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[BOLT] Add support for .plt.sec and refactor PLT-reading code
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Summary:
A binary can contain multiple PLT sections with different name and
attributes (such as an entry size). Extend the support to .plt.sec and
refactor the code to make future extensions simpler.
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@maksfb @rafaelauler
maksfb authored and rafaelauler committed Jul 6, 2021
1 parent 8679f32 commit a54a30b
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Showing 4 changed files with 914 additions and 81 deletions.
116 changes: 47 additions & 69 deletions bolt/src/RewriteInstance.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1025,10 +1025,11 @@ void RewriteInstance::discoverFileObjects() {

FileSymRefs[Address] = Symbol;

// Symbols that will be registered by disassemblePLT()
if ((PLTSection && PLTSection->getAddress() == Address) ||
(PLTGOTSection && PLTGOTSection->getAddress() == Address)) {
continue;
// Skip section symbols that will be registered by disassemblePLT().
if ((cantFail(Symbol.getType()) == SymbolRef::ST_Debug)) {
ErrorOr<BinarySection &> BSection = BC->getSectionForAddress(Address);
if (BSection && getPLTSectionInfo(BSection->getName()))
continue;
}

/// It is possible we are seeing a globalized local. LLVM might treat it as
Expand Down Expand Up @@ -1332,97 +1333,76 @@ void RewriteInstance::discoverFileObjects() {
}

void RewriteInstance::disassemblePLT() {
// Used to analyze both the .plt section (most common) and the less common
// .plt.got created by the BFD linker.
auto analyzeOnePLTSection = [&](BinarySection &Section,
const BinarySection &RelocsSection,
uint64_t RelocType, uint64_t EntrySize) {
auto analyzeOnePLTSection = [&](BinarySection &Section, uint64_t EntrySize) {
const uint64_t PLTAddress = Section.getAddress();
StringRef PLTContents = Section.getContents();
ArrayRef<uint8_t> PLTData(
reinterpret_cast<const uint8_t *>(PLTContents.data()),
Section.getSize());
const unsigned PtrSize = BC->AsmInfo->getCodePointerSize();

// Runtime linker will put a value of an external symbol at the location
// referenced by the relocation. Map the address to the name of the symbol.
std::unordered_map<uint64_t, StringRef> RelAddrToNameMap;
for (const RelocationRef &Rel :
RelocsSection.getSectionRef().relocations()) {
if (Rel.getType() != RelocType)
continue;
const auto SymbolIter = Rel.getSymbol();
assert(SymbolIter != InputFile->symbol_end() &&
"non-null symbol expected");
RelAddrToNameMap[Rel.getOffset()] = cantFail((*SymbolIter).getName());
}

for (uint64_t Offset = 0; Offset < Section.getSize(); Offset += EntrySize) {
for (uint64_t EntryOffset = 0; EntryOffset + EntrySize <= Section.getSize();
EntryOffset += EntrySize) {
uint64_t InstrOffset = EntryOffset;
uint64_t InstrSize;
MCInst Instruction;
const uint64_t InstrAddr = PLTAddress + Offset;
if (!BC->DisAsm->getInstruction(Instruction, InstrSize,
PLTData.slice(Offset), InstrAddr,
nulls())) {
errs() << "BOLT-ERROR: unable to disassemble instruction in PLT "
"section "
<< Section.getName() << " at offset 0x"
<< Twine::utohexstr(Offset) << '\n';
exit(1);
while (InstrOffset < EntryOffset + EntrySize) {
uint64_t InstrAddr = PLTAddress + InstrOffset;
if (!BC->DisAsm->getInstruction(Instruction, InstrSize,
PLTData.slice(InstrOffset), InstrAddr,
nulls())) {
errs() << "BOLT-ERROR: unable to disassemble instruction in PLT "
"section "
<< Section.getName() << " at offset 0x"
<< Twine::utohexstr(InstrOffset) << '\n';
exit(1);
}

if (BC->MIB->isIndirectBranch(Instruction))
break;

InstrOffset += InstrSize;
}

if (!BC->MIB->isIndirectBranch(Instruction))
if (InstrOffset + InstrSize > EntryOffset + EntrySize)
continue;

uint64_t TargetAddress;
if (!BC->MIB->evaluateMemOperandTarget(Instruction, TargetAddress,
InstrAddr, InstrSize)) {
PLTAddress + InstrOffset,
InstrSize)) {
errs() << "BOLT-ERROR: error evaluating PLT instruction at offset 0x"
<< Twine::utohexstr(InstrAddr) << '\n';
<< Twine::utohexstr(PLTAddress + InstrOffset) << '\n';
exit(1);
}

auto NI = RelAddrToNameMap.find(TargetAddress);
if (NI == RelAddrToNameMap.end())
const Relocation *Rel = BC->getDynamicRelocationAt(TargetAddress);
if (!Rel || !Rel->Symbol)
continue;

StringRef SymbolName = NI->second;
BinaryFunction *BF =
BC->createBinaryFunction(SymbolName.str() + "@PLT", Section,
InstrAddr, 0, EntrySize, PLTAlignment);
BinaryFunction *BF = BC->createBinaryFunction(
Rel->Symbol->getName().str() + "@PLT", Section,
PLTAddress + EntryOffset, 0, EntrySize, Section.getAlignment());
MCSymbol *TargetSymbol =
BC->registerNameAtAddress(SymbolName.str() + "@GOT",
TargetAddress, PtrSize, PLTAlignment);
BC->registerNameAtAddress(Rel->Symbol->getName().str() + "@GOT",
TargetAddress, PtrSize, PtrSize);
BF->setPLTSymbol(TargetSymbol);
}
};

if (PLTSection) {
// Pseudo function for the start of PLT. The table could have a matching
// FDE that we want to match to pseudo function.
BinaryFunction *BF = BC->createBinaryFunction(
"__BOLT_PLT_PSEUDO", *PLTSection, PLTSection->getAddress(), 0, PLTSize,
PLTAlignment);
BF->setPseudo(true);
if (RelaPLTSection) {
analyzeOnePLTSection(*PLTSection, *RelaPLTSection,
ELF::R_X86_64_JUMP_SLOT, PLTSize);
}
}
for (BinarySection &Section : BC->allocatableSections()) {
const PLTSectionInfo *PLTSI = getPLTSectionInfo(Section.getName());
if (!PLTSI)
continue;

if (PLTGOTSection) {
if (RelaDynSection) {
analyzeOnePLTSection(*PLTGOTSection, *RelaDynSection,
ELF::R_X86_64_GLOB_DAT, /*Size=*/8);
}
// If we did not register any function at PLTGOT start, we may be missing
// relocs. Add a function at the start to mark this section.
if (BC->getBinaryFunctions().find(PLTGOTSection->getAddress()) ==
analyzeOnePLTSection(Section, PLTSI->EntrySize);
// If we did not register any function at the start of the section,
// then it must be a general PLT entry. Add a function at the location.
if (BC->getBinaryFunctions().find(Section.getAddress()) ==
BC->getBinaryFunctions().end()) {
BinaryFunction *BF =
BC->createBinaryFunction("__BOLT_PLTGOT_PSEUDO", *PLTGOTSection,
PLTGOTSection->getAddress(), 0,
/*SymbolSize*/ 8, PLTAlignment);
BinaryFunction *BF = BC->createBinaryFunction(
"__BOLT_PSEUDO_" + Section.getName().str(), Section,
Section.getAddress(), 0, PLTSI->EntrySize, Section.getAlignment());
BF->setPseudo(true);
}
}
Expand Down Expand Up @@ -1606,9 +1586,7 @@ void RewriteInstance::readSpecialSections() {
HasTextRelocations = (bool)BC->getUniqueSectionByName(".rela.text");
LSDASection = BC->getUniqueSectionByName(".gcc_except_table");
EHFrameSection = BC->getUniqueSectionByName(".eh_frame");
PLTSection = BC->getUniqueSectionByName(".plt");
GOTPLTSection = BC->getUniqueSectionByName(".got.plt");
PLTGOTSection = BC->getUniqueSectionByName(".plt.got");
RelaPLTSection = BC->getUniqueSectionByName(".rela.plt");
RelaDynSection = BC->getUniqueSectionByName(".rela.dyn");
BuildIDSection = BC->getUniqueSectionByName(".note.gnu.build-id");
Expand Down
54 changes: 42 additions & 12 deletions bolt/src/RewriteInstance.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -380,10 +380,6 @@ class RewriteInstance {
/// Alignment value used for .eh_frame_hdr.
static constexpr uint64_t EHFrameHdrAlign = 4;

// TODO: these are platform (x86, aarch64) specific.
static constexpr uint64_t PLTSize = 16;
static constexpr uint16_t PLTAlignment = 16;

/// String to be added before the original section name.
///
/// When BOLT creates a new section with the same name as the one in the
Expand Down Expand Up @@ -448,24 +444,58 @@ class RewriteInstance {
/// Number of local symbols in newly written symbol table.
uint64_t NumLocalSymbols{0};

/// Information on special Procedure Linkage Table sections. There are
/// multiple variants generated by different linkers.
struct PLTSectionInfo {
const char *Name;
uint64_t EntrySize;
};

/// Different types of X86-64 PLT sections.
const PLTSectionInfo X86_64_PLTSections[4] = {
{ ".plt", 16 },
{ ".plt.got", 8 },
{ ".plt.sec", 16 },
{ nullptr, 0 }
};

/// AArch64 PLT sections.
const PLTSectionInfo AArch64_PLTSections[2] = {
{ ".plt", 16 },
{ nullptr, 0 }
};

/// Return PLT information for a section with \p SectionName or nullptr
/// if the section is not PLT.
const PLTSectionInfo *getPLTSectionInfo(StringRef SectionName) {
const PLTSectionInfo *PLTSI = nullptr;
switch (BC->TheTriple->getArch()) {
default:
break;
case Triple::x86_64:
PLTSI = X86_64_PLTSections;
break;
case Triple::aarch64:
PLTSI = AArch64_PLTSections;
break;
}
for (; PLTSI && PLTSI->Name; ++PLTSI) {
if (SectionName == PLTSI->Name)
return PLTSI;
}
return nullptr;
}

/// Exception handling and stack unwinding information in this binary.
ErrorOr<BinarySection &> LSDASection{std::errc::bad_address};
ErrorOr<BinarySection &> EHFrameSection{std::errc::bad_address};

/// .plt section.
ErrorOr<BinarySection &> PLTSection{std::errc::bad_address};

/// .got.plt sections.
///
/// Contains jump slots (addresses) indirectly referenced by
/// instructions in .plt section.
ErrorOr<BinarySection &> GOTPLTSection{std::errc::bad_address};

/// .plt.got section (#clowntown).
///
/// A section sometimes generated by BFD linker.
ErrorOr<BinarySection &> PLTGOTSection{std::errc::bad_address};

/// .rela.plt section.
///
/// Contains relocations against .got.plt.
Expand Down
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