ArrayBoundCheckerV2.cpp revision 6bcf27bb9a4b5c3f79cb44c0e4654a6d7619ad89 
1//== ArrayBoundCheckerV2.cpp ------------------------------------*- 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 file defines ArrayBoundCheckerV2, which is a path-sensitive check
11// which looks for an out-of-bound array element access.
12//
13//===----------------------------------------------------------------------===//
14
15#include "ClangSACheckers.h"
16#include "clang/AST/CharUnits.h"
17#include "clang/StaticAnalyzer/Core/BugReporter/BugType.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/PathSensitive/ExprEngine.h"
22#include "llvm/ADT/SmallString.h"
23#include "llvm/Support/raw_ostream.h"
24
25using namespace clang;
26using namespace ento;
27
28namespace {
29class ArrayBoundCheckerV2 :
30    public Checker<check::Location> {
31  mutable std::unique_ptr<BuiltinBug> BT;
32
33  enum OOB_Kind { OOB_Precedes, OOB_Excedes, OOB_Tainted };
34
35  void reportOOB(CheckerContext &C, ProgramStateRef errorState,
36                 OOB_Kind kind) const;
37
38public:
39  void checkLocation(SVal l, bool isLoad, const Stmt*S,
40                     CheckerContext &C) const;
41};
42
43// FIXME: Eventually replace RegionRawOffset with this class.
44class RegionRawOffsetV2 {
45private:
46  const SubRegion *baseRegion;
47  SVal byteOffset;
48
49  RegionRawOffsetV2()
50    : baseRegion(nullptr), byteOffset(UnknownVal()) {}
51
52public:
53  RegionRawOffsetV2(const SubRegion* base, SVal offset)
54    : baseRegion(base), byteOffset(offset) {}
55
56  NonLoc getByteOffset() const { return byteOffset.castAs<NonLoc>(); }
57  const SubRegion *getRegion() const { return baseRegion; }
58
59  static RegionRawOffsetV2 computeOffset(ProgramStateRef state,
60                                         SValBuilder &svalBuilder,
61                                         SVal location);
62
63  void dump() const;
64  void dumpToStream(raw_ostream &os) const;
65};
66}
67
68static SVal computeExtentBegin(SValBuilder &svalBuilder,
69                               const MemRegion *region) {
70  while (true)
71    switch (region->getKind()) {
72      default:
73        return svalBuilder.makeZeroArrayIndex();
74      case MemRegion::SymbolicRegionKind:
75        // FIXME: improve this later by tracking symbolic lower bounds
76        // for symbolic regions.
77        return UnknownVal();
78      case MemRegion::ElementRegionKind:
79        region = cast<SubRegion>(region)->getSuperRegion();
80        continue;
81    }
82}
83
84void ArrayBoundCheckerV2::checkLocation(SVal location, bool isLoad,
85                                        const Stmt* LoadS,
86                                        CheckerContext &checkerContext) const {
87
88  // NOTE: Instead of using ProgramState::assumeInBound(), we are prototyping
89  // some new logic here that reasons directly about memory region extents.
90  // Once that logic is more mature, we can bring it back to assumeInBound()
91  // for all clients to use.
92  //
93  // The algorithm we are using here for bounds checking is to see if the
94  // memory access is within the extent of the base region.  Since we
95  // have some flexibility in defining the base region, we can achieve
96  // various levels of conservatism in our buffer overflow checking.
97  ProgramStateRef state = checkerContext.getState();
98  ProgramStateRef originalState = state;
99
100  SValBuilder &svalBuilder = checkerContext.getSValBuilder();
101  const RegionRawOffsetV2 &rawOffset =
102    RegionRawOffsetV2::computeOffset(state, svalBuilder, location);
103
104  if (!rawOffset.getRegion())
105    return;
106
107  // CHECK LOWER BOUND: Is byteOffset < extent begin?
108  //  If so, we are doing a load/store
109  //  before the first valid offset in the memory region.
110
111  SVal extentBegin = computeExtentBegin(svalBuilder, rawOffset.getRegion());
112
113  if (Optional<NonLoc> NV = extentBegin.getAs<NonLoc>()) {
114    SVal lowerBound =
115        svalBuilder.evalBinOpNN(state, BO_LT, rawOffset.getByteOffset(), *NV,
116                                svalBuilder.getConditionType());
117
118    Optional<NonLoc> lowerBoundToCheck = lowerBound.getAs<NonLoc>();
119    if (!lowerBoundToCheck)
120      return;
121
122    ProgramStateRef state_precedesLowerBound, state_withinLowerBound;
123    std::tie(state_precedesLowerBound, state_withinLowerBound) =
124      state->assume(*lowerBoundToCheck);
125
126    // Are we constrained enough to definitely precede the lower bound?
127    if (state_precedesLowerBound && !state_withinLowerBound) {
128      reportOOB(checkerContext, state_precedesLowerBound, OOB_Precedes);
129      return;
130    }
131
132    // Otherwise, assume the constraint of the lower bound.
133    assert(state_withinLowerBound);
134    state = state_withinLowerBound;
135  }
136
137  do {
138    // CHECK UPPER BOUND: Is byteOffset >= extent(baseRegion)?  If so,
139    // we are doing a load/store after the last valid offset.
140    DefinedOrUnknownSVal extentVal =
141      rawOffset.getRegion()->getExtent(svalBuilder);
142    if (!extentVal.getAs<NonLoc>())
143      break;
144
145    SVal upperbound
146      = svalBuilder.evalBinOpNN(state, BO_GE, rawOffset.getByteOffset(),
147                                extentVal.castAs<NonLoc>(),
148                                svalBuilder.getConditionType());
149
150    Optional<NonLoc> upperboundToCheck = upperbound.getAs<NonLoc>();
151    if (!upperboundToCheck)
152      break;
153
154    ProgramStateRef state_exceedsUpperBound, state_withinUpperBound;
155    std::tie(state_exceedsUpperBound, state_withinUpperBound) =
156      state->assume(*upperboundToCheck);
157
158    // If we are under constrained and the index variables are tainted, report.
159    if (state_exceedsUpperBound && state_withinUpperBound) {
160      if (state->isTainted(rawOffset.getByteOffset()))
161        reportOOB(checkerContext, state_exceedsUpperBound, OOB_Tainted);
162        return;
163    }
164
165    // If we are constrained enough to definitely exceed the upper bound, report.
166    if (state_exceedsUpperBound) {
167      assert(!state_withinUpperBound);
168      reportOOB(checkerContext, state_exceedsUpperBound, OOB_Excedes);
169      return;
170    }
171
172    assert(state_withinUpperBound);
173    state = state_withinUpperBound;
174  }
175  while (false);
176
177  if (state != originalState)
178    checkerContext.addTransition(state);
179}
180
181void ArrayBoundCheckerV2::reportOOB(CheckerContext &checkerContext,
182                                    ProgramStateRef errorState,
183                                    OOB_Kind kind) const {
184
185  ExplodedNode *errorNode = checkerContext.generateSink(errorState);
186  if (!errorNode)
187    return;
188
189  if (!BT)
190    BT.reset(new BuiltinBug(this, "Out-of-bound access"));
191
192  // FIXME: This diagnostics are preliminary.  We should get far better
193  // diagnostics for explaining buffer overruns.
194
195  SmallString<256> buf;
196  llvm::raw_svector_ostream os(buf);
197  os << "Out of bound memory access ";
198  switch (kind) {
199  case OOB_Precedes:
200    os << "(accessed memory precedes memory block)";
201    break;
202  case OOB_Excedes:
203    os << "(access exceeds upper limit of memory block)";
204    break;
205  case OOB_Tainted:
206    os << "(index is tainted)";
207    break;
208  }
209
210  checkerContext.emitReport(new BugReport(*BT, os.str(), errorNode));
211}
212
213void RegionRawOffsetV2::dump() const {
214  dumpToStream(llvm::errs());
215}
216
217void RegionRawOffsetV2::dumpToStream(raw_ostream &os) const {
218  os << "raw_offset_v2{" << getRegion() << ',' << getByteOffset() << '}';
219}
220
221// FIXME: Merge with the implementation of the same method in Store.cpp
222static bool IsCompleteType(ASTContext &Ctx, QualType Ty) {
223  if (const RecordType *RT = Ty->getAs<RecordType>()) {
224    const RecordDecl *D = RT->getDecl();
225    if (!D->getDefinition())
226      return false;
227  }
228
229  return true;
230}
231
232
233// Lazily computes a value to be used by 'computeOffset'.  If 'val'
234// is unknown or undefined, we lazily substitute '0'.  Otherwise,
235// return 'val'.
236static inline SVal getValue(SVal val, SValBuilder &svalBuilder) {
237  return val.getAs<UndefinedVal>() ? svalBuilder.makeArrayIndex(0) : val;
238}
239
240// Scale a base value by a scaling factor, and return the scaled
241// value as an SVal.  Used by 'computeOffset'.
242static inline SVal scaleValue(ProgramStateRef state,
243                              NonLoc baseVal, CharUnits scaling,
244                              SValBuilder &sb) {
245  return sb.evalBinOpNN(state, BO_Mul, baseVal,
246                        sb.makeArrayIndex(scaling.getQuantity()),
247                        sb.getArrayIndexType());
248}
249
250// Add an SVal to another, treating unknown and undefined values as
251// summing to UnknownVal.  Used by 'computeOffset'.
252static SVal addValue(ProgramStateRef state, SVal x, SVal y,
253                     SValBuilder &svalBuilder) {
254  // We treat UnknownVals and UndefinedVals the same here because we
255  // only care about computing offsets.
256  if (x.isUnknownOrUndef() || y.isUnknownOrUndef())
257    return UnknownVal();
258
259  return svalBuilder.evalBinOpNN(state, BO_Add, x.castAs<NonLoc>(),
260                                 y.castAs<NonLoc>(),
261                                 svalBuilder.getArrayIndexType());
262}
263
264/// Compute a raw byte offset from a base region.  Used for array bounds
265/// checking.
266RegionRawOffsetV2 RegionRawOffsetV2::computeOffset(ProgramStateRef state,
267                                                   SValBuilder &svalBuilder,
268                                                   SVal location)
269{
270  const MemRegion *region = location.getAsRegion();
271  SVal offset = UndefinedVal();
272
273  while (region) {
274    switch (region->getKind()) {
275      default: {
276        if (const SubRegion *subReg = dyn_cast<SubRegion>(region)) {
277          offset = getValue(offset, svalBuilder);
278          if (!offset.isUnknownOrUndef())
279            return RegionRawOffsetV2(subReg, offset);
280        }
281        return RegionRawOffsetV2();
282      }
283      case MemRegion::ElementRegionKind: {
284        const ElementRegion *elemReg = cast<ElementRegion>(region);
285        SVal index = elemReg->getIndex();
286        if (!index.getAs<NonLoc>())
287          return RegionRawOffsetV2();
288        QualType elemType = elemReg->getElementType();
289        // If the element is an incomplete type, go no further.
290        ASTContext &astContext = svalBuilder.getContext();
291        if (!IsCompleteType(astContext, elemType))
292          return RegionRawOffsetV2();
293
294        // Update the offset.
295        offset = addValue(state,
296                          getValue(offset, svalBuilder),
297                          scaleValue(state,
298                          index.castAs<NonLoc>(),
299                          astContext.getTypeSizeInChars(elemType),
300                          svalBuilder),
301                          svalBuilder);
302
303        if (offset.isUnknownOrUndef())
304          return RegionRawOffsetV2();
305
306        region = elemReg->getSuperRegion();
307        continue;
308      }
309    }
310  }
311  return RegionRawOffsetV2();
312}
313
314void ento::registerArrayBoundCheckerV2(CheckerManager &mgr) {
315  mgr.registerChecker<ArrayBoundCheckerV2>();
316}
317