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/StaticAnalyzer/Core/Checker.h"
19 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
20 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
21 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
22 #include "clang/AST/CharUnits.h"
23 #include "llvm/ADT/SmallString.h"
24 #include "llvm/ADT/STLExtras.h"
25
26 using namespace clang;
27 using namespace ento;
28
29 namespace {
30 class VLASizeChecker : public Checker< check::PreStmt<DeclStmt> > {
31 mutable OwningPtr<BugType> BT;
32 enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Tainted };
33
34 void reportBug(VLASize_Kind Kind,
35 const Expr *SizeE,
36 ProgramStateRef State,
37 CheckerContext &C) const;
38 public:
39 void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
40 };
41 } // end anonymous namespace
42
reportBug(VLASize_Kind Kind,const Expr * SizeE,ProgramStateRef State,CheckerContext & C) const43 void VLASizeChecker::reportBug(VLASize_Kind Kind,
44 const Expr *SizeE,
45 ProgramStateRef State,
46 CheckerContext &C) const {
47 // Generate an error node.
48 ExplodedNode *N = C.generateSink(State);
49 if (!N)
50 return;
51
52 if (!BT)
53 BT.reset(new BuiltinBug("Dangerous variable-length array (VLA) declaration"));
54
55 SmallString<256> buf;
56 llvm::raw_svector_ostream os(buf);
57 os << "Declared variable-length array (VLA) ";
58 switch (Kind) {
59 case VLA_Garbage:
60 os << "uses a garbage value as its size";
61 break;
62 case VLA_Zero:
63 os << "has zero size";
64 break;
65 case VLA_Tainted:
66 os << "has tainted size";
67 break;
68 }
69
70 BugReport *report = new BugReport(*BT, os.str(), N);
71 report->addRange(SizeE->getSourceRange());
72 bugreporter::trackNullOrUndefValue(N, SizeE, *report);
73 C.EmitReport(report);
74 return;
75 }
76
checkPreStmt(const DeclStmt * DS,CheckerContext & C) const77 void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
78 if (!DS->isSingleDecl())
79 return;
80
81 const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
82 if (!VD)
83 return;
84
85 ASTContext &Ctx = C.getASTContext();
86 const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType());
87 if (!VLA)
88 return;
89
90 // FIXME: Handle multi-dimensional VLAs.
91 const Expr *SE = VLA->getSizeExpr();
92 ProgramStateRef state = C.getState();
93 SVal sizeV = state->getSVal(SE, C.getLocationContext());
94
95 if (sizeV.isUndef()) {
96 reportBug(VLA_Garbage, SE, state, C);
97 return;
98 }
99
100 // See if the size value is known. It can't be undefined because we would have
101 // warned about that already.
102 if (sizeV.isUnknown())
103 return;
104
105 // Check if the size is tainted.
106 if (state->isTainted(sizeV)) {
107 reportBug(VLA_Tainted, SE, 0, C);
108 return;
109 }
110
111 // Check if the size is zero.
112 DefinedSVal sizeD = cast<DefinedSVal>(sizeV);
113
114 ProgramStateRef stateNotZero, stateZero;
115 llvm::tie(stateNotZero, stateZero) = state->assume(sizeD);
116
117 if (stateZero && !stateNotZero) {
118 reportBug(VLA_Zero, SE, stateZero, C);
119 return;
120 }
121
122 // From this point on, assume that the size is not zero.
123 state = stateNotZero;
124
125 // VLASizeChecker is responsible for defining the extent of the array being
126 // declared. We do this by multiplying the array length by the element size,
127 // then matching that with the array region's extent symbol.
128
129 // Convert the array length to size_t.
130 SValBuilder &svalBuilder = C.getSValBuilder();
131 QualType SizeTy = Ctx.getSizeType();
132 NonLoc ArrayLength = cast<NonLoc>(svalBuilder.evalCast(sizeD, SizeTy,
133 SE->getType()));
134
135 // Get the element size.
136 CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
137 SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
138
139 // Multiply the array length by the element size.
140 SVal ArraySizeVal = svalBuilder.evalBinOpNN(state, BO_Mul, ArrayLength,
141 cast<NonLoc>(EleSizeVal), SizeTy);
142
143 // Finally, assume that the array's extent matches the given size.
144 const LocationContext *LC = C.getLocationContext();
145 DefinedOrUnknownSVal Extent =
146 state->getRegion(VD, LC)->getExtent(svalBuilder);
147 DefinedOrUnknownSVal ArraySize = cast<DefinedOrUnknownSVal>(ArraySizeVal);
148 DefinedOrUnknownSVal sizeIsKnown =
149 svalBuilder.evalEQ(state, Extent, ArraySize);
150 state = state->assume(sizeIsKnown, true);
151
152 // Assume should not fail at this point.
153 assert(state);
154
155 // Remember our assumptions!
156 C.addTransition(state);
157 }
158
registerVLASizeChecker(CheckerManager & mgr)159 void ento::registerVLASizeChecker(CheckerManager &mgr) {
160 mgr.registerChecker<VLASizeChecker>();
161 }
162