1 //== ReturnUndefChecker.cpp -------------------------------------*- 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 file defines ReturnUndefChecker, which is a path-sensitive
11 // check which looks for undefined or garbage values being returned to the
12 // caller.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #include "ClangSACheckers.h"
17 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
18 #include "clang/StaticAnalyzer/Core/Checker.h"
19 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
20 #include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
21 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
22 
23 using namespace clang;
24 using namespace ento;
25 
26 namespace {
27 class ReturnUndefChecker : public Checker< check::PreStmt<ReturnStmt> > {
28   mutable std::unique_ptr<BuiltinBug> BT_Undef;
29   mutable std::unique_ptr<BuiltinBug> BT_NullReference;
30 
31   void emitUndef(CheckerContext &C, const Expr *RetE) const;
32   void checkReference(CheckerContext &C, const Expr *RetE,
33                       DefinedOrUnknownSVal RetVal) const;
34 public:
35   void checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const;
36 };
37 }
38 
checkPreStmt(const ReturnStmt * RS,CheckerContext & C) const39 void ReturnUndefChecker::checkPreStmt(const ReturnStmt *RS,
40                                       CheckerContext &C) const {
41   const Expr *RetE = RS->getRetValue();
42   if (!RetE)
43     return;
44   SVal RetVal = C.getSVal(RetE);
45 
46   const StackFrameContext *SFC = C.getStackFrame();
47   QualType RT = CallEvent::getDeclaredResultType(SFC->getDecl());
48 
49   if (RetVal.isUndef()) {
50     // "return;" is modeled to evaluate to an UndefinedVal. Allow UndefinedVal
51     // to be returned in functions returning void to support this pattern:
52     //   void foo() {
53     //     return;
54     //   }
55     //   void test() {
56     //     return foo();
57     //   }
58     if (!RT.isNull() && RT->isVoidType())
59       return;
60 
61     // Not all blocks have explicitly-specified return types; if the return type
62     // is not available, but the return value expression has 'void' type, assume
63     // Sema already checked it.
64     if (RT.isNull() && isa<BlockDecl>(SFC->getDecl()) &&
65         RetE->getType()->isVoidType())
66       return;
67 
68     emitUndef(C, RetE);
69     return;
70   }
71 
72   if (RT.isNull())
73     return;
74 
75   if (RT->isReferenceType()) {
76     checkReference(C, RetE, RetVal.castAs<DefinedOrUnknownSVal>());
77     return;
78   }
79 }
80 
emitBug(CheckerContext & C,BuiltinBug & BT,const Expr * RetE,const Expr * TrackingE=nullptr)81 static void emitBug(CheckerContext &C, BuiltinBug &BT, const Expr *RetE,
82                     const Expr *TrackingE = nullptr) {
83   ExplodedNode *N = C.generateSink();
84   if (!N)
85     return;
86 
87   BugReport *Report = new BugReport(BT, BT.getDescription(), N);
88 
89   Report->addRange(RetE->getSourceRange());
90   bugreporter::trackNullOrUndefValue(N, TrackingE ? TrackingE : RetE, *Report);
91 
92   C.emitReport(Report);
93 }
94 
emitUndef(CheckerContext & C,const Expr * RetE) const95 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 
checkReference(CheckerContext & C,const Expr * RetE,DefinedOrUnknownSVal RetVal) const103 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 
registerReturnUndefChecker(CheckerManager & mgr)121 void ento::registerReturnUndefChecker(CheckerManager &mgr) {
122   mgr.registerChecker<ReturnUndefChecker>();
123 }
124