lib/Checker/MallocChecker.cpp

//=== MallocChecker.cpp - A malloc/free checker -------------------*- C++ -*--//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines malloc/free checker, which checks for potential memory
// leaks, double free, and use-after-free problems.
//
//===----------------------------------------------------------------------===//

#include "GRExprEngineExperimentalChecks.h"
#include "clang/Checker/BugReporter/BugType.h"
#include "clang/Checker/PathSensitive/CheckerVisitor.h"
#include "clang/Checker/PathSensitive/GRState.h"
#include "clang/Checker/PathSensitive/GRStateTrait.h"
#include "clang/Checker/PathSensitive/SymbolManager.h"
#include "llvm/ADT/ImmutableMap.h"
using namespace clang;

namespace {

class RefState {
  enum Kind { AllocateUnchecked, AllocateFailed, Released, Escaped,
              Relinquished } K;
  const Stmt *S;

public:
  RefState(Kind k, const Stmt *s) : K(k), S(s) {}

  bool isAllocated() const { return K == AllocateUnchecked; }
  bool isFailed() const { return K == AllocateFailed; }
  bool isReleased() const { return K == Released; }
  bool isEscaped() const { return K == Escaped; }
  bool isRelinquished() const { return K == Relinquished; }

  bool operator==(const RefState &X) const {
    return K == X.K && S == X.S;
  }

  static RefState getAllocateUnchecked(const Stmt *s) { 
    return RefState(AllocateUnchecked, s); 
  }
  static RefState getAllocateFailed() {
    return RefState(AllocateFailed, 0);
  }
  static RefState getReleased(const Stmt *s) { return RefState(Released, s); }
  static RefState getEscaped(const Stmt *s) { return RefState(Escaped, s); }
  static RefState getRelinquished(const Stmt *s) {
    return RefState(Relinquished, s);
  }

  void Profile(llvm::FoldingSetNodeID &ID) const {
    ID.AddInteger(K);
    ID.AddPointer(S);
  }
};

class RegionState {};

class MallocChecker : public CheckerVisitor<MallocChecker> {
  BuiltinBug *BT_DoubleFree;
  BuiltinBug *BT_Leak;
  BuiltinBug *BT_UseFree;
  BuiltinBug *BT_UseRelinquished;
  BuiltinBug *BT_BadFree;
  IdentifierInfo *II_malloc, *II_free, *II_realloc, *II_calloc;

public:
  MallocChecker() 
    : BT_DoubleFree(0), BT_Leak(0), BT_UseFree(0), BT_UseRelinquished(0),
      BT_BadFree(0),
      II_malloc(0), II_free(0), II_realloc(0), II_calloc(0) {}
  static void *getTag();
  bool EvalCallExpr(CheckerContext &C, const CallExpr *CE);
  void EvalDeadSymbols(CheckerContext &C, SymbolReaper &SymReaper);
  void EvalEndPath(GREndPathNodeBuilder &B, void *tag, GRExprEngine &Eng);
  void PreVisitReturnStmt(CheckerContext &C, const ReturnStmt *S);
  const GRState *EvalAssume(const GRState *state, SVal Cond, bool Assumption,
                            bool *respondsToCallback);
  void VisitLocation(CheckerContext &C, const Stmt *S, SVal l);
  virtual void PreVisitBind(CheckerContext &C, const Stmt *AssignE,
                            const Stmt *StoreE, SVal location,
                            SVal val);

private:
  void MallocMem(CheckerContext &C, const CallExpr *CE);
  void MallocMemReturnsAttr(CheckerContext &C, const CallExpr *CE,
                            const OwnershipAttr* Att);
  const GRState *MallocMemAux(CheckerContext &C, const CallExpr *CE,
                              const Expr *SizeEx, SVal Init,
                              const GRState *state) {
    return MallocMemAux(C, CE, state->getSVal(SizeEx), Init, state);
  }
  const GRState *MallocMemAux(CheckerContext &C, const CallExpr *CE,
                              SVal SizeEx, SVal Init,
                              const GRState *state);

  void FreeMem(CheckerContext &C, const CallExpr *CE);
  void FreeMemAttr(CheckerContext &C, const CallExpr *CE, const OwnershipAttr* Att);
  const GRState *FreeMemAux(CheckerContext &C, const CallExpr *CE,
                            const GRState *state, unsigned Num, bool Hold);

  void ReallocMem(CheckerContext &C, const CallExpr *CE);
  void CallocMem(CheckerContext &C, const CallExpr *CE);
  
  bool SummarizeValue(llvm::raw_ostream& os, SVal V);
  bool SummarizeRegion(llvm::raw_ostream& os, const MemRegion *MR);
  void ReportBadFree(CheckerContext &C, SVal ArgVal, SourceRange range);
};
} // end anonymous namespace

typedef llvm::ImmutableMap<SymbolRef, RefState> RegionStateTy;

namespace clang {
  template <>
  struct GRStateTrait<RegionState> 
    : public GRStatePartialTrait<llvm::ImmutableMap<SymbolRef, RefState> > {
    static void *GDMIndex() { return MallocChecker::getTag(); }
  };
}

void clang::RegisterMallocChecker(GRExprEngine &Eng) {
  Eng.registerCheck(new MallocChecker());
}

void *MallocChecker::getTag() {
  static int x;
  return &x;
}

bool MallocChecker::EvalCallExpr(CheckerContext &C, const CallExpr *CE) {
  const GRState *state = C.getState();
  const Expr *Callee = CE->getCallee();
  SVal L = state->getSVal(Callee);

  const FunctionDecl *FD = L.getAsFunctionDecl();
  if (!FD)
    return false;

  ASTContext &Ctx = C.getASTContext();
  if (!II_malloc)
    II_malloc = &Ctx.Idents.get("malloc");
  if (!II_free)
    II_free = &Ctx.Idents.get("free");
  if (!II_realloc)
    II_realloc = &Ctx.Idents.get("realloc");
  if (!II_calloc)
    II_calloc = &Ctx.Idents.get("calloc");

  if (FD->getIdentifier() == II_malloc) {
    MallocMem(C, CE);
    return true;
  }

  if (FD->getIdentifier() == II_free) {
    FreeMem(C, CE);
    return true;
  }

  if (FD->getIdentifier() == II_realloc) {
    ReallocMem(C, CE);
    return true;
  }

  if (FD->getIdentifier() == II_calloc) {
    CallocMem(C, CE);
    return true;
  }

  // Check all the attributes, if there are any.
  // There can be multiple of these attributes.
  bool rv = false;
  if (FD->hasAttrs()) {
    for (const Attr *attr = FD->getAttrs(); attr; attr = attr->getNext()) {
      if(const OwnershipAttr* Att = dyn_cast<OwnershipAttr>(attr)) {
        switch (Att->getKind()) {
        case OwnershipAttr::Returns: {
          MallocMemReturnsAttr(C, CE, Att);
          rv = true;
          break;
        }
        case OwnershipAttr::Takes:
        case OwnershipAttr::Holds: {
          FreeMemAttr(C, CE, Att);
          rv = true;
          break;
        }
        default:
          break;
        }
      }
    }
  }
  return rv;
}

void MallocChecker::MallocMem(CheckerContext &C, const CallExpr *CE) {
  const GRState *state = MallocMemAux(C, CE, CE->getArg(0), UndefinedVal(),
                                      C.getState());
  C.addTransition(state);
}

void MallocChecker::MallocMemReturnsAttr(CheckerContext &C, const CallExpr *CE,
                                         const OwnershipAttr* Att) {
  if (!Att->isModule("malloc"))
    return;

  const unsigned *I = Att->begin(), *E = Att->end();
  if (I != E) {
    const GRState *state =
        MallocMemAux(C, CE, CE->getArg(*I), UndefinedVal(), C.getState());
    C.addTransition(state);
    return;
  }
  const GRState *state = MallocMemAux(C, CE, UnknownVal(), UndefinedVal(),
                                        C.getState());
  C.addTransition(state);
}

const GRState *MallocChecker::MallocMemAux(CheckerContext &C,  
                                           const CallExpr *CE,
                                           SVal Size, SVal Init,
                                           const GRState *state) {
  unsigned Count = C.getNodeBuilder().getCurrentBlockCount();
  ValueManager &ValMgr = C.getValueManager();

  // Set the return value.
  SVal RetVal = ValMgr.getConjuredSymbolVal(NULL, CE, CE->getType(), Count);
  state = state->BindExpr(CE, RetVal);

  // Fill the region with the initialization value.
  state = state->bindDefault(RetVal, Init);

  // Set the region's extent equal to the Size parameter.
  const SymbolicRegion *R = cast<SymbolicRegion>(RetVal.getAsRegion());
  DefinedOrUnknownSVal Extent = R->getExtent(ValMgr);
  DefinedOrUnknownSVal DefinedSize = cast<DefinedOrUnknownSVal>(Size);

  SValuator &SVator = ValMgr.getSValuator();
  DefinedOrUnknownSVal ExtentMatchesSize =
    SVator.EvalEQ(state, Extent, DefinedSize);
  state = state->Assume(ExtentMatchesSize, true);

  SymbolRef Sym = RetVal.getAsLocSymbol();
  assert(Sym);
  // Set the symbol's state to Allocated.
  return state->set<RegionState>(Sym, RefState::getAllocateUnchecked(CE));
}

void MallocChecker::FreeMem(CheckerContext &C, const CallExpr *CE) {
  const GRState *state = FreeMemAux(C, CE, C.getState(), 0, false);

  if (state)
    C.addTransition(state);
}

void MallocChecker::FreeMemAttr(CheckerContext &C, const CallExpr *CE,
                                     const OwnershipAttr* Att) {
  if (!Att->isModule("malloc"))
    return;

  for (const unsigned *I = Att->begin(), *E = Att->end(); I != E; ++I) {
    const GRState *state =
        FreeMemAux(C, CE, C.getState(), *I, Att->isKind(OwnershipAttr::Holds));
  if (state)
    C.addTransition(state);
  }
}

const GRState *MallocChecker::FreeMemAux(CheckerContext &C, const CallExpr *CE,
                                         const GRState *state, unsigned Num,
                                         bool Hold) {
  const Expr *ArgExpr = CE->getArg(Num);
  SVal ArgVal = state->getSVal(ArgExpr);

  DefinedOrUnknownSVal location = cast<DefinedOrUnknownSVal>(ArgVal);

  // Check for null dereferences.
  if (!isa<Loc>(location))
    return state;

  // FIXME: Technically using 'Assume' here can result in a path
  //  bifurcation.  In such cases we need to return two states, not just one.
  const GRState *notNullState, *nullState;
  llvm::tie(notNullState, nullState) = state->Assume(location);

  // The explicit NULL case, no operation is performed.
  if (nullState && !notNullState)
    return nullState;

  assert(notNullState);

  // Unknown values could easily be okay
  // Undefined values are handled elsewhere
  if (ArgVal.isUnknownOrUndef())
    return notNullState;

  const MemRegion *R = ArgVal.getAsRegion();
  
  // Nonlocs can't be freed, of course.
  // Non-region locations (labels and fixed addresses) also shouldn't be freed.
  if (!R) {
    ReportBadFree(C, ArgVal, ArgExpr->getSourceRange());
    return NULL;
  }
  
  R = R->StripCasts();
  
  // Blocks might show up as heap data, but should not be free()d
  if (isa<BlockDataRegion>(R)) {
    ReportBadFree(C, ArgVal, ArgExpr->getSourceRange());
    return NULL;
  }
  
  const MemSpaceRegion *MS = R->getMemorySpace();
  
  // Parameters, locals, statics, and globals shouldn't be freed.
  if (!(isa<UnknownSpaceRegion>(MS) || isa<HeapSpaceRegion>(MS))) {
    // FIXME: at the time this code was written, malloc() regions were
    // represented by conjured symbols, which are all in UnknownSpaceRegion.
    // This means that there isn't actually anything from HeapSpaceRegion
    // that should be freed, even though we allow it here.
    // Of course, free() can work on memory allocated outside the current
    // function, so UnknownSpaceRegion is always a possibility.
    // False negatives are better than false positives.
    
    ReportBadFree(C, ArgVal, ArgExpr->getSourceRange());
    return NULL;
  }
  
  const SymbolicRegion *SR = dyn_cast<SymbolicRegion>(R);
  // Various cases could lead to non-symbol values here.
  // For now, ignore them.
  if (!SR)
    return notNullState;

  SymbolRef Sym = SR->getSymbol();
  const RefState *RS = state->get<RegionState>(Sym);

  // If the symbol has not been tracked, return. This is possible when free() is
  // called on a pointer that does not get its pointee directly from malloc(). 
  // Full support of this requires inter-procedural analysis.
  if (!RS)
    return notNullState;

  // Check double free.
  if (RS->isReleased()) {
    if (ExplodedNode *N = C.GenerateSink()) {
      if (!BT_DoubleFree)
        BT_DoubleFree
          = new BuiltinBug("Double free",
                         "Try to free a memory block that has been released");
      // FIXME: should find where it's freed last time.
      BugReport *R = new BugReport(*BT_DoubleFree, 
                                   BT_DoubleFree->getDescription(), N);
      C.EmitReport(R);
    }
    return NULL;
  }

  // Normal free.
  if (Hold)
    return notNullState->set<RegionState>(Sym, RefState::getRelinquished(CE));
  return notNullState->set<RegionState>(Sym, RefState::getReleased(CE));
}

bool MallocChecker::SummarizeValue(llvm::raw_ostream& os, SVal V) {
  if (nonloc::ConcreteInt *IntVal = dyn_cast<nonloc::ConcreteInt>(&V))
    os << "an integer (" << IntVal->getValue() << ")";
  else if (loc::ConcreteInt *ConstAddr = dyn_cast<loc::ConcreteInt>(&V))
    os << "a constant address (" << ConstAddr->getValue() << ")";
  else if (loc::GotoLabel *Label = dyn_cast<loc::GotoLabel>(&V))
    os << "the address of the label '"
       << Label->getLabel()->getID()->getName()
       << "'";
  else
    return false;
  
  return true;
}

bool MallocChecker::SummarizeRegion(llvm::raw_ostream& os,
                                    const MemRegion *MR) {
  switch (MR->getKind()) {
  case MemRegion::FunctionTextRegionKind: {
    const FunctionDecl *FD = cast<FunctionTextRegion>(MR)->getDecl();
    if (FD)
      os << "the address of the function '" << FD << "'";
    else
      os << "the address of a function";
    return true;
  }
  case MemRegion::BlockTextRegionKind:
    os << "block text";
    return true;
  case MemRegion::BlockDataRegionKind:
    // FIXME: where the block came from?
    os << "a block";
    return true;
  default: {
    const MemSpaceRegion *MS = MR->getMemorySpace();
    
    switch (MS->getKind()) {
    case MemRegion::StackLocalsSpaceRegionKind: {
      const VarRegion *VR = dyn_cast<VarRegion>(MR);
      const VarDecl *VD;
      if (VR)
        VD = VR->getDecl();
      else
        VD = NULL;
      
      if (VD)
        os << "the address of the local variable '" << VD->getName() << "'";
      else
        os << "the address of a local stack variable";
      return true;
    }
    case MemRegion::StackArgumentsSpaceRegionKind: {
      const VarRegion *VR = dyn_cast<VarRegion>(MR);
      const VarDecl *VD;
      if (VR)
        VD = VR->getDecl();
      else
        VD = NULL;
      
      if (VD)
        os << "the address of the parameter '" << VD->getName() << "'";
      else
        os << "the address of a parameter";
      return true;
    }
    case MemRegion::NonStaticGlobalSpaceRegionKind:
    case MemRegion::StaticGlobalSpaceRegionKind: {
      const VarRegion *VR = dyn_cast<VarRegion>(MR);
      const VarDecl *VD;
      if (VR)
        VD = VR->getDecl();
      else
        VD = NULL;
      
      if (VD) {
        if (VD->isStaticLocal())
          os << "the address of the static variable '" << VD->getName() << "'";
        else
          os << "the address of the global variable '" << VD->getName() << "'";
      } else
        os << "the address of a global variable";
      return true;
    }
    default:
      return false;
    }
  }
  }
}

void MallocChecker::ReportBadFree(CheckerContext &C, SVal ArgVal,
                                  SourceRange range) {
  if (ExplodedNode *N = C.GenerateSink()) {
    if (!BT_BadFree)
      BT_BadFree = new BuiltinBug("Bad free");
    
    llvm::SmallString<100> buf;
    llvm::raw_svector_ostream os(buf);
    
    const MemRegion *MR = ArgVal.getAsRegion();
    if (MR) {
      while (const ElementRegion *ER = dyn_cast<ElementRegion>(MR))
        MR = ER->getSuperRegion();
      
      // Special case for alloca()
      if (isa<AllocaRegion>(MR))
        os << "Argument to free() was allocated by alloca(), not malloc()";
      else {
        os << "Argument to free() is ";
        if (SummarizeRegion(os, MR))
          os << ", which is not memory allocated by malloc()";
        else
          os << "not memory allocated by malloc()";
      }
    } else {
      os << "Argument to free() is ";
      if (SummarizeValue(os, ArgVal))
        os << ", which is not memory allocated by malloc()";
      else
        os << "not memory allocated by malloc()";
    }
    
    EnhancedBugReport *R = new EnhancedBugReport(*BT_BadFree, os.str(), N);
    R->addRange(range);
    C.EmitReport(R);
  }
}

void MallocChecker::ReallocMem(CheckerContext &C, const CallExpr *CE) {
  const GRState *state = C.getState();
  const Expr *Arg0 = CE->getArg(0);
  DefinedOrUnknownSVal Arg0Val=cast<DefinedOrUnknownSVal>(state->getSVal(Arg0));

  ValueManager &ValMgr = C.getValueManager();
  SValuator &SVator = C.getSValuator();

  DefinedOrUnknownSVal PtrEQ = SVator.EvalEQ(state, Arg0Val, ValMgr.makeNull());

  // If the ptr is NULL, the call is equivalent to malloc(size).
  if (const GRState *stateEqual = state->Assume(PtrEQ, true)) {
    // Hack: set the NULL symbolic region to released to suppress false warning.
    // In the future we should add more states for allocated regions, e.g., 
    // CheckedNull, CheckedNonNull.
    
    SymbolRef Sym = Arg0Val.getAsLocSymbol();
    if (Sym)
      stateEqual = stateEqual->set<RegionState>(Sym, RefState::getReleased(CE));

    const GRState *stateMalloc = MallocMemAux(C, CE, CE->getArg(1), 
                                              UndefinedVal(), stateEqual);
    C.addTransition(stateMalloc);
  }

  if (const GRState *stateNotEqual = state->Assume(PtrEQ, false)) {
    const Expr *Arg1 = CE->getArg(1);
    DefinedOrUnknownSVal Arg1Val = 
      cast<DefinedOrUnknownSVal>(stateNotEqual->getSVal(Arg1));
    DefinedOrUnknownSVal SizeZero = SVator.EvalEQ(stateNotEqual, Arg1Val,
                                      ValMgr.makeIntValWithPtrWidth(0, false));

    if (const GRState *stateSizeZero = stateNotEqual->Assume(SizeZero, true)) {
      const GRState *stateFree = FreeMemAux(C, CE, stateSizeZero, 0, false);
      if (stateFree)
        C.addTransition(stateFree->BindExpr(CE, UndefinedVal(), true));
    }

    if (const GRState *stateSizeNotZero=stateNotEqual->Assume(SizeZero,false)) {
      const GRState *stateFree = FreeMemAux(C, CE, stateSizeNotZero, 0, false);
      if (stateFree) {
        // FIXME: We should copy the content of the original buffer.
        const GRState *stateRealloc = MallocMemAux(C, CE, CE->getArg(1), 
                                                   UnknownVal(), stateFree);
        C.addTransition(stateRealloc);
      }
    }
  }
}

void MallocChecker::CallocMem(CheckerContext &C, const CallExpr *CE) {
  const GRState *state = C.getState();
  
  ValueManager &ValMgr = C.getValueManager();
  SValuator &SVator = C.getSValuator();

  SVal Count = state->getSVal(CE->getArg(0));
  SVal EleSize = state->getSVal(CE->getArg(1));
  SVal TotalSize = SVator.EvalBinOp(state, BinaryOperator::Mul, Count, EleSize,
                                    ValMgr.getContext().getSizeType());
  
  SVal Zero = ValMgr.makeZeroVal(ValMgr.getContext().CharTy);

  state = MallocMemAux(C, CE, TotalSize, Zero, state);
  C.addTransition(state);
}

void MallocChecker::EvalDeadSymbols(CheckerContext &C,SymbolReaper &SymReaper) {
  for (SymbolReaper::dead_iterator I = SymReaper.dead_begin(),
         E = SymReaper.dead_end(); I != E; ++I) {
    SymbolRef Sym = *I;
    const GRState *state = C.getState();
    const RefState *RS = state->get<RegionState>(Sym);
    if (!RS)
      return;

    if (RS->isAllocated()) {
      if (ExplodedNode *N = C.GenerateSink()) {
        if (!BT_Leak)
          BT_Leak = new BuiltinBug("Memory leak",
                     "Allocated memory never released. Potential memory leak.");
        // FIXME: where it is allocated.
        BugReport *R = new BugReport(*BT_Leak, BT_Leak->getDescription(), N);
        C.EmitReport(R);
      }
    }
  }
}

void MallocChecker::EvalEndPath(GREndPathNodeBuilder &B, void *tag,
                                GRExprEngine &Eng) {
  SaveAndRestore<bool> OldHasGen(B.HasGeneratedNode);
  const GRState *state = B.getState();
  typedef llvm::ImmutableMap<SymbolRef, RefState> SymMap;
  SymMap M = state->get<RegionState>();

  for (SymMap::iterator I = M.begin(), E = M.end(); I != E; ++I) {
    RefState RS = I->second;
    if (RS.isAllocated()) {
      ExplodedNode *N = B.generateNode(state, tag, B.getPredecessor());
      if (N) {
        if (!BT_Leak)
          BT_Leak = new BuiltinBug("Memory leak",
                     "Allocated memory never released. Potential memory leak.");
        BugReport *R = new BugReport(*BT_Leak, BT_Leak->getDescription(), N);
        Eng.getBugReporter().EmitReport(R);
      }
    }
  }
}

void MallocChecker::PreVisitReturnStmt(CheckerContext &C, const ReturnStmt *S) {
  const Expr *RetE = S->getRetValue();
  if (!RetE)
    return;

  const GRState *state = C.getState();

  SymbolRef Sym = state->getSVal(RetE).getAsSymbol();

  if (!Sym)
    return;

  const RefState *RS = state->get<RegionState>(Sym);
  if (!RS)
    return;

  // FIXME: check other cases.
  if (RS->isAllocated())
    state = state->set<RegionState>(Sym, RefState::getEscaped(S));

  C.addTransition(state);
}

const GRState *MallocChecker::EvalAssume(const GRState *state, SVal Cond, 
                                         bool Assumption,
                                         bool * /* respondsToCallback */) {
  // If a symblic region is assumed to NULL, set its state to AllocateFailed.
  // FIXME: should also check symbols assumed to non-null.

  RegionStateTy RS = state->get<RegionState>();

  for (RegionStateTy::iterator I = RS.begin(), E = RS.end(); I != E; ++I) {
    if (state->getSymVal(I.getKey()))
      state = state->set<RegionState>(I.getKey(),RefState::getAllocateFailed());
  }

  return state;
}

// Check if the location is a freed symbolic region.
void MallocChecker::VisitLocation(CheckerContext &C, const Stmt *S, SVal l) {
  SymbolRef Sym = l.getLocSymbolInBase();
  if (Sym) {
    const RefState *RS = C.getState()->get<RegionState>(Sym);
    if (RS && RS->isReleased()) {
      if (ExplodedNode *N = C.GenerateNode()) {
        if (!BT_UseFree)
          BT_UseFree = new BuiltinBug("Use dynamically allocated memory after"
                                      " it is freed.");

        BugReport *R = new BugReport(*BT_UseFree, BT_UseFree->getDescription(),
                                     N);
        C.EmitReport(R);
      }
    }
  }
}

void MallocChecker::PreVisitBind(CheckerContext &C,
                                 const Stmt *AssignE,
                                 const Stmt *StoreE,
                                 SVal location,
                                 SVal val) {
  // The PreVisitBind implements the same algorithm as already used by the 
  // Objective C ownership checker: if the pointer escaped from this scope by 
  // assignment, let it go.  However, assigning to fields of a stack-storage 
  // structure does not transfer ownership.

  const GRState *state = C.getState();
  DefinedOrUnknownSVal l = cast<DefinedOrUnknownSVal>(location);

  // Check for null dereferences.
  if (!isa<Loc>(l))
    return;

  // Before checking if the state is null, check if 'val' has a RefState.
  // Only then should we check for null and bifurcate the state.
  SymbolRef Sym = val.getLocSymbolInBase();
  if (Sym) {
    if (const RefState *RS = state->get<RegionState>(Sym)) {
      // If ptr is NULL, no operation is performed.
      const GRState *notNullState, *nullState;
      llvm::tie(notNullState, nullState) = state->Assume(l);

      // Generate a transition for 'nullState' to record the assumption
      // that the state was null.
      if (nullState)
        C.addTransition(nullState);

      if (!notNullState)
        return;

      if (RS->isAllocated()) {
        // Something we presently own is being assigned somewhere.
        const MemRegion *AR = location.getAsRegion();
        if (!AR)
          return;
        AR = AR->StripCasts()->getBaseRegion();
        do {
          // If it is on the stack, we still own it.
          if (AR->hasStackNonParametersStorage())
            break;

          // If the state can't represent this binding, we still own it.
          if (notNullState == (notNullState->bindLoc(cast<Loc>(location),
                                                     UnknownVal())))
            break;

          // We no longer own this pointer.
          notNullState =
            notNullState->set<RegionState>(Sym,
                                           RefState::getRelinquished(StoreE));
        }
        while (false);
      }
      C.addTransition(notNullState);
    }
  }
}