text: //== ArrayBoundCheckerV2.cpp ------------------------------------*- C++ -*--==//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file defines ArrayBoundCheckerV2, which is a path-sensitive check
  // which looks for an out-of-bound array element access.
  //
  //===----------------------------------------------------------------------===//
  
  #include "ClangSACheckers.h"
  #include "clang/AST/CharUnits.h"
  #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
  #include "clang/StaticAnalyzer/Core/Checker.h"
  #include "clang/StaticAnalyzer/Core/CheckerManager.h"
  #include "clang/StaticAnalyzer/Core/PathSensitive/APSIntType.h"
  #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
  #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
  #include "llvm/ADT/SmallString.h"
  #include "llvm/Support/raw_ostream.h"
  
  using namespace clang;
  using namespace ento;
  
  namespace {
  class ArrayBoundCheckerV2 :
      public Checker<check::Location> {
    mutable std::unique_ptr<BuiltinBug> BT;
  
    enum OOB_Kind { OOB_Precedes, OOB_Excedes, OOB_Tainted };
  
    void reportOOB(CheckerContext &C, ProgramStateRef errorState, OOB_Kind kind,
                   std::unique_ptr<BugReporterVisitor> Visitor = nullptr) const;
  
  public:
    void checkLocation(SVal l, bool isLoad, const Stmt*S,
                       CheckerContext &C) const;
  };
  
  // FIXME: Eventually replace RegionRawOffset with this class.
  class RegionRawOffsetV2 {
  private:
    const SubRegion *baseRegion;
    SVal byteOffset;
  
    RegionRawOffsetV2()
      : baseRegion(nullptr), byteOffset(UnknownVal()) {}
  
  public:
    RegionRawOffsetV2(const SubRegion* base, SVal offset)
      : baseRegion(base), byteOffset(offset) {}
  
    NonLoc getByteOffset() const { return byteOffset.castAs<NonLoc>(); }
    const SubRegion *getRegion() const { return baseRegion; }
  
    static RegionRawOffsetV2 computeOffset(ProgramStateRef state,
                                           SValBuilder &svalBuilder,
                                           SVal location);
  
    void dump() const;
    void dumpToStream(raw_ostream &os) const;
  };
  }
  
  static SVal computeExtentBegin(SValBuilder &svalBuilder,
                                 const MemRegion *region) {
    const MemSpaceRegion *SR = region->getMemorySpace();
    if (SR->getKind() == MemRegion::UnknownSpaceRegionKind)
      return UnknownVal();
    else
      return svalBuilder.makeZeroArrayIndex();
  }
  
  // TODO: once the constraint manager is smart enough to handle non simplified
  // symbolic expressions remove this function. Note that this can not be used in
  // the constraint manager as is, since this does not handle overflows. It is
  // safe to assume, however, that memory offsets will not overflow.
  static std::pair<NonLoc, nonloc::ConcreteInt>
  getSimplifiedOffsets(NonLoc offset, nonloc::ConcreteInt extent,
                       SValBuilder &svalBuilder) {
    Optional<nonloc::SymbolVal> SymVal = offset.getAs<nonloc::SymbolVal>();
    if (SymVal && SymVal->isExpression()) {
      if (const SymIntExpr *SIE = dyn_cast<SymIntExpr>(SymVal->getSymbol())) {
        llvm::APSInt constant =
            APSIntType(extent.getValue()).convert(SIE->getRHS());
        switch (SIE->getOpcode()) {
        case BO_Mul:
          // The constant should never be 0 here, since it the result of scaling
          // based on the size of a type which is never 0.
          if ((extent.getValue() % constant) != 0)
            return std::pair<NonLoc, nonloc::ConcreteInt>(offset, extent);
          else
            return getSimplifiedOffsets(
                nonloc::SymbolVal(SIE->getLHS()),
                svalBuilder.makeIntVal(extent.getValue() / constant),
                svalBuilder);
        case BO_Add:
          return getSimplifiedOffsets(
              nonloc::SymbolVal(SIE->getLHS()),
              svalBuilder.makeIntVal(extent.getValue() - constant), svalBuilder);
        default:
          break;
        }
      }
    }
  
    return std::pair<NonLoc, nonloc::ConcreteInt>(offset, extent);
  }
  
  void ArrayBoundCheckerV2::checkLocation(SVal location, bool isLoad,
                                          const Stmt* LoadS,
                                          CheckerContext &checkerContext) const {
  
    // NOTE: Instead of using ProgramState::assumeInBound(), we are prototyping
    // some new logic here that reasons directly about memory region extents.
    // Once that logic is more mature, we can bring it back to assumeInBound()
    // for all clients to use.
    //
    // The algorithm we are using here for bounds checking is to see if the
    // memory access is within the extent of the base region.  Since we
    // have some flexibility in defining the base region, we can achieve
    // various levels of conservatism in our buffer overflow checking.
    ProgramStateRef state = checkerContext.getState();
  
    SValBuilder &svalBuilder = checkerContext.getSValBuilder();
    const RegionRawOffsetV2 &rawOffset =
      RegionRawOffsetV2::computeOffset(state, svalBuilder, location);
  
    if (!rawOffset.getRegion())
      return;
  
    NonLoc rawOffsetVal = rawOffset.getByteOffset();
  
    // CHECK LOWER BOUND: Is byteOffset < extent begin?
    //  If so, we are doing a load/store
    //  before the first valid offset in the memory region.
  
    SVal extentBegin = computeExtentBegin(svalBuilder, rawOffset.getRegion());
  
    if (Optional<NonLoc> NV = extentBegin.getAs<NonLoc>()) {
      if (NV->getAs<nonloc::ConcreteInt>()) {
        std::pair<NonLoc, nonloc::ConcreteInt> simplifiedOffsets =
            getSimplifiedOffsets(rawOffset.getByteOffset(),
                                 NV->castAs<nonloc::ConcreteInt>(),
                                 svalBuilder);
        rawOffsetVal = simplifiedOffsets.first;
        *NV = simplifiedOffsets.second;
      }
  
      SVal lowerBound = svalBuilder.evalBinOpNN(state, BO_LT, rawOffsetVal, *NV,
                                                svalBuilder.getConditionType());
  
      Optional<NonLoc> lowerBoundToCheck = lowerBound.getAs<NonLoc>();
      if (!lowerBoundToCheck)
        return;
  
      ProgramStateRef state_precedesLowerBound, state_withinLowerBound;
      std::tie(state_precedesLowerBound, state_withinLowerBound) =
        state->assume(*lowerBoundToCheck);
  
      // Are we constrained enough to definitely precede the lower bound?
      if (state_precedesLowerBound && !state_withinLowerBound) {
        reportOOB(checkerContext, state_precedesLowerBound, OOB_Precedes);
        return;
      }
  
      // Otherwise, assume the constraint of the lower bound.
      assert(state_withinLowerBound);
      state = state_withinLowerBound;
    }
  
    do {
      // CHECK UPPER BOUND: Is byteOffset >= extent(baseRegion)?  If so,
      // we are doing a load/store after the last valid offset.
      DefinedOrUnknownSVal extentVal =
        rawOffset.getRegion()->getExtent(svalBuilder);
      if (!extentVal.getAs<NonLoc>())
        break;
  
      if (extentVal.getAs<nonloc::ConcreteInt>()) {
        std::pair<NonLoc, nonloc::ConcreteInt> simplifiedOffsets =
            getSimplifiedOffsets(rawOffset.getByteOffset(),
                                 extentVal.castAs<nonloc::ConcreteInt>(),
                                 svalBuilder);
        rawOffsetVal = simplifiedOffsets.first;
        extentVal = simplifiedOffsets.second;
      }
  
      SVal upperbound = svalBuilder.evalBinOpNN(state, BO_GE, rawOffsetVal,
                                                extentVal.castAs<NonLoc>(),
                                                svalBuilder.getConditionType());
  
      Optional<NonLoc> upperboundToCheck = upperbound.getAs<NonLoc>();
      if (!upperboundToCheck)
        break;
  
      ProgramStateRef state_exceedsUpperBound, state_withinUpperBound;
      std::tie(state_exceedsUpperBound, state_withinUpperBound) =
        state->assume(*upperboundToCheck);
  
      // If we are under constrained and the index variables are tainted, report.
      if (state_exceedsUpperBound && state_withinUpperBound) {
        SVal ByteOffset = rawOffset.getByteOffset();
        if (state->isTainted(ByteOffset)) {
          reportOOB(checkerContext, state_exceedsUpperBound, OOB_Tainted,
                    llvm::make_unique<TaintBugVisitor>(ByteOffset));
          return;
        }
      } else if (state_exceedsUpperBound) {
        // If we are constrained enough to definitely exceed the upper bound,
        // report.
        assert(!state_withinUpperBound);
        reportOOB(checkerContext, state_exceedsUpperBound, OOB_Excedes);
        return;
      }
  
      assert(state_withinUpperBound);
      state = state_withinUpperBound;
    }
    while (false);
  
    checkerContext.addTransition(state);
  }
  
  void ArrayBoundCheckerV2::reportOOB(
      CheckerContext &checkerContext, ProgramStateRef errorState, OOB_Kind kind,
      std::unique_ptr<BugReporterVisitor> Visitor) const {
  
    ExplodedNode *errorNode = checkerContext.generateErrorNode(errorState);
    if (!errorNode)
      return;
  
    if (!BT)
      BT.reset(new BuiltinBug(this, "Out-of-bound access"));
  
    // FIXME: This diagnostics are preliminary.  We should get far better
    // diagnostics for explaining buffer overruns.
  
    SmallString<256> buf;
    llvm::raw_svector_ostream os(buf);
    os << "Out of bound memory access ";
    switch (kind) {
    case OOB_Precedes:
      os << "(accessed memory precedes memory block)";
      break;
    case OOB_Excedes:
      os << "(access exceeds upper limit of memory block)";
      break;
    case OOB_Tainted:
      os << "(index is tainted)";
      break;
    }
  
    auto BR = llvm::make_unique<BugReport>(*BT, os.str(), errorNode);
    BR->addVisitor(std::move(Visitor));
    checkerContext.emitReport(std::move(BR));
  }
  
  #ifndef NDEBUG
  LLVM_DUMP_METHOD void RegionRawOffsetV2::dump() const {
    dumpToStream(llvm::errs());
  }
  
  void RegionRawOffsetV2::dumpToStream(raw_ostream &os) const {
    os << "raw_offset_v2{" << getRegion() << ',' << getByteOffset() << '}';
  }
  #endif
  
  // Lazily computes a value to be used by 'computeOffset'.  If 'val'
  // is unknown or undefined, we lazily substitute '0'.  Otherwise,
  // return 'val'.
  static inline SVal getValue(SVal val, SValBuilder &svalBuilder) {
    return val.getAs<UndefinedVal>() ? svalBuilder.makeArrayIndex(0) : val;
  }
  
  // Scale a base value by a scaling factor, and return the scaled
  // value as an SVal.  Used by 'computeOffset'.
  static inline SVal scaleValue(ProgramStateRef state,
                                NonLoc baseVal, CharUnits scaling,
                                SValBuilder &sb) {
    return sb.evalBinOpNN(state, BO_Mul, baseVal,
                          sb.makeArrayIndex(scaling.getQuantity()),
                          sb.getArrayIndexType());
  }
  
  // Add an SVal to another, treating unknown and undefined values as
  // summing to UnknownVal.  Used by 'computeOffset'.
  static SVal addValue(ProgramStateRef state, SVal x, SVal y,
                       SValBuilder &svalBuilder) {
    // We treat UnknownVals and UndefinedVals the same here because we
    // only care about computing offsets.
    if (x.isUnknownOrUndef() || y.isUnknownOrUndef())
      return UnknownVal();
  
    return svalBuilder.evalBinOpNN(state, BO_Add, x.castAs<NonLoc>(),
                                   y.castAs<NonLoc>(),
                                   svalBuilder.getArrayIndexType());
  }
  
  /// Compute a raw byte offset from a base region.  Used for array bounds
  /// checking.
  RegionRawOffsetV2 RegionRawOffsetV2::computeOffset(ProgramStateRef state,
                                                     SValBuilder &svalBuilder,
                                                     SVal location)
  {
    const MemRegion *region = location.getAsRegion();
    SVal offset = UndefinedVal();
  
    while (region) {
      switch (region->getKind()) {
        default: {
          if (const SubRegion *subReg = dyn_cast<SubRegion>(region)) {
            offset = getValue(offset, svalBuilder);
            if (!offset.isUnknownOrUndef())
              return RegionRawOffsetV2(subReg, offset);
          }
          return RegionRawOffsetV2();
        }
        case MemRegion::ElementRegionKind: {
          const ElementRegion *elemReg = cast<ElementRegion>(region);
          SVal index = elemReg->getIndex();
          if (!index.getAs<NonLoc>())
            return RegionRawOffsetV2();
          QualType elemType = elemReg->getElementType();
          // If the element is an incomplete type, go no further.
          ASTContext &astContext = svalBuilder.getContext();
          if (elemType->isIncompleteType())
            return RegionRawOffsetV2();
  
          // Update the offset.
          offset = addValue(state,
                            getValue(offset, svalBuilder),
                            scaleValue(state,
                            index.castAs<NonLoc>(),
                            astContext.getTypeSizeInChars(elemType),
                            svalBuilder),
                            svalBuilder);
  
          if (offset.isUnknownOrUndef())
            return RegionRawOffsetV2();
  
          region = elemReg->getSuperRegion();
          continue;
        }
      }
    }
    return RegionRawOffsetV2();
  }
  
  void ento::registerArrayBoundCheckerV2(CheckerManager &mgr) {
    mgr.registerChecker<ArrayBoundCheckerV2>();
  }
  