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/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.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 using namespace taint;
30 
31 namespace {
32 class VLASizeChecker : public Checker< check::PreStmt<DeclStmt> > {
33   mutable std::unique_ptr<BugType> BT;
34   enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Tainted, VLA_Negative };
35 
36   void reportBug(VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State,
37                  CheckerContext &C,
38                  std::unique_ptr<BugReporterVisitor> Visitor = nullptr) const;
39 
40 public:
41   void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
42 };
43 } // end anonymous namespace
44 
45 void VLASizeChecker::reportBug(
46     VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State,
47     CheckerContext &C, std::unique_ptr<BugReporterVisitor> Visitor) 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 = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), N);
76   report->addVisitor(std::move(Visitor));
77   report->addRange(SizeE->getSourceRange());
78   bugreporter::trackExpressionValue(N, SizeE, *report);
79   C.emitReport(std::move(report));
80 }
81 
82 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 = C.getSVal(SE);
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 (isTainted(state, sizeV)) {
112     reportBug(VLA_Tainted, SE, nullptr, C,
113               std::make_unique<TaintBugVisitor>(sizeV));
114     return;
115   }
116 
117   // Check if the size is zero.
118   DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
119 
120   ProgramStateRef stateNotZero, stateZero;
121   std::tie(stateNotZero, stateZero) = state->assume(sizeD);
122 
123   if (stateZero && !stateNotZero) {
124     reportBug(VLA_Zero, SE, stateZero, C);
125     return;
126   }
127 
128   // From this point on, assume that the size is not zero.
129   state = stateNotZero;
130 
131   // VLASizeChecker is responsible for defining the extent of the array being
132   // declared. We do this by multiplying the array length by the element size,
133   // then matching that with the array region's extent symbol.
134 
135   // Check if the size is negative.
136   SValBuilder &svalBuilder = C.getSValBuilder();
137 
138   QualType Ty = SE->getType();
139   DefinedOrUnknownSVal Zero = svalBuilder.makeZeroVal(Ty);
140 
141   SVal LessThanZeroVal = svalBuilder.evalBinOp(state, BO_LT, sizeD, Zero, Ty);
142   if (Optional<DefinedSVal> LessThanZeroDVal =
143         LessThanZeroVal.getAs<DefinedSVal>()) {
144     ConstraintManager &CM = C.getConstraintManager();
145     ProgramStateRef StatePos, StateNeg;
146 
147     std::tie(StateNeg, StatePos) = CM.assumeDual(state, *LessThanZeroDVal);
148     if (StateNeg && !StatePos) {
149       reportBug(VLA_Negative, SE, state, C);
150       return;
151     }
152     state = StatePos;
153   }
154 
155   // Convert the array length to size_t.
156   QualType SizeTy = Ctx.getSizeType();
157   NonLoc ArrayLength =
158       svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
159 
160   // Get the element size.
161   CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
162   SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
163 
164   // Multiply the array length by the element size.
165   SVal ArraySizeVal = svalBuilder.evalBinOpNN(
166       state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
167 
168   // Finally, assume that the array's extent matches the given size.
169   const LocationContext *LC = C.getLocationContext();
170   DefinedOrUnknownSVal Extent =
171     state->getRegion(VD, LC)->getExtent(svalBuilder);
172   DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
173   DefinedOrUnknownSVal sizeIsKnown =
174     svalBuilder.evalEQ(state, Extent, ArraySize);
175   state = state->assume(sizeIsKnown, true);
176 
177   // Assume should not fail at this point.
178   assert(state);
179 
180   // Remember our assumptions!
181   C.addTransition(state);
182 }
183 
184 void ento::registerVLASizeChecker(CheckerManager &mgr) {
185   mgr.registerChecker<VLASizeChecker>();
186 }
187 
188 bool ento::shouldRegisterVLASizeChecker(const LangOptions &LO) {
189   return true;
190 }
191