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 OwningPtr<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("Dangerous variable-length array (VLA) declaration"));
55
56 SmallString<256> buf;
57 llvm::raw_svector_ostream os(buf);
58 os << "Declared variable-length array (VLA) ";
59 switch (Kind) {
60 case VLA_Garbage:
61 os << "uses a garbage value as its size";
62 break;
63 case VLA_Zero:
64 os << "has zero size";
65 break;
66 case VLA_Tainted:
67 os << "has tainted size";
68 break;
69 }
70
71 BugReport *report = new BugReport(*BT, os.str(), N);
72 report->addRange(SizeE->getSourceRange());
73 bugreporter::trackNullOrUndefValue(N, SizeE, *report);
74 C.emitReport(report);
75 return;
76 }
77
checkPreStmt(const DeclStmt * DS,CheckerContext & C) const78 void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
79 if (!DS->isSingleDecl())
80 return;
81
82 const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
83 if (!VD)
84 return;
85
86 ASTContext &Ctx = C.getASTContext();
87 const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType());
88 if (!VLA)
89 return;
90
91 // FIXME: Handle multi-dimensional VLAs.
92 const Expr *SE = VLA->getSizeExpr();
93 ProgramStateRef state = C.getState();
94 SVal sizeV = state->getSVal(SE, C.getLocationContext());
95
96 if (sizeV.isUndef()) {
97 reportBug(VLA_Garbage, SE, state, C);
98 return;
99 }
100
101 // See if the size value is known. It can't be undefined because we would have
102 // warned about that already.
103 if (sizeV.isUnknown())
104 return;
105
106 // Check if the size is tainted.
107 if (state->isTainted(sizeV)) {
108 reportBug(VLA_Tainted, SE, 0, C);
109 return;
110 }
111
112 // Check if the size is zero.
113 DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
114
115 ProgramStateRef stateNotZero, stateZero;
116 llvm::tie(stateNotZero, stateZero) = state->assume(sizeD);
117
118 if (stateZero && !stateNotZero) {
119 reportBug(VLA_Zero, SE, stateZero, C);
120 return;
121 }
122
123 // From this point on, assume that the size is not zero.
124 state = stateNotZero;
125
126 // VLASizeChecker is responsible for defining the extent of the array being
127 // declared. We do this by multiplying the array length by the element size,
128 // then matching that with the array region's extent symbol.
129
130 // Convert the array length to size_t.
131 SValBuilder &svalBuilder = C.getSValBuilder();
132 QualType SizeTy = Ctx.getSizeType();
133 NonLoc ArrayLength =
134 svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
135
136 // Get the element size.
137 CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
138 SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
139
140 // Multiply the array length by the element size.
141 SVal ArraySizeVal = svalBuilder.evalBinOpNN(
142 state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
143
144 // Finally, assume that the array's extent matches the given size.
145 const LocationContext *LC = C.getLocationContext();
146 DefinedOrUnknownSVal Extent =
147 state->getRegion(VD, LC)->getExtent(svalBuilder);
148 DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
149 DefinedOrUnknownSVal sizeIsKnown =
150 svalBuilder.evalEQ(state, Extent, ArraySize);
151 state = state->assume(sizeIsKnown, true);
152
153 // Assume should not fail at this point.
154 assert(state);
155
156 // Remember our assumptions!
157 C.addTransition(state);
158 }
159
registerVLASizeChecker(CheckerManager & mgr)160 void ento::registerVLASizeChecker(CheckerManager &mgr) {
161 mgr.registerChecker<VLASizeChecker>();
162 }
163