ELF_x86_64.cpp

Go to the documentation of this file.

//===---- ELF_x86_64.cpp -JIT linker implementation for ELF/x86-64 ----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// ELF/x86-64 jit-link implementation.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/ExecutionEngine/JITLink/ELF_x86_64.h"
#include "llvm/ExecutionEngine/JITLink/DWARFRecordSectionSplitter.h"
#include "llvm/ExecutionEngine/JITLink/JITLink.h"
#include "llvm/ExecutionEngine/JITLink/TableManager.h"
#include "llvm/ExecutionEngine/JITLink/x86_64.h"
#include "llvm/Object/ELFObjectFile.h"
#include "llvm/Support/Endian.h"
 
#include "DefineExternalSectionStartAndEndSymbols.h"
#include "EHFrameSupportImpl.h"
#include "ELFLinkGraphBuilder.h"
#include "JITLinkGeneric.h"
 
#define DEBUG_TYPE "jitlink"
 
using namespace llvm;
using namespace llvm::jitlink;
 
namespace {
 
constexpr StringRef ELFGOTSymbolName = "_GLOBAL_OFFSET_TABLE_";
constexpr StringRef ELFTLSInfoSectionName = "$__TLSINFO";
 
class TLSInfoTableManager_ELF_x86_64
    : public TableManager<TLSInfoTableManager_ELF_x86_64> {
public:
  static const uint8_t TLSInfoEntryContent[16];
 
  static StringRef getSectionName() { return ELFTLSInfoSectionName; }
 
  bool visitEdge(LinkGraph &G, Block *B, Edge &E) {
    if (E.getKind() == x86_64::RequestTLSDescInGOTAndTransformToDelta32) {
      LLVM_DEBUG({
        dbgs() << "  Fixing " << G.getEdgeKindName(E.getKind()) << " edge at "
               << formatv("{0:x}", B->getFixupAddress(E)) << " ("
               << formatv("{0:x}", B->getAddress()) << " + "
               << formatv("{0:x}", E.getOffset()) << ")\n";
      });
      E.setKind(x86_64::Delta32);
      E.setTarget(getEntryForTarget(G, E.getTarget()));
      return true;
    }
    return false;
  }
 
  Symbol &createEntry(LinkGraph &G, Symbol &Target) {
    // the TLS Info entry's key value will be written by the fixTLVSectionByName
    // pass, so create mutable content.
    auto &TLSInfoEntry = G.createMutableContentBlock(
        getTLSInfoSection(G), G.allocateContent(getTLSInfoEntryContent()),
        orc::ExecutorAddr(), 8, 0);
    TLSInfoEntry.addEdge(x86_64::Pointer64, 8, Target, 0);
    return G.addAnonymousSymbol(TLSInfoEntry, 0, 16, false, false);
  }
 
private:
  Section &getTLSInfoSection(LinkGraph &G) {
    if (!TLSInfoTable)
      TLSInfoTable =
          &G.createSection(ELFTLSInfoSectionName, orc::MemProt::Read);
    return *TLSInfoTable;
  }
 
  ArrayRef<char> getTLSInfoEntryContent() const {
    return {reinterpret_cast<const char *>(TLSInfoEntryContent),
            sizeof(TLSInfoEntryContent)};
  }
 
  Section *TLSInfoTable = nullptr;
};
 
const uint8_t TLSInfoTableManager_ELF_x86_64::TLSInfoEntryContent[16] = {
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, /*pthread key */
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00  /*data address*/
};
 
Error buildTables_ELF_x86_64(LinkGraph &G) {
  LLVM_DEBUG(dbgs() << "Visiting edges in graph:\n");
 
  x86_64::GOTTableManager GOT;
  x86_64::PLTTableManager PLT(GOT);
  TLSInfoTableManager_ELF_x86_64 TLSInfo;
  visitExistingEdges(G, GOT, PLT, TLSInfo);
  return Error::success();
}
} // namespace
 
namespace llvm {
namespace jitlink {
 
// This should become a template as the ELFFile is so a lot of this could become
// generic
class ELFLinkGraphBuilder_x86_64 : public ELFLinkGraphBuilder<object::ELF64LE> {
private:
  using ELFT = object::ELF64LE;
 
  enum ELFX86RelocationKind : Edge::Kind {
    Branch32 = Edge::FirstRelocation,
    Pointer32Signed,
    Pointer64,
    PCRel32,
    PCRel32GOTLoad,
    PCRel32GOTLoadRelaxable,
    PCRel32REXGOTLoadRelaxable,
    PCRel32TLV,
    PCRel64GOT,
    GOTOFF64,
    GOT64,
    Delta64,
  };
 
  static Expected<ELFX86RelocationKind> getRelocationKind(const uint32_t Type) {
    switch (Type) {
    case ELF::R_X86_64_32S:
      return ELFX86RelocationKind::Pointer32Signed;
    case ELF::R_X86_64_PC32:
      return ELFX86RelocationKind::PCRel32;
    case ELF::R_X86_64_PC64:
    case ELF::R_X86_64_GOTPC64:
      return ELFX86RelocationKind::Delta64;
    case ELF::R_X86_64_64:
      return ELFX86RelocationKind::Pointer64;
    case ELF::R_X86_64_GOTPCREL:
      return ELFX86RelocationKind::PCRel32GOTLoad;
    case ELF::R_X86_64_GOTPCRELX:
      return ELFX86RelocationKind::PCRel32GOTLoadRelaxable;
    case ELF::R_X86_64_REX_GOTPCRELX:
      return ELFX86RelocationKind::PCRel32REXGOTLoadRelaxable;
    case ELF::R_X86_64_GOTPCREL64:
      return ELFX86RelocationKind::PCRel64GOT;
    case ELF::R_X86_64_GOT64:
      return ELFX86RelocationKind::GOT64;
    case ELF::R_X86_64_GOTOFF64:
      return ELFX86RelocationKind::GOTOFF64;
    case ELF::R_X86_64_PLT32:
      return ELFX86RelocationKind::Branch32;
    case ELF::R_X86_64_TLSGD:
      return ELFX86RelocationKind::PCRel32TLV;
    }
    return make_error<JITLinkError>(
        "Unsupported x86-64 relocation type " + formatv("{0:d}: ", Type) +
        object::getELFRelocationTypeName(ELF::EM_X86_64, Type));
  }
 
  Error addRelocations() override {
    LLVM_DEBUG(dbgs() << "Processing relocations:\n");
 
    using Base = ELFLinkGraphBuilder<ELFT>;
    using Self = ELFLinkGraphBuilder_x86_64;
    for (const auto &RelSect : Base::Sections) {
      // Validate the section to read relocation entries from.
      if (RelSect.sh_type == ELF::SHT_REL)
        return make_error<StringError>(
            "No SHT_REL in valid x64 ELF object files",
            inconvertibleErrorCode());
 
      if (Error Err = Base::forEachRelaRelocation(RelSect, this,
                                                  &Self::addSingleRelocation))
        return Err;
    }
 
    return Error::success();
  }
 
  Error addSingleRelocation(const typename ELFT::Rela &Rel,
                            const typename ELFT::Shdr &FixupSection,
                            Block &BlockToFix) {
    using Base = ELFLinkGraphBuilder<ELFT>;
 
    uint32_t SymbolIndex = Rel.getSymbol(false);
    auto ObjSymbol = Base::Obj.getRelocationSymbol(Rel, Base::SymTabSec);
    if (!ObjSymbol)
      return ObjSymbol.takeError();
 
    Symbol *GraphSymbol = Base::getGraphSymbol(SymbolIndex);
    if (!GraphSymbol)
      return make_error<StringError>(
          formatv("Could not find symbol at given index, did you add it to "
                  "JITSymbolTable? index: {0}, shndx: {1} Size of table: {2}",
                  SymbolIndex, (*ObjSymbol)->st_shndx,
                  Base::GraphSymbols.size()),
          inconvertibleErrorCode());
 
    // Validate the relocation kind.
    auto ELFRelocKind = getRelocationKind(Rel.getType(false));
    if (!ELFRelocKind)
      return ELFRelocKind.takeError();
 
    int64_t Addend = Rel.r_addend;
    Edge::Kind Kind = Edge::Invalid;
    switch (*ELFRelocKind) {
    case PCRel32:
      Kind = x86_64::Delta32;
      break;
    case Delta64:
      Kind = x86_64::Delta64;
      break;
    case Pointer32Signed:
      Kind = x86_64::Pointer32Signed;
      break;
    case Pointer64:
      Kind = x86_64::Pointer64;
      break;
    case PCRel32GOTLoad: {
      Kind = x86_64::RequestGOTAndTransformToDelta32;
      break;
    }
    case PCRel32REXGOTLoadRelaxable: {
      Kind = x86_64::RequestGOTAndTransformToPCRel32GOTLoadREXRelaxable;
      Addend = 0;
      break;
    }
    case PCRel32TLV: {
      Kind = x86_64::RequestTLSDescInGOTAndTransformToDelta32;
      break;
    }
    case PCRel32GOTLoadRelaxable: {
      Kind = x86_64::RequestGOTAndTransformToPCRel32GOTLoadRelaxable;
      Addend = 0;
      break;
    }
    case PCRel64GOT: {
      Kind = x86_64::RequestGOTAndTransformToDelta64;
      break;
    }
    case GOT64: {
      Kind = x86_64::RequestGOTAndTransformToDelta64FromGOT;
      break;
    }
    case GOTOFF64: {
      Kind = x86_64::Delta64FromGOT;
      break;
    }
    case Branch32: {
      Kind = x86_64::BranchPCRel32;
      // BranchPCRel32 implicitly handles the '-4' PC adjustment, so we have to
      // adjust the addend by '+4' to compensate.
      Addend += 4;
      break;
    }
    }
 
    auto FixupAddress = orc::ExecutorAddr(FixupSection.sh_addr) + Rel.r_offset;
    Edge::OffsetT Offset = FixupAddress - BlockToFix.getAddress();
    Edge GE(Kind, Offset, *GraphSymbol, Addend);
    LLVM_DEBUG({
      dbgs() << "    ";
      printEdge(dbgs(), BlockToFix, GE, x86_64::getEdgeKindName(Kind));
      dbgs() << "\n";
    });
 
    BlockToFix.addEdge(std::move(GE));
    return Error::success();
  }
 
public:
  ELFLinkGraphBuilder_x86_64(StringRef FileName,
                             const object::ELFFile<object::ELF64LE> &Obj)
      : ELFLinkGraphBuilder(Obj, Triple("x86_64-unknown-linux"), FileName,
                            x86_64::getEdgeKindName) {}
};
 
class ELFJITLinker_x86_64 : public JITLinker<ELFJITLinker_x86_64> {
  friend class JITLinker<ELFJITLinker_x86_64>;
 
public:
  ELFJITLinker_x86_64(std::unique_ptr<JITLinkContext> Ctx,
                      std::unique_ptr<LinkGraph> G,
                      PassConfiguration PassConfig)
      : JITLinker(std::move(Ctx), std::move(G), std::move(PassConfig)) {
    getPassConfig().PostAllocationPasses.push_back(
        [this](LinkGraph &G) { return getOrCreateGOTSymbol(G); });
  }
 
private:
  Symbol *GOTSymbol = nullptr;
 
  Error getOrCreateGOTSymbol(LinkGraph &G) {
    auto DefineExternalGOTSymbolIfPresent =
        createDefineExternalSectionStartAndEndSymbolsPass(
            [&](LinkGraph &LG, Symbol &Sym) -> SectionRangeSymbolDesc {
              if (Sym.getName() == ELFGOTSymbolName)
                if (auto *GOTSection = G.findSectionByName(
                        x86_64::GOTTableManager::getSectionName())) {
                  GOTSymbol = &Sym;
                  return {*GOTSection, true};
                }
              return {};
            });
 
    // Try to attach _GLOBAL_OFFSET_TABLE_ to the GOT if it's defined as an
    // external.
    if (auto Err = DefineExternalGOTSymbolIfPresent(G))
      return Err;
 
    // If we succeeded then we're done.
    if (GOTSymbol)
      return Error::success();
 
    // Otherwise look for a GOT section: If it already has a start symbol we'll
    // record it, otherwise we'll create our own.
    // If there's a GOT section but we didn't find an external GOT symbol...
    if (auto *GOTSection =
            G.findSectionByName(x86_64::GOTTableManager::getSectionName())) {
 
      // Check for an existing defined symbol.
      for (auto *Sym : GOTSection->symbols())
        if (Sym->getName() == ELFGOTSymbolName) {
          GOTSymbol = Sym;
          return Error::success();
        }
 
      // If there's no defined symbol then create one.
      SectionRange SR(*GOTSection);
      if (SR.empty())
        GOTSymbol =
            &G.addAbsoluteSymbol(ELFGOTSymbolName, orc::ExecutorAddr(), 0,
                                 Linkage::Strong, Scope::Local, true);
      else
        GOTSymbol =
            &G.addDefinedSymbol(*SR.getFirstBlock(), 0, ELFGOTSymbolName, 0,
                                Linkage::Strong, Scope::Local, false, true);
    }
 
    return Error::success();
  }
 
  Error applyFixup(LinkGraph &G, Block &B, const Edge &E) const {
    return x86_64::applyFixup(G, B, E, GOTSymbol);
  }
};
 
Expected<std::unique_ptr<LinkGraph>>
createLinkGraphFromELFObject_x86_64(MemoryBufferRef ObjectBuffer) {
  LLVM_DEBUG({
    dbgs() << "Building jitlink graph for new input "
           << ObjectBuffer.getBufferIdentifier() << "...\n";
  });
 
  auto ELFObj = object::ObjectFile::createELFObjectFile(ObjectBuffer);
  if (!ELFObj)
    return ELFObj.takeError();
 
  auto &ELFObjFile = cast<object::ELFObjectFile<object::ELF64LE>>(**ELFObj);
  return ELFLinkGraphBuilder_x86_64((*ELFObj)->getFileName(),
                                    ELFObjFile.getELFFile())
      .buildGraph();
}
 
static SectionRangeSymbolDesc
identifyELFSectionStartAndEndSymbols(LinkGraph &G, Symbol &Sym) {
  constexpr StringRef StartSymbolPrefix = "__start";
  constexpr StringRef EndSymbolPrefix = "__end";
 
  auto SymName = Sym.getName();
  if (SymName.startswith(StartSymbolPrefix)) {
    if (auto *Sec =
            G.findSectionByName(SymName.drop_front(StartSymbolPrefix.size())))
      return {*Sec, true};
  } else if (SymName.startswith(EndSymbolPrefix)) {
    if (auto *Sec =
            G.findSectionByName(SymName.drop_front(EndSymbolPrefix.size())))
      return {*Sec, false};
  }
  return {};
}
 
void link_ELF_x86_64(std::unique_ptr<LinkGraph> G,
                     std::unique_ptr<JITLinkContext> Ctx) {
  PassConfiguration Config;
 
  if (Ctx->shouldAddDefaultTargetPasses(G->getTargetTriple())) {
 
    Config.PrePrunePasses.push_back(DWARFRecordSectionSplitter(".eh_frame"));
    Config.PrePrunePasses.push_back(EHFrameEdgeFixer(
        ".eh_frame", x86_64::PointerSize, x86_64::Pointer32, x86_64::Pointer64,
        x86_64::Delta32, x86_64::Delta64, x86_64::NegDelta32));
    Config.PrePrunePasses.push_back(EHFrameNullTerminator(".eh_frame"));
 
    // Construct a JITLinker and run the link function.
    // Add a mark-live pass.
    if (auto MarkLive = Ctx->getMarkLivePass(G->getTargetTriple()))
      Config.PrePrunePasses.push_back(std::move(MarkLive));
    else
      Config.PrePrunePasses.push_back(markAllSymbolsLive);
 
    // Add an in-place GOT/Stubs/TLSInfoEntry build pass.
    Config.PostPrunePasses.push_back(buildTables_ELF_x86_64);
 
    // Resolve any external section start / end symbols.
    Config.PostAllocationPasses.push_back(
        createDefineExternalSectionStartAndEndSymbolsPass(
            identifyELFSectionStartAndEndSymbols));
 
    // Add GOT/Stubs optimizer pass.
    Config.PreFixupPasses.push_back(x86_64::optimizeGOTAndStubAccesses);
  }
 
  if (auto Err = Ctx->modifyPassConfig(*G, Config))
    return Ctx->notifyFailed(std::move(Err));
 
  ELFJITLinker_x86_64::link(std::move(Ctx), std::move(G), std::move(Config));
}
} // end namespace jitlink
} // end namespace llvm