clang/lib/Checker/CStringChecker.cpp

//= CStringChecker.h - Checks calls to C string functions ----------*- C++ -*-//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This defines CStringChecker, which is an assortment of checks on calls
// to functions in <string.h>.
//
//===----------------------------------------------------------------------===//

#include "GRExprEngineExperimentalChecks.h"
#include "clang/Checker/BugReporter/BugType.h"
#include "clang/Checker/PathSensitive/CheckerVisitor.h"
#include "llvm/ADT/StringSwitch.h"

using namespace clang;

namespace {
class CStringChecker : public CheckerVisitor<CStringChecker> {
  BugType *BT_Null, *BT_Bounds, *BT_Overlap, *BT_NotCString;
public:
  CStringChecker()
  : BT_Null(0), BT_Bounds(0), BT_Overlap(0), BT_NotCString(0) {}
  static void *getTag() { static int tag; return &tag; }

  bool EvalCallExpr(CheckerContext &C, const CallExpr *CE);

  typedef void (CStringChecker::*FnCheck)(CheckerContext &, const CallExpr *);

  void EvalMemcpy(CheckerContext &C, const CallExpr *CE);
  void EvalMemmove(CheckerContext &C, const CallExpr *CE);
  void EvalBcopy(CheckerContext &C, const CallExpr *CE);
  void EvalCopyCommon(CheckerContext &C, const GRState *state,
                      const Expr *Size, const Expr *Source, const Expr *Dest,
                      bool Restricted = false);

  void EvalMemcmp(CheckerContext &C, const CallExpr *CE);

  void EvalStrlen(CheckerContext &C, const CallExpr *CE);

  // Utility methods
  std::pair<const GRState*, const GRState*>
  AssumeZero(CheckerContext &C, const GRState *state, SVal V, QualType Ty);

  SVal GetCStringLength(CheckerContext &C, const GRState *state,
                        const Expr *Ex, SVal Buf);

  bool SummarizeRegion(llvm::raw_ostream& os, ASTContext& Ctx,
                       const MemRegion *MR);

  // Re-usable checks
  const GRState *CheckNonNull(CheckerContext &C, const GRState *state,
                               const Expr *S, SVal l);
  const GRState *CheckLocation(CheckerContext &C, const GRState *state,
                               const Expr *S, SVal l);
  const GRState *CheckBufferAccess(CheckerContext &C, const GRState *state,
                                   const Expr *Size,
                                   const Expr *FirstBuf,
                                   const Expr *SecondBuf = NULL);
  const GRState *CheckOverlap(CheckerContext &C, const GRState *state,
                              const Expr *Size, const Expr *First,
                              const Expr *Second);
  void EmitOverlapBug(CheckerContext &C, const GRState *state,
                      const Stmt *First, const Stmt *Second);
};
} //end anonymous namespace

void clang::RegisterCStringChecker(GRExprEngine &Eng) {
  Eng.registerCheck(new CStringChecker());
}

//===----------------------------------------------------------------------===//
// Individual checks and utility methods.
//===----------------------------------------------------------------------===//

std::pair<const GRState*, const GRState*>
CStringChecker::AssumeZero(CheckerContext &C, const GRState *state, SVal V,
                           QualType Ty) {
  DefinedSVal *Val = dyn_cast<DefinedSVal>(&V);
  if (!Val)
    return std::pair<const GRState*, const GRState *>(state, state);

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

  DefinedOrUnknownSVal Zero = ValMgr.makeZeroVal(Ty);
  DefinedOrUnknownSVal ValIsZero = SV.EvalEQ(state, *Val, Zero);

  return state->Assume(ValIsZero);
}

const GRState *CStringChecker::CheckNonNull(CheckerContext &C,
                                            const GRState *state,
                                            const Expr *S, SVal l) {
  // If a previous check has failed, propagate the failure.
  if (!state)
    return NULL;

  const GRState *stateNull, *stateNonNull;
  llvm::tie(stateNull, stateNonNull) = AssumeZero(C, state, l, S->getType());

  if (stateNull && !stateNonNull) {
    ExplodedNode *N = C.GenerateSink(stateNull);
    if (!N)
      return NULL;

    if (!BT_Null)
      BT_Null = new BuiltinBug("API",
        "Null pointer argument in call to byte string function");

    // Generate a report for this bug.
    BuiltinBug *BT = static_cast<BuiltinBug*>(BT_Null);
    EnhancedBugReport *report = new EnhancedBugReport(*BT,
                                                      BT->getDescription(), N);

    report->addRange(S->getSourceRange());
    report->addVisitorCreator(bugreporter::registerTrackNullOrUndefValue, S);
    C.EmitReport(report);
    return NULL;
  }

  // From here on, assume that the value is non-null.
  assert(stateNonNull);
  return stateNonNull;
}

// FIXME: This was originally copied from ArrayBoundChecker.cpp. Refactor?
const GRState *CStringChecker::CheckLocation(CheckerContext &C,
                                             const GRState *state,
                                             const Expr *S, SVal l) {
  // If a previous check has failed, propagate the failure.
  if (!state)
    return NULL;

  // Check for out of bound array element access.
  const MemRegion *R = l.getAsRegion();
  if (!R)
    return state;

  const ElementRegion *ER = dyn_cast<ElementRegion>(R);
  if (!ER)
    return state;

  assert(ER->getValueType() == C.getASTContext().CharTy &&
    "CheckLocation should only be called with char* ElementRegions");

  // Get the size of the array.
  const SubRegion *Super = cast<SubRegion>(ER->getSuperRegion());
  ValueManager &ValMgr = C.getValueManager();
  SVal Extent = ValMgr.convertToArrayIndex(Super->getExtent(ValMgr));
  DefinedOrUnknownSVal Size = cast<DefinedOrUnknownSVal>(Extent);

  // Get the index of the accessed element.
  DefinedOrUnknownSVal &Idx = cast<DefinedOrUnknownSVal>(ER->getIndex());

  const GRState *StInBound = state->AssumeInBound(Idx, Size, true);
  const GRState *StOutBound = state->AssumeInBound(Idx, Size, false);
  if (StOutBound && !StInBound) {
    ExplodedNode *N = C.GenerateSink(StOutBound);
    if (!N)
      return NULL;

    if (!BT_Bounds)
      BT_Bounds = new BuiltinBug("Out-of-bound array access",
        "Byte string function accesses out-of-bound array element "
        "(buffer overflow)");

    // FIXME: It would be nice to eventually make this diagnostic more clear,
    // e.g., by referencing the original declaration or by saying *why* this
    // reference is outside the range.

    // Generate a report for this bug.
    BuiltinBug *BT = static_cast<BuiltinBug*>(BT_Bounds);
    RangedBugReport *report = new RangedBugReport(*BT, BT->getDescription(), N);

    report->addRange(S->getSourceRange());
    C.EmitReport(report);
    return NULL;
  }
  
  // Array bound check succeeded.  From this point forward the array bound
  // should always succeed.
  return StInBound;
}

const GRState *CStringChecker::CheckBufferAccess(CheckerContext &C,
                                                 const GRState *state,
                                                 const Expr *Size,
                                                 const Expr *FirstBuf,
                                                 const Expr *SecondBuf) {
  // If a previous check has failed, propagate the failure.
  if (!state)
    return NULL;

  ValueManager &VM = C.getValueManager();
  SValuator &SV = VM.getSValuator();
  ASTContext &Ctx = C.getASTContext();

  QualType SizeTy = Ctx.getSizeType();
  QualType PtrTy = Ctx.getPointerType(Ctx.CharTy);

  // Check that the first buffer is non-null.
  SVal BufVal = state->getSVal(FirstBuf);
  state = CheckNonNull(C, state, FirstBuf, BufVal);
  if (!state)
    return NULL;

  // Get the access length and make sure it is known.
  SVal LengthVal = state->getSVal(Size);
  NonLoc *Length = dyn_cast<NonLoc>(&LengthVal);
  if (!Length)
    return state;

  // Compute the offset of the last element to be accessed: size-1.
  NonLoc One = cast<NonLoc>(VM.makeIntVal(1, SizeTy));
  NonLoc LastOffset = cast<NonLoc>(SV.EvalBinOpNN(state, BinaryOperator::Sub,
                                                  *Length, One, SizeTy));

  // Check that the first buffer is sufficently long.
  SVal BufStart = SV.EvalCast(BufVal, PtrTy, FirstBuf->getType());
  if (Loc *BufLoc = dyn_cast<Loc>(&BufStart)) {
    SVal BufEnd = SV.EvalBinOpLN(state, BinaryOperator::Add, *BufLoc,
                                 LastOffset, PtrTy);
    state = CheckLocation(C, state, FirstBuf, BufEnd);

    // If the buffer isn't large enough, abort.
    if (!state)
      return NULL;
  }

  // If there's a second buffer, check it as well.
  if (SecondBuf) {
    BufVal = state->getSVal(SecondBuf);
    state = CheckNonNull(C, state, SecondBuf, BufVal);
    if (!state)
      return NULL;

    BufStart = SV.EvalCast(BufVal, PtrTy, SecondBuf->getType());
    if (Loc *BufLoc = dyn_cast<Loc>(&BufStart)) {
      SVal BufEnd = SV.EvalBinOpLN(state, BinaryOperator::Add, *BufLoc,
                                   LastOffset, PtrTy);
      state = CheckLocation(C, state, SecondBuf, BufEnd);
    }
  }

  // Large enough or not, return this state!
  return state;
}

const GRState *CStringChecker::CheckOverlap(CheckerContext &C,
                                            const GRState *state,
                                            const Expr *Size,
                                            const Expr *First,
                                            const Expr *Second) {
  // Do a simple check for overlap: if the two arguments are from the same
  // buffer, see if the end of the first is greater than the start of the second
  // or vice versa.

  // If a previous check has failed, propagate the failure.
  if (!state)
    return NULL;

  ValueManager &VM = state->getStateManager().getValueManager();
  SValuator &SV = VM.getSValuator();
  ASTContext &Ctx = VM.getContext();
  const GRState *stateTrue, *stateFalse;

  // Get the buffer values and make sure they're known locations.
  SVal FirstVal = state->getSVal(First);
  SVal SecondVal = state->getSVal(Second);

  Loc *FirstLoc = dyn_cast<Loc>(&FirstVal);
  if (!FirstLoc)
    return state;

  Loc *SecondLoc = dyn_cast<Loc>(&SecondVal);
  if (!SecondLoc)
    return state;

  // Are the two values the same?
  DefinedOrUnknownSVal EqualTest = SV.EvalEQ(state, *FirstLoc, *SecondLoc);
  llvm::tie(stateTrue, stateFalse) = state->Assume(EqualTest);

  if (stateTrue && !stateFalse) {
    // If the values are known to be equal, that's automatically an overlap.
    EmitOverlapBug(C, stateTrue, First, Second);
    return NULL;
  }

  // Assume the two expressions are not equal.
  assert(stateFalse);
  state = stateFalse;

  // Which value comes first?
  QualType CmpTy = Ctx.IntTy;
  SVal Reverse = SV.EvalBinOpLL(state, BinaryOperator::GT,
                                *FirstLoc, *SecondLoc, CmpTy);
  DefinedOrUnknownSVal *ReverseTest = dyn_cast<DefinedOrUnknownSVal>(&Reverse);
  if (!ReverseTest)
    return state;

  llvm::tie(stateTrue, stateFalse) = state->Assume(*ReverseTest);

  if (stateTrue) {
    if (stateFalse) {
      // If we don't know which one comes first, we can't perform this test.
      return state;
    } else {
      // Switch the values so that FirstVal is before SecondVal.
      Loc *tmpLoc = FirstLoc;
      FirstLoc = SecondLoc;
      SecondLoc = tmpLoc;

      // Switch the Exprs as well, so that they still correspond.
      const Expr *tmpExpr = First;
      First = Second;
      Second = tmpExpr;
    }
  }

  // Get the length, and make sure it too is known.
  SVal LengthVal = state->getSVal(Size);
  NonLoc *Length = dyn_cast<NonLoc>(&LengthVal);
  if (!Length)
    return state;

  // Convert the first buffer's start address to char*.
  // Bail out if the cast fails.
  QualType CharPtrTy = Ctx.getPointerType(Ctx.CharTy);
  SVal FirstStart = SV.EvalCast(*FirstLoc, CharPtrTy, First->getType());
  Loc *FirstStartLoc = dyn_cast<Loc>(&FirstStart);
  if (!FirstStartLoc)
    return state;

  // Compute the end of the first buffer. Bail out if THAT fails.
  SVal FirstEnd = SV.EvalBinOpLN(state, BinaryOperator::Add,
                                 *FirstStartLoc, *Length, CharPtrTy);
  Loc *FirstEndLoc = dyn_cast<Loc>(&FirstEnd);
  if (!FirstEndLoc)
    return state;

  // Is the end of the first buffer past the start of the second buffer?
  SVal Overlap = SV.EvalBinOpLL(state, BinaryOperator::GT,
                                *FirstEndLoc, *SecondLoc, CmpTy);
  DefinedOrUnknownSVal *OverlapTest = dyn_cast<DefinedOrUnknownSVal>(&Overlap);
  if (!OverlapTest)
    return state;

  llvm::tie(stateTrue, stateFalse) = state->Assume(*OverlapTest);

  if (stateTrue && !stateFalse) {
    // Overlap!
    EmitOverlapBug(C, stateTrue, First, Second);
    return NULL;
  }

  // Assume the two expressions don't overlap.
  assert(stateFalse);
  return stateFalse;
}

void CStringChecker::EmitOverlapBug(CheckerContext &C, const GRState *state,
                                    const Stmt *First, const Stmt *Second) {
  ExplodedNode *N = C.GenerateSink(state);
  if (!N)
    return;

  if (!BT_Overlap)
    BT_Overlap = new BugType("Unix API", "Improper arguments");

  // Generate a report for this bug.
  RangedBugReport *report = 
    new RangedBugReport(*BT_Overlap,
      "Arguments must not be overlapping buffers", N);
  report->addRange(First->getSourceRange());
  report->addRange(Second->getSourceRange());

  C.EmitReport(report);
}

SVal CStringChecker::GetCStringLength(CheckerContext &C, const GRState *state,
                                      const Expr *Ex, SVal Buf) {
  const MemRegion *MR = Buf.getAsRegion();
  if (!MR) {
    // If we can't get a region, see if it's something we /know/ isn't a
    // C string. In the context of locations, the only time we can issue such
    // a warning is for labels.
    if (loc::GotoLabel *Label = dyn_cast<loc::GotoLabel>(&Buf)) {
      ExplodedNode *N = C.GenerateSink(state);
      if (N) {
        if (!BT_NotCString)
          BT_NotCString = new BuiltinBug("API",
            "Argument is not a null-terminated string.");

        llvm::SmallString<120> buf;
        llvm::raw_svector_ostream os(buf);
        os << "Argument to byte string function is the address of the label '"
           << Label->getLabel()->getID()->getName()
           << "', which is not a null-terminated string";

        // Generate a report for this bug.
        EnhancedBugReport *report = new EnhancedBugReport(*BT_NotCString,
                                                          os.str(), N);

        report->addRange(Ex->getSourceRange());
        C.EmitReport(report);        
      }

      return UndefinedVal();
    }

    // If it's not a region and not a label, it may be a constant location,
    // or it may be unknown. Just conjure a value as usual (see end of method).

  } else {
    // If we have a region, strip casts from it and see if we can figure out
    // its length. For anything we can't figure out, just conjure a value as
    // usual (see end of method).
    MR = MR->StripCasts();

    switch (MR->getKind()) {
    case MemRegion::StringRegionKind: {
      ValueManager &ValMgr = C.getValueManager();
      ASTContext &Ctx = ValMgr.getContext();
      const StringLiteral *Str = cast<StringRegion>(MR)->getStringLiteral();

      // Non-constant string literals may have been changed, so only return a
      // known value if we know the literal is constant.
      if (Str->getType().isConstant(Ctx)) {
        QualType SizeTy = Ctx.getSizeType();
        return ValMgr.makeIntVal(Str->getByteLength(), SizeTy);        
      }

      // FIXME: Handle the non-constant case. For now, just treat it like any
      // other initialized region.
      // FALL-THROUGH
    }
    case MemRegion::SymbolicRegionKind:
    case MemRegion::AllocaRegionKind:
    case MemRegion::VarRegionKind:
    case MemRegion::FieldRegionKind:
    case MemRegion::ObjCIvarRegionKind:
      // FIXME: These need to be tracked!
      break;
    case MemRegion::CompoundLiteralRegionKind:
      // FIXME: Can we track this? Is it necessary?
      break;
    case MemRegion::ElementRegionKind:
      // FIXME: How can we handle this? It's not good enough to subtract the
      // offset from the base string length; consider "123\x00567" and &a[5].
      break;
    default: {
      // Other regions (mostly non-data) can't have a reliable C string length.
      // In this case, an error is emitted and UndefinedVal is returned.
      // The caller should always be prepared to handle this case.
      ExplodedNode *N = C.GenerateSink(state);
      if (N) {
        if (!BT_NotCString)
          BT_NotCString = new BuiltinBug("API",
            "Argument is not a null-terminated string.");

        llvm::SmallString<120> buf;
        llvm::raw_svector_ostream os(buf);

        os << "Argument to byte string function is ";

        if (SummarizeRegion(os, C.getASTContext(), MR)) {
          os << ", which is not a null-terminated string";
        } else {
          os << "not a null-terminated string";
        }

        // Generate a report for this bug.
        EnhancedBugReport *report = new EnhancedBugReport(*BT_NotCString,
                                                          os.str(), N);

        report->addRange(Ex->getSourceRange());
        C.EmitReport(report);        
      }

      return UndefinedVal();
    }
    }
  }

  // If we can't track a certain region's C string length, or if we can't get a
  // region from the SVal, conjure a value, for use in later constraints.
  unsigned Count = C.getNodeBuilder().getCurrentBlockCount();
  ValueManager &ValMgr = C.getValueManager();
  QualType SizeTy = ValMgr.getContext().getSizeType();
  return ValMgr.getConjuredSymbolVal(getTag(), Ex, SizeTy, Count);
}

bool CStringChecker::SummarizeRegion(llvm::raw_ostream& os, ASTContext& Ctx,
                                     const MemRegion *MR) {
  const TypedRegion *TR = dyn_cast<TypedRegion>(MR);
  if (!TR)
    return false;

  switch (TR->getKind()) {
  case MemRegion::FunctionTextRegionKind: {
    const FunctionDecl *FD = cast<FunctionTextRegion>(TR)->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:
    os << "a block";
    return true;
  case MemRegion::CXXThisRegionKind:
  case MemRegion::CXXObjectRegionKind:
    os << "a C++ object of type " << TR->getValueType().getAsString();
    return true;
  case MemRegion::VarRegionKind:
    os << "a variable of type" << TR->getValueType().getAsString();
    return true;
  case MemRegion::FieldRegionKind:
    os << "a field of type " << TR->getValueType().getAsString();
    return true;
  case MemRegion::ObjCIvarRegionKind:
    os << "an instance variable of type " << TR->getValueType().getAsString();
    return true;
  default:
    return false;
  }
}

//===----------------------------------------------------------------------===//
// Evaluation of individual function calls.
//===----------------------------------------------------------------------===//

void CStringChecker::EvalCopyCommon(CheckerContext &C, const GRState *state,
                                    const Expr *Size, const Expr *Dest,
                                    const Expr *Source, bool Restricted) {
  // See if the size argument is zero.
  SVal SizeVal = state->getSVal(Size);
  QualType SizeTy = Size->getType();

  const GRState *StZeroSize, *StNonZeroSize;
  llvm::tie(StZeroSize, StNonZeroSize) = AssumeZero(C, state, SizeVal, SizeTy);

  // If the size is zero, there won't be any actual memory access.
  if (StZeroSize)
    C.addTransition(StZeroSize);

  // If the size can be nonzero, we have to check the other arguments.
  if (StNonZeroSize) {
    state = StNonZeroSize;
    state = CheckBufferAccess(C, state, Size, Dest, Source);
    if (Restricted)
      state = CheckOverlap(C, state, Size, Dest, Source);
    if (state)
      C.addTransition(state);
  }
}


void CStringChecker::EvalMemcpy(CheckerContext &C, const CallExpr *CE) {
  // void *memcpy(void *restrict dst, const void *restrict src, size_t n);
  // The return value is the address of the destination buffer.
  const Expr *Dest = CE->getArg(0);
  const GRState *state = C.getState();
  state = state->BindExpr(CE, state->getSVal(Dest));
  EvalCopyCommon(C, state, CE->getArg(2), Dest, CE->getArg(1), true);
}

void CStringChecker::EvalMemmove(CheckerContext &C, const CallExpr *CE) {
  // void *memmove(void *dst, const void *src, size_t n);
  // The return value is the address of the destination buffer.
  const Expr *Dest = CE->getArg(0);
  const GRState *state = C.getState();
  state = state->BindExpr(CE, state->getSVal(Dest));
  EvalCopyCommon(C, state, CE->getArg(2), Dest, CE->getArg(1));
}

void CStringChecker::EvalBcopy(CheckerContext &C, const CallExpr *CE) {
  // void bcopy(const void *src, void *dst, size_t n);
  EvalCopyCommon(C, C.getState(), CE->getArg(2), CE->getArg(1), CE->getArg(0));
}

void CStringChecker::EvalMemcmp(CheckerContext &C, const CallExpr *CE) {
  // int memcmp(const void *s1, const void *s2, size_t n);
  const Expr *Left = CE->getArg(0);
  const Expr *Right = CE->getArg(1);
  const Expr *Size = CE->getArg(2);

  const GRState *state = C.getState();
  ValueManager &ValMgr = C.getValueManager();
  SValuator &SV = ValMgr.getSValuator();

  // See if the size argument is zero.
  SVal SizeVal = state->getSVal(Size);
  QualType SizeTy = Size->getType();

  const GRState *StZeroSize, *StNonZeroSize;
  llvm::tie(StZeroSize, StNonZeroSize) = AssumeZero(C, state, SizeVal, SizeTy);

  // If the size can be zero, the result will be 0 in that case, and we don't
  // have to check either of the buffers.
  if (StZeroSize) {
    state = StZeroSize;
    state = state->BindExpr(CE, ValMgr.makeZeroVal(CE->getType()));
    C.addTransition(state);
  }

  // If the size can be nonzero, we have to check the other arguments.
  if (StNonZeroSize) {
    state = StNonZeroSize;

    // If we know the two buffers are the same, we know the result is 0.
    // First, get the two buffers' addresses. Another checker will have already
    // made sure they're not undefined.
    DefinedOrUnknownSVal LV = cast<DefinedOrUnknownSVal>(state->getSVal(Left));
    DefinedOrUnknownSVal RV = cast<DefinedOrUnknownSVal>(state->getSVal(Right));

    // See if they are the same.
    DefinedOrUnknownSVal SameBuf = SV.EvalEQ(state, LV, RV);
    const GRState *StSameBuf, *StNotSameBuf;
    llvm::tie(StSameBuf, StNotSameBuf) = state->Assume(SameBuf);

    // If the two arguments might be the same buffer, we know the result is zero,
    // and we only need to check one size.
    if (StSameBuf) {
      state = StSameBuf;
      state = CheckBufferAccess(C, state, Size, Left);
      if (state) {
        state = StSameBuf->BindExpr(CE, ValMgr.makeZeroVal(CE->getType()));
        C.addTransition(state); 
      }
    }

    // If the two arguments might be different buffers, we have to check the
    // size of both of them.
    if (StNotSameBuf) {
      state = StNotSameBuf;
      state = CheckBufferAccess(C, state, Size, Left, Right);
      if (state) {
        // The return value is the comparison result, which we don't know.
        unsigned Count = C.getNodeBuilder().getCurrentBlockCount();
        SVal CmpV = ValMgr.getConjuredSymbolVal(NULL, CE, CE->getType(), Count);
        state = state->BindExpr(CE, CmpV);
        C.addTransition(state);
      }
    }
  }
}

void CStringChecker::EvalStrlen(CheckerContext &C, const CallExpr *CE) {
  // size_t strlen(const char *s);
  const GRState *state = C.getState();
  const Expr *Arg = CE->getArg(0);
  SVal ArgVal = state->getSVal(Arg);

  // Check that the argument is non-null.
  state = CheckNonNull(C, state, Arg, ArgVal);

  if (state) {
    // Figure out what the length is, making sure the argument is a C string
    // (or something similar to a C string). If the argument is valid, the
    // length will be defined, and we can then set the return value.
    SVal StrLen = GetCStringLength(C, state, Arg, ArgVal);
    if (!StrLen.isUndef()) {
      state = state->BindExpr(CE, StrLen);
      C.addTransition(state);
    }
  }
}

//===----------------------------------------------------------------------===//
// The driver method.
//===----------------------------------------------------------------------===//

bool CStringChecker::EvalCallExpr(CheckerContext &C, const CallExpr *CE) {
  // Get the callee.  All the functions we care about are C functions
  // with simple identifiers.
  const GRState *state = C.getState();
  const Expr *Callee = CE->getCallee();
  const FunctionDecl *FD = state->getSVal(Callee).getAsFunctionDecl();

  if (!FD)
    return false;

  // Get the name of the callee. If it's a builtin, strip off the prefix.
  llvm::StringRef Name = FD->getName();
  if (Name.startswith("__builtin_"))
    Name = Name.substr(10);

  FnCheck EvalFunction = llvm::StringSwitch<FnCheck>(Name)
    .Cases("memcpy", "__memcpy_chk", &CStringChecker::EvalMemcpy)
    .Cases("memcmp", "bcmp", &CStringChecker::EvalMemcmp)
    .Cases("memmove", "__memmove_chk", &CStringChecker::EvalMemmove)
    .Case("strlen", &CStringChecker::EvalStrlen)
    .Case("bcopy", &CStringChecker::EvalBcopy)
    .Default(NULL);

  // If the callee isn't a string function, let another checker handle it.
  if (!EvalFunction)
    return false;

  // Check and evaluate the call.
  (this->*EvalFunction)(C, CE);
  return true;
}