clang 10.0.0git

ConversionChecker.cpp
Go to the documentation of this file.
1 //=== ConversionChecker.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 // Check that there is no loss of sign/precision in assignments, comparisons
10 // and multiplications.
11 //
12 // ConversionChecker uses path sensitive analysis to determine possible values
13 // of expressions. A warning is reported when:
14 // * a negative value is implicitly converted to an unsigned value in an
15 // assignment, comparison or multiplication.
16 // * assignment / initialization when the source value is greater than the max
17 // value of the target integer type
18 // * assignment / initialization when the source integer is above the range
19 // where the target floating point type can represent all integers
20 //
21 // Many compilers and tools have similar checks that are based on semantic
22 // analysis. Those checks are sound but have poor precision. ConversionChecker
23 // is an alternative to those checks.
24 //
25 //===----------------------------------------------------------------------===//
26 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
27 #include "clang/AST/ParentMap.h"
28 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
29 #include "clang/StaticAnalyzer/Core/Checker.h"
30 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
31 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
32 #include "llvm/ADT/APFloat.h"
33
34 #include <climits>
35
36 using namespace clang;
37 using namespace ento;
38
39 namespace {
40 class ConversionChecker : public Checker<check::PreStmt<ImplicitCastExpr>> {
41 public:
42 void checkPreStmt(const ImplicitCastExpr *Cast, CheckerContext &C) const;
43
44 private:
45 mutable std::unique_ptr<BuiltinBug> BT;
46
47 bool isLossOfPrecision(const ImplicitCastExpr *Cast, QualType DestType,
48 CheckerContext &C) const;
49
50 bool isLossOfSign(const ImplicitCastExpr *Cast, CheckerContext &C) const;
51
52 void reportBug(ExplodedNode *N, CheckerContext &C, const char Msg[]) const;
53 };
54 }
55
56 void ConversionChecker::checkPreStmt(const ImplicitCastExpr *Cast,
57 CheckerContext &C) const {
58 // TODO: For now we only warn about DeclRefExpr, to avoid noise. Warn for
59 // calculations also.
60 if (!isa<DeclRefExpr>(Cast->IgnoreParenImpCasts()))
61 return;
62
63 // Don't warn for loss of sign/precision in macros.
64 if (Cast->getExprLoc().isMacroID())
65 return;
66
67 // Get Parent.
68 const ParentMap &PM = C.getLocationContext()->getParentMap();
69 const Stmt *Parent = PM.getParent(Cast);
70 if (!Parent)
71 return;
72
73 bool LossOfSign = false;
74 bool LossOfPrecision = false;
75
76 // Loss of sign/precision in binary operation.
77 if (const auto *B = dyn_cast<BinaryOperator>(Parent)) {
78 BinaryOperator::Opcode Opc = B->getOpcode();
79 if (Opc == BO_Assign) {
80 LossOfSign = isLossOfSign(Cast, C);
81 LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
82 } else if (Opc == BO_AddAssign || Opc == BO_SubAssign) {
83 // No loss of sign.
84 LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
85 } else if (Opc == BO_MulAssign) {
86 LossOfSign = isLossOfSign(Cast, C);
87 LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
88 } else if (Opc == BO_DivAssign || Opc == BO_RemAssign) {
89 LossOfSign = isLossOfSign(Cast, C);
90 // No loss of precision.
91 } else if (Opc == BO_AndAssign) {
92 LossOfSign = isLossOfSign(Cast, C);
93 // No loss of precision.
94 } else if (Opc == BO_OrAssign || Opc == BO_XorAssign) {
95 LossOfSign = isLossOfSign(Cast, C);
96 LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
97 } else if (B->isRelationalOp() || B->isMultiplicativeOp()) {
98 LossOfSign = isLossOfSign(Cast, C);
99 }
100 } else if (isa<DeclStmt>(Parent)) {
101 LossOfSign = isLossOfSign(Cast, C);
102 LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
103 }
104
105 if (LossOfSign || LossOfPrecision) {
106 // Generate an error node.
107 ExplodedNode *N = C.generateNonFatalErrorNode(C.getState());
108 if (!N)
109 return;
110 if (LossOfSign)
111 reportBug(N, C, "Loss of sign in implicit conversion");
112 if (LossOfPrecision)
113 reportBug(N, C, "Loss of precision in implicit conversion");
114 }
115 }
116
117 void ConversionChecker::reportBug(ExplodedNode *N, CheckerContext &C,
118 const char Msg[]) const {
119 if (!BT)
120 BT.reset(
121 new BuiltinBug(this, "Conversion", "Possible loss of sign/precision."));
122
123 // Generate a report for this bug.
124 auto R = std::make_unique<PathSensitiveBugReport>(*BT, Msg, N);
125 C.emitReport(std::move(R));
126 }
127
128 bool ConversionChecker::isLossOfPrecision(const ImplicitCastExpr *Cast,
129 QualType DestType,
130 CheckerContext &C) const {
131 // Don't warn about explicit loss of precision.
132 if (Cast->isEvaluatable(C.getASTContext()))
133 return false;
134
135 QualType SubType = Cast->IgnoreParenImpCasts()->getType();
136
137 if (!DestType->isRealType() || !SubType->isIntegerType())
138 return false;
139
140 const bool isFloat = DestType->isFloatingType();
141
142 const auto &AC = C.getASTContext();
143
144 // We will find the largest RepresentsUntilExp value such that the DestType
145 // can exactly represent all nonnegative integers below 2^RepresentsUntilExp.
146 unsigned RepresentsUntilExp;
147
148 if (isFloat) {
149 const llvm::fltSemantics &Sema = AC.getFloatTypeSemantics(DestType);
150 RepresentsUntilExp = llvm::APFloat::semanticsPrecision(Sema);
151 } else {
152 RepresentsUntilExp = AC.getIntWidth(DestType);
153 if (RepresentsUntilExp == 1) {
154 // This is just casting a number to bool, probably not a bug.
155 return false;
156 }
157 if (DestType->isSignedIntegerType())
158 RepresentsUntilExp--;
159 }
160
161 if (RepresentsUntilExp >= sizeof(unsigned long long) * CHAR_BIT) {
162 // Avoid overflow in our later calculations.
163 return false;
164 }
165
166 unsigned CorrectedSrcWidth = AC.getIntWidth(SubType);
167 if (SubType->isSignedIntegerType())
168 CorrectedSrcWidth--;
169
170 if (RepresentsUntilExp >= CorrectedSrcWidth) {
171 // Simple case: the destination can store all values of the source type.
172 return false;
173 }
174
175 unsigned long long MaxVal = 1ULL << RepresentsUntilExp;
176 if (isFloat) {
177 // If this is a floating point type, it can also represent MaxVal exactly.
178 MaxVal++;
179 }
180 return C.isGreaterOrEqual(Cast->getSubExpr(), MaxVal);
181 // TODO: maybe also check negative values with too large magnitude.
182 }
183
184 bool ConversionChecker::isLossOfSign(const ImplicitCastExpr *Cast,
185 CheckerContext &C) const {
186 QualType CastType = Cast->getType();
187 QualType SubType = Cast->IgnoreParenImpCasts()->getType();
188
189 if (!CastType->isUnsignedIntegerType() || !SubType->isSignedIntegerType())
190 return false;
191
192 return C.isNegative(Cast->getSubExpr());
193 }
194
195 void ento::registerConversionChecker(CheckerManager &mgr) {
196 mgr.registerChecker<ConversionChecker>();
197 }
198
199 bool ento::shouldRegisterConversionChecker(const LangOptions &LO) {
200 return true;
201 }
ento
clang::interp::Cast
bool Cast(InterpState &S, CodePtr OpPC)
Definition: Interp.h:800
clang::QualType
A (possibly-)qualified type.
Definition: Type.h:654
clang::ParentMap
Definition: ParentMap.h:20
clang::Stmt
Stmt - This represents one statement.
Definition: Stmt.h:66
clang::ParentMap::getParent
Stmt * getParent(Stmt *) const
Definition: ParentMap.cpp:134
BugType.h
clang::Type::isUnsignedIntegerType
bool isUnsignedIntegerType() const
Return true if this is an integer type that is unsigned, according to C99 6.2.5p6 [which returns true...
Definition: Type.cpp:1968
Checker.h
CheckerManager.h
clang::CastExpr::getSubExpr
Expr * getSubExpr()
Definition: Expr.h:3202
clang::LangOptions
Keeps track of the various options that can be enabled, which controls the dialect of C or C++ that i...
Definition: LangOptions.h:53
clang::BinaryOperatorKind
BinaryOperatorKind
Definition: OperationKinds.h:25
CHAR_BIT
#define CHAR_BIT
Definition: limits.h:63
Parent
NodeId Parent
Definition: ASTDiff.cpp:191
clang::Sema
Sema - This implements semantic analysis and AST building for C.
Definition: Sema.h:336
ParentMap.h
clang::Type::isSignedIntegerType
bool isSignedIntegerType() const
Return true if this is an integer type that is signed, according to C99 6.2.5p4 [char, signed char, short, int, long..], or an enum decl which has a signed representation.
Definition: Type.cpp:1928
clang::Expr::getType
QualType getType() const
Definition: Expr.h:137
BuiltinCheckerRegistration.h
clang::ImplicitCastExpr
ImplicitCastExpr - Allows us to explicitly represent implicit type conversions, which have no direct ...
Definition: Expr.h:3274
clang::Expr::getExprLoc
SourceLocation getExprLoc() const LLVM_READONLY
getExprLoc - Return the preferred location for the arrow when diagnosing a problem with a generic exp...
Definition: Expr.cpp:224
clang::Type::isRealType
bool isRealType() const
Definition: Type.cpp:2027
clang
Dataflow Directional Tag Classes.
Definition: CFGReachabilityAnalysis.h:21
clang::Expr::IgnoreParenImpCasts
Expr * IgnoreParenImpCasts() LLVM_READONLY
Skip past any parentheses and implicit casts which might surround this expression until reaching a fi...
Definition: Expr.cpp:2990
clang::SourceLocation::isMacroID
bool isMacroID() const
Definition: SourceLocation.h:99
clang::Type::isIntegerType
bool isIntegerType() const
isIntegerType() does not include complex integers (a GCC extension).
Definition: Type.h:6811
clang::Expr::isEvaluatable
bool isEvaluatable(const ASTContext &Ctx, SideEffectsKind AllowSideEffects=SE_NoSideEffects) const
isEvaluatable - Call EvaluateAsRValue to see if this expression can be constant folded without side-e...
Definition: ExprConstant.cpp:13946
clang::Type::isFloatingType
bool isFloatingType() const
Definition: Type.cpp:2005
CheckerContext.h
CastType
CastType
Definition: SemaCast.cpp:45