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
27using namespace clang;
28using namespace ento;
29
30namespace {
31class 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;
39public:
40  void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
41};
42} // end anonymous namespace
43
44void 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}
80
81void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
82  if (!DS->isSingleDecl())
83    return;
84
85  const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
86  if (!VD)
87    return;
88
89  ASTContext &Ctx = C.getASTContext();
90  const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType());
91  if (!VLA)
92    return;
93
94  // FIXME: Handle multi-dimensional VLAs.
95  const Expr *SE = VLA->getSizeExpr();
96  ProgramStateRef state = C.getState();
97  SVal sizeV = state->getSVal(SE, C.getLocationContext());
98
99  if (sizeV.isUndef()) {
100    reportBug(VLA_Garbage, SE, state, C);
101    return;
102  }
103
104  // See if the size value is known. It can't be undefined because we would have
105  // warned about that already.
106  if (sizeV.isUnknown())
107    return;
108
109  // Check if the size is tainted.
110  if (state->isTainted(sizeV)) {
111    reportBug(VLA_Tainted, SE, nullptr, C);
112    return;
113  }
114
115  // Check if the size is zero.
116  DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
117
118  ProgramStateRef stateNotZero, stateZero;
119  std::tie(stateNotZero, stateZero) = state->assume(sizeD);
120
121  if (stateZero && !stateNotZero) {
122    reportBug(VLA_Zero, SE, stateZero, C);
123    return;
124  }
125
126  // From this point on, assume that the size is not zero.
127  state = stateNotZero;
128
129  // VLASizeChecker is responsible for defining the extent of the array being
130  // declared. We do this by multiplying the array length by the element size,
131  // then matching that with the array region's extent symbol.
132
133  // Check if the size is negative.
134  SValBuilder &svalBuilder = C.getSValBuilder();
135
136  QualType Ty = SE->getType();
137  DefinedOrUnknownSVal Zero = svalBuilder.makeZeroVal(Ty);
138
139  SVal LessThanZeroVal = svalBuilder.evalBinOp(state, BO_LT, sizeD, Zero, Ty);
140  if (Optional<DefinedSVal> LessThanZeroDVal =
141        LessThanZeroVal.getAs<DefinedSVal>()) {
142    ConstraintManager &CM = C.getConstraintManager();
143    ProgramStateRef StatePos, StateNeg;
144
145    std::tie(StateNeg, StatePos) = CM.assumeDual(state, *LessThanZeroDVal);
146    if (StateNeg && !StatePos) {
147      reportBug(VLA_Negative, SE, state, C);
148      return;
149    }
150    state = StatePos;
151  }
152
153  // Convert the array length to size_t.
154  QualType SizeTy = Ctx.getSizeType();
155  NonLoc ArrayLength =
156      svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
157
158  // Get the element size.
159  CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
160  SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
161
162  // Multiply the array length by the element size.
163  SVal ArraySizeVal = svalBuilder.evalBinOpNN(
164      state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
165
166  // Finally, assume that the array's extent matches the given size.
167  const LocationContext *LC = C.getLocationContext();
168  DefinedOrUnknownSVal Extent =
169    state->getRegion(VD, LC)->getExtent(svalBuilder);
170  DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
171  DefinedOrUnknownSVal sizeIsKnown =
172    svalBuilder.evalEQ(state, Extent, ArraySize);
173  state = state->assume(sizeIsKnown, true);
174
175  // Assume should not fail at this point.
176  assert(state);
177
178  // Remember our assumptions!
179  C.addTransition(state);
180}
181
182void ento::registerVLASizeChecker(CheckerManager &mgr) {
183  mgr.registerChecker<VLASizeChecker>();
184}
185