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
14#include "clang/AST/CharUnits.h"
15#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.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#include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h"
21
22using namespace clang;
23using namespace ento;
24
25namespace {
26class CastSizeChecker : public Checker< check::PreStmt<CastExpr> > {
27  mutable std::unique_ptr<BuiltinBug> BT;
28
29public:
30  void checkPreStmt(const CastExpr *CE, CheckerContext &C) const;
31};
32}
33
34/// Check if we are casting to a struct with a flexible array at the end.
35/// \code
36/// struct foo {
37///   size_t len;
38///   struct bar data[];
39/// };
40/// \endcode
41/// or
42/// \code
43/// struct foo {
44///   size_t len;
45///   struct bar data[0];
46/// }
47/// \endcode
48/// In these cases it is also valid to allocate size of struct foo + a multiple
49/// of struct bar.
50static bool evenFlexibleArraySize(ASTContext &Ctx, CharUnits RegionSize,
51                                  CharUnits TypeSize, QualType ToPointeeTy) {
52  const RecordType *RT = ToPointeeTy->getAs<RecordType>();
53  if (!RT)
54    return false;
55
56  const RecordDecl *RD = RT->getDecl();
57  RecordDecl::field_iterator Iter(RD->field_begin());
58  RecordDecl::field_iterator End(RD->field_end());
59  const FieldDecl *Last = nullptr;
60  for (; Iter != End; ++Iter)
61    Last = *Iter;
62  assert(Last && "empty structs should already be handled");
63
64  const Type *ElemType = Last->getType()->getArrayElementTypeNoTypeQual();
65  CharUnits FlexSize;
66  if (const ConstantArrayType *ArrayTy =
67        Ctx.getAsConstantArrayType(Last->getType())) {
68    FlexSize = Ctx.getTypeSizeInChars(ElemType);
69    if (ArrayTy->getSize() == 1 && TypeSize > FlexSize)
70      TypeSize -= FlexSize;
71    else if (ArrayTy->getSize() != 0)
72      return false;
73  } else if (RD->hasFlexibleArrayMember()) {
74    FlexSize = Ctx.getTypeSizeInChars(ElemType);
75  } else {
76    return false;
77  }
78
79  if (FlexSize.isZero())
80    return false;
81
82  CharUnits Left = RegionSize - TypeSize;
83  if (Left.isNegative())
84    return false;
85
86  return Left % FlexSize == 0;
87}
88
89void CastSizeChecker::checkPreStmt(const CastExpr *CE,CheckerContext &C) const {
90  const Expr *E = CE->getSubExpr();
91  ASTContext &Ctx = C.getASTContext();
92  QualType ToTy = Ctx.getCanonicalType(CE->getType());
93  const PointerType *ToPTy = dyn_cast<PointerType>(ToTy.getTypePtr());
94
95  if (!ToPTy)
96    return;
97
98  QualType ToPointeeTy = ToPTy->getPointeeType();
99
100  // Only perform the check if 'ToPointeeTy' is a complete type.
101  if (ToPointeeTy->isIncompleteType())
102    return;
103
104  ProgramStateRef state = C.getState();
105  const MemRegion *R = C.getSVal(E).getAsRegion();
106  if (!R)
107    return;
108
109  const SymbolicRegion *SR = dyn_cast<SymbolicRegion>(R);
110  if (!SR)
111    return;
112
113  SValBuilder &svalBuilder = C.getSValBuilder();
114
115  DefinedOrUnknownSVal Size = getDynamicExtent(state, SR, svalBuilder);
116  const llvm::APSInt *SizeInt = svalBuilder.getKnownValue(state, Size);
117  if (!SizeInt)
118    return;
119
120  CharUnits regionSize = CharUnits::fromQuantity(SizeInt->getZExtValue());
121  CharUnits typeSize = C.getASTContext().getTypeSizeInChars(ToPointeeTy);
122
123  // Ignore void, and a few other un-sizeable types.
124  if (typeSize.isZero())
125    return;
126
127  if (regionSize % typeSize == 0)
128    return;
129
130  if (evenFlexibleArraySize(Ctx, regionSize, typeSize, ToPointeeTy))
131    return;
132
133  if (ExplodedNode *errorNode = C.generateErrorNode()) {
134    if (!BT)
135      BT.reset(new BuiltinBug(this, "Cast region with wrong size.",
136                                    "Cast a region whose size is not a multiple"
137                                    " of the destination type size."));
138    auto R = std::make_unique<PathSensitiveBugReport>(*BT, BT->getDescription(),
139                                                      errorNode);
140    R->addRange(CE->getSourceRange());
141    C.emitReport(std::move(R));
142  }
143}
144
145void ento::registerCastSizeChecker(CheckerManager &mgr) {
146  mgr.registerChecker<CastSizeChecker>();
147}
148
149bool ento::shouldRegisterCastSizeChecker(const CheckerManager &mgr) {
150  // PR31226: C++ is more complicated than what this checker currently supports.
151  // There are derived-to-base casts, there are different rules for 0-size
152  // structures, no flexible arrays, etc.
153  // FIXME: Disabled on C++ for now.
154  const LangOptions &LO = mgr.getLangOpts();
155  return !LO.CPlusPlus;
156}
157