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Add support for SHT_RELA in LGC ElfLinker (#2856)
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Currently LLVM generates relocations in an ELF SHT_REL section. To
allow for changing the compiler to use SHT_RELA instead, this patch
updates ElfLinker to support both section types.
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@jayfoad
jayfoad authored Nov 29, 2023
1 parent 13bb140 commit bbef90e
Showing 1 changed file with 39 additions and 10 deletions.
49 changes: 39 additions & 10 deletions lgc/elfLinker/ElfLinker.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -94,7 +94,7 @@ class OutputSection {

// Add a relocation to the output elf
void addRelocation(object::ELFRelocationRef relocRef, StringRef id, unsigned int relocSectionOffset,
unsigned int targetSectionOffset);
unsigned int targetSectionOffset, unsigned sectType);

// Get the output file offset of a particular input section in the output section
uint64_t getOutputOffset(unsigned inputIdx) { return m_offset + m_inputSections[inputIdx].offset; }
Expand Down Expand Up @@ -180,6 +180,7 @@ class ElfLinkerImpl final : public ElfLinker {
StringRef getStrings() { return m_strings; }
SmallVectorImpl<ELF::Elf64_Sym> &getSymbols() { return m_symbols; }
SmallVectorImpl<ELF::Elf64_Rel> &getRelocations() { return m_relocations; }
SmallVectorImpl<ELF::Elf64_Rela> &getRelocationsA() { return m_relocationsA; }
void setStringTableIndex(unsigned index) { m_ehdr.e_shstrndx = index; }
StringRef getNotes() { return m_notes; }

Expand Down Expand Up @@ -229,6 +230,7 @@ class ElfLinkerImpl final : public ElfLinker {
SmallVector<OutputSection, 4> m_outputSections; // Output sections
SmallVector<ELF::Elf64_Sym, 8> m_symbols; // Symbol table
SmallVector<ELF::Elf64_Rel, 8> m_relocations; // Relocations
SmallVector<ELF::Elf64_Rela, 8> m_relocationsA; // Relocations with explicit addend
StringMap<unsigned> m_symbolMap; // Map from name to symbol index
std::string m_strings; // Strings for string table
StringMap<unsigned> m_stringMap; // Map from string to string table index
Expand Down Expand Up @@ -447,16 +449,22 @@ bool ElfLinkerImpl::link(raw_pwrite_stream &outStream) {
// The creation of the relocation section is delayed below so we can create it only if there is at least one
// relocation.
bool relSectionCreated = false;
bool relaSectionCreated = false;

// Allocate input sections to output sections.
for (auto &elfInput : m_elfInputs) {
for (const object::SectionRef &section : elfInput.objectFile->sections()) {
unsigned sectType = object::ELFSectionRef(section).getType();
if (sectType == ELF::SHT_REL || sectType == ELF::SHT_RELA) {
if (!relSectionCreated && section.relocation_begin() != section.relocation_end()) {
if (sectType == ELF::SHT_REL) {
if (!relSectionCreated && !section.relocations().empty()) {
m_outputSections.push_back(OutputSection(this, ".rel.text", ELF::SHT_REL));
relSectionCreated = true;
}
} else if (sectType == ELF::SHT_RELA) {
if (!relaSectionCreated && !section.relocations().empty()) {
m_outputSections.push_back(OutputSection(this, ".rela.text", ELF::SHT_RELA));
relaSectionCreated = true;
}
} else if (sectType == ELF::SHT_PROGBITS) {
// Put same-named sections together (excluding symbol table, string table, reloc sections).
StringRef name = cantFail(section.getName());
Expand Down Expand Up @@ -547,15 +555,15 @@ bool ElfLinkerImpl::link(raw_pwrite_stream &outStream) {
if (targetSectionIdx != UINT_MAX) {
(void)(textSectionIdx);
assert(targetSectionIdx == textSectionIdx && "We assume all relocations are applied to the text section");
assert(sectType == ELF::SHT_REL && "We do not output a RELA section yet");
object::SectionRef relocSection = *cantFail(reloc.getSymbol()->getSection());
unsigned relocSectionId = UINT_MAX;
unsigned relocIdxInSection = UINT_MAX;
std::tie(relocSectionId, relocIdxInSection) = findInputSection(elfInput, relocSection);
uint64_t relocSectionOffset = m_outputSections[relocSectionId].getOutputOffset(relocIdxInSection);
uint64_t targetSectionOffset = m_outputSections[targetSectionIdx].getOutputOffset(targetIdxInSection);
StringRef id = sys::path::filename(elfInput.objectFile->getFileName());
m_outputSections[relocSectionId].addRelocation(reloc, id, relocSectionOffset, targetSectionOffset);
m_outputSections[relocSectionId].addRelocation(reloc, id, relocSectionOffset, targetSectionOffset,
sectType);
}
}
}
Expand Down Expand Up @@ -965,8 +973,7 @@ void OutputSection::addSymbol(const object::ELFSymbolRef &elfSymRef, unsigned in

// Add a relocation to the output elf
void OutputSection::addRelocation(object::ELFRelocationRef relocRef, StringRef id, unsigned int relocSectionOffset,
unsigned int targetSectionOffset) {
ELF::Elf64_Rel newReloc = {};
unsigned int targetSectionOffset, unsigned sectType) {
object::ELFSymbolRef relocSymRef(*relocRef.getSymbol());
std::string rodataSymName = cantFail(relocSymRef.getName()).str();
rodataSymName += ".";
Expand All @@ -984,9 +991,19 @@ void OutputSection::addRelocation(object::ELFRelocationRef relocRef, StringRef i
rodataSymIdx = m_linker->getSymbols().size();
m_linker->getSymbols().push_back(newSym);
}
newReloc.setSymbolAndType(rodataSymIdx, relocRef.getType());
newReloc.r_offset = targetSectionOffset + relocRef.getOffset();
m_linker->getRelocations().push_back(newReloc);
if (sectType == ELF::SHT_REL) {
ELF::Elf64_Rel newReloc = {};
newReloc.setSymbolAndType(rodataSymIdx, relocRef.getType());
newReloc.r_offset = targetSectionOffset + relocRef.getOffset();
m_linker->getRelocations().push_back(newReloc);
} else {
assert(sectType == ELF::SHT_RELA);
ELF::Elf64_Rela newReloc = {};
newReloc.setSymbolAndType(rodataSymIdx, relocRef.getType());
newReloc.r_offset = targetSectionOffset + relocRef.getOffset();
newReloc.r_addend = cantFail(relocRef.getAddend());
m_linker->getRelocationsA().push_back(newReloc);
}
}

// =====================================================================================================================
Expand Down Expand Up @@ -1037,6 +1054,18 @@ void OutputSection::write(raw_pwrite_stream &outStream, ELF::Elf64_Shdr *shdr) {
return;
}

if (m_type == ELF::SHT_RELA) {
ArrayRef<ELF::Elf64_Rela> relocations = m_linker->getRelocationsA();
shdr->sh_type = m_type;
shdr->sh_size = relocations.size() * sizeof(ELF::Elf64_Rela);
shdr->sh_entsize = sizeof(ELF::Elf64_Rela);
shdr->sh_link = 2; // Section index of symbol table
shdr->sh_info = 3; // Section index of the .text section
outStream << StringRef(reinterpret_cast<const char *>(relocations.data()),
relocations.size() * sizeof(ELF::Elf64_Rela));
return;
}

if (m_inputSections.empty())
return;

Expand Down

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