1 //===- EnumCastOutOfRangeChecker.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 // The EnumCastOutOfRangeChecker is responsible for checking integer to
10 // enumeration casts that could result in undefined values. This could happen
11 // if the value that we cast from is out of the value range of the enumeration.
12 // Reference:
13 // [ISO/IEC 14882-2014] ISO/IEC 14882-2014.
14 //   Programming Languages — C++, Fourth Edition. 2014.
15 // C++ Standard, [dcl.enum], in paragraph 8, which defines the range of an enum
16 // C++ Standard, [expr.static.cast], paragraph 10, which defines the behaviour
17 //   of casting an integer value that is out of range
18 // SEI CERT C++ Coding Standard, INT50-CPP. Do not cast to an out-of-range
19 //   enumeration value
20 //===----------------------------------------------------------------------===//
21 
22 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
23 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
24 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
25 
26 using namespace clang;
27 using namespace ento;
28 
29 namespace {
30 // This evaluator checks two SVals for equality. The first SVal is provided via
31 // the constructor, the second is the parameter of the overloaded () operator.
32 // It uses the in-built ConstraintManager to resolve the equlity to possible or
33 // not possible ProgramStates.
34 class ConstraintBasedEQEvaluator {
35   const DefinedOrUnknownSVal CompareValue;
36   const ProgramStateRef PS;
37   SValBuilder &SVB;
38 
39 public:
40   ConstraintBasedEQEvaluator(CheckerContext &C,
41                              const DefinedOrUnknownSVal CompareValue)
42       : CompareValue(CompareValue), PS(C.getState()), SVB(C.getSValBuilder()) {}
43 
44   bool operator()(const llvm::APSInt &EnumDeclInitValue) {
45     DefinedOrUnknownSVal EnumDeclValue = SVB.makeIntVal(EnumDeclInitValue);
46     DefinedOrUnknownSVal ElemEqualsValueToCast =
47         SVB.evalEQ(PS, EnumDeclValue, CompareValue);
48 
49     return static_cast<bool>(PS->assume(ElemEqualsValueToCast, true));
50   }
51 };
52 
53 // This checker checks CastExpr statements.
54 // If the value provided to the cast is one of the values the enumeration can
55 // represent, the said value matches the enumeration. If the checker can
56 // establish the impossibility of matching it gives a warning.
57 // Being conservative, it does not warn if there is slight possibility the
58 // value can be matching.
59 class EnumCastOutOfRangeChecker : public Checker<check::PreStmt<CastExpr>> {
60   mutable std::unique_ptr<BuiltinBug> EnumValueCastOutOfRange;
61   void reportWarning(CheckerContext &C) const;
62 
63 public:
64   void checkPreStmt(const CastExpr *CE, CheckerContext &C) const;
65 };
66 
67 using EnumValueVector = llvm::SmallVector<llvm::APSInt, 6>;
68 
69 // Collects all of the values an enum can represent (as SVals).
70 EnumValueVector getDeclValuesForEnum(const EnumDecl *ED) {
71   EnumValueVector DeclValues(
72       std::distance(ED->enumerator_begin(), ED->enumerator_end()));
73   llvm::transform(ED->enumerators(), DeclValues.begin(),
74                  [](const EnumConstantDecl *D) { return D->getInitVal(); });
75   return DeclValues;
76 }
77 } // namespace
78 
79 void EnumCastOutOfRangeChecker::reportWarning(CheckerContext &C) const {
80   if (const ExplodedNode *N = C.generateNonFatalErrorNode()) {
81     if (!EnumValueCastOutOfRange)
82       EnumValueCastOutOfRange.reset(
83           new BuiltinBug(this, "Enum cast out of range",
84                          "The value provided to the cast expression is not in "
85                          "the valid range of values for the enum"));
86     C.emitReport(std::make_unique<PathSensitiveBugReport>(
87         *EnumValueCastOutOfRange, EnumValueCastOutOfRange->getDescription(),
88         N));
89   }
90 }
91 
92 void EnumCastOutOfRangeChecker::checkPreStmt(const CastExpr *CE,
93                                              CheckerContext &C) const {
94 
95   // Only perform enum range check on casts where such checks are valid.  For
96   // all other cast kinds (where enum range checks are unnecessary or invalid),
97   // just return immediately.  TODO: The set of casts allowed for enum range
98   // checking may be incomplete.  Better to add a missing cast kind to enable a
99   // missing check than to generate false negatives and have to remove those
100   // later.
101   switch (CE->getCastKind()) {
102   case CK_IntegralCast:
103     break;
104 
105   default:
106     return;
107     break;
108   }
109 
110   // Get the value of the expression to cast.
111   const llvm::Optional<DefinedOrUnknownSVal> ValueToCast =
112       C.getSVal(CE->getSubExpr()).getAs<DefinedOrUnknownSVal>();
113 
114   // If the value cannot be reasoned about (not even a DefinedOrUnknownSVal),
115   // don't analyze further.
116   if (!ValueToCast)
117     return;
118 
119   const QualType T = CE->getType();
120   // Check whether the cast type is an enum.
121   if (!T->isEnumeralType())
122     return;
123 
124   // If the cast is an enum, get its declaration.
125   // If the isEnumeralType() returned true, then the declaration must exist
126   // even if it is a stub declaration. It is up to the getDeclValuesForEnum()
127   // function to handle this.
128   const EnumDecl *ED = T->castAs<EnumType>()->getDecl();
129 
130   EnumValueVector DeclValues = getDeclValuesForEnum(ED);
131   // Check if any of the enum values possibly match.
132   bool PossibleValueMatch = llvm::any_of(
133       DeclValues, ConstraintBasedEQEvaluator(C, *ValueToCast));
134 
135   // If there is no value that can possibly match any of the enum values, then
136   // warn.
137   if (!PossibleValueMatch)
138     reportWarning(C);
139 }
140 
141 void ento::registerEnumCastOutOfRangeChecker(CheckerManager &mgr) {
142   mgr.registerChecker<EnumCastOutOfRangeChecker>();
143 }
144 
145 bool ento::shouldRegisterEnumCastOutOfRangeChecker(const CheckerManager &mgr) {
146   return true;
147 }
148