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      1 //=== CastSizeChecker.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 // CastSizeChecker checks when casting a malloc'ed symbolic region to type T,
     11 // whether the size of the symbolic region is a multiple of the size of T.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 #include "ClangSACheckers.h"
     15 #include "clang/AST/CharUnits.h"
     16 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
     17 #include "clang/StaticAnalyzer/Core/Checker.h"
     18 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
     19 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
     20 
     21 using namespace clang;
     22 using namespace ento;
     23 
     24 namespace {
     25 class CastSizeChecker : public Checker< check::PreStmt<CastExpr> > {
     26   mutable std::unique_ptr<BuiltinBug> BT;
     27 
     28 public:
     29   void checkPreStmt(const CastExpr *CE, CheckerContext &C) const;
     30 };
     31 }
     32 
     33 /// Check if we are casting to a struct with a flexible array at the end.
     34 /// \code
     35 /// struct foo {
     36 ///   size_t len;
     37 ///   struct bar data[];
     38 /// };
     39 /// \endcode
     40 /// or
     41 /// \code
     42 /// struct foo {
     43 ///   size_t len;
     44 ///   struct bar data[0];
     45 /// }
     46 /// \endcode
     47 /// In these cases it is also valid to allocate size of struct foo + a multiple
     48 /// of struct bar.
     49 static bool evenFlexibleArraySize(ASTContext &Ctx, CharUnits RegionSize,
     50                                   CharUnits TypeSize, QualType ToPointeeTy) {
     51   const RecordType *RT = ToPointeeTy->getAs<RecordType>();
     52   if (!RT)
     53     return false;
     54 
     55   const RecordDecl *RD = RT->getDecl();
     56   RecordDecl::field_iterator Iter(RD->field_begin());
     57   RecordDecl::field_iterator End(RD->field_end());
     58   const FieldDecl *Last = nullptr;
     59   for (; Iter != End; ++Iter)
     60     Last = *Iter;
     61   assert(Last && "empty structs should already be handled");
     62 
     63   const Type *ElemType = Last->getType()->getArrayElementTypeNoTypeQual();
     64   CharUnits FlexSize;
     65   if (const ConstantArrayType *ArrayTy =
     66         Ctx.getAsConstantArrayType(Last->getType())) {
     67     FlexSize = Ctx.getTypeSizeInChars(ElemType);
     68     if (ArrayTy->getSize() == 1 && TypeSize > FlexSize)
     69       TypeSize -= FlexSize;
     70     else if (ArrayTy->getSize() != 0)
     71       return false;
     72   } else if (RD->hasFlexibleArrayMember()) {
     73     FlexSize = Ctx.getTypeSizeInChars(ElemType);
     74   } else {
     75     return false;
     76   }
     77 
     78   if (FlexSize.isZero())
     79     return false;
     80 
     81   CharUnits Left = RegionSize - TypeSize;
     82   if (Left.isNegative())
     83     return false;
     84 
     85   if (Left % FlexSize == 0)
     86     return true;
     87 
     88   return false;
     89 }
     90 
     91 void CastSizeChecker::checkPreStmt(const CastExpr *CE,CheckerContext &C) const {
     92   const Expr *E = CE->getSubExpr();
     93   ASTContext &Ctx = C.getASTContext();
     94   QualType ToTy = Ctx.getCanonicalType(CE->getType());
     95   const PointerType *ToPTy = dyn_cast<PointerType>(ToTy.getTypePtr());
     96 
     97   if (!ToPTy)
     98     return;
     99 
    100   QualType ToPointeeTy = ToPTy->getPointeeType();
    101 
    102   // Only perform the check if 'ToPointeeTy' is a complete type.
    103   if (ToPointeeTy->isIncompleteType())
    104     return;
    105 
    106   ProgramStateRef state = C.getState();
    107   const MemRegion *R = state->getSVal(E, C.getLocationContext()).getAsRegion();
    108   if (!R)
    109     return;
    110 
    111   const SymbolicRegion *SR = dyn_cast<SymbolicRegion>(R);
    112   if (!SR)
    113     return;
    114 
    115   SValBuilder &svalBuilder = C.getSValBuilder();
    116   SVal extent = SR->getExtent(svalBuilder);
    117   const llvm::APSInt *extentInt = svalBuilder.getKnownValue(state, extent);
    118   if (!extentInt)
    119     return;
    120 
    121   CharUnits regionSize = CharUnits::fromQuantity(extentInt->getSExtValue());
    122   CharUnits typeSize = C.getASTContext().getTypeSizeInChars(ToPointeeTy);
    123 
    124   // Ignore void, and a few other un-sizeable types.
    125   if (typeSize.isZero())
    126     return;
    127 
    128   if (regionSize % typeSize == 0)
    129     return;
    130 
    131   if (evenFlexibleArraySize(Ctx, regionSize, typeSize, ToPointeeTy))
    132     return;
    133 
    134   if (ExplodedNode *errorNode = C.generateSink()) {
    135     if (!BT)
    136       BT.reset(new BuiltinBug(this, "Cast region with wrong size.",
    137                                     "Cast a region whose size is not a multiple"
    138                                     " of the destination type size."));
    139     BugReport *R = new BugReport(*BT, BT->getDescription(), errorNode);
    140     R->addRange(CE->getSourceRange());
    141     C.emitReport(R);
    142   }
    143 }
    144 
    145 void ento::registerCastSizeChecker(CheckerManager &mgr) {
    146   mgr.registerChecker<CastSizeChecker>();
    147 }
    148