| //===- Relocations.cpp ----------------------------------------------------===// |
| // |
| // The LLVM Linker |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file contains platform-independent functions to process relocations. |
| // I'll describe the overview of this file here. |
| // |
| // Simple relocations are easy to handle for the linker. For example, |
| // for R_X86_64_PC64 relocs, the linker just has to fix up locations |
| // with the relative offsets to the target symbols. It would just be |
| // reading records from relocation sections and applying them to output. |
| // |
| // But not all relocations are that easy to handle. For example, for |
| // R_386_GOTOFF relocs, the linker has to create new GOT entries for |
| // symbols if they don't exist, and fix up locations with GOT entry |
| // offsets from the beginning of GOT section. So there is more than |
| // fixing addresses in relocation processing. |
| // |
| // ELF defines a large number of complex relocations. |
| // |
| // The functions in this file analyze relocations and do whatever needs |
| // to be done. It includes, but not limited to, the following. |
| // |
| // - create GOT/PLT entries |
| // - create new relocations in .dynsym to let the dynamic linker resolve |
| // them at runtime (since ELF supports dynamic linking, not all |
| // relocations can be resolved at link-time) |
| // - create COPY relocs and reserve space in .bss |
| // - replace expensive relocs (in terms of runtime cost) with cheap ones |
| // - error out infeasible combinations such as PIC and non-relative relocs |
| // |
| // Note that the functions in this file don't actually apply relocations |
| // because it doesn't know about the output file nor the output file buffer. |
| // It instead stores Relocation objects to InputSection's Relocations |
| // vector to let it apply later in InputSection::writeTo. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "Relocations.h" |
| #include "Config.h" |
| #include "OutputSections.h" |
| #include "SymbolTable.h" |
| #include "Target.h" |
| #include "Thunks.h" |
| |
| #include "llvm/Support/Endian.h" |
| #include "llvm/Support/raw_ostream.h" |
| |
| using namespace llvm; |
| using namespace llvm::ELF; |
| using namespace llvm::object; |
| using namespace llvm::support::endian; |
| |
| namespace lld { |
| namespace elf { |
| |
| static bool refersToGotEntry(RelExpr Expr) { |
| return Expr == R_GOT || Expr == R_GOT_OFF || Expr == R_MIPS_GOT_LOCAL_PAGE || |
| Expr == R_MIPS_GOT_OFF || Expr == R_MIPS_TLSGD || |
| Expr == R_MIPS_TLSLD || Expr == R_GOT_PAGE_PC || Expr == R_GOT_PC || |
| Expr == R_GOT_FROM_END || Expr == R_TLSGD || Expr == R_TLSGD_PC || |
| Expr == R_TLSDESC || Expr == R_TLSDESC_PAGE; |
| } |
| |
| static bool isPreemptible(const SymbolBody &Body, uint32_t Type) { |
| // In case of MIPS GP-relative relocations always resolve to a definition |
| // in a regular input file, ignoring the one-definition rule. So we, |
| // for example, should not attempt to create a dynamic relocation even |
| // if the target symbol is preemptible. There are two two MIPS GP-relative |
| // relocations R_MIPS_GPREL16 and R_MIPS_GPREL32. But only R_MIPS_GPREL16 |
| // can be against a preemptible symbol. |
| // To get MIPS relocation type we apply 0xff mask. In case of O32 ABI all |
| // relocation types occupy eight bit. In case of N64 ABI we extract first |
| // relocation from 3-in-1 packet because only the first relocation can |
| // be against a real symbol. |
| if (Config->EMachine == EM_MIPS && (Type & 0xff) == R_MIPS_GPREL16) |
| return false; |
| return Body.isPreemptible(); |
| } |
| |
| // This function is similar to the `handleTlsRelocation`. ARM and MIPS do not |
| // support any relaxations for TLS relocations so by factoring out ARM and MIPS |
| // handling in to the separate function we can simplify the code and do not |
| // pollute `handleTlsRelocation` by ARM and MIPS `ifs` statements. |
| // FIXME: The ARM implementation always adds the module index dynamic |
| // relocation even for non-preemptible symbols in applications. For static |
| // linking support we must either resolve the module index relocation at static |
| // link time, or hard code the module index (1) for the application in the GOT. |
| template <class ELFT> |
| static unsigned handleNoRelaxTlsRelocation(uint32_t Type, SymbolBody &Body, |
| InputSectionBase<ELFT> &C, |
| typename ELFT::uint Offset, |
| typename ELFT::uint Addend, |
| RelExpr Expr) { |
| if (Expr == R_MIPS_TLSLD || Expr == R_TLSLD_PC) { |
| if (Out<ELFT>::Got->addTlsIndex() && |
| (Config->Pic || Config->EMachine == EM_ARM)) |
| Out<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, Out<ELFT>::Got, |
| Out<ELFT>::Got->getTlsIndexOff(), false, |
| nullptr, 0}); |
| C.Relocations.push_back({Expr, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| typedef typename ELFT::uint uintX_t; |
| if (Target->isTlsGlobalDynamicRel(Type)) { |
| if (Out<ELFT>::Got->addDynTlsEntry(Body) && |
| (Body.isPreemptible() || Config->EMachine == EM_ARM)) { |
| uintX_t Off = Out<ELFT>::Got->getGlobalDynOffset(Body); |
| Out<ELFT>::RelaDyn->addReloc( |
| {Target->TlsModuleIndexRel, Out<ELFT>::Got, Off, false, &Body, 0}); |
| if (Body.isPreemptible()) |
| Out<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, Out<ELFT>::Got, |
| Off + (uintX_t)sizeof(uintX_t), false, |
| &Body, 0}); |
| } |
| C.Relocations.push_back({Expr, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| return 0; |
| } |
| |
| // Returns the number of relocations processed. |
| template <class ELFT> |
| static unsigned handleTlsRelocation(uint32_t Type, SymbolBody &Body, |
| InputSectionBase<ELFT> &C, |
| typename ELFT::uint Offset, |
| typename ELFT::uint Addend, RelExpr Expr) { |
| if (!(C.getSectionHdr()->sh_flags & SHF_ALLOC)) |
| return 0; |
| |
| if (!Body.isTls()) |
| return 0; |
| |
| typedef typename ELFT::uint uintX_t; |
| |
| if (Config->EMachine == EM_MIPS || Config->EMachine == EM_ARM) |
| return handleNoRelaxTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, |
| Expr); |
| |
| if ((Expr == R_TLSDESC || Expr == R_TLSDESC_PAGE || Expr == R_HINT) && |
| Config->Shared) { |
| if (Out<ELFT>::Got->addDynTlsEntry(Body)) { |
| uintX_t Off = Out<ELFT>::Got->getGlobalDynOffset(Body); |
| Out<ELFT>::RelaDyn->addReloc( |
| {Target->TlsDescRel, Out<ELFT>::Got, Off, false, &Body, 0}); |
| } |
| if (Expr != R_HINT) |
| C.Relocations.push_back({Expr, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| |
| if (Expr == R_TLSLD_PC || Expr == R_TLSLD) { |
| // Local-Dynamic relocs can be relaxed to Local-Exec. |
| if (!Config->Shared) { |
| C.Relocations.push_back( |
| {R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Body}); |
| return 2; |
| } |
| if (Out<ELFT>::Got->addTlsIndex()) |
| Out<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, Out<ELFT>::Got, |
| Out<ELFT>::Got->getTlsIndexOff(), false, |
| nullptr, 0}); |
| C.Relocations.push_back({Expr, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| |
| // Local-Dynamic relocs can be relaxed to Local-Exec. |
| if (Target->isTlsLocalDynamicRel(Type) && !Config->Shared) { |
| C.Relocations.push_back( |
| {R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| |
| if (Expr == R_TLSDESC_PAGE || Expr == R_TLSDESC || Expr == R_HINT || |
| Target->isTlsGlobalDynamicRel(Type)) { |
| if (Config->Shared) { |
| if (Out<ELFT>::Got->addDynTlsEntry(Body)) { |
| uintX_t Off = Out<ELFT>::Got->getGlobalDynOffset(Body); |
| Out<ELFT>::RelaDyn->addReloc( |
| {Target->TlsModuleIndexRel, Out<ELFT>::Got, Off, false, &Body, 0}); |
| |
| // If the symbol is preemptible we need the dynamic linker to write |
| // the offset too. |
| if (isPreemptible(Body, Type)) |
| Out<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, Out<ELFT>::Got, |
| Off + (uintX_t)sizeof(uintX_t), false, |
| &Body, 0}); |
| } |
| C.Relocations.push_back({Expr, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| |
| // Global-Dynamic relocs can be relaxed to Initial-Exec or Local-Exec |
| // depending on the symbol being locally defined or not. |
| if (isPreemptible(Body, Type)) { |
| C.Relocations.push_back( |
| {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_IE), Type, |
| Offset, Addend, &Body}); |
| if (!Body.isInGot()) { |
| Out<ELFT>::Got->addEntry(Body); |
| Out<ELFT>::RelaDyn->addReloc({Target->TlsGotRel, Out<ELFT>::Got, |
| Body.getGotOffset<ELFT>(), false, &Body, |
| 0}); |
| } |
| return Target->TlsGdRelaxSkip; |
| } |
| C.Relocations.push_back( |
| {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_LE), Type, |
| Offset, Addend, &Body}); |
| return Target->TlsGdRelaxSkip; |
| } |
| |
| // Initial-Exec relocs can be relaxed to Local-Exec if the symbol is locally |
| // defined. |
| if (Target->isTlsInitialExecRel(Type) && !Config->Shared && |
| !isPreemptible(Body, Type)) { |
| C.Relocations.push_back( |
| {R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Body}); |
| return 1; |
| } |
| return 0; |
| } |
| |
| template <endianness E> static int16_t readSignedLo16(const uint8_t *Loc) { |
| return read32<E>(Loc) & 0xffff; |
| } |
| |
| template <class RelTy> |
| static uint32_t getMipsPairType(const RelTy *Rel, const SymbolBody &Sym) { |
| switch (Rel->getType(Config->Mips64EL)) { |
| case R_MIPS_HI16: |
| return R_MIPS_LO16; |
| case R_MIPS_GOT16: |
| return Sym.isLocal() ? R_MIPS_LO16 : R_MIPS_NONE; |
| case R_MIPS_PCHI16: |
| return R_MIPS_PCLO16; |
| case R_MICROMIPS_HI16: |
| return R_MICROMIPS_LO16; |
| default: |
| return R_MIPS_NONE; |
| } |
| } |
| |
| template <class ELFT, class RelTy> |
| static int32_t findMipsPairedAddend(const uint8_t *Buf, const uint8_t *BufLoc, |
| SymbolBody &Sym, const RelTy *Rel, |
| const RelTy *End) { |
| uint32_t SymIndex = Rel->getSymbol(Config->Mips64EL); |
| uint32_t Type = getMipsPairType(Rel, Sym); |
| |
| // Some MIPS relocations use addend calculated from addend of the relocation |
| // itself and addend of paired relocation. ABI requires to compute such |
| // combined addend in case of REL relocation record format only. |
| // See p. 4-17 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf |
| if (RelTy::IsRela || Type == R_MIPS_NONE) |
| return 0; |
| |
| for (const RelTy *RI = Rel; RI != End; ++RI) { |
| if (RI->getType(Config->Mips64EL) != Type) |
| continue; |
| if (RI->getSymbol(Config->Mips64EL) != SymIndex) |
| continue; |
| const endianness E = ELFT::TargetEndianness; |
| return ((read32<E>(BufLoc) & 0xffff) << 16) + |
| readSignedLo16<E>(Buf + RI->r_offset); |
| } |
| warn("can't find matching " + getRelName(Type) + " relocation for " + |
| getRelName(Rel->getType(Config->Mips64EL))); |
| return 0; |
| } |
| |
| // True if non-preemptable symbol always has the same value regardless of where |
| // the DSO is loaded. |
| template <class ELFT> static bool isAbsolute(const SymbolBody &Body) { |
| if (Body.isUndefined()) |
| return !Body.isLocal() && Body.symbol()->isWeak(); |
| if (const auto *DR = dyn_cast<DefinedRegular<ELFT>>(&Body)) |
| return DR->Section == nullptr; // Absolute symbol. |
| return false; |
| } |
| |
| static bool needsPlt(RelExpr Expr) { |
| return Expr == R_PLT_PC || Expr == R_PPC_PLT_OPD || Expr == R_PLT || |
| Expr == R_PLT_PAGE_PC || Expr == R_THUNK_PLT_PC; |
| } |
| |
| // True if this expression is of the form Sym - X, where X is a position in the |
| // file (PC, or GOT for example). |
| static bool isRelExpr(RelExpr Expr) { |
| return Expr == R_PC || Expr == R_GOTREL || Expr == R_GOTREL_FROM_END || |
| Expr == R_PAGE_PC || Expr == R_RELAX_GOT_PC || Expr == R_THUNK_PC || |
| Expr == R_THUNK_PLT_PC; |
| } |
| |
| template <class ELFT> |
| static bool isStaticLinkTimeConstant(RelExpr E, uint32_t Type, |
| const SymbolBody &Body) { |
| // These expressions always compute a constant |
| if (E == R_SIZE || E == R_GOT_FROM_END || E == R_GOT_OFF || |
| E == R_MIPS_GOT_LOCAL_PAGE || E == R_MIPS_GOT_OFF || E == R_MIPS_TLSGD || |
| E == R_GOT_PAGE_PC || E == R_GOT_PC || E == R_PLT_PC || E == R_TLSGD_PC || |
| E == R_TLSGD || E == R_PPC_PLT_OPD || E == R_TLSDESC_PAGE || |
| E == R_HINT || E == R_THUNK_PC || E == R_THUNK_PLT_PC) |
| return true; |
| |
| // These never do, except if the entire file is position dependent or if |
| // only the low bits are used. |
| if (E == R_GOT || E == R_PLT || E == R_TLSDESC) |
| return Target->usesOnlyLowPageBits(Type) || !Config->Pic; |
| |
| if (isPreemptible(Body, Type)) |
| return false; |
| |
| if (!Config->Pic) |
| return true; |
| |
| bool AbsVal = isAbsolute<ELFT>(Body) || Body.isTls(); |
| bool RelE = isRelExpr(E); |
| if (AbsVal && !RelE) |
| return true; |
| if (!AbsVal && RelE) |
| return true; |
| |
| // Relative relocation to an absolute value. This is normally unrepresentable, |
| // but if the relocation refers to a weak undefined symbol, we allow it to |
| // resolve to the image base. This is a little strange, but it allows us to |
| // link function calls to such symbols. Normally such a call will be guarded |
| // with a comparison, which will load a zero from the GOT. |
| if (AbsVal && RelE) { |
| if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak()) |
| return true; |
| error("relocation " + getRelName(Type) + |
| " cannot refer to absolute symbol " + Body.getName()); |
| return true; |
| } |
| |
| return Target->usesOnlyLowPageBits(Type); |
| } |
| |
| static RelExpr toPlt(RelExpr Expr) { |
| if (Expr == R_PPC_OPD) |
| return R_PPC_PLT_OPD; |
| if (Expr == R_PC) |
| return R_PLT_PC; |
| if (Expr == R_PAGE_PC) |
| return R_PLT_PAGE_PC; |
| if (Expr == R_ABS) |
| return R_PLT; |
| return Expr; |
| } |
| |
| static RelExpr fromPlt(RelExpr Expr) { |
| // We decided not to use a plt. Optimize a reference to the plt to a |
| // reference to the symbol itself. |
| if (Expr == R_PLT_PC) |
| return R_PC; |
| if (Expr == R_PPC_PLT_OPD) |
| return R_PPC_OPD; |
| if (Expr == R_PLT) |
| return R_ABS; |
| return Expr; |
| } |
| |
| template <class ELFT> static uint32_t getAlignment(SharedSymbol<ELFT> *SS) { |
| typedef typename ELFT::uint uintX_t; |
| |
| uintX_t SecAlign = SS->file()->getSection(SS->Sym)->sh_addralign; |
| uintX_t SymValue = SS->Sym.st_value; |
| int TrailingZeros = |
| std::min(countTrailingZeros(SecAlign), countTrailingZeros(SymValue)); |
| return 1 << TrailingZeros; |
| } |
| |
| // Reserve space in .bss for copy relocation. |
| template <class ELFT> static void addCopyRelSymbol(SharedSymbol<ELFT> *SS) { |
| typedef typename ELFT::uint uintX_t; |
| typedef typename ELFT::Sym Elf_Sym; |
| |
| // Copy relocation against zero-sized symbol doesn't make sense. |
| uintX_t SymSize = SS->template getSize<ELFT>(); |
| if (SymSize == 0) |
| fatal("cannot create a copy relocation for symbol " + SS->getName()); |
| |
| uintX_t Alignment = getAlignment(SS); |
| uintX_t Off = alignTo(Out<ELFT>::Bss->getSize(), Alignment); |
| Out<ELFT>::Bss->setSize(Off + SymSize); |
| Out<ELFT>::Bss->updateAlignment(Alignment); |
| uintX_t Shndx = SS->Sym.st_shndx; |
| uintX_t Value = SS->Sym.st_value; |
| // Look through the DSO's dynamic symbol table for aliases and create a |
| // dynamic symbol for each one. This causes the copy relocation to correctly |
| // interpose any aliases. |
| for (const Elf_Sym &S : SS->file()->getElfSymbols(true)) { |
| if (S.st_shndx != Shndx || S.st_value != Value) |
| continue; |
| auto *Alias = dyn_cast_or_null<SharedSymbol<ELFT>>( |
| Symtab<ELFT>::X->find(check(S.getName(SS->file()->getStringTable())))); |
| if (!Alias) |
| continue; |
| Alias->OffsetInBss = Off; |
| Alias->NeedsCopyOrPltAddr = true; |
| Alias->symbol()->IsUsedInRegularObj = true; |
| } |
| Out<ELFT>::RelaDyn->addReloc( |
| {Target->CopyRel, Out<ELFT>::Bss, SS->OffsetInBss, false, SS, 0}); |
| } |
| |
| template <class ELFT> |
| static StringRef getSymbolName(const elf::ObjectFile<ELFT> &File, |
| SymbolBody &Body) { |
| if (Body.isLocal() && Body.getNameOffset()) |
| return File.getStringTable().data() + Body.getNameOffset(); |
| if (!Body.isLocal()) |
| return Body.getName(); |
| return ""; |
| } |
| |
| template <class ELFT> |
| static RelExpr adjustExpr(const elf::ObjectFile<ELFT> &File, SymbolBody &Body, |
| bool IsWrite, RelExpr Expr, uint32_t Type, |
| const uint8_t *Data) { |
| bool Preemptible = isPreemptible(Body, Type); |
| if (Body.isGnuIFunc()) { |
| Expr = toPlt(Expr); |
| } else if (!Preemptible) { |
| if (needsPlt(Expr)) |
| Expr = fromPlt(Expr); |
| if (Expr == R_GOT_PC) |
| Expr = Target->adjustRelaxExpr(Type, Data, Expr); |
| } |
| Expr = Target->getThunkExpr(Expr, Type, File, Body); |
| |
| if (IsWrite || isStaticLinkTimeConstant<ELFT>(Expr, Type, Body)) |
| return Expr; |
| |
| // This relocation would require the dynamic linker to write a value to read |
| // only memory. We can hack around it if we are producing an executable and |
| // the refered symbol can be preemepted to refer to the executable. |
| if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) { |
| StringRef Name = getSymbolName(File, Body); |
| error("can't create dynamic relocation " + getRelName(Type) + |
| " against " + (Name.empty() ? "readonly segment" : "symbol " + Name)); |
| return Expr; |
| } |
| if (Body.getVisibility() != STV_DEFAULT) { |
| error("cannot preempt symbol " + Body.getName()); |
| return Expr; |
| } |
| if (Body.isObject()) { |
| // Produce a copy relocation. |
| auto *B = cast<SharedSymbol<ELFT>>(&Body); |
| if (!B->needsCopy()) |
| addCopyRelSymbol(B); |
| return Expr; |
| } |
| if (Body.isFunc()) { |
| // This handles a non PIC program call to function in a shared library. In |
| // an ideal world, we could just report an error saying the relocation can |
| // overflow at runtime. In the real world with glibc, crt1.o has a |
| // R_X86_64_PC32 pointing to libc.so. |
| // |
| // The general idea on how to handle such cases is to create a PLT entry and |
| // use that as the function value. |
| // |
| // For the static linking part, we just return a plt expr and everything |
| // else will use the the PLT entry as the address. |
| // |
| // The remaining problem is making sure pointer equality still works. We |
| // need the help of the dynamic linker for that. We let it know that we have |
| // a direct reference to a so symbol by creating an undefined symbol with a |
| // non zero st_value. Seeing that, the dynamic linker resolves the symbol to |
| // the value of the symbol we created. This is true even for got entries, so |
| // pointer equality is maintained. To avoid an infinite loop, the only entry |
| // that points to the real function is a dedicated got entry used by the |
| // plt. That is identified by special relocation types (R_X86_64_JUMP_SLOT, |
| // R_386_JMP_SLOT, etc). |
| Body.NeedsCopyOrPltAddr = true; |
| return toPlt(Expr); |
| } |
| error("symbol " + Body.getName() + " is missing type"); |
| |
| return Expr; |
| } |
| |
| template <class ELFT, class RelTy> |
| static typename ELFT::uint computeAddend(const elf::ObjectFile<ELFT> &File, |
| const uint8_t *SectionData, |
| const RelTy *End, const RelTy &RI, |
| RelExpr Expr, SymbolBody &Body) { |
| typedef typename ELFT::uint uintX_t; |
| |
| uint32_t Type = RI.getType(Config->Mips64EL); |
| uintX_t Addend = getAddend<ELFT>(RI); |
| const uint8_t *BufLoc = SectionData + RI.r_offset; |
| if (!RelTy::IsRela) |
| Addend += Target->getImplicitAddend(BufLoc, Type); |
| if (Config->EMachine == EM_MIPS) { |
| Addend += findMipsPairedAddend<ELFT>(SectionData, BufLoc, Body, &RI, End); |
| if (Type == R_MIPS_LO16 && Expr == R_PC) |
| // R_MIPS_LO16 expression has R_PC type iif the target is _gp_disp |
| // symbol. In that case we should use the following formula for |
| // calculation "AHL + GP - P + 4". Let's add 4 right here. |
| // For details see p. 4-19 at |
| // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf |
| Addend += 4; |
| if (Expr == R_GOTREL) { |
| Addend -= MipsGPOffset; |
| if (Body.isLocal()) |
| Addend += File.getMipsGp0(); |
| } |
| } |
| if (Config->Pic && Config->EMachine == EM_PPC64 && Type == R_PPC64_TOC) |
| Addend += getPPC64TocBase(); |
| return Addend; |
| } |
| |
| // The reason we have to do this early scan is as follows |
| // * To mmap the output file, we need to know the size |
| // * For that, we need to know how many dynamic relocs we will have. |
| // It might be possible to avoid this by outputting the file with write: |
| // * Write the allocated output sections, computing addresses. |
| // * Apply relocations, recording which ones require a dynamic reloc. |
| // * Write the dynamic relocations. |
| // * Write the rest of the file. |
| // This would have some drawbacks. For example, we would only know if .rela.dyn |
| // is needed after applying relocations. If it is, it will go after rw and rx |
| // sections. Given that it is ro, we will need an extra PT_LOAD. This |
| // complicates things for the dynamic linker and means we would have to reserve |
| // space for the extra PT_LOAD even if we end up not using it. |
| template <class ELFT, class RelTy> |
| static void scanRelocs(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels) { |
| typedef typename ELFT::uint uintX_t; |
| |
| bool IsWrite = C.getSectionHdr()->sh_flags & SHF_WRITE; |
| |
| auto AddDyn = [=](const DynamicReloc<ELFT> &Reloc) { |
| Out<ELFT>::RelaDyn->addReloc(Reloc); |
| }; |
| |
| const elf::ObjectFile<ELFT> &File = *C.getFile(); |
| ArrayRef<uint8_t> SectionData = C.Data; |
| const uint8_t *Buf = SectionData.begin(); |
| |
| ArrayRef<EhSectionPiece> Pieces; |
| if (auto *Eh = dyn_cast<EhInputSection<ELFT>>(&C)) |
| Pieces = Eh->Pieces; |
| |
| ArrayRef<EhSectionPiece>::iterator PieceI = Pieces.begin(); |
| ArrayRef<EhSectionPiece>::iterator PieceE = Pieces.end(); |
| |
| for (auto I = Rels.begin(), E = Rels.end(); I != E; ++I) { |
| const RelTy &RI = *I; |
| SymbolBody &Body = File.getRelocTargetSym(RI); |
| uint32_t Type = RI.getType(Config->Mips64EL); |
| |
| RelExpr Expr = Target->getRelExpr(Type, Body); |
| bool Preemptible = isPreemptible(Body, Type); |
| Expr = adjustExpr(File, Body, IsWrite, Expr, Type, Buf + RI.r_offset); |
| if (HasError) |
| continue; |
| |
| // Skip a relocation that points to a dead piece |
| // in a eh_frame section. |
| while (PieceI != PieceE && |
| (PieceI->InputOff + PieceI->size() <= RI.r_offset)) |
| ++PieceI; |
| |
| // Compute the offset of this section in the output section. We do it here |
| // to try to compute it only once. |
| uintX_t Offset; |
| if (PieceI != PieceE) { |
| assert(PieceI->InputOff <= RI.r_offset && "Relocation not in any piece"); |
| if (PieceI->OutputOff == (size_t)-1) |
| continue; |
| Offset = PieceI->OutputOff + RI.r_offset - PieceI->InputOff; |
| } else { |
| Offset = RI.r_offset; |
| } |
| |
| // This relocation does not require got entry, but it is relative to got and |
| // needs it to be created. Here we request for that. |
| if (Expr == R_GOTONLY_PC || Expr == R_GOTONLY_PC_FROM_END || |
| Expr == R_GOTREL || Expr == R_GOTREL_FROM_END || Expr == R_PPC_TOC) |
| Out<ELFT>::Got->HasGotOffRel = true; |
| |
| uintX_t Addend = computeAddend(File, Buf, E, RI, Expr, Body); |
| |
| if (unsigned Processed = |
| handleTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr)) { |
| I += (Processed - 1); |
| continue; |
| } |
| |
| // Ignore "hint" relocation because it is for optional code optimization. |
| if (Expr == R_HINT) |
| continue; |
| |
| if (needsPlt(Expr) || Expr == R_THUNK_ABS || Expr == R_THUNK_PC || |
| Expr == R_THUNK_PLT_PC || refersToGotEntry(Expr) || |
| !isPreemptible(Body, Type)) { |
| // If the relocation points to something in the file, we can process it. |
| bool Constant = isStaticLinkTimeConstant<ELFT>(Expr, Type, Body); |
| |
| // If the output being produced is position independent, the final value |
| // is still not known. In that case we still need some help from the |
| // dynamic linker. We can however do better than just copying the incoming |
| // relocation. We can process some of it and and just ask the dynamic |
| // linker to add the load address. |
| if (!Constant) |
| AddDyn({Target->RelativeRel, &C, Offset, true, &Body, Addend}); |
| |
| // If the produced value is a constant, we just remember to write it |
| // when outputting this section. We also have to do it if the format |
| // uses Elf_Rel, since in that case the written value is the addend. |
| if (Constant || !RelTy::IsRela) |
| C.Relocations.push_back({Expr, Type, Offset, Addend, &Body}); |
| } else { |
| // We don't know anything about the finaly symbol. Just ask the dynamic |
| // linker to handle the relocation for us. |
| AddDyn({Target->getDynRel(Type), &C, Offset, false, &Body, Addend}); |
| // MIPS ABI turns using of GOT and dynamic relocations inside out. |
| // While regular ABI uses dynamic relocations to fill up GOT entries |
| // MIPS ABI requires dynamic linker to fills up GOT entries using |
| // specially sorted dynamic symbol table. This affects even dynamic |
| // relocations against symbols which do not require GOT entries |
| // creation explicitly, i.e. do not have any GOT-relocations. So if |
| // a preemptible symbol has a dynamic relocation we anyway have |
| // to create a GOT entry for it. |
| // If a non-preemptible symbol has a dynamic relocation against it, |
| // dynamic linker takes it st_value, adds offset and writes down |
| // result of the dynamic relocation. In case of preemptible symbol |
| // dynamic linker performs symbol resolution, writes the symbol value |
| // to the GOT entry and reads the GOT entry when it needs to perform |
| // a dynamic relocation. |
| // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf p.4-19 |
| if (Config->EMachine == EM_MIPS) |
| Out<ELFT>::Got->addMipsEntry(Body, Addend, Expr); |
| continue; |
| } |
| |
| // At this point we are done with the relocated position. Some relocations |
| // also require us to create a got or plt entry. |
| |
| // If a relocation needs PLT, we create a PLT and a GOT slot for the symbol. |
| if (needsPlt(Expr)) { |
| if (Body.isInPlt()) |
| continue; |
| Out<ELFT>::Plt->addEntry(Body); |
| |
| uint32_t Rel; |
| if (Body.isGnuIFunc() && !Preemptible) |
| Rel = Target->IRelativeRel; |
| else |
| Rel = Target->PltRel; |
| |
| Out<ELFT>::GotPlt->addEntry(Body); |
| Out<ELFT>::RelaPlt->addReloc({Rel, Out<ELFT>::GotPlt, |
| Body.getGotPltOffset<ELFT>(), !Preemptible, |
| &Body, 0}); |
| continue; |
| } |
| |
| if (refersToGotEntry(Expr)) { |
| if (Config->EMachine == EM_MIPS) { |
| // MIPS ABI has special rules to process GOT entries and doesn't |
| // require relocation entries for them. A special case is TLS |
| // relocations. In that case dynamic loader applies dynamic |
| // relocations to initialize TLS GOT entries. |
| // 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 |
| Out<ELFT>::Got->addMipsEntry(Body, Addend, Expr); |
| if (Body.isTls() && Body.isPreemptible()) |
| AddDyn({Target->TlsGotRel, Out<ELFT>::Got, Body.getGotOffset<ELFT>(), |
| false, &Body, 0}); |
| continue; |
| } |
| |
| if (Body.isInGot()) |
| continue; |
| |
| Out<ELFT>::Got->addEntry(Body); |
| if (Preemptible || (Config->Pic && !isAbsolute<ELFT>(Body))) { |
| uint32_t DynType; |
| if (Body.isTls()) |
| DynType = Target->TlsGotRel; |
| else if (Preemptible) |
| DynType = Target->GotRel; |
| else |
| DynType = Target->RelativeRel; |
| AddDyn({DynType, Out<ELFT>::Got, Body.getGotOffset<ELFT>(), |
| !Preemptible, &Body, 0}); |
| } |
| continue; |
| } |
| } |
| } |
| |
| template <class ELFT> |
| void scanRelocations(InputSectionBase<ELFT> &S, |
| const typename ELFT::Shdr &RelSec) { |
| ELFFile<ELFT> &EObj = S.getFile()->getObj(); |
| if (RelSec.sh_type == SHT_RELA) |
| scanRelocs(S, EObj.relas(&RelSec)); |
| else |
| scanRelocs(S, EObj.rels(&RelSec)); |
| } |
| |
| template <class ELFT, class RelTy> |
| static void createThunks(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels) { |
| const elf::ObjectFile<ELFT> &File = *C.getFile(); |
| for (const RelTy &Rel : Rels) { |
| SymbolBody &Body = File.getRelocTargetSym(Rel); |
| uint32_t Type = Rel.getType(Config->Mips64EL); |
| RelExpr Expr = Target->getRelExpr(Type, Body); |
| if (!isPreemptible(Body, Type) && needsPlt(Expr)) |
| Expr = fromPlt(Expr); |
| Expr = Target->getThunkExpr(Expr, Type, File, Body); |
| // Some targets might require creation of thunks for relocations. |
| // Now we support only MIPS which requires LA25 thunk to call PIC |
| // code from non-PIC one, and ARM which requires interworking. |
| if (Expr == R_THUNK_ABS || Expr == R_THUNK_PC || Expr == R_THUNK_PLT_PC) { |
| auto *Sec = cast<InputSection<ELFT>>(&C); |
| addThunk<ELFT>(Type, Body, *Sec); |
| } |
| } |
| } |
| |
| template <class ELFT> |
| void createThunks(InputSectionBase<ELFT> &S, |
| const typename ELFT::Shdr &RelSec) { |
| ELFFile<ELFT> &EObj = S.getFile()->getObj(); |
| if (RelSec.sh_type == SHT_RELA) |
| createThunks(S, EObj.relas(&RelSec)); |
| else |
| createThunks(S, EObj.rels(&RelSec)); |
| } |
| |
| template void scanRelocations<ELF32LE>(InputSectionBase<ELF32LE> &, |
| const ELF32LE::Shdr &); |
| template void scanRelocations<ELF32BE>(InputSectionBase<ELF32BE> &, |
| const ELF32BE::Shdr &); |
| template void scanRelocations<ELF64LE>(InputSectionBase<ELF64LE> &, |
| const ELF64LE::Shdr &); |
| template void scanRelocations<ELF64BE>(InputSectionBase<ELF64BE> &, |
| const ELF64BE::Shdr &); |
| |
| template void createThunks<ELF32LE>(InputSectionBase<ELF32LE> &, |
| const ELF32LE::Shdr &); |
| template void createThunks<ELF32BE>(InputSectionBase<ELF32BE> &, |
| const ELF32BE::Shdr &); |
| template void createThunks<ELF64LE>(InputSectionBase<ELF64LE> &, |
| const ELF64LE::Shdr &); |
| template void createThunks<ELF64BE>(InputSectionBase<ELF64BE> &, |
| const ELF64BE::Shdr &); |
| } |
| } |