1 //=== VLASizeChecker.cpp - Undefined dereference checker --------*- C++ -*-===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This defines VLASizeChecker, a builtin check in ExprEngine that 11 // performs checks for declaration of VLA of undefined or zero size. 12 // In addition, VLASizeChecker is responsible for defining the extent 13 // of the MemRegion that represents a VLA. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #include "ClangSACheckers.h" 18 #include "clang/AST/CharUnits.h" 19 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h" 20 #include "clang/StaticAnalyzer/Core/Checker.h" 21 #include "clang/StaticAnalyzer/Core/CheckerManager.h" 22 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h" 23 #include "llvm/ADT/STLExtras.h" 24 #include "llvm/ADT/SmallString.h" 25 #include "llvm/Support/raw_ostream.h" 26 27 using namespace clang; 28 using namespace ento; 29 30 namespace { 31 class VLASizeChecker : public Checker< check::PreStmt<DeclStmt> > { 32 mutable std::unique_ptr<BugType> BT; 33 enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Tainted, VLA_Negative }; 34 35 void reportBug(VLASize_Kind Kind, 36 const Expr *SizeE, 37 ProgramStateRef State, 38 CheckerContext &C) const; 39 public: 40 void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const; 41 }; 42 } // end anonymous namespace 43 44 void VLASizeChecker::reportBug(VLASize_Kind Kind, 45 const Expr *SizeE, 46 ProgramStateRef State, 47 CheckerContext &C) const { 48 // Generate an error node. 49 ExplodedNode *N = C.generateErrorNode(State); 50 if (!N) 51 return; 52 53 if (!BT) 54 BT.reset(new BuiltinBug( 55 this, "Dangerous variable-length array (VLA) declaration")); 56 57 SmallString<256> buf; 58 llvm::raw_svector_ostream os(buf); 59 os << "Declared variable-length array (VLA) "; 60 switch (Kind) { 61 case VLA_Garbage: 62 os << "uses a garbage value as its size"; 63 break; 64 case VLA_Zero: 65 os << "has zero size"; 66 break; 67 case VLA_Tainted: 68 os << "has tainted size"; 69 break; 70 case VLA_Negative: 71 os << "has negative size"; 72 break; 73 } 74 75 auto report = llvm::make_unique<BugReport>(*BT, os.str(), N); 76 report->addRange(SizeE->getSourceRange()); 77 bugreporter::trackNullOrUndefValue(N, SizeE, *report); 78 C.emitReport(std::move(report)); 79 return; 80 } 81 82 void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const { 83 if (!DS->isSingleDecl()) 84 return; 85 86 const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl()); 87 if (!VD) 88 return; 89 90 ASTContext &Ctx = C.getASTContext(); 91 const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType()); 92 if (!VLA) 93 return; 94 95 // FIXME: Handle multi-dimensional VLAs. 96 const Expr *SE = VLA->getSizeExpr(); 97 ProgramStateRef state = C.getState(); 98 SVal sizeV = state->getSVal(SE, C.getLocationContext()); 99 100 if (sizeV.isUndef()) { 101 reportBug(VLA_Garbage, SE, state, C); 102 return; 103 } 104 105 // See if the size value is known. It can't be undefined because we would have 106 // warned about that already. 107 if (sizeV.isUnknown()) 108 return; 109 110 // Check if the size is tainted. 111 if (state->isTainted(sizeV)) { 112 reportBug(VLA_Tainted, SE, nullptr, C); 113 return; 114 } 115 116 // Check if the size is zero. 117 DefinedSVal sizeD = sizeV.castAs<DefinedSVal>(); 118 119 ProgramStateRef stateNotZero, stateZero; 120 std::tie(stateNotZero, stateZero) = state->assume(sizeD); 121 122 if (stateZero && !stateNotZero) { 123 reportBug(VLA_Zero, SE, stateZero, C); 124 return; 125 } 126 127 // From this point on, assume that the size is not zero. 128 state = stateNotZero; 129 130 // VLASizeChecker is responsible for defining the extent of the array being 131 // declared. We do this by multiplying the array length by the element size, 132 // then matching that with the array region's extent symbol. 133 134 // Check if the size is negative. 135 SValBuilder &svalBuilder = C.getSValBuilder(); 136 137 QualType Ty = SE->getType(); 138 DefinedOrUnknownSVal Zero = svalBuilder.makeZeroVal(Ty); 139 140 SVal LessThanZeroVal = svalBuilder.evalBinOp(state, BO_LT, sizeD, Zero, Ty); 141 if (Optional<DefinedSVal> LessThanZeroDVal = 142 LessThanZeroVal.getAs<DefinedSVal>()) { 143 ConstraintManager &CM = C.getConstraintManager(); 144 ProgramStateRef StatePos, StateNeg; 145 146 std::tie(StateNeg, StatePos) = CM.assumeDual(state, *LessThanZeroDVal); 147 if (StateNeg && !StatePos) { 148 reportBug(VLA_Negative, SE, state, C); 149 return; 150 } 151 state = StatePos; 152 } 153 154 // Convert the array length to size_t. 155 QualType SizeTy = Ctx.getSizeType(); 156 NonLoc ArrayLength = 157 svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>(); 158 159 // Get the element size. 160 CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType()); 161 SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy); 162 163 // Multiply the array length by the element size. 164 SVal ArraySizeVal = svalBuilder.evalBinOpNN( 165 state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy); 166 167 // Finally, assume that the array's extent matches the given size. 168 const LocationContext *LC = C.getLocationContext(); 169 DefinedOrUnknownSVal Extent = 170 state->getRegion(VD, LC)->getExtent(svalBuilder); 171 DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>(); 172 DefinedOrUnknownSVal sizeIsKnown = 173 svalBuilder.evalEQ(state, Extent, ArraySize); 174 state = state->assume(sizeIsKnown, true); 175 176 // Assume should not fail at this point. 177 assert(state); 178 179 // Remember our assumptions! 180 C.addTransition(state); 181 } 182 183 void ento::registerVLASizeChecker(CheckerManager &mgr) { 184 mgr.registerChecker<VLASizeChecker>(); 185 } 186
