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 const BugType BT{this, "Cast region with wrong size."};
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 if (!ElemType)
66 return false;
67 CharUnits FlexSize;
68 if (const ConstantArrayType *ArrayTy =
69 Ctx.getAsConstantArrayType(T: Last->getType())) {
70 FlexSize = Ctx.getTypeSizeInChars(T: ElemType);
71 if (ArrayTy->getSize() == 1 && TypeSize > FlexSize)
72 TypeSize -= FlexSize;
73 else if (!ArrayTy->isZeroSize())
74 return false;
75 } else if (RD->hasFlexibleArrayMember()) {
76 FlexSize = Ctx.getTypeSizeInChars(T: ElemType);
77 } else {
78 return false;
79 }
80
81 if (FlexSize.isZero())
82 return false;
83
84 CharUnits Left = RegionSize - TypeSize;
85 if (Left.isNegative())
86 return false;
87
88 return Left % FlexSize == 0;
89}
90
91void CastSizeChecker::checkPreStmt(const CastExpr *CE,CheckerContext &C) const {
92 const Expr *E = CE->getSubExpr();
93 ASTContext &Ctx = C.getASTContext();
94 QualType ToTy = Ctx.getCanonicalType(T: CE->getType());
95 const PointerType *ToPTy = dyn_cast<PointerType>(Val: ToTy.getTypePtr());
96
97 if (!ToPTy)
98 return;
99
100 QualType ToPointeeTy = ToPTy->getPointeeType();
101
102 // Only perform the check if 'ToPointeeTy' is a complete type.
103 if (ToPointeeTy->isIncompleteType())
104 return;
105
106 ProgramStateRef state = C.getState();
107 const MemRegion *R = C.getSVal(S: E).getAsRegion();
108 if (!R)
109 return;
110
111 const SymbolicRegion *SR = dyn_cast<SymbolicRegion>(Val: R);
112 if (!SR)
113 return;
114
115 SValBuilder &svalBuilder = C.getSValBuilder();
116
117 DefinedOrUnknownSVal Size = getDynamicExtent(State: state, MR: SR, SVB&: svalBuilder);
118 const llvm::APSInt *SizeInt = svalBuilder.getKnownValue(state, val: Size);
119 if (!SizeInt)
120 return;
121
122 CharUnits regionSize = CharUnits::fromQuantity(Quantity: SizeInt->getZExtValue());
123 CharUnits typeSize = C.getASTContext().getTypeSizeInChars(T: ToPointeeTy);
124
125 // Ignore void, and a few other un-sizeable types.
126 if (typeSize.isZero())
127 return;
128
129 if (regionSize % typeSize == 0)
130 return;
131
132 if (evenFlexibleArraySize(Ctx, RegionSize: regionSize, TypeSize: typeSize, ToPointeeTy))
133 return;
134
135 if (ExplodedNode *errorNode = C.generateErrorNode()) {
136 constexpr llvm::StringLiteral Msg =
137 "Cast a region whose size is not a multiple of the destination type "
138 "size.";
139 auto R = std::make_unique<PathSensitiveBugReport>(args: BT, args: Msg, args&: errorNode);
140 R->addRange(R: CE->getSourceRange());
141 C.emitReport(R: 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