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      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/StaticAnalyzer/Core/Checker.h"
     17 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
     18 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
     19 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
     20 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
     21 #include "clang/AST/CharUnits.h"
     22 #include "llvm/ADT/SmallString.h"
     23 #include "llvm/ADT/STLExtras.h"
     24 
     25 using namespace clang;
     26 using namespace ento;
     27 
     28 namespace {
     29 class ArrayBoundCheckerV2 :
     30     public Checker<check::Location> {
     31   mutable OwningPtr<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 
     38 public:
     39   void checkLocation(SVal l, bool isLoad, const Stmt*S,
     40                      CheckerContext &C) const;
     41 };
     42 
     43 // FIXME: Eventually replace RegionRawOffset with this class.
     44 class RegionRawOffsetV2 {
     45 private:
     46   const SubRegion *baseRegion;
     47   SVal byteOffset;
     48 
     49   RegionRawOffsetV2()
     50     : baseRegion(0), byteOffset(UnknownVal()) {}
     51 
     52 public:
     53   RegionRawOffsetV2(const SubRegion* base, SVal offset)
     54     : baseRegion(base), byteOffset(offset) {}
     55 
     56   NonLoc getByteOffset() const { return cast<NonLoc>(byteOffset); }
     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 
     68 static 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 
     84 void 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 (isa<NonLoc>(extentBegin)) {
    114     SVal lowerBound
    115       = svalBuilder.evalBinOpNN(state, BO_LT, rawOffset.getByteOffset(),
    116                                 cast<NonLoc>(extentBegin),
    117                                 svalBuilder.getConditionType());
    118 
    119     NonLoc *lowerBoundToCheck = dyn_cast<NonLoc>(&lowerBound);
    120     if (!lowerBoundToCheck)
    121       return;
    122 
    123     ProgramStateRef state_precedesLowerBound, state_withinLowerBound;
    124     llvm::tie(state_precedesLowerBound, state_withinLowerBound) =
    125       state->assume(*lowerBoundToCheck);
    126 
    127     // Are we constrained enough to definitely precede the lower bound?
    128     if (state_precedesLowerBound && !state_withinLowerBound) {
    129       reportOOB(checkerContext, state_precedesLowerBound, OOB_Precedes);
    130       return;
    131     }
    132 
    133     // Otherwise, assume the constraint of the lower bound.
    134     assert(state_withinLowerBound);
    135     state = state_withinLowerBound;
    136   }
    137 
    138   do {
    139     // CHECK UPPER BOUND: Is byteOffset >= extent(baseRegion)?  If so,
    140     // we are doing a load/store after the last valid offset.
    141     DefinedOrUnknownSVal extentVal =
    142       rawOffset.getRegion()->getExtent(svalBuilder);
    143     if (!isa<NonLoc>(extentVal))
    144       break;
    145 
    146     SVal upperbound
    147       = svalBuilder.evalBinOpNN(state, BO_GE, rawOffset.getByteOffset(),
    148                                 cast<NonLoc>(extentVal),
    149                                 svalBuilder.getConditionType());
    150 
    151     NonLoc *upperboundToCheck = dyn_cast<NonLoc>(&upperbound);
    152     if (!upperboundToCheck)
    153       break;
    154 
    155     ProgramStateRef state_exceedsUpperBound, state_withinUpperBound;
    156     llvm::tie(state_exceedsUpperBound, state_withinUpperBound) =
    157       state->assume(*upperboundToCheck);
    158 
    159     // If we are under constrained and the index variables are tainted, report.
    160     if (state_exceedsUpperBound && state_withinUpperBound) {
    161       if (state->isTainted(rawOffset.getByteOffset()))
    162         reportOOB(checkerContext, state_exceedsUpperBound, OOB_Tainted);
    163         return;
    164     }
    165 
    166     // If we are constrained enough to definitely exceed the upper bound, report.
    167     if (state_exceedsUpperBound) {
    168       assert(!state_withinUpperBound);
    169       reportOOB(checkerContext, state_exceedsUpperBound, OOB_Excedes);
    170       return;
    171     }
    172 
    173     assert(state_withinUpperBound);
    174     state = state_withinUpperBound;
    175   }
    176   while (false);
    177 
    178   if (state != originalState)
    179     checkerContext.addTransition(state);
    180 }
    181 
    182 void ArrayBoundCheckerV2::reportOOB(CheckerContext &checkerContext,
    183                                     ProgramStateRef errorState,
    184                                     OOB_Kind kind) const {
    185 
    186   ExplodedNode *errorNode = checkerContext.generateSink(errorState);
    187   if (!errorNode)
    188     return;
    189 
    190   if (!BT)
    191     BT.reset(new BuiltinBug("Out-of-bound access"));
    192 
    193   // FIXME: This diagnostics are preliminary.  We should get far better
    194   // diagnostics for explaining buffer overruns.
    195 
    196   SmallString<256> buf;
    197   llvm::raw_svector_ostream os(buf);
    198   os << "Out of bound memory access ";
    199   switch (kind) {
    200   case OOB_Precedes:
    201     os << "(accessed memory precedes memory block)";
    202     break;
    203   case OOB_Excedes:
    204     os << "(access exceeds upper limit of memory block)";
    205     break;
    206   case OOB_Tainted:
    207     os << "(index is tainted)";
    208     break;
    209   }
    210 
    211   checkerContext.EmitReport(new BugReport(*BT, os.str(), errorNode));
    212 }
    213 
    214 void RegionRawOffsetV2::dump() const {
    215   dumpToStream(llvm::errs());
    216 }
    217 
    218 void RegionRawOffsetV2::dumpToStream(raw_ostream &os) const {
    219   os << "raw_offset_v2{" << getRegion() << ',' << getByteOffset() << '}';
    220 }
    221 
    222 // FIXME: Merge with the implementation of the same method in Store.cpp
    223 static bool IsCompleteType(ASTContext &Ctx, QualType Ty) {
    224   if (const RecordType *RT = Ty->getAs<RecordType>()) {
    225     const RecordDecl *D = RT->getDecl();
    226     if (!D->getDefinition())
    227       return false;
    228   }
    229 
    230   return true;
    231 }
    232 
    233 
    234 // Lazily computes a value to be used by 'computeOffset'.  If 'val'
    235 // is unknown or undefined, we lazily substitute '0'.  Otherwise,
    236 // return 'val'.
    237 static inline SVal getValue(SVal val, SValBuilder &svalBuilder) {
    238   return isa<UndefinedVal>(val) ? svalBuilder.makeArrayIndex(0) : val;
    239 }
    240 
    241 // Scale a base value by a scaling factor, and return the scaled
    242 // value as an SVal.  Used by 'computeOffset'.
    243 static inline SVal scaleValue(ProgramStateRef state,
    244                               NonLoc baseVal, CharUnits scaling,
    245                               SValBuilder &sb) {
    246   return sb.evalBinOpNN(state, BO_Mul, baseVal,
    247                         sb.makeArrayIndex(scaling.getQuantity()),
    248                         sb.getArrayIndexType());
    249 }
    250 
    251 // Add an SVal to another, treating unknown and undefined values as
    252 // summing to UnknownVal.  Used by 'computeOffset'.
    253 static SVal addValue(ProgramStateRef state, SVal x, SVal y,
    254                      SValBuilder &svalBuilder) {
    255   // We treat UnknownVals and UndefinedVals the same here because we
    256   // only care about computing offsets.
    257   if (x.isUnknownOrUndef() || y.isUnknownOrUndef())
    258     return UnknownVal();
    259 
    260   return svalBuilder.evalBinOpNN(state, BO_Add,
    261                                  cast<NonLoc>(x), cast<NonLoc>(y),
    262                                  svalBuilder.getArrayIndexType());
    263 }
    264 
    265 /// Compute a raw byte offset from a base region.  Used for array bounds
    266 /// checking.
    267 RegionRawOffsetV2 RegionRawOffsetV2::computeOffset(ProgramStateRef state,
    268                                                    SValBuilder &svalBuilder,
    269                                                    SVal location)
    270 {
    271   const MemRegion *region = location.getAsRegion();
    272   SVal offset = UndefinedVal();
    273 
    274   while (region) {
    275     switch (region->getKind()) {
    276       default: {
    277         if (const SubRegion *subReg = dyn_cast<SubRegion>(region)) {
    278           offset = getValue(offset, svalBuilder);
    279           if (!offset.isUnknownOrUndef())
    280             return RegionRawOffsetV2(subReg, offset);
    281         }
    282         return RegionRawOffsetV2();
    283       }
    284       case MemRegion::ElementRegionKind: {
    285         const ElementRegion *elemReg = cast<ElementRegion>(region);
    286         SVal index = elemReg->getIndex();
    287         if (!isa<NonLoc>(index))
    288           return RegionRawOffsetV2();
    289         QualType elemType = elemReg->getElementType();
    290         // If the element is an incomplete type, go no further.
    291         ASTContext &astContext = svalBuilder.getContext();
    292         if (!IsCompleteType(astContext, elemType))
    293           return RegionRawOffsetV2();
    294 
    295         // Update the offset.
    296         offset = addValue(state,
    297                           getValue(offset, svalBuilder),
    298                           scaleValue(state,
    299                           cast<NonLoc>(index),
    300                           astContext.getTypeSizeInChars(elemType),
    301                           svalBuilder),
    302                           svalBuilder);
    303 
    304         if (offset.isUnknownOrUndef())
    305           return RegionRawOffsetV2();
    306 
    307         region = elemReg->getSuperRegion();
    308         continue;
    309       }
    310     }
    311   }
    312   return RegionRawOffsetV2();
    313 }
    314 
    315 
    316 void ento::registerArrayBoundCheckerV2(CheckerManager &mgr) {
    317   mgr.registerChecker<ArrayBoundCheckerV2>();
    318 }
    319