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  return;
80}
81
82void 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 = state->getSVal(SE, C.getLocationContext());
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 (state->isTainted(sizeV)) {
112    reportBug(VLA_Tainted, SE, nullptr, C);
113    return;
114  }
115
116  // Check if the size is zero.
117  DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
118
119  ProgramStateRef stateNotZero, stateZero;
120  std::tie(stateNotZero, stateZero) = state->assume(sizeD);
121
122  if (stateZero && !stateNotZero) {
123    reportBug(VLA_Zero, SE, stateZero, C);
124    return;
125  }
126
127  // From this point on, assume that the size is not zero.
128  state = stateNotZero;
129
130  // VLASizeChecker is responsible for defining the extent of the array being
131  // declared. We do this by multiplying the array length by the element size,
132  // then matching that with the array region's extent symbol.
133
134  // Check if the size is negative.
135  SValBuilder &svalBuilder = C.getSValBuilder();
136
137  QualType Ty = SE->getType();
138  DefinedOrUnknownSVal Zero = svalBuilder.makeZeroVal(Ty);
139
140  SVal LessThanZeroVal = svalBuilder.evalBinOp(state, BO_LT, sizeD, Zero, Ty);
141  if (Optional<DefinedSVal> LessThanZeroDVal =
142        LessThanZeroVal.getAs<DefinedSVal>()) {
143    ConstraintManager &CM = C.getConstraintManager();
144    ProgramStateRef StatePos, StateNeg;
145
146    std::tie(StateNeg, StatePos) = CM.assumeDual(state, *LessThanZeroDVal);
147    if (StateNeg && !StatePos) {
148      reportBug(VLA_Negative, SE, state, C);
149      return;
150    }
151    state = StatePos;
152  }
153
154  // Convert the array length to size_t.
155  QualType SizeTy = Ctx.getSizeType();
156  NonLoc ArrayLength =
157      svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
158
159  // Get the element size.
160  CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
161  SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
162
163  // Multiply the array length by the element size.
164  SVal ArraySizeVal = svalBuilder.evalBinOpNN(
165      state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
166
167  // Finally, assume that the array's extent matches the given size.
168  const LocationContext *LC = C.getLocationContext();
169  DefinedOrUnknownSVal Extent =
170    state->getRegion(VD, LC)->getExtent(svalBuilder);
171  DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
172  DefinedOrUnknownSVal sizeIsKnown =
173    svalBuilder.evalEQ(state, Extent, ArraySize);
174  state = state->assume(sizeIsKnown, true);
175
176  // Assume should not fail at this point.
177  assert(state);
178
179  // Remember our assumptions!
180  C.addTransition(state);
181}
182
183void ento::registerVLASizeChecker(CheckerManager &mgr) {
184  mgr.registerChecker<VLASizeChecker>();
185}
186