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 
22 using namespace clang;
23 using namespace ento;
24 
25 namespace {
26 class CastSizeChecker : public Checker< check::PreStmt<CastExpr> > {
27   const BugType BT{this, "Cast region with wrong size."};
28 
29 public:
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.
50 static 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 
89 void 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     constexpr llvm::StringLiteral Msg =
135         "Cast a region whose size is not a multiple of the destination type "
136         "size.";
137     auto R = std::make_unique<PathSensitiveBugReport>(BT, Msg, errorNode);
138     R->addRange(CE->getSourceRange());
139     C.emitReport(std::move(R));
140   }
141 }
142 
143 void ento::registerCastSizeChecker(CheckerManager &mgr) {
144   mgr.registerChecker<CastSizeChecker>();
145 }
146 
147 bool ento::shouldRegisterCastSizeChecker(const CheckerManager &mgr) {
148   // PR31226: C++ is more complicated than what this checker currently supports.
149   // There are derived-to-base casts, there are different rules for 0-size
150   // structures, no flexible arrays, etc.
151   // FIXME: Disabled on C++ for now.
152   const LangOptions &LO = mgr.getLangOpts();
153   return !LO.CPlusPlus;
154 }
155