CastSizeChecker.cpp revision 360784 
1//=== CastSizeChecker.cpp ---------------------------------------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// CastSizeChecker checks when casting a malloc'ed symbolic region to type T,
10// whether the size of the symbolic region is a multiple of the size of T.
11//
12//===----------------------------------------------------------------------===//
13#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
14#include "clang/AST/CharUnits.h"
15#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
16#include "clang/StaticAnalyzer/Core/Checker.h"
17#include "clang/StaticAnalyzer/Core/CheckerManager.h"
18#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
19
20using namespace clang;
21using namespace ento;
22
23namespace {
24class CastSizeChecker : public Checker< check::PreStmt<CastExpr> > {
25  mutable std::unique_ptr<BuiltinBug> BT;
26
27public:
28  void checkPreStmt(const CastExpr *CE, CheckerContext &C) const;
29};
30}
31
32/// Check if we are casting to a struct with a flexible array at the end.
33/// \code
34/// struct foo {
35///   size_t len;
36///   struct bar data[];
37/// };
38/// \endcode
39/// or
40/// \code
41/// struct foo {
42///   size_t len;
43///   struct bar data[0];
44/// }
45/// \endcode
46/// In these cases it is also valid to allocate size of struct foo + a multiple
47/// of struct bar.
48static bool evenFlexibleArraySize(ASTContext &Ctx, CharUnits RegionSize,
49                                  CharUnits TypeSize, QualType ToPointeeTy) {
50  const RecordType *RT = ToPointeeTy->getAs<RecordType>();
51  if (!RT)
52    return false;
53
54  const RecordDecl *RD = RT->getDecl();
55  RecordDecl::field_iterator Iter(RD->field_begin());
56  RecordDecl::field_iterator End(RD->field_end());
57  const FieldDecl *Last = nullptr;
58  for (; Iter != End; ++Iter)
59    Last = *Iter;
60  assert(Last && "empty structs should already be handled");
61
62  const Type *ElemType = Last->getType()->getArrayElementTypeNoTypeQual();
63  CharUnits FlexSize;
64  if (const ConstantArrayType *ArrayTy =
65        Ctx.getAsConstantArrayType(Last->getType())) {
66    FlexSize = Ctx.getTypeSizeInChars(ElemType);
67    if (ArrayTy->getSize() == 1 && TypeSize > FlexSize)
68      TypeSize -= FlexSize;
69    else if (ArrayTy->getSize() != 0)
70      return false;
71  } else if (RD->hasFlexibleArrayMember()) {
72    FlexSize = Ctx.getTypeSizeInChars(ElemType);
73  } else {
74    return false;
75  }
76
77  if (FlexSize.isZero())
78    return false;
79
80  CharUnits Left = RegionSize - TypeSize;
81  if (Left.isNegative())
82    return false;
83
84  return Left % FlexSize == 0;
85}
86
87void CastSizeChecker::checkPreStmt(const CastExpr *CE,CheckerContext &C) const {
88  const Expr *E = CE->getSubExpr();
89  ASTContext &Ctx = C.getASTContext();
90  QualType ToTy = Ctx.getCanonicalType(CE->getType());
91  const PointerType *ToPTy = dyn_cast<PointerType>(ToTy.getTypePtr());
92
93  if (!ToPTy)
94    return;
95
96  QualType ToPointeeTy = ToPTy->getPointeeType();
97
98  // Only perform the check if 'ToPointeeTy' is a complete type.
99  if (ToPointeeTy->isIncompleteType())
100    return;
101
102  ProgramStateRef state = C.getState();
103  const MemRegion *R = C.getSVal(E).getAsRegion();
104  if (!R)
105    return;
106
107  const SymbolicRegion *SR = dyn_cast<SymbolicRegion>(R);
108  if (!SR)
109    return;
110
111  SValBuilder &svalBuilder = C.getSValBuilder();
112  SVal extent = SR->getExtent(svalBuilder);
113  const llvm::APSInt *extentInt = svalBuilder.getKnownValue(state, extent);
114  if (!extentInt)
115    return;
116
117  CharUnits regionSize = CharUnits::fromQuantity(extentInt->getSExtValue());
118  CharUnits typeSize = C.getASTContext().getTypeSizeInChars(ToPointeeTy);
119
120  // Ignore void, and a few other un-sizeable types.
121  if (typeSize.isZero())
122    return;
123
124  if (regionSize % typeSize == 0)
125    return;
126
127  if (evenFlexibleArraySize(Ctx, regionSize, typeSize, ToPointeeTy))
128    return;
129
130  if (ExplodedNode *errorNode = C.generateErrorNode()) {
131    if (!BT)
132      BT.reset(new BuiltinBug(this, "Cast region with wrong size.",
133                                    "Cast a region whose size is not a multiple"
134                                    " of the destination type size."));
135    auto R = std::make_unique<PathSensitiveBugReport>(*BT, BT->getDescription(),
136                                                      errorNode);
137    R->addRange(CE->getSourceRange());
138    C.emitReport(std::move(R));
139  }
140}
141
142void ento::registerCastSizeChecker(CheckerManager &mgr) {
143  mgr.registerChecker<CastSizeChecker>();
144}
145
146bool ento::shouldRegisterCastSizeChecker(const LangOptions &LO) {
147  // PR31226: C++ is more complicated than what this checker currently supports.
148  // There are derived-to-base casts, there are different rules for 0-size
149  // structures, no flexible arrays, etc.
150  // FIXME: Disabled on C++ for now.
151  return !LO.CPlusPlus;
152}
153