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llvm-project/lld/ELF/Relocations.cpp

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//===- 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`. MIPS does not support
// any relaxations for TLS relocations so by factoring out MIPS handling into
// the separate function we can simplify the code and does not pollute
// `handleTlsRelocation` by MIPS `ifs` statements.
template <class ELFT>
static unsigned
handleMipsTlsRelocation(uint32_t Type, SymbolBody &Body,
InputSectionBase<ELFT> &C, typename ELFT::uint Offset,
typename ELFT::uint Addend, RelExpr Expr) {
if (Expr == R_MIPS_TLSLD) {
if (Out<ELFT>::Got->addTlsIndex() && Config->Pic)
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;
}
if (Target->isTlsGlobalDynamicRel(Type)) {
if (Out<ELFT>::Got->addDynTlsEntry(Body) && Body.isPreemptible()) {
typedef typename ELFT::uint uintX_t;
uintX_t Off = Out<ELFT>::Got->getGlobalDynOffset(Body);
Out<ELFT>::RelaDyn->addReloc(
{Target->TlsModuleIndexRel, Out<ELFT>::Got, Off, false, &Body, 0});
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)
return handleMipsTlsRelocation<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);
}
warning("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 &);
}
}
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