1) << ((Sym.Hash >> Shift2) % C);
writeUint(Buf + I * Config->Wordsize, Val);
}
}
void GnuHashTableSection::writeHashTable(uint8_t *Buf) {
uint32_t *Buckets = reinterpret_cast<uint32_t *>(Buf);
uint32_t OldBucket = -1;
uint32_t *Values = Buckets + NBuckets;
for (auto I = Symbols.begin(), E = Symbols.end(); I != E; ++I) {
// Write a hash value. It represents a sequence of chains that share the
// same hash modulo value. The last element of each chain is terminated by
// LSB 1.
uint32_t Hash = I->Hash;
bool IsLastInChain = (I + 1) == E || I->BucketIdx != (I + 1)->BucketIdx;
Hash = IsLastInChain ? Hash | 1 : Hash & ~1;
write32(Values++, Hash);
if (I->BucketIdx == OldBucket)
continue;
// Write a hash bucket. Hash buckets contain indices in the following hash
// value table.
write32(Buckets + I->BucketIdx, I->Sym->DynsymIndex);
OldBucket = I->BucketIdx;
}
}
static uint32_t hashGnu(StringRef Name) {
uint32_t H = 5381;
for (uint8_t C : Name)
H = (H << 5) + H + C;
return H;
}
// Add symbols to this symbol hash table. Note that this function
// destructively sort a given vector -- which is needed because
// GNU-style hash table places some sorting requirements.
void GnuHashTableSection::addSymbols(std::vector<SymbolTableEntry> &V) {
// We cannot use 'auto' for Mid because GCC 6.1 cannot deduce
// its type correctly.
std::vector<SymbolTableEntry>::iterator Mid =
std::stable_partition(V.begin(), V.end(), [](const SymbolTableEntry &S) {
return !S.Sym->isDefined();
});
// We chose load factor 4 for the on-disk hash table. For each hash
// collision, the dynamic linker will compare a uint32_t hash value.
// Since the integer comparison is quite fast, we believe we can
// make the load factor even larger. 4 is just a conservative choice.
//
// Note that we don't want to create a zero-sized hash table because
// Android loader as of 2018 doesn't like a .gnu.hash containing such
// table. If that's the case, we create a hash table with one unused
// dummy slot.
NBuckets = std::max<size_t>((V.end() - Mid) / 4, 1);
if (Mid == V.end())
return;
for (SymbolTableEntry &Ent : llvm::make_range(Mid, V.end())) {
Symbol *B = Ent.Sym;
uint32_t Hash = hashGnu(B->getName());
uint32_t BucketIdx = Hash % NBuckets;
Symbols.push_back({B, Ent.StrTabOffset, Hash, BucketIdx});
}
std::stable_sort(
Symbols.begin(), Symbols.end(),
[](const Entry &L, const Entry &R) { return L.BucketIdx < R.BucketIdx; });
V.erase(Mid, V.end());
for (const Entry &Ent : Symbols)
V.push_back({Ent.Sym, Ent.StrTabOffset});
}
HashTableSection::HashTableSection()
: SyntheticSection(SHF_ALLOC, SHT_HASH, 4, ".hash") {
this->Entsize = 4;
}
void HashTableSection::finalizeContents() {
getParent()->Link = In.DynSymTab->getParent()->SectionIndex;
unsigned NumEntries = 2; // nbucket and nchain.
NumEntries += In.DynSymTab->getNumSymbols(); // The chain entries.
// Create as many buckets as there are symbols.
NumEntries += In.DynSymTab->getNumSymbols();
this->Size = NumEntries * 4;
}
void HashTableSection::writeTo(uint8_t *Buf) {
// See comment in GnuHashTableSection::writeTo.
memset(Buf, 0, Size);
unsigned NumSymbols = In.DynSymTab->getNumSymbols();
uint32_t *P = reinterpret_cast<uint32_t *>(Buf);
write32(P++, NumSymbols); // nbucket
write32(P++, NumSymbols); // nchain
uint32_t *Buckets = P;
uint32_t *Chains = P + NumSymbols;
for (const SymbolTableEntry &S : In.DynSymTab->getSymbols()) {
Symbol *Sym = S.Sym;
StringRef Name = Sym->getName();
unsigned I = Sym->DynsymIndex;
uint32_t Hash = hashSysV(Name) % NumSymbols;
Chains[I] = Buckets[Hash];
write32(Buckets + Hash, I);
}
}
// On PowerPC64 the lazy symbol resolvers go into the `global linkage table`
// in the .glink section, rather then the typical .plt section.
PltSection::PltSection(bool IsIplt)
: SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS, 16,
Config->EMachine == EM_PPC64 ? ".glink" : ".plt"),
HeaderSize(IsIplt ? 0 : Target->PltHeaderSize), IsIplt(IsIplt) {
// The PLT needs to be writable on SPARC as the dynamic linker will
// modify the instructions in the PLT entries.
if (Config->EMachine == EM_SPARCV9)
this->Flags |= SHF_WRITE;
}
void PltSection::writeTo(uint8_t *Buf) {
// At beginning of PLT but not the IPLT, we have code to call the dynamic
// linker to resolve dynsyms at runtime. Write such code.
if (!IsIplt)
Target->writePltHeader(Buf);
size_t Off = HeaderSize;
// The IPlt is immediately after the Plt, account for this in RelOff
unsigned PltOff = getPltRelocOff();
for (auto &I : Entries) {
const Symbol *B = I.first;
unsigned RelOff = I.second + PltOff;
uint64_t Got = B->getGotPltVA();
uint64_t Plt = this->getVA() + Off;
Target->writePlt(Buf + Off, Got, Plt, B->PltIndex, RelOff);
Off += Target->PltEntrySize;
}
}
template <class ELFT> void PltSection::addEntry(Symbol &Sym) {
Sym.PltIndex = Entries.size();
RelocationBaseSection *PltRelocSection = In.RelaPlt;
if (IsIplt) {
PltRelocSection = In.RelaIplt;
Sym.IsInIplt = true;
}
unsigned RelOff =
static_cast<RelocationSection<ELFT> *>(PltRelocSection)->getRelocOffset();
Entries.push_back(std::make_pair(&Sym, RelOff));
}
size_t PltSection::getSize() const {
return HeaderSize + Entries.size() * Target->PltEntrySize;
}
// Some architectures such as additional symbols in the PLT section. For
// example ARM uses mapping symbols to aid disassembly
void PltSection::addSymbols() {
// The PLT may have symbols defined for the Header, the IPLT has no header
if (!IsIplt)
Target->addPltHeaderSymbols(*this);
size_t Off = HeaderSize;
for (size_t I = 0; I < Entries.size(); ++I) {
Target->addPltSymbols(*this, Off);
Off += Target->PltEntrySize;
}
}
unsigned PltSection::getPltRelocOff() const {
return IsIplt ? In.Plt->getSize() : 0;
}
// The string hash function for .gdb_index.
static uint32_t computeGdbHash(StringRef S) {
uint32_t H = 0;
for (uint8_t C : S)
H = H * 67 + toLower(C) - 113;
return H;
}
GdbIndexSection::GdbIndexSection()
: SyntheticSection(0, SHT_PROGBITS, 1, ".gdb_index") {}
// Returns the desired size of an on-disk hash table for a .gdb_index section.
// There's a tradeoff between size and collision rate. We aim 75% utilization.
size_t GdbIndexSection::computeSymtabSize() const {
return std::max<size_t>(NextPowerOf2(Symbols.size() * 4 / 3), 1024);
}
// Compute the output section size.
void GdbIndexSection::initOutputSize() {
Size = sizeof(GdbIndexHeader) + computeSymtabSize() * 8;
for (GdbChunk &Chunk : Chunks)
Size += Chunk.CompilationUnits.size() * 16 + Chunk.AddressAreas.size() * 20;
// Add the constant pool size if exists.
if (!Symbols.empty()) {
GdbSymbol &Sym = Symbols.back();
Size += Sym.NameOff + Sym.Name.size() + 1;
}
}
static std::vector<InputSection *> getDebugInfoSections() {
std::vector<InputSection *> Ret;
for (InputSectionBase *S : InputSections)
if (InputSection *IS = dyn_cast<InputSection>(S))
if (IS->Name == ".debug_info")
Ret.push_back(IS);
return Ret;
}
static std::vector<GdbIndexSection::CuEntry> readCuList(DWARFContext &Dwarf) {
std::vector<GdbIndexSection::CuEntry> Ret;
for (std::unique_ptr<DWARFUnit> &Cu : Dwarf.compile_units())
Ret.push_back({Cu->getOffset(), Cu->getLength() + 4});
return Ret;
}
static std::vector<GdbIndexSection::AddressEntry>
readAddressAreas(DWARFContext &Dwarf, InputSection *Sec) {
std::vector<GdbIndexSection::AddressEntry> Ret;
uint32_t CuIdx = 0;
for (std::unique_ptr<DWARFUnit> &Cu : Dwarf.compile_units()) {
DWARFAddressRangesVector Ranges;
Cu->collectAddressRanges(Ranges);
ArrayRef<InputSectionBase *> Sections = Sec->File->getSections();
for (DWARFAddressRange &R : Ranges) {
InputSectionBase *S = Sections[R.SectionIndex];
if (!S || S == &InputSection::Discarded || !S->Live)
continue;
// Range list with zero size has no effect.
if (R.LowPC == R.HighPC)
continue;
auto *IS = cast<InputSection>(S);
uint64_t Offset = IS->getOffsetInFile();
Ret.push_back({IS, R.LowPC - Offset, R.HighPC - Offset, CuIdx});
}
++CuIdx;
}
return Ret;
}
static std::vector<GdbIndexSection::NameTypeEntry>
readPubNamesAndTypes(DWARFContext &Dwarf, uint32_t Idx) {
StringRef Sec1 = Dwarf.getDWARFObj().getGnuPubNamesSection();
StringRef Sec2 = Dwarf.getDWARFObj().getGnuPubTypesSection();
std::vector<GdbIndexSection::NameTypeEntry> Ret;
for (StringRef Sec : {Sec1, Sec2}) {
DWARFDebugPubTable Table(Sec, Config->IsLE, true);
for (const DWARFDebugPubTable::Set &Set : Table.getData())
for (const DWARFDebugPubTable::Entry &Ent : Set.Entries)
Ret.push_back({{Ent.Name, computeGdbHash(Ent.Name)},
(Ent.Descriptor.toBits() << 24) | Idx});
}
return Ret;
}
// Create a list of symbols from a given list of symbol names and types
// by uniquifying them by name.
static std::vector<GdbIndexSection::GdbSymbol>
createSymbols(ArrayRef<std::vector<GdbIndexSection::NameTypeEntry>> NameTypes) {
typedef GdbIndexSection::GdbSymbol GdbSymbol;
typedef GdbIndexSection::NameTypeEntry NameTypeEntry;
// The number of symbols we will handle in this function is of the order
// of millions for very large executables, so we use multi-threading to
// speed it up.
size_t NumShards = 32;
size_t Concurrency = 1;
if (ThreadsEnabled)
Concurrency =
std::min<size_t>(PowerOf2Floor(hardware_concurrency()), NumShards);
// A sharded map to uniquify symbols by name.
std::vector<DenseMap<CachedHashStringRef, size_t>> Map(NumShards);
size_t Shift = 32 - countTrailingZeros(NumShards);
// Instantiate GdbSymbols while uniqufying them by name.
std::vector<std::vector<GdbSymbol>> Symbols(NumShards);
parallelForEachN(0, Concurrency, [&](size_t ThreadId) {
for (ArrayRef<NameTypeEntry> Entries : NameTypes) {
for (const NameTypeEntry &Ent : Entries) {
size_t ShardId = Ent.Name.hash() >> Shift;
if ((ShardId & (Concurrency - 1)) != ThreadId)
continue;
size_t &Idx = Map[ShardId][Ent.Name];
if (Idx) {
Symbols[ShardId][Idx - 1].CuVector.push_back(Ent.Type);
continue;
}
Idx = Symbols[ShardId].size() + 1;
Symbols[ShardId].push_back({Ent.Name, {Ent.Type}, 0, 0});
}
}
});
size_t NumSymbols = 0;
for (ArrayRef<GdbSymbol> V : Symbols)
NumSymbols += V.size();
// The return type is a flattened vector, so we'll copy each vector
// contents to Ret.
std::vector<GdbSymbol> Ret;
Ret.reserve(NumSymbols);
for (std::vector<GdbSymbol> &Vec : Symbols)
for (GdbSymbol &Sym : Vec)
Ret.push_back(std::move(Sym));
// CU vectors and symbol names are adjacent in the output file.
// We can compute their offsets in the output file now.
size_t Off = 0;
for (GdbSymbol &Sym : Ret) {
Sym.CuVectorOff = Off;
Off += (Sym.CuVector.size() + 1) * 4;
}
for (GdbSymbol &Sym : Ret) {
Sym.NameOff = Off;
Off += Sym.Name.size() + 1;
}
return Ret;
}
// Returns a newly-created .gdb_index section.
template <class ELFT> GdbIndexSection *GdbIndexSection::create() {
std::vector<InputSection *> Sections = getDebugInfoSections();
// .debug_gnu_pub{names,types} are useless in executables.
// They are present in input object files solely for creating
// a .gdb_index. So we can remove them from the output.
for (InputSectionBase *S : InputSections)
if (S->Name == ".debug_gnu_pubnames" || S->Name == ".debug_gnu_pubtypes")
S->Live = false;
std::vector<GdbChunk> Chunks(Sections.size());
std::vector<std::vector<NameTypeEntry>> NameTypes(Sections.size());
parallelForEachN(0, Sections.size(), [&](size_t I) {
ObjFile<ELFT> *File = Sections[I]->getFile<ELFT>();
DWARFContext Dwarf(make_unique<LLDDwarfObj<ELFT>>(File));
Chunks[I].Sec = Sections[I];
Chunks[I].CompilationUnits = readCuList(Dwarf);
Chunks[I].AddressAreas = readAddressAreas(Dwarf, Sections[I]);
NameTypes[I] = readPubNamesAndTypes(Dwarf, I);
});
auto *Ret = make<GdbIndexSection>();
Ret->Chunks = std::move(Chunks);
Ret->Symbols = createSymbols(NameTypes);
Ret->initOutputSize();
return Ret;
}
void GdbIndexSection::writeTo(uint8_t *Buf) {
// Write the header.
auto *Hdr = reinterpret_cast<GdbIndexHeader *>(Buf);
uint8_t *Start = Buf;
Hdr->Version = 7;
Buf += sizeof(*Hdr);
// Write the CU list.
Hdr->CuListOff = Buf - Start;
for (GdbChunk &Chunk : Chunks) {
for (CuEntry &Cu : Chunk.CompilationUnits) {
write64le(Buf, Chunk.Sec->OutSecOff + Cu.CuOffset);
write64le(Buf + 8, Cu.CuLength);
Buf += 16;
}
}
// Write the address area.
Hdr->CuTypesOff = Buf - Start;
Hdr->AddressAreaOff = Buf - Start;
uint32_t CuOff = 0;
for (GdbChunk &Chunk : Chunks) {
for (AddressEntry &E : Chunk.AddressAreas) {
uint64_t BaseAddr = E.Section->getVA(0);
write64le(Buf, BaseAddr + E.LowAddress);
write64le(Buf + 8, BaseAddr + E.HighAddress);
write32le(Buf + 16, E.CuIndex + CuOff);
Buf += 20;
}
CuOff += Chunk.CompilationUnits.size();
}
// Write the on-disk open-addressing hash table containing symbols.
Hdr->SymtabOff = Buf - Start;
size_t SymtabSize = computeSymtabSize();
uint32_t Mask = SymtabSize - 1;
for (GdbSymbol &Sym : Symbols) {
uint32_t H = Sym.Name.hash();
uint32_t I = H & Mask;
uint32_t Step = ((H * 17) & Mask) | 1;
while (read32le(Buf + I * 8))
I = (I + Step) & Mask;
write32le(Buf + I * 8, Sym.NameOff);
write32le(Buf + I * 8 + 4, Sym.CuVectorOff);
}
Buf += SymtabSize * 8;
// Write the string pool.
Hdr->ConstantPoolOff = Buf - Start;
parallelForEach(Symbols, [&](GdbSymbol &Sym) {
memcpy(Buf + Sym.NameOff, Sym.Name.data(), Sym.Name.size());
});
// Write the CU vectors.
for (GdbSymbol &Sym : Symbols) {
write32le(Buf, Sym.CuVector.size());
Buf += 4;
for (uint32_t Val : Sym.CuVector) {
write32le(Buf, Val);
Buf += 4;
}
}
}
bool GdbIndexSection::empty() const { return !Out::DebugInfo; }
EhFrameHeader::EhFrameHeader()
: SyntheticSection(SHF_ALLOC, SHT_PROGBITS, 4, ".eh_frame_hdr") {}
// .eh_frame_hdr contains a binary search table of pointers to FDEs.
// Each entry of the search table consists of two values,
// the starting PC from where FDEs covers, and the FDE's address.
// It is sorted by PC.
void EhFrameHeader::writeTo(uint8_t *Buf) {
typedef EhFrameSection::FdeData FdeData;
std::vector<FdeData> Fdes = In.EhFrame->getFdeData();
Buf[0] = 1;
Buf[1] = DW_EH_PE_pcrel | DW_EH_PE_sdata4;
Buf[2] = DW_EH_PE_udata4;
Buf[3] = DW_EH_PE_datarel | DW_EH_PE_sdata4;
write32(Buf + 4, In.EhFrame->getParent()->Addr - this->getVA() - 4);
write32(Buf + 8, Fdes.size());
Buf += 12;
for (FdeData &Fde : Fdes) {
write32(Buf, Fde.PcRel);
write32(Buf + 4, Fde.FdeVARel);
Buf += 8;
}
}
size_t EhFrameHeader::getSize() const {
// .eh_frame_hdr has a 12 bytes header followed by an array of FDEs.
return 12 + In.EhFrame->NumFdes * 8;
}
bool EhFrameHeader::empty() const { return In.EhFrame->empty(); }
VersionDefinitionSection::VersionDefinitionSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_verdef, sizeof(uint32_t),
".gnu.version_d") {}
static StringRef getFileDefName() {
if (!Config->SoName.empty())
return Config->SoName;
return Config->OutputFile;
}
void VersionDefinitionSection::finalizeContents() {
FileDefNameOff = In.DynStrTab->addString(getFileDefName());
for (VersionDefinition &V : Config->VersionDefinitions)
V.NameOff = In.DynStrTab->addString(V.Name);
getParent()->Link = In.DynStrTab->getParent()->SectionIndex;
// sh_info should be set to the number of definitions. This fact is missed in
// documentation, but confirmed by binutils community:
// https://sourceware.org/ml/binutils/2014-11/msg00355.html
getParent()->Info = getVerDefNum();
}
void VersionDefinitionSection::writeOne(uint8_t *Buf, uint32_t Index,
StringRef Name, size_t NameOff) {
uint16_t Flags = Index == 1 ? VER_FLG_BASE : 0;
// Write a verdef.
write16(Buf, 1); // vd_version
write16(Buf + 2, Flags); // vd_flags
write16(Buf + 4, Index); // vd_ndx
write16(Buf + 6, 1); // vd_cnt
write32(Buf + 8, hashSysV(Name)); // vd_hash
write32(Buf + 12, 20); // vd_aux
write32(Buf + 16, 28); // vd_next
// Write a veraux.
write32(Buf + 20, NameOff); // vda_name
write32(Buf + 24, 0); // vda_next
}
void VersionDefinitionSection::writeTo(uint8_t *Buf) {
writeOne(Buf, 1, getFileDefName(), FileDefNameOff);
for (VersionDefinition &V : Config->VersionDefinitions) {
Buf += EntrySize;
writeOne(Buf, V.Id, V.Name, V.NameOff);
}
// Need to terminate the last version definition.
write32(Buf + 16, 0); // vd_next
}
size_t VersionDefinitionSection::getSize() const {
return EntrySize * getVerDefNum();
}
// .gnu.version is a table where each entry is 2 byte long.
template <class ELFT>
VersionTableSection<ELFT>::VersionTableSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_versym, sizeof(uint16_t),
".gnu.version") {
this->Entsize = 2;
}
template <class ELFT> void VersionTableSection<ELFT>::finalizeContents() {
// At the moment of june 2016 GNU docs does not mention that sh_link field
// should be set, but Sun docs do. Also readelf relies on this field.
getParent()->Link = In.DynSymTab->getParent()->SectionIndex;
}
template <class ELFT> size_t VersionTableSection<ELFT>::getSize() const {
return (In.DynSymTab->getSymbols().size() + 1) * 2;
}
template <class ELFT> void VersionTableSection<ELFT>::writeTo(uint8_t *Buf) {
Buf += 2;
for (const SymbolTableEntry &S : In.DynSymTab->getSymbols()) {
write16(Buf, S.Sym->VersionId);
Buf += 2;
}
}
template <class ELFT> bool VersionTableSection<ELFT>::empty() const {
return !In.VerDef && InX<ELFT>::VerNeed->empty();
}
template <class ELFT>
VersionNeedSection<ELFT>::VersionNeedSection()
: SyntheticSection(SHF_ALLOC, SHT_GNU_verneed, sizeof(uint32_t),
".gnu.version_r") {
// Identifiers in verneed section start at 2 because 0 and 1 are reserved
// for VER_NDX_LOCAL and VER_NDX_GLOBAL.
// First identifiers are reserved by verdef section if it exist.
NextIndex = getVerDefNum() + 1;
}
template <class ELFT> void VersionNeedSection<ELFT>::addSymbol(Symbol *SS) {
auto &File = cast<SharedFile<ELFT>>(*SS->File);
if (SS->VerdefIndex == VER_NDX_GLOBAL) {
SS->VersionId = VER_NDX_GLOBAL;
return;
}
// If we don't already know that we need an Elf_Verneed for this DSO, prepare
// to create one by adding it to our needed list and creating a dynstr entry
// for the soname.
if (File.VerdefMap.empty())
Needed.push_back({&File, In.DynStrTab->addString(File.SoName)});
const typename ELFT::Verdef *Ver = File.Verdefs[SS->VerdefIndex];
typename SharedFile<ELFT>::NeededVer &NV = File.VerdefMap[Ver];
// If we don't already know that we need an Elf_Vernaux for this Elf_Verdef,
// prepare to create one by allocating a version identifier and creating a
// dynstr entry for the version name.
if (NV.Index == 0) {
NV.StrTab = In.DynStrTab->addString(File.getStringTable().data() +
Ver->getAux()->vda_name);
NV.Index = NextIndex++;
}
SS->VersionId = NV.Index;
}
template <class ELFT> void VersionNeedSection<ELFT>::writeTo(uint8_t *Buf) {
// The Elf_Verneeds need to appear first, followed by the Elf_Vernauxs.
auto *Verneed = reinterpret_cast<Elf_Verneed *>(Buf);
auto *Vernaux = reinterpret_cast<Elf_Vernaux *>(Verneed + Needed.size());
for (std::pair<SharedFile<ELFT> *, size_t> &P : Needed) {
// Create an Elf_Verneed for this DSO.
Verneed->vn_version = 1;
Verneed->vn_cnt = P.first->VerdefMap.size();
Verneed->vn_file = P.second;
Verneed->vn_aux =
reinterpret_cast<char *>(Vernaux) - reinterpret_cast<char *>(Verneed);
Verneed->vn_next = sizeof(Elf_Verneed);
++Verneed;
// Create the Elf_Vernauxs for this Elf_Verneed. The loop iterates over
// VerdefMap, which will only contain references to needed version
// definitions. Each Elf_Vernaux is based on the information contained in
// the Elf_Verdef in the source DSO. This loop iterates over a std::map of
// pointers, but is deterministic because the pointers refer to Elf_Verdef
// data structures within a single input file.
for (auto &NV : P.first->VerdefMap) {
Vernaux->vna_hash = NV.first->vd_hash;
Vernaux->vna_flags = 0;
Vernaux->vna_other = NV.second.Index;
Vernaux->vna_name = NV.second.StrTab;
Vernaux->vna_next = sizeof(Elf_Vernaux);
++Vernaux;
}
Vernaux[-1].vna_next = 0;
}
Verneed[-1].vn_next = 0;
}
template <class ELFT> void VersionNeedSection<ELFT>::finalizeContents() {
getParent()->Link = In.DynStrTab->getParent()->SectionIndex;
getParent()->Info = Needed.size();
}
template <class ELFT> size_t VersionNeedSection<ELFT>::getSize() const {
unsigned Size = Needed.size() * sizeof(Elf_Verneed);
for (const std::pair<SharedFile<ELFT> *, size_t> &P : Needed)
Size += P.first->VerdefMap.size() * sizeof(Elf_Vernaux);
return Size;
}
template <class ELFT> bool VersionNeedSection<ELFT>::empty() const {
return getNeedNum() == 0;
}
void MergeSyntheticSection::addSection(MergeInputSection *MS) {
MS->Parent = this;
Sections.push_back(MS);
}
MergeTailSection::MergeTailSection(StringRef Name, uint32_t Type,
uint64_t Flags, uint32_t Alignment)
: MergeSyntheticSection(Name, Type, Flags, Alignment),
Builder(StringTableBuilder::RAW, Alignment) {}
size_t MergeTailSection::getSize() const { return Builder.getSize(); }
void MergeTailSection::writeTo(uint8_t *Buf) { Builder.write(Buf); }
void MergeTailSection::finalizeContents() {
// Add all string pieces to the string table builder to create section
// contents.
for (MergeInputSection *Sec : Sections)
for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I)
if (Sec->Pieces[I].Live)
Builder.add(Sec->getData(I));
// Fix the string table content. After this, the contents will never change.
Builder.finalize();
// finalize() fixed tail-optimized strings, so we can now get
// offsets of strings. Get an offset for each string and save it
// to a corresponding StringPiece for easy access.
for (MergeInputSection *Sec : Sections)
for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I)
if (Sec->Pieces[I].Live)
Sec->Pieces[I].OutputOff = Builder.getOffset(Sec->getData(I));
}
void MergeNoTailSection::writeTo(uint8_t *Buf) {
for (size_t I = 0; I < NumShards; ++I)
Shards[I].write(Buf + ShardOffsets[I]);
}
// This function is very hot (i.e. it can take several seconds to finish)
// because sometimes the number of inputs is in an order of magnitude of
// millions. So, we use multi-threading.
//
// For any strings S and T, we know S is not mergeable with T if S's hash
// value is different from T's. If that's the case, we can safely put S and
// T into different string builders without worrying about merge misses.
// We do it in parallel.
void MergeNoTailSection::finalizeContents() {
// Initializes string table builders.
for (size_t I = 0; I < NumShards; ++I)
Shards.emplace_back(StringTableBuilder::RAW, Alignment);
// Concurrency level. Must be a power of 2 to avoid expensive modulo
// operations in the following tight loop.
size_t Concurrency = 1;
if (ThreadsEnabled)
Concurrency =
std::min<size_t>(PowerOf2Floor(hardware_concurrency()), NumShards);
// Add section pieces to the builders.
parallelForEachN(0, Concurrency, [&](size_t ThreadId) {
for (MergeInputSection *Sec : Sections) {
for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I) {
size_t ShardId = getShardId(Sec->Pieces[I].Hash);
if ((ShardId & (Concurrency - 1)) == ThreadId && Sec->Pieces[I].Live)
Sec->Pieces[I].OutputOff = Shards[ShardId].add(Sec->getData(I));
}
}
});
// Compute an in-section offset for each shard.
size_t Off = 0;
for (size_t I = 0; I < NumShards; ++I) {
Shards[I].finalizeInOrder();
if (Shards[I].getSize() > 0)
Off = alignTo(Off, Alignment);
ShardOffsets[I] = Off;
Off += Shards[I].getSize();
}
Size = Off;
// So far, section pieces have offsets from beginning of shards, but
// we want offsets from beginning of the whole section. Fix them.
parallelForEach(Sections, [&](MergeInputSection *Sec) {
for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I)
if (Sec->Pieces[I].Live)
Sec->Pieces[I].OutputOff +=
ShardOffsets[getShardId(Sec->Pieces[I].Hash)];
});
}
static MergeSyntheticSection *createMergeSynthetic(StringRef Name,
uint32_t Type,
uint64_t Flags,
uint32_t Alignment) {
bool ShouldTailMerge = (Flags & SHF_STRINGS) && Config->Optimize >= 2;
if (ShouldTailMerge)
return make<MergeTailSection>(Name, Type, Flags, Alignment);
return make<MergeNoTailSection>(Name, Type, Flags, Alignment);
}
template <class ELFT> void elf::splitSections() {
// splitIntoPieces needs to be called on each MergeInputSection
// before calling finalizeContents().
parallelForEach(InputSections, [](InputSectionBase *Sec) {
if (auto *S = dyn_cast<MergeInputSection>(Sec))
S->splitIntoPieces();
else if (auto *Eh = dyn_cast<EhInputSection>(Sec))
Eh->split<ELFT>();
});
}
// This function scans over the inputsections to create mergeable
// synthetic sections.
//
// It removes MergeInputSections from the input section array and adds
// new synthetic sections at the location of the first input section
// that it replaces. It then finalizes each synthetic section in order
// to compute an output offset for each piece of each input section.
void elf::mergeSections() {
std::vector<MergeSyntheticSection *> MergeSections;
for (InputSectionBase *&S : InputSections) {
MergeInputSection *MS = dyn_cast<MergeInputSection>(S);
if (!MS)
continue;
// We do not want to handle sections that are not alive, so just remove
// them instead of trying to merge.
if (!MS->Live) {
S = nullptr;
continue;
}
StringRef OutsecName = getOutputSectionName(MS);
uint32_t Alignment = std::max<uint32_t>(MS->Alignment, MS->Entsize);
auto I = llvm::find_if(MergeSections, [=](MergeSyntheticSection *Sec) {
// While we could create a single synthetic section for two different
// values of Entsize, it is better to take Entsize into consideration.
//
// With a single synthetic section no two pieces with different Entsize
// could be equal, so we may as well have two sections.
//
// Using Entsize in here also allows us to propagate it to the synthetic
// section.
return Sec->Name == OutsecName && Sec->Flags == MS->Flags &&
Sec->Entsize == MS->Entsize && Sec->Alignment == Alignment;
});
if (I == MergeSections.end()) {
MergeSyntheticSection *Syn =
createMergeSynthetic(OutsecName, MS->Type, MS->Flags, Alignment);
MergeSections.push_back(Syn);
I = std::prev(MergeSections.end());
S = Syn;
Syn->Entsize = MS->Entsize;
} else {
S = nullptr;
}
(*I)->addSection(MS);
}
for (auto *MS : MergeSections)
MS->finalizeContents();
std::vector<InputSectionBase *> &V = InputSections;
V.erase(std::remove(V.begin(), V.end(), nullptr), V.end());
}
MipsRldMapSection::MipsRldMapSection()
: SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS, Config->Wordsize,
".rld_map") {}
ARMExidxSentinelSection::ARMExidxSentinelSection()
: SyntheticSection(SHF_ALLOC | SHF_LINK_ORDER, SHT_ARM_EXIDX,
Config->Wordsize, ".ARM.exidx") {}
// Write a terminating sentinel entry to the end of the .ARM.exidx table.
// This section will have been sorted last in the .ARM.exidx table.
// This table entry will have the form:
// | PREL31 upper bound of code that has exception tables | EXIDX_CANTUNWIND |
// The sentinel must have the PREL31 value of an address higher than any
// address described by any other table entry.
void ARMExidxSentinelSection::writeTo(uint8_t *Buf) {
assert(Highest);
uint64_t S = Highest->getVA(Highest->getSize());
uint64_t P = getVA();
Target->relocateOne(Buf, R_ARM_PREL31, S - P);
write32le(Buf + 4, 1);
}
// The sentinel has to be removed if there are no other .ARM.exidx entries.
bool ARMExidxSentinelSection::empty() const {
for (InputSection *IS : getInputSections(getParent()))
if (!isa<ARMExidxSentinelSection>(IS))
return false;
return true;
}
bool ARMExidxSentinelSection::classof(const SectionBase *D) {
return D->kind() == InputSectionBase::Synthetic && D->Type == SHT_ARM_EXIDX;
}
ThunkSection::ThunkSection(OutputSection *OS, uint64_t Off)
: SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS,
Config->Wordsize, ".text.thunk") {
this->Parent = OS;
this->OutSecOff = Off;
}
void ThunkSection::addThunk(Thunk *T) {
Thunks.push_back(T);
T->addSymbols(*this);
}
void ThunkSection::writeTo(uint8_t *Buf) {
for (Thunk *T : Thunks)
T->writeTo(Buf + T->Offset);
}
InputSection *ThunkSection::getTargetInputSection() const {
if (Thunks.empty())
return nullptr;
const Thunk *T = Thunks.front();
return T->getTargetInputSection();
}
bool ThunkSection::assignOffsets() {
uint64_t Off = 0;
for (Thunk *T : Thunks) {
Off = alignTo(Off, T->Alignment);
T->setOffset(Off);
uint32_t Size = T->size();
T->getThunkTargetSym()->Size = Size;
Off += Size;
}
bool Changed = Off != Size;
Size = Off;
return Changed;
}
InStruct elf::In;
template GdbIndexSection *GdbIndexSection::create<ELF32LE>();
template GdbIndexSection *GdbIndexSection::create<ELF32BE>();
template GdbIndexSection *GdbIndexSection::create<ELF64LE>();
template GdbIndexSection *GdbIndexSection::create<ELF64BE>();
template void elf::splitSections<ELF32LE>();
template void elf::splitSections<ELF32BE>();
template void elf::splitSections<ELF64LE>();
template void elf::splitSections<ELF64BE>();
template void EhFrameSection::addSection<ELF32LE>(InputSectionBase *);
template void EhFrameSection::addSection<ELF32BE>(InputSectionBase *);
template void EhFrameSection::addSection<ELF64LE>(InputSectionBase *);
template void EhFrameSection::addSection<ELF64BE>(InputSectionBase *);
template void PltSection::addEntry<ELF32LE>(Symbol &Sym);
template void PltSection::addEntry<ELF32BE>(Symbol &Sym);
template void PltSection::addEntry<ELF64LE>(Symbol &Sym);
template void PltSection::addEntry<ELF64BE>(Symbol &Sym);
template void MipsGotSection::build<ELF32LE>();
template void MipsGotSection::build<ELF32BE>();
template void MipsGotSection::build<ELF64LE>();
template void MipsGotSection::build<ELF64BE>();
template class elf::MipsAbiFlagsSection<ELF32LE>;
template class elf::MipsAbiFlagsSection<ELF32BE>;
template class elf::MipsAbiFlagsSection<ELF64LE>;
template class elf::MipsAbiFlagsSection<ELF64BE>;
template class elf::MipsOptionsSection<ELF32LE>;
template class elf::MipsOptionsSection<ELF32BE>;
template class elf::MipsOptionsSection<ELF64LE>;
template class elf::MipsOptionsSection<ELF64BE>;
template class elf::MipsReginfoSection<ELF32LE>;
template class elf::MipsReginfoSection<ELF32BE>;
template class elf::MipsReginfoSection<ELF64LE>;
template class elf::MipsReginfoSection<ELF64BE>;
template class elf::DynamicSection<ELF32LE>;
template class elf::DynamicSection<ELF32BE>;
template class elf::DynamicSection<ELF64LE>;
template class elf::DynamicSection<ELF64BE>;
template class elf::RelocationSection<ELF32LE>;
template class elf::RelocationSection<ELF32BE>;
template class elf::RelocationSection<ELF64LE>;
template class elf::RelocationSection<ELF64BE>;
template class elf::AndroidPackedRelocationSection<ELF32LE>;
template class elf::AndroidPackedRelocationSection<ELF32BE>;
template class elf::AndroidPackedRelocationSection<ELF64LE>;
template class elf::AndroidPackedRelocationSection<ELF64BE>;
template class elf::RelrSection<ELF32LE>;
template class elf::RelrSection<ELF32BE>;
template class elf::RelrSection<ELF64LE>;
template class elf::RelrSection<ELF64BE>;
template class elf::SymbolTableSection<ELF32LE>;
template class elf::SymbolTableSection<ELF32BE>;
template class elf::SymbolTableSection<ELF64LE>;
template class elf::SymbolTableSection<ELF64BE>;
template class elf::VersionTableSection<ELF32LE>;
template class elf::VersionTableSection<ELF32BE>;
template class elf::VersionTableSection<ELF64LE>;
template class elf::VersionTableSection<ELF64BE>;
template class elf::VersionNeedSection<ELF32LE>;
template class elf::VersionNeedSection<ELF32BE>;
template class elf::VersionNeedSection<ELF64LE>;
template class elf::VersionNeedSection<ELF64BE>;
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