1 //=== VLASizeChecker.cpp - Undefined dereference checker --------*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This defines VLASizeChecker, a builtin check in ExprEngine that 10 // performs checks for declaration of VLA of undefined or zero size. 11 // In addition, VLASizeChecker is responsible for defining the extent 12 // of the MemRegion that represents a VLA. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "Taint.h" 17 #include "clang/AST/CharUnits.h" 18 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.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 "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h" 24 #include "llvm/ADT/STLExtras.h" 25 #include "llvm/ADT/SmallString.h" 26 #include "llvm/Support/raw_ostream.h" 27 28 using namespace clang; 29 using namespace ento; 30 using namespace taint; 31 32 namespace { 33 class VLASizeChecker 34 : public Checker<check::PreStmt<DeclStmt>, 35 check::PreStmt<UnaryExprOrTypeTraitExpr>> { 36 mutable std::unique_ptr<BugType> BT; 37 enum VLASize_Kind { 38 VLA_Garbage, 39 VLA_Zero, 40 VLA_Tainted, 41 VLA_Negative, 42 VLA_Overflow 43 }; 44 45 /// Check a VLA for validity. 46 /// Every dimension of the array and the total size is checked for validity. 47 /// Returns null or a new state where the size is validated. 48 /// 'ArraySize' will contain SVal that refers to the total size (in char) 49 /// of the array. 50 ProgramStateRef checkVLA(CheckerContext &C, ProgramStateRef State, 51 const VariableArrayType *VLA, SVal &ArraySize) const; 52 /// Check a single VLA index size expression for validity. 53 ProgramStateRef checkVLAIndexSize(CheckerContext &C, ProgramStateRef State, 54 const Expr *SizeE) const; 55 56 void reportBug(VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State, 57 CheckerContext &C, 58 std::unique_ptr<BugReporterVisitor> Visitor = nullptr) const; 59 60 public: 61 void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const; 62 void checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE, 63 CheckerContext &C) const; 64 }; 65 } // end anonymous namespace 66 67 ProgramStateRef VLASizeChecker::checkVLA(CheckerContext &C, 68 ProgramStateRef State, 69 const VariableArrayType *VLA, 70 SVal &ArraySize) const { 71 assert(VLA && "Function should be called with non-null VLA argument."); 72 73 const VariableArrayType *VLALast = nullptr; 74 llvm::SmallVector<const Expr *, 2> VLASizes; 75 76 // Walk over the VLAs for every dimension until a non-VLA is found. 77 // There is a VariableArrayType for every dimension (fixed or variable) until 78 // the most inner array that is variably modified. 79 // Dimension sizes are collected into 'VLASizes'. 'VLALast' is set to the 80 // innermost VLA that was encountered. 81 // In "int vla[x][2][y][3]" this will be the array for index "y" (with type 82 // int[3]). 'VLASizes' contains 'x', '2', and 'y'. 83 while (VLA) { 84 const Expr *SizeE = VLA->getSizeExpr(); 85 State = checkVLAIndexSize(C, State, SizeE); 86 if (!State) 87 return nullptr; 88 VLASizes.push_back(SizeE); 89 VLALast = VLA; 90 VLA = C.getASTContext().getAsVariableArrayType(VLA->getElementType()); 91 }; 92 assert(VLALast && 93 "Array should have at least one variably-modified dimension."); 94 95 ASTContext &Ctx = C.getASTContext(); 96 SValBuilder &SVB = C.getSValBuilder(); 97 CanQualType SizeTy = Ctx.getSizeType(); 98 uint64_t SizeMax = 99 SVB.getBasicValueFactory().getMaxValue(SizeTy).getZExtValue(); 100 101 // Get the element size. 102 CharUnits EleSize = Ctx.getTypeSizeInChars(VLALast->getElementType()); 103 NonLoc ArrSize = 104 SVB.makeIntVal(EleSize.getQuantity(), SizeTy).castAs<NonLoc>(); 105 106 // Try to calculate the known real size of the array in KnownSize. 107 uint64_t KnownSize = 0; 108 if (const llvm::APSInt *KV = SVB.getKnownValue(State, ArrSize)) 109 KnownSize = KV->getZExtValue(); 110 111 for (const Expr *SizeE : VLASizes) { 112 auto SizeD = C.getSVal(SizeE).castAs<DefinedSVal>(); 113 // Convert the array length to size_t. 114 NonLoc IndexLength = 115 SVB.evalCast(SizeD, SizeTy, SizeE->getType()).castAs<NonLoc>(); 116 // Multiply the array length by the element size. 117 SVal Mul = SVB.evalBinOpNN(State, BO_Mul, ArrSize, IndexLength, SizeTy); 118 if (auto MulNonLoc = Mul.getAs<NonLoc>()) 119 ArrSize = *MulNonLoc; 120 else 121 // Extent could not be determined. 122 return State; 123 124 if (const llvm::APSInt *IndexLVal = SVB.getKnownValue(State, IndexLength)) { 125 // Check if the array size will overflow. 126 // Size overflow check does not work with symbolic expressions because a 127 // overflow situation can not be detected easily. 128 uint64_t IndexL = IndexLVal->getZExtValue(); 129 // FIXME: See https://reviews.llvm.org/D80903 for discussion of 130 // some difference in assume and getKnownValue that leads to 131 // unexpected behavior. Just bail on IndexL == 0 at this point. 132 if (IndexL == 0) 133 return nullptr; 134 135 if (KnownSize <= SizeMax / IndexL) { 136 KnownSize *= IndexL; 137 } else { 138 // Array size does not fit into size_t. 139 reportBug(VLA_Overflow, SizeE, State, C); 140 return nullptr; 141 } 142 } else { 143 KnownSize = 0; 144 } 145 } 146 147 ArraySize = ArrSize; 148 149 return State; 150 } 151 152 ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C, 153 ProgramStateRef State, 154 const Expr *SizeE) const { 155 SVal SizeV = C.getSVal(SizeE); 156 157 if (SizeV.isUndef()) { 158 reportBug(VLA_Garbage, SizeE, State, C); 159 return nullptr; 160 } 161 162 // See if the size value is known. It can't be undefined because we would have 163 // warned about that already. 164 if (SizeV.isUnknown()) 165 return nullptr; 166 167 // Check if the size is tainted. 168 if (isTainted(State, SizeV)) { 169 reportBug(VLA_Tainted, SizeE, nullptr, C, 170 std::make_unique<TaintBugVisitor>(SizeV)); 171 return nullptr; 172 } 173 174 // Check if the size is zero. 175 DefinedSVal SizeD = SizeV.castAs<DefinedSVal>(); 176 177 ProgramStateRef StateNotZero, StateZero; 178 std::tie(StateNotZero, StateZero) = State->assume(SizeD); 179 180 if (StateZero && !StateNotZero) { 181 reportBug(VLA_Zero, SizeE, StateZero, C); 182 return nullptr; 183 } 184 185 // From this point on, assume that the size is not zero. 186 State = StateNotZero; 187 188 // Check if the size is negative. 189 SValBuilder &SVB = C.getSValBuilder(); 190 191 QualType SizeTy = SizeE->getType(); 192 DefinedOrUnknownSVal Zero = SVB.makeZeroVal(SizeTy); 193 194 SVal LessThanZeroVal = SVB.evalBinOp(State, BO_LT, SizeD, Zero, SizeTy); 195 if (Optional<DefinedSVal> LessThanZeroDVal = 196 LessThanZeroVal.getAs<DefinedSVal>()) { 197 ConstraintManager &CM = C.getConstraintManager(); 198 ProgramStateRef StatePos, StateNeg; 199 200 std::tie(StateNeg, StatePos) = CM.assumeDual(State, *LessThanZeroDVal); 201 if (StateNeg && !StatePos) { 202 reportBug(VLA_Negative, SizeE, State, C); 203 return nullptr; 204 } 205 State = StatePos; 206 } 207 208 return State; 209 } 210 211 void VLASizeChecker::reportBug( 212 VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State, 213 CheckerContext &C, std::unique_ptr<BugReporterVisitor> Visitor) const { 214 // Generate an error node. 215 ExplodedNode *N = C.generateErrorNode(State); 216 if (!N) 217 return; 218 219 if (!BT) 220 BT.reset(new BuiltinBug( 221 this, "Dangerous variable-length array (VLA) declaration")); 222 223 SmallString<256> buf; 224 llvm::raw_svector_ostream os(buf); 225 os << "Declared variable-length array (VLA) "; 226 switch (Kind) { 227 case VLA_Garbage: 228 os << "uses a garbage value as its size"; 229 break; 230 case VLA_Zero: 231 os << "has zero size"; 232 break; 233 case VLA_Tainted: 234 os << "has tainted size"; 235 break; 236 case VLA_Negative: 237 os << "has negative size"; 238 break; 239 case VLA_Overflow: 240 os << "has too large size"; 241 break; 242 } 243 244 auto report = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), N); 245 report->addVisitor(std::move(Visitor)); 246 report->addRange(SizeE->getSourceRange()); 247 bugreporter::trackExpressionValue(N, SizeE, *report); 248 C.emitReport(std::move(report)); 249 } 250 251 void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const { 252 if (!DS->isSingleDecl()) 253 return; 254 255 ASTContext &Ctx = C.getASTContext(); 256 SValBuilder &SVB = C.getSValBuilder(); 257 ProgramStateRef State = C.getState(); 258 QualType TypeToCheck; 259 260 const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl()); 261 262 if (VD) 263 TypeToCheck = VD->getType().getCanonicalType(); 264 else if (const auto *TND = dyn_cast<TypedefNameDecl>(DS->getSingleDecl())) 265 TypeToCheck = TND->getUnderlyingType().getCanonicalType(); 266 else 267 return; 268 269 const VariableArrayType *VLA = Ctx.getAsVariableArrayType(TypeToCheck); 270 if (!VLA) 271 return; 272 273 // Check the VLA sizes for validity. 274 275 SVal ArraySize; 276 277 State = checkVLA(C, State, VLA, ArraySize); 278 if (!State) 279 return; 280 281 auto ArraySizeNL = ArraySize.getAs<NonLoc>(); 282 if (!ArraySizeNL) { 283 // Array size could not be determined but state may contain new assumptions. 284 C.addTransition(State); 285 return; 286 } 287 288 // VLASizeChecker is responsible for defining the extent of the array. 289 if (VD) { 290 State = 291 setDynamicExtent(State, State->getRegion(VD, C.getLocationContext()), 292 ArraySize.castAs<DefinedOrUnknownSVal>(), SVB); 293 } 294 295 // Remember our assumptions! 296 C.addTransition(State); 297 } 298 299 void VLASizeChecker::checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE, 300 CheckerContext &C) const { 301 // Want to check for sizeof. 302 if (UETTE->getKind() != UETT_SizeOf) 303 return; 304 305 // Ensure a type argument. 306 if (!UETTE->isArgumentType()) 307 return; 308 309 const VariableArrayType *VLA = C.getASTContext().getAsVariableArrayType( 310 UETTE->getTypeOfArgument().getCanonicalType()); 311 // Ensure that the type is a VLA. 312 if (!VLA) 313 return; 314 315 ProgramStateRef State = C.getState(); 316 SVal ArraySize; 317 State = checkVLA(C, State, VLA, ArraySize); 318 if (!State) 319 return; 320 321 C.addTransition(State); 322 } 323 324 void ento::registerVLASizeChecker(CheckerManager &mgr) { 325 mgr.registerChecker<VLASizeChecker>(); 326 } 327 328 bool ento::shouldRegisterVLASizeChecker(const CheckerManager &mgr) { 329 return true; 330 } 331