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
reportBug(VLASize_Kind Kind,const Expr * SizeE,ProgramStateRef State,CheckerContext & C) const44 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
checkPreStmt(const DeclStmt * DS,CheckerContext & C) const82 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
registerVLASizeChecker(CheckerManager & mgr)183 void ento::registerVLASizeChecker(CheckerManager &mgr) {
184 mgr.registerChecker<VLASizeChecker>();
185 }
186