summaryrefslogtreecommitdiff
path: root/lib/MC/ELFObjectWriter.cpp
diff options
context:
space:
mode:
Diffstat (limited to 'lib/MC/ELFObjectWriter.cpp')
-rw-r--r--lib/MC/ELFObjectWriter.cpp386
1 files changed, 226 insertions, 160 deletions
diff --git a/lib/MC/ELFObjectWriter.cpp b/lib/MC/ELFObjectWriter.cpp
index 4ebfc4de55..d8f03fcc58 100644
--- a/lib/MC/ELFObjectWriter.cpp
+++ b/lib/MC/ELFObjectWriter.cpp
@@ -76,6 +76,27 @@ public:
uint8_t other, uint32_t shndx, bool Reserved);
};
+struct ELFRelocationEntry {
+ uint64_t Offset; // Where is the relocation.
+ bool UseSymbol; // Relocate with a symbol, not the section.
+ union {
+ const MCSymbol *Symbol; // The symbol to relocate with.
+ const MCSectionData *Section; // The section to relocate with.
+ };
+ unsigned Type; // The type of the relocation.
+ uint64_t Addend; // The addend to use.
+
+ ELFRelocationEntry(uint64_t Offset, const MCSymbol *Symbol, unsigned Type,
+ uint64_t Addend)
+ : Offset(Offset), UseSymbol(true), Symbol(Symbol), Type(Type),
+ Addend(Addend) {}
+
+ ELFRelocationEntry(uint64_t Offset, const MCSectionData *Section,
+ unsigned Type, uint64_t Addend)
+ : Offset(Offset), UseSymbol(false), Section(Section), Type(Type),
+ Addend(Addend) {}
+};
+
class ELFObjectWriter : public MCObjectWriter {
FragmentWriter FWriter;
@@ -125,8 +146,8 @@ class ELFObjectWriter : public MCObjectWriter {
SmallPtrSet<const MCSymbol *, 16> WeakrefUsedInReloc;
DenseMap<const MCSymbol *, const MCSymbol *> Renames;
- llvm::DenseMap<const MCSectionData*,
- std::vector<ELFRelocationEntry> > Relocations;
+ llvm::DenseMap<const MCSectionData *, std::vector<ELFRelocationEntry>>
+ Relocations;
DenseMap<const MCSection*, uint64_t> SectionStringTableIndex;
/// @}
@@ -153,27 +174,7 @@ class ELFObjectWriter : public MCObjectWriter {
unsigned ShstrtabIndex;
- const MCSymbol *SymbolToReloc(const MCAssembler &Asm,
- const MCValue &Target,
- const MCFragment &F,
- const MCFixup &Fixup,
- bool IsPCRel) const;
-
// TargetObjectWriter wrappers.
- const MCSymbol *ExplicitRelSym(const MCAssembler &Asm,
- const MCValue &Target,
- const MCFragment &F,
- const MCFixup &Fixup,
- bool IsPCRel) const {
- return TargetObjectWriter->ExplicitRelSym(Asm, Target, F, Fixup, IsPCRel);
- }
- const MCSymbol *undefinedExplicitRelSym(const MCValue &Target,
- const MCFixup &Fixup,
- bool IsPCRel) const {
- return TargetObjectWriter->undefinedExplicitRelSym(Target, Fixup,
- IsPCRel);
- }
-
bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
bool hasRelocationAddend() const {
return TargetObjectWriter->hasRelocationAddend();
@@ -213,9 +214,14 @@ class ELFObjectWriter : public MCObjectWriter {
const MCAsmLayout &Layout,
SectionIndexMapTy &SectionIndexMap);
+ bool shouldRelocateWithSymbol(const MCSymbolRefExpr *RefA,
+ const MCSymbolData *SD, uint64_t C,
+ unsigned Type) const;
+
void RecordRelocation(const MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup,
- MCValue Target, uint64_t &FixedValue) override;
+ MCValue Target, bool &IsPCRel,
+ uint64_t &FixedValue) override;
uint64_t getSymbolIndexInSymbolTable(const MCAssembler &Asm,
const MCSymbol *S);
@@ -716,146 +722,186 @@ void ELFObjectWriter::WriteSymbolTable(MCDataFragment *SymtabF,
}
}
-const MCSymbol *ELFObjectWriter::SymbolToReloc(const MCAssembler &Asm,
- const MCValue &Target,
- const MCFragment &F,
- const MCFixup &Fixup,
- bool IsPCRel) const {
- const MCSymbol &Symbol = Target.getSymA()->getSymbol();
- const MCSymbol &ASymbol = Symbol.AliasedSymbol();
- const MCSymbol *Renamed = Renames.lookup(&Symbol);
- const MCSymbolData &SD = Asm.getSymbolData(Symbol);
-
- if (ASymbol.isUndefined()) {
- if (Renamed)
- return Renamed;
- return undefinedExplicitRelSym(Target, Fixup, IsPCRel);
- }
-
- if (SD.isExternal()) {
- if (Renamed)
- return Renamed;
- return &Symbol;
- }
-
- const MCSectionELF &Section =
- static_cast<const MCSectionELF&>(ASymbol.getSection());
- const SectionKind secKind = Section.getKind();
+// It is always valid to create a relocation with a symbol. It is preferable
+// to use a relocation with a section if that is possible. Using the section
+// allows us to omit some local symbols from the symbol table.
+bool ELFObjectWriter::shouldRelocateWithSymbol(const MCSymbolRefExpr *RefA,
+ const MCSymbolData *SD,
+ uint64_t C,
+ unsigned Type) const {
+ // A PCRel relocation to an absolute value has no symbol (or section). We
+ // represent that with a relocation to a null section.
+ if (!RefA)
+ return false;
- if (secKind.isBSS())
- return ExplicitRelSym(Asm, Target, F, Fixup, IsPCRel);
+ MCSymbolRefExpr::VariantKind Kind = RefA->getKind();
+ switch (Kind) {
+ default:
+ break;
+ // The .odp creation emits a relocation against the symbol ".TOC." which
+ // create a R_PPC64_TOC relocation. However the relocation symbol name
+ // in final object creation should be NULL, since the symbol does not
+ // really exist, it is just the reference to TOC base for the current
+ // object file. Since the symbol is undefined, returning false results
+ // in a relocation with a null section which is the desired result.
+ case MCSymbolRefExpr::VK_PPC_TOCBASE:
+ return false;
- if (secKind.isThreadLocal()) {
- if (Renamed)
- return Renamed;
- return &Symbol;
+ // These VariantKind cause the relocation to refer to something other than
+ // the symbol itself, like a linker generated table. Since the address of
+ // symbol is not relevant, we cannot replace the symbol with the
+ // section and patch the difference in the addend.
+ case MCSymbolRefExpr::VK_GOT:
+ case MCSymbolRefExpr::VK_PLT:
+ case MCSymbolRefExpr::VK_GOTPCREL:
+ case MCSymbolRefExpr::VK_Mips_GOT:
+ case MCSymbolRefExpr::VK_PPC_GOT_LO:
+ case MCSymbolRefExpr::VK_PPC_GOT_HI:
+ case MCSymbolRefExpr::VK_PPC_GOT_HA:
+ return true;
}
- MCSymbolRefExpr::VariantKind Kind = Target.getSymA()->getKind();
- const MCSectionELF &Sec2 =
- static_cast<const MCSectionELF&>(F.getParent()->getSection());
+ // An undefined symbol is not in any section, so the relocation has to point
+ // to the symbol itself.
+ const MCSymbol &Sym = SD->getSymbol();
+ if (Sym.isUndefined())
+ return true;
- if (&Sec2 != &Section &&
- (Kind == MCSymbolRefExpr::VK_PLT ||
- Kind == MCSymbolRefExpr::VK_GOTPCREL ||
- Kind == MCSymbolRefExpr::VK_GOTOFF)) {
- if (Renamed)
- return Renamed;
- return &Symbol;
+ unsigned Binding = MCELF::GetBinding(*SD);
+ switch(Binding) {
+ default:
+ llvm_unreachable("Invalid Binding");
+ case ELF::STB_LOCAL:
+ break;
+ case ELF::STB_WEAK:
+ // If the symbol is weak, it might be overridden by a symbol in another
+ // file. The relocation has to point to the symbol so that the linker
+ // can update it.
+ return true;
+ case ELF::STB_GLOBAL:
+ // Global ELF symbols can be preempted by the dynamic linker. The relocation
+ // has to point to the symbol for a reason analogous to the STB_WEAK case.
+ return true;
}
- if (Section.getFlags() & ELF::SHF_MERGE) {
- if (Target.getConstant() == 0)
- return ExplicitRelSym(Asm, Target, F, Fixup, IsPCRel);
- if (Renamed)
- return Renamed;
- return &Symbol;
+ // If a relocation points to a mergeable section, we have to be careful.
+ // If the offset is zero, a relocation with the section will encode the
+ // same information. With a non-zero offset, the situation is different.
+ // For example, a relocation can point 42 bytes past the end of a string.
+ // If we change such a relocation to use the section, the linker would think
+ // that it pointed to another string and subtracting 42 at runtime will
+ // produce the wrong value.
+ auto &Sec = cast<MCSectionELF>(Sym.getSection());
+ unsigned Flags = Sec.getFlags();
+ if (Flags & ELF::SHF_MERGE) {
+ if (C != 0)
+ return true;
}
- return ExplicitRelSym(Asm, Target, F, Fixup, IsPCRel);
+ // Most TLS relocations use a got, so they need the symbol. Even those that
+ // are just an offset (@tpoff), require a symbol in some linkers (gold,
+ // but not bfd ld).
+ if (Flags & ELF::SHF_TLS)
+ return true;
+ if (TargetObjectWriter->needsRelocateWithSymbol(Type))
+ return true;
+ return false;
}
-
void ELFObjectWriter::RecordRelocation(const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
+ bool &IsPCRel,
uint64_t &FixedValue) {
- int64_t Addend = 0;
- int Index = 0;
- int64_t Value = Target.getConstant();
- const MCSymbol *RelocSymbol = NULL;
-
- bool IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
- if (!Target.isAbsolute()) {
- const MCSymbol &Symbol = Target.getSymA()->getSymbol();
- const MCSymbol &ASymbol = Symbol.AliasedSymbol();
- RelocSymbol = SymbolToReloc(Asm, Target, *Fragment, Fixup, IsPCRel);
-
- if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
- const MCSymbol &SymbolB = RefB->getSymbol();
- MCSymbolData &SDB = Asm.getSymbolData(SymbolB);
- IsPCRel = true;
-
- if (!SDB.getFragment())
- Asm.getContext().FatalError(
- Fixup.getLoc(),
- Twine("symbol '") + SymbolB.getName() +
- "' can not be undefined in a subtraction expression");
-
- // Offset of the symbol in the section
- int64_t a = Layout.getSymbolOffset(&SDB);
-
- // Offset of the relocation in the section
- int64_t b = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
- Value += b - a;
- }
+ const MCSectionData *FixupSection = Fragment->getParent();
+ uint64_t C = Target.getConstant();
+ uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
+
+ if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
+ assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
+ "Should not have constructed this");
+
+ // Let A, B and C being the components of Target and R be the location of
+ // the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
+ // If it is pcrel, we want to compute (A - B + C - R).
+
+ // In general, ELF has no relocations for -B. It can only represent (A + C)
+ // or (A + C - R). If B = R + K and the relocation is not pcrel, we can
+ // replace B to implement it: (A - R - K + C)
+ if (IsPCRel)
+ Asm.getContext().FatalError(
+ Fixup.getLoc(),
+ "No relocation available to represent this relative expression");
+
+ const MCSymbol &SymB = RefB->getSymbol();
+
+ if (SymB.isUndefined())
+ Asm.getContext().FatalError(
+ Fixup.getLoc(),
+ Twine("symbol '") + SymB.getName() +
+ "' can not be undefined in a subtraction expression");
+
+ assert(!SymB.isAbsolute() && "Should have been folded");
+ const MCSection &SecB = SymB.getSection();
+ if (&SecB != &FixupSection->getSection())
+ Asm.getContext().FatalError(
+ Fixup.getLoc(), "Cannot represent a difference across sections");
+
+ const MCSymbolData &SymBD = Asm.getSymbolData(SymB);
+ uint64_t SymBOffset = Layout.getSymbolOffset(&SymBD);
+ uint64_t K = SymBOffset - FixupOffset;
+ IsPCRel = true;
+ C -= K;
+ }
- if (!RelocSymbol) {
- MCSymbolData &SD = Asm.getSymbolData(ASymbol);
- MCFragment *F = SD.getFragment();
+ // We either rejected the fixup or folded B into C at this point.
+ const MCSymbolRefExpr *RefA = Target.getSymA();
+ const MCSymbol *SymA = RefA ? &RefA->getSymbol() : nullptr;
+ const MCSymbolData *SymAD = SymA ? &Asm.getSymbolData(*SymA) : nullptr;
- if (F) {
- Index = F->getParent()->getOrdinal() + 1;
- // Offset of the symbol in the section
- Value += Layout.getSymbolOffset(&SD);
- } else {
- Index = 0;
- }
- } else {
- if (Target.getSymA()->getKind() == MCSymbolRefExpr::VK_WEAKREF)
- WeakrefUsedInReloc.insert(RelocSymbol);
- else
- UsedInReloc.insert(RelocSymbol);
- Index = -1;
- }
- Addend = Value;
- if (hasRelocationAddend())
- Value = 0;
+ unsigned Type = GetRelocType(Target, Fixup, IsPCRel);
+ bool RelocateWithSymbol = shouldRelocateWithSymbol(RefA, SymAD, C, Type);
+ if (!RelocateWithSymbol && SymA && !SymA->isUndefined())
+ C += Layout.getSymbolOffset(SymAD);
+
+ uint64_t Addend = 0;
+ if (hasRelocationAddend()) {
+ Addend = C;
+ C = 0;
}
- FixedValue = Value;
- unsigned Type = GetRelocType(Target, Fixup, IsPCRel);
- MCSymbolRefExpr::VariantKind Modifier = Target.isAbsolute() ?
- MCSymbolRefExpr::VK_None : Target.getSymA()->getKind();
+ FixedValue = C;
+
+ // FIXME: What is this!?!?
+ MCSymbolRefExpr::VariantKind Modifier =
+ RefA ? RefA->getKind() : MCSymbolRefExpr::VK_None;
if (RelocNeedsGOT(Modifier))
NeedsGOT = true;
- uint64_t RelocOffset = Layout.getFragmentOffset(Fragment) +
- Fixup.getOffset();
+ if (!RelocateWithSymbol) {
+ const MCSection *SecA =
+ (SymA && !SymA->isUndefined()) ? &SymA->getSection() : nullptr;
+ const MCSectionData *SecAD = SecA ? &Asm.getSectionData(*SecA) : nullptr;
+ ELFRelocationEntry Rec(FixupOffset, SecAD, Type, Addend);
+ Relocations[FixupSection].push_back(Rec);
+ return;
+ }
- if (!hasRelocationAddend())
- Addend = 0;
+ if (SymA) {
+ if (const MCSymbol *R = Renames.lookup(SymA))
+ SymA = R;
- if (is64Bit())
- assert(isInt<64>(Addend));
- else
- assert(isInt<32>(Addend));
-
- ELFRelocationEntry ERE(RelocOffset, Index, Type, RelocSymbol, Addend, Fixup);
- Relocations[Fragment->getParent()].push_back(ERE);
+ if (RefA->getKind() == MCSymbolRefExpr::VK_WEAKREF)
+ WeakrefUsedInReloc.insert(SymA);
+ else
+ UsedInReloc.insert(SymA);
+ }
+ ELFRelocationEntry Rec(FixupOffset, SymA, Type, Addend);
+ Relocations[FixupSection].push_back(Rec);
+ return;
}
@@ -1154,51 +1200,71 @@ void ELFObjectWriter::WriteSecHdrEntry(uint32_t Name, uint32_t Type,
WriteWord(EntrySize); // sh_entsize
}
+// ELF doesn't require relocations to be in any order. We sort by the r_offset,
+// just to match gnu as for easier comparison. The use type is an arbitrary way
+// of making the sort deterministic.
+static int cmpRel(const ELFRelocationEntry *AP, const ELFRelocationEntry *BP) {
+ const ELFRelocationEntry &A = *AP;
+ const ELFRelocationEntry &B = *BP;
+ if (A.Offset != B.Offset)
+ return B.Offset - A.Offset;
+ if (B.Type != A.Type)
+ return A.Type - B.Type;
+ llvm_unreachable("ELFRelocs might be unstable!");
+}
+
+static void sortRelocs(const MCAssembler &Asm,
+ std::vector<ELFRelocationEntry> &Relocs) {
+ array_pod_sort(Relocs.begin(), Relocs.end(), cmpRel);
+}
+
void ELFObjectWriter::WriteRelocationsFragment(const MCAssembler &Asm,
MCDataFragment *F,
const MCSectionData *SD) {
std::vector<ELFRelocationEntry> &Relocs = Relocations[SD];
- // Sort the relocation entries. Most targets just sort by r_offset, but some
- // (e.g., MIPS) have additional constraints.
- TargetObjectWriter->sortRelocs(Asm, Relocs);
+ sortRelocs(Asm, Relocs);
for (unsigned i = 0, e = Relocs.size(); i != e; ++i) {
- ELFRelocationEntry entry = Relocs[e - i - 1];
+ const ELFRelocationEntry &Entry = Relocs[e - i - 1];
+
+ unsigned Index;
+ if (Entry.UseSymbol) {
+ Index = getSymbolIndexInSymbolTable(Asm, Entry.Symbol);
+ } else {
+ const MCSectionData *Sec = Entry.Section;
+ if (Sec)
+ Index = Sec->getOrdinal() + FileSymbolData.size() +
+ LocalSymbolData.size() + 1;
+ else
+ Index = 0;
+ }
- if (!entry.Index)
- ;
- // FIXME: this is most likely a bug if index overflows.
- else if (entry.Index < 0)
- entry.Index = getSymbolIndexInSymbolTable(Asm, entry.Symbol);
- else
- entry.Index += FileSymbolData.size() + LocalSymbolData.size();
if (is64Bit()) {
- write(*F, entry.r_offset);
+ write(*F, Entry.Offset);
if (TargetObjectWriter->isN64()) {
- write(*F, uint32_t(entry.Index));
+ write(*F, uint32_t(Index));
- write(*F, TargetObjectWriter->getRSsym(entry.Type));
- write(*F, TargetObjectWriter->getRType3(entry.Type));
- write(*F, TargetObjectWriter->getRType2(entry.Type));
- write(*F, TargetObjectWriter->getRType(entry.Type));
- }
- else {
+ write(*F, TargetObjectWriter->getRSsym(Entry.Type));
+ write(*F, TargetObjectWriter->getRType3(Entry.Type));
+ write(*F, TargetObjectWriter->getRType2(Entry.Type));
+ write(*F, TargetObjectWriter->getRType(Entry.Type));
+ } else {
struct ELF::Elf64_Rela ERE64;
- ERE64.setSymbolAndType(entry.Index, entry.Type);
+ ERE64.setSymbolAndType(Index, Entry.Type);
write(*F, ERE64.r_info);
}
if (hasRelocationAddend())
- write(*F, entry.r_addend);
+ write(*F, Entry.Addend);
} else {
- write(*F, uint32_t(entry.r_offset));
+ write(*F, uint32_t(Entry.Offset));
struct ELF::Elf32_Rela ERE32;
- ERE32.setSymbolAndType(entry.Index, entry.Type);
+ ERE32.setSymbolAndType(Index, Entry.Type);
write(*F, ERE32.r_info);
if (hasRelocationAddend())
- write(*F, uint32_t(entry.r_addend));
+ write(*F, uint32_t(Entry.Addend));
}
}
}
generated by cgit v1.2.3 (git 2.25.1) at 2024-06-02 01:50:19 +0000