1 //== ReturnUndefChecker.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 // This file defines ReturnUndefChecker, which is a path-sensitive
10 // check which looks for undefined or garbage values being returned to the
11 // caller.
12 //
13 //===----------------------------------------------------------------------===//
14 
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/CallEvent.h"
20 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
21 
22 using namespace clang;
23 using namespace ento;
24 
25 namespace {
26 class ReturnUndefChecker : public Checker< check::PreStmt<ReturnStmt> > {
27   mutable std::unique_ptr<BuiltinBug> BT_Undef;
28   mutable std::unique_ptr<BuiltinBug> BT_NullReference;
29 
30   void emitUndef(CheckerContext &C, const Expr *RetE) const;
31   void checkReference(CheckerContext &C, const Expr *RetE,
32                       DefinedOrUnknownSVal RetVal) const;
33 public:
34   void checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const;
35 };
36 }
37 
38 void ReturnUndefChecker::checkPreStmt(const ReturnStmt *RS,
39                                       CheckerContext &C) const {
40   const Expr *RetE = RS->getRetValue();
41   if (!RetE)
42     return;
43   SVal RetVal = C.getSVal(RetE);
44 
45   const StackFrameContext *SFC = C.getStackFrame();
46   QualType RT = CallEvent::getDeclaredResultType(SFC->getDecl());
47 
48   if (RetVal.isUndef()) {
49     // "return;" is modeled to evaluate to an UndefinedVal. Allow UndefinedVal
50     // to be returned in functions returning void to support this pattern:
51     //   void foo() {
52     //     return;
53     //   }
54     //   void test() {
55     //     return foo();
56     //   }
57     if (!RT.isNull() && RT->isVoidType())
58       return;
59 
60     // Not all blocks have explicitly-specified return types; if the return type
61     // is not available, but the return value expression has 'void' type, assume
62     // Sema already checked it.
63     if (RT.isNull() && isa<BlockDecl>(SFC->getDecl()) &&
64         RetE->getType()->isVoidType())
65       return;
66 
67     emitUndef(C, RetE);
68     return;
69   }
70 
71   if (RT.isNull())
72     return;
73 
74   if (RT->isReferenceType()) {
75     checkReference(C, RetE, RetVal.castAs<DefinedOrUnknownSVal>());
76     return;
77   }
78 }
79 
80 static void emitBug(CheckerContext &C, BuiltinBug &BT, const Expr *RetE,
81                     const Expr *TrackingE = nullptr) {
82   ExplodedNode *N = C.generateErrorNode();
83   if (!N)
84     return;
85 
86   auto Report =
87       std::make_unique<PathSensitiveBugReport>(BT, BT.getDescription(), N);
88 
89   Report->addRange(RetE->getSourceRange());
90   bugreporter::trackExpressionValue(N, TrackingE ? TrackingE : RetE, *Report);
91 
92   C.emitReport(std::move(Report));
93 }
94 
95 void ReturnUndefChecker::emitUndef(CheckerContext &C, const Expr *RetE) const {
96   if (!BT_Undef)
97     BT_Undef.reset(
98         new BuiltinBug(this, "Garbage return value",
99                        "Undefined or garbage value returned to caller"));
100   emitBug(C, *BT_Undef, RetE);
101 }
102 
103 void ReturnUndefChecker::checkReference(CheckerContext &C, const Expr *RetE,
104                                         DefinedOrUnknownSVal RetVal) const {
105   ProgramStateRef StNonNull, StNull;
106   std::tie(StNonNull, StNull) = C.getState()->assume(RetVal);
107 
108   if (StNonNull) {
109     // Going forward, assume the location is non-null.
110     C.addTransition(StNonNull);
111     return;
112   }
113 
114   // The return value is known to be null. Emit a bug report.
115   if (!BT_NullReference)
116     BT_NullReference.reset(new BuiltinBug(this, "Returning null reference"));
117 
118   emitBug(C, *BT_NullReference, RetE, bugreporter::getDerefExpr(RetE));
119 }
120 
121 void ento::registerReturnUndefChecker(CheckerManager &mgr) {
122   mgr.registerChecker<ReturnUndefChecker>();
123 }
124 
125 bool ento::shouldRegisterReturnUndefChecker(const CheckerManager &mgr) {
126   return true;
127 }
128