text: //=== VLASizeChecker.cpp - Undefined dereference checker --------*- C++ -*-===//
  //
  //                     The LLVM37 Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This defines VLASizeChecker, a builtin check in ExprEngine that 
  // performs checks for declaration of VLA of undefined or zero size.
  // In addition, VLASizeChecker is responsible for defining the extent
  // of the MemRegion that represents a VLA.
  //
  //===----------------------------------------------------------------------===//
  
  #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/CheckerContext.h"
  #include "llvm37/ADT/STLExtras.h"
  #include "llvm37/ADT/SmallString.h"
  #include "llvm37/Support/raw_ostream.h"
  
  using namespace clang;
  using namespace ento;
  
  namespace {
  class VLASizeChecker : public Checker< check::PreStmt<DeclStmt> > {
    mutable std::unique_ptr<BugType> BT;
    enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Tainted, VLA_Negative };
  
    void reportBug(VLASize_Kind Kind,
                   const Expr *SizeE,
                   ProgramStateRef State,
                   CheckerContext &C) const;
  public:
    void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
  };
  } // end anonymous namespace
  
  void VLASizeChecker::reportBug(VLASize_Kind Kind,
                                 const Expr *SizeE,
                                 ProgramStateRef State,
                                 CheckerContext &C) const {
    // Generate an error node.
    ExplodedNode *N = C.generateSink(State);
    if (!N)
      return;
  
    if (!BT)
      BT.reset(new BuiltinBug(
          this, "Dangerous variable-length array (VLA) declaration"));
  
    SmallString<256> buf;
    llvm37::raw_svector_ostream os(buf);
    os << "Declared variable-length array (VLA) ";
    switch (Kind) {
    case VLA_Garbage:
      os << "uses a garbage value as its size";
      break;
    case VLA_Zero:
      os << "has zero size";
      break;
    case VLA_Tainted:
      os << "has tainted size";
      break;
    case VLA_Negative:
      os << "has negative size";
      break;
    }
  
    auto report = llvm37::make_unique<BugReport>(*BT, os.str(), N);
    report->addRange(SizeE->getSourceRange());
    bugreporter::trackNullOrUndefValue(N, SizeE, *report);
    C.emitReport(std::move(report));
    return;
  }
  
  void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
    if (!DS->isSingleDecl())
      return;
    
    const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
    if (!VD)
      return;
  
    ASTContext &Ctx = C.getASTContext();
    const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType());
    if (!VLA)
      return;
  
    // FIXME: Handle multi-dimensional VLAs.
    const Expr *SE = VLA->getSizeExpr();
    ProgramStateRef state = C.getState();
    SVal sizeV = state->getSVal(SE, C.getLocationContext());
  
    if (sizeV.isUndef()) {
      reportBug(VLA_Garbage, SE, state, C);
      return;
    }
  
    // See if the size value is known. It can't be undefined because we would have
    // warned about that already.
    if (sizeV.isUnknown())
      return;
    
    // Check if the size is tainted.
    if (state->isTainted(sizeV)) {
      reportBug(VLA_Tainted, SE, nullptr, C);
      return;
    }
  
    // Check if the size is zero.
    DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
  
    ProgramStateRef stateNotZero, stateZero;
    std::tie(stateNotZero, stateZero) = state->assume(sizeD);
  
    if (stateZero && !stateNotZero) {
      reportBug(VLA_Zero, SE, stateZero, C);
      return;
    }
   
    // From this point on, assume that the size is not zero.
    state = stateNotZero;
  
    // VLASizeChecker is responsible for defining the extent of the array being
    // declared. We do this by multiplying the array length by the element size,
    // then matching that with the array region's extent symbol.
  
    // Check if the size is negative.
    SValBuilder &svalBuilder = C.getSValBuilder();
  
    QualType Ty = SE->getType();
    DefinedOrUnknownSVal Zero = svalBuilder.makeZeroVal(Ty);
  
    SVal LessThanZeroVal = svalBuilder.evalBinOp(state, BO_LT, sizeD, Zero, Ty);
    if (Optional<DefinedSVal> LessThanZeroDVal =
          LessThanZeroVal.getAs<DefinedSVal>()) {
      ConstraintManager &CM = C.getConstraintManager();
      ProgramStateRef StatePos, StateNeg;
  
      std::tie(StateNeg, StatePos) = CM.assumeDual(state, *LessThanZeroDVal);
      if (StateNeg && !StatePos) {
        reportBug(VLA_Negative, SE, state, C);
        return;
      }
      state = StatePos;
    }
  
    // Convert the array length to size_t.
    QualType SizeTy = Ctx.getSizeType();
    NonLoc ArrayLength =
        svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
  
    // Get the element size.
    CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
    SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
  
    // Multiply the array length by the element size.
    SVal ArraySizeVal = svalBuilder.evalBinOpNN(
        state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
  
    // Finally, assume that the array's extent matches the given size.
    const LocationContext *LC = C.getLocationContext();
    DefinedOrUnknownSVal Extent =
      state->getRegion(VD, LC)->getExtent(svalBuilder);
    DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
    DefinedOrUnknownSVal sizeIsKnown =
      svalBuilder.evalEQ(state, Extent, ArraySize);
    state = state->assume(sizeIsKnown, true);
  
    // Assume should not fail at this point.
    assert(state);
  
    // Remember our assumptions!
    C.addTransition(state);
  }
  
  void ento::registerVLASizeChecker(CheckerManager &mgr) {
    mgr.registerChecker<VLASizeChecker>();
  }
  