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 };
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.generateSink(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 }
71
72 BugReport *report = new BugReport(*BT, os.str(), N);
73 report->addRange(SizeE->getSourceRange());
74 bugreporter::trackNullOrUndefValue(N, SizeE, *report);
75 C.emitReport(report);
76 return;
77 }
78
checkPreStmt(const DeclStmt * DS,CheckerContext & C) const79 void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
80 if (!DS->isSingleDecl())
81 return;
82
83 const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
84 if (!VD)
85 return;
86
87 ASTContext &Ctx = C.getASTContext();
88 const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType());
89 if (!VLA)
90 return;
91
92 // FIXME: Handle multi-dimensional VLAs.
93 const Expr *SE = VLA->getSizeExpr();
94 ProgramStateRef state = C.getState();
95 SVal sizeV = state->getSVal(SE, C.getLocationContext());
96
97 if (sizeV.isUndef()) {
98 reportBug(VLA_Garbage, SE, state, C);
99 return;
100 }
101
102 // See if the size value is known. It can't be undefined because we would have
103 // warned about that already.
104 if (sizeV.isUnknown())
105 return;
106
107 // Check if the size is tainted.
108 if (state->isTainted(sizeV)) {
109 reportBug(VLA_Tainted, SE, nullptr, C);
110 return;
111 }
112
113 // Check if the size is zero.
114 DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
115
116 ProgramStateRef stateNotZero, stateZero;
117 std::tie(stateNotZero, stateZero) = state->assume(sizeD);
118
119 if (stateZero && !stateNotZero) {
120 reportBug(VLA_Zero, SE, stateZero, C);
121 return;
122 }
123
124 // From this point on, assume that the size is not zero.
125 state = stateNotZero;
126
127 // VLASizeChecker is responsible for defining the extent of the array being
128 // declared. We do this by multiplying the array length by the element size,
129 // then matching that with the array region's extent symbol.
130
131 // Convert the array length to size_t.
132 SValBuilder &svalBuilder = C.getSValBuilder();
133 QualType SizeTy = Ctx.getSizeType();
134 NonLoc ArrayLength =
135 svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
136
137 // Get the element size.
138 CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
139 SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
140
141 // Multiply the array length by the element size.
142 SVal ArraySizeVal = svalBuilder.evalBinOpNN(
143 state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
144
145 // Finally, assume that the array's extent matches the given size.
146 const LocationContext *LC = C.getLocationContext();
147 DefinedOrUnknownSVal Extent =
148 state->getRegion(VD, LC)->getExtent(svalBuilder);
149 DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
150 DefinedOrUnknownSVal sizeIsKnown =
151 svalBuilder.evalEQ(state, Extent, ArraySize);
152 state = state->assume(sizeIsKnown, true);
153
154 // Assume should not fail at this point.
155 assert(state);
156
157 // Remember our assumptions!
158 C.addTransition(state);
159 }
160
registerVLASizeChecker(CheckerManager & mgr)161 void ento::registerVLASizeChecker(CheckerManager &mgr) {
162 mgr.registerChecker<VLASizeChecker>();
163 }
164