1//== TrustNonnullChecker.cpp --------- API nullability modeling -*- C++ -*--==//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This checker adds nullability-related assumptions:
11//
12// 1. Methods annotated with _Nonnull
13// which come from system headers actually return a non-null pointer.
14//
15// 2. NSDictionary key is non-null after the keyword subscript operation
16// on read if and only if the resulting expression is non-null.
17//
18// 3. NSMutableDictionary index is non-null after a write operation.
19//
20//===----------------------------------------------------------------------===//
21
22#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
23#include "clang/Analysis/SelectorExtras.h"
24#include "clang/StaticAnalyzer/Core/Checker.h"
25#include "clang/StaticAnalyzer/Core/CheckerManager.h"
26#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
27#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
28#include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
29
30using namespace clang;
31using namespace ento;
32
33/// Records implications between symbols.
34/// The semantics is:
35/// (antecedent != 0) => (consequent != 0)
36/// These implications are then read during the evaluation of the assumption,
37/// and the appropriate antecedents are applied.
38REGISTER_MAP_WITH_PROGRAMSTATE(NonNullImplicationMap, SymbolRef, SymbolRef)
39
40/// The semantics is:
41/// (antecedent == 0) => (consequent == 0)
42REGISTER_MAP_WITH_PROGRAMSTATE(NullImplicationMap, SymbolRef, SymbolRef)
43
44namespace {
45
46class TrustNonnullChecker : public Checker<check::PostCall,
47 check::PostObjCMessage,
48 check::DeadSymbols,
49 eval::Assume> {
50 // Do not try to iterate over symbols with higher complexity.
51 static unsigned constexpr ComplexityThreshold = 10;
52 Selector ObjectForKeyedSubscriptSel;
53 Selector ObjectForKeySel;
54 Selector SetObjectForKeyedSubscriptSel;
55 Selector SetObjectForKeySel;
56
57public:
58 TrustNonnullChecker(ASTContext &Ctx)
59 : ObjectForKeyedSubscriptSel(
60 getKeywordSelector(Ctx, "objectForKeyedSubscript")),
61 ObjectForKeySel(getKeywordSelector(Ctx, "objectForKey")),
62 SetObjectForKeyedSubscriptSel(
63 getKeywordSelector(Ctx, "setObject", "forKeyedSubscript")),
64 SetObjectForKeySel(getKeywordSelector(Ctx, "setObject", "forKey")) {}
65
66 ProgramStateRef evalAssume(ProgramStateRef State,
67 SVal Cond,
68 bool Assumption) const {
69 const SymbolRef CondS = Cond.getAsSymbol();
70 if (!CondS || CondS->computeComplexity() > ComplexityThreshold)
71 return State;
72
73 for (auto B=CondS->symbol_begin(), E=CondS->symbol_end(); B != E; ++B) {
74 const SymbolRef Antecedent = *B;
75 State = addImplication(Antecedent, State, true);
76 State = addImplication(Antecedent, State, false);
77 }
78
79 return State;
80 }
81
82 void checkPostCall(const CallEvent &Call, CheckerContext &C) const {
83 // Only trust annotations for system headers for non-protocols.
84 if (!Call.isInSystemHeader())
85 return;
86
87 ProgramStateRef State = C.getState();
88
89 if (isNonNullPtr(Call, C))
90 if (auto L = Call.getReturnValue().getAs<Loc>())
91 State = State->assume(*L, /*Assumption=*/true);
92
93 C.addTransition(State);
94 }
95
96 void checkPostObjCMessage(const ObjCMethodCall &Msg,
97 CheckerContext &C) const {
98 const ObjCInterfaceDecl *ID = Msg.getReceiverInterface();
99 if (!ID)
100 return;
101
102 ProgramStateRef State = C.getState();
103
104 // Index to setter for NSMutableDictionary is assumed to be non-null,
105 // as an exception is thrown otherwise.
106 if (interfaceHasSuperclass(ID, "NSMutableDictionary") &&
107 (Msg.getSelector() == SetObjectForKeyedSubscriptSel ||
108 Msg.getSelector() == SetObjectForKeySel)) {
109 if (auto L = Msg.getArgSVal(1).getAs<Loc>())
110 State = State->assume(*L, /*Assumption=*/true);
111 }
112
113 // Record an implication: index is non-null if the output is non-null.
114 if (interfaceHasSuperclass(ID, "NSDictionary") &&
115 (Msg.getSelector() == ObjectForKeyedSubscriptSel ||
116 Msg.getSelector() == ObjectForKeySel)) {
117 SymbolRef ArgS = Msg.getArgSVal(0).getAsSymbol();
118 SymbolRef RetS = Msg.getReturnValue().getAsSymbol();
119
120 if (ArgS && RetS) {
121 // Emulate an implication: the argument is non-null if
122 // the return value is non-null.
123 State = State->set<NonNullImplicationMap>(RetS, ArgS);
124
125 // Conversely, when the argument is null, the return value
126 // is definitely null.
127 State = State->set<NullImplicationMap>(ArgS, RetS);
128 }
129 }
130
131 C.addTransition(State);
132 }
133
134 void checkDeadSymbols(SymbolReaper &SymReaper, CheckerContext &C) const {
135 ProgramStateRef State = C.getState();
136
137 State = dropDeadFromGDM<NullImplicationMap>(SymReaper, State);
138 State = dropDeadFromGDM<NonNullImplicationMap>(SymReaper, State);
139
140 C.addTransition(State);
141 }
142
143private:
144
145 /// \returns State with GDM \p MapName where all dead symbols were
146 // removed.
147 template <typename MapName>
148 ProgramStateRef dropDeadFromGDM(SymbolReaper &SymReaper,
149 ProgramStateRef State) const {
150 for (const std::pair<SymbolRef, SymbolRef> &P : State->get<MapName>())
151 if (!SymReaper.isLive(P.first) || !SymReaper.isLive(P.second))
152 State = State->remove<MapName>(P.first);
153 return State;
154 }
155
156 /// \returns Whether we trust the result of the method call to be
157 /// a non-null pointer.
158 bool isNonNullPtr(const CallEvent &Call, CheckerContext &C) const {
159 QualType ExprRetType = Call.getResultType();
160 if (!ExprRetType->isAnyPointerType())
161 return false;
162
163 if (getNullabilityAnnotation(ExprRetType) == Nullability::Nonnull)
164 return true;
165
166 // The logic for ObjC instance method calls is more complicated,
167 // as the return value is nil when the receiver is nil.
168 if (!isa<ObjCMethodCall>(&Call))
169 return false;
170
171 const auto *MCall = cast<ObjCMethodCall>(&Call);
172 const ObjCMethodDecl *MD = MCall->getDecl();
173
174 // Distrust protocols.
175 if (isa<ObjCProtocolDecl>(MD->getDeclContext()))
176 return false;
177
178 QualType DeclRetType = MD->getReturnType();
179 if (getNullabilityAnnotation(DeclRetType) != Nullability::Nonnull)
180 return false;
181
182 // For class messages it is sufficient for the declaration to be
183 // annotated _Nonnull.
184 if (!MCall->isInstanceMessage())
185 return true;
186
187 // Alternatively, the analyzer could know that the receiver is not null.
188 SVal Receiver = MCall->getReceiverSVal();
189 ConditionTruthVal TV = C.getState()->isNonNull(Receiver);
190 if (TV.isConstrainedTrue())
191 return true;
192
193 return false;
194 }
195
196 /// \return Whether \p ID has a superclass by the name \p ClassName.
197 bool interfaceHasSuperclass(const ObjCInterfaceDecl *ID,
198 StringRef ClassName) const {
199 if (ID->getIdentifier()->getName() == ClassName)
200 return true;
201
202 if (const ObjCInterfaceDecl *Super = ID->getSuperClass())
203 return interfaceHasSuperclass(Super, ClassName);
204
205 return false;
206 }
207
208
209 /// \return a state with an optional implication added (if exists)
210 /// from a map of recorded implications.
211 /// If \p Negated is true, checks NullImplicationMap, and assumes
212 /// the negation of \p Antecedent.
213 /// Checks NonNullImplicationMap and assumes \p Antecedent otherwise.
214 ProgramStateRef addImplication(SymbolRef Antecedent,
215 ProgramStateRef InputState,
216 bool Negated) const {
217 if (!InputState)
218 return nullptr;
219 SValBuilder &SVB = InputState->getStateManager().getSValBuilder();
220 const SymbolRef *Consequent =
221 Negated ? InputState->get<NonNullImplicationMap>(Antecedent)
222 : InputState->get<NullImplicationMap>(Antecedent);
223 if (!Consequent)
224 return InputState;
225
226 SVal AntecedentV = SVB.makeSymbolVal(Antecedent);
227 ProgramStateRef State = InputState;
228
229 if ((Negated && InputState->isNonNull(AntecedentV).isConstrainedTrue())
230 || (!Negated && InputState->isNull(AntecedentV).isConstrainedTrue())) {
231 SVal ConsequentS = SVB.makeSymbolVal(*Consequent);
232 State = InputState->assume(ConsequentS.castAs<DefinedSVal>(), Negated);
233 if (!State)
234 return nullptr;
235
236 // Drop implications from the map.
237 if (Negated) {
238 State = State->remove<NonNullImplicationMap>(Antecedent);
239 State = State->remove<NullImplicationMap>(*Consequent);
240 } else {
241 State = State->remove<NullImplicationMap>(Antecedent);
242 State = State->remove<NonNullImplicationMap>(*Consequent);
243 }
244 }
245
246 return State;
247 }
248};
249
250} // end empty namespace
251
252
253void ento::registerTrustNonnullChecker(CheckerManager &Mgr) {
254 Mgr.registerChecker<TrustNonnullChecker>(Mgr.getASTContext());
255}