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Completely rewrite ELFObjectWriter::RecordRelocation.

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I started trying to fix a small issue, but this code has seen a small fix too
many.

The old code was fairly convoluted. Some of the issues it had:

* It failed to check if a symbol difference was in the some section when
  converting a relocation to pcrel.
* It failed to check if the relocation was already pcrel.
* The pcrel value computation was wrong in some cases (relocation-pc.s)
* It was missing quiet a few cases where it should not convert symbol
  relocations to section relocations, leaving the backends to patch it up.
* It would not propagate the fact that it had changed a relocation to pcrel,
  requiring a quiet nasty work around in ARM.
* It was missing comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205076 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Rafael Espindola
2014-03-29 06:26:49 +00:00
parent 44b2b9dc1a
commit 224dbf4aec
34 changed files with 353 additions and 697 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/MC/ELFObjectWriter.cpp
+222 -156
View File
@@ -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);
// 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 (ASymbol.isUndefined()) {
if (Renamed)
return Renamed;
return undefinedExplicitRelSym(Target, 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;
// 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;
}
if (SD.isExternal()) {
if (Renamed)
return Renamed;
return &Symbol;
// 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;
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;
}
const MCSectionELF &Section =
static_cast<const MCSectionELF&>(ASymbol.getSection());
const SectionKind secKind = Section.getKind();
if (secKind.isBSS())
return ExplicitRelSym(Asm, Target, F, Fixup, IsPCRel);
if (secKind.isThreadLocal()) {
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;
}
MCSymbolRefExpr::VariantKind Kind = Target.getSymA()->getKind();
const MCSectionELF &Sec2 =
static_cast<const MCSectionELF&>(F.getParent()->getSection());
if (&Sec2 != &Section &&
(Kind == MCSymbolRefExpr::VK_PLT ||
Kind == MCSymbolRefExpr::VK_GOTPCREL ||
Kind == MCSymbolRefExpr::VK_GOTOFF)) {
if (Renamed)
return Renamed;
return &Symbol;
}
if (Section.getFlags() & ELF::SHF_MERGE) {
if (Target.getConstant() == 0)
return ExplicitRelSym(Asm, Target, F, Fixup, IsPCRel);
if (Renamed)
return Renamed;
return &Symbol;
}
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;
const MCSectionData *FixupSection = Fragment->getParent();
uint64_t C = Target.getConstant();
uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
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()) {
assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
"Should not have constructed this");
if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
const MCSymbol &SymbolB = RefB->getSymbol();
MCSymbolData &SDB = Asm.getSymbolData(SymbolB);
IsPCRel = true;
// 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).
if (!SDB.getFragment())
Asm.getContext().FatalError(
Fixup.getLoc(),
Twine("symbol '") + SymbolB.getName() +
"' can not be undefined in a subtraction expression");
// 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");
// Offset of the symbol in the section
int64_t a = Layout.getSymbolOffset(&SDB);
const MCSymbol &SymB = RefB->getSymbol();
// Offset of the relocation in the section
int64_t b = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
Value += b - a;
}
if (SymB.isUndefined())
Asm.getContext().FatalError(
Fixup.getLoc(),
Twine("symbol '") + SymB.getName() +
"' can not be undefined in a subtraction expression");
if (!RelocSymbol) {
MCSymbolData &SD = Asm.getSymbolData(ASymbol);
MCFragment *F = SD.getFragment();
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");
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;
const MCSymbolData &SymBD = Asm.getSymbolData(SymB);
uint64_t SymBOffset = Layout.getSymbolOffset(&SymBD);
uint64_t K = SymBOffset - FixupOffset;
IsPCRel = true;
C -= K;
}
FixedValue = Value;
// 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;
unsigned Type = GetRelocType(Target, Fixup, IsPCRel);
MCSymbolRefExpr::VariantKind Modifier = Target.isAbsolute() ?
MCSymbolRefExpr::VK_None : Target.getSymA()->getKind();
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 = 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));
}
}
}