1 //=== UndefResultChecker.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 defines UndefResultChecker, a builtin check in ExprEngine that
10 // performs checks for undefined results of non-assignment binary operators.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
15 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
16 #include "clang/StaticAnalyzer/Core/Checker.h"
17 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
18 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
19 #include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h"
20 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
21 #include "llvm/ADT/SmallString.h"
22 #include "llvm/Support/raw_ostream.h"
23 
24 using namespace clang;
25 using namespace ento;
26 
27 namespace {
28 class UndefResultChecker
29   : public Checker< check::PostStmt<BinaryOperator> > {
30 
31   mutable std::unique_ptr<BugType> BT;
32 
33 public:
34   void checkPostStmt(const BinaryOperator *B, CheckerContext &C) const;
35 };
36 } // end anonymous namespace
37 
isArrayIndexOutOfBounds(CheckerContext & C,const Expr * Ex)38 static bool isArrayIndexOutOfBounds(CheckerContext &C, const Expr *Ex) {
39   ProgramStateRef state = C.getState();
40 
41   if (!isa<ArraySubscriptExpr>(Ex))
42     return false;
43 
44   SVal Loc = C.getSVal(Ex);
45   if (!Loc.isValid())
46     return false;
47 
48   const MemRegion *MR = Loc.castAs<loc::MemRegionVal>().getRegion();
49   const ElementRegion *ER = dyn_cast<ElementRegion>(MR);
50   if (!ER)
51     return false;
52 
53   DefinedOrUnknownSVal Idx = ER->getIndex().castAs<DefinedOrUnknownSVal>();
54   DefinedOrUnknownSVal ElementCount = getDynamicElementCount(
55       state, ER->getSuperRegion(), C.getSValBuilder(), ER->getValueType());
56   ProgramStateRef StInBound = state->assumeInBound(Idx, ElementCount, true);
57   ProgramStateRef StOutBound = state->assumeInBound(Idx, ElementCount, false);
58   return StOutBound && !StInBound;
59 }
60 
isShiftOverflow(const BinaryOperator * B,CheckerContext & C)61 static bool isShiftOverflow(const BinaryOperator *B, CheckerContext &C) {
62   return C.isGreaterOrEqual(
63       B->getRHS(), C.getASTContext().getIntWidth(B->getLHS()->getType()));
64 }
65 
isLeftShiftResultUnrepresentable(const BinaryOperator * B,CheckerContext & C)66 static bool isLeftShiftResultUnrepresentable(const BinaryOperator *B,
67                                              CheckerContext &C) {
68   SValBuilder &SB = C.getSValBuilder();
69   ProgramStateRef State = C.getState();
70   const llvm::APSInt *LHS = SB.getKnownValue(State, C.getSVal(B->getLHS()));
71   const llvm::APSInt *RHS = SB.getKnownValue(State, C.getSVal(B->getRHS()));
72   assert(LHS && RHS && "Values unknown, inconsistent state");
73   return (unsigned)RHS->getZExtValue() > LHS->countLeadingZeros();
74 }
75 
checkPostStmt(const BinaryOperator * B,CheckerContext & C) const76 void UndefResultChecker::checkPostStmt(const BinaryOperator *B,
77                                        CheckerContext &C) const {
78   if (C.getSVal(B).isUndef()) {
79 
80     // Do not report assignments of uninitialized values inside swap functions.
81     // This should allow to swap partially uninitialized structs
82     // (radar://14129997)
83     if (const FunctionDecl *EnclosingFunctionDecl =
84         dyn_cast<FunctionDecl>(C.getStackFrame()->getDecl()))
85       if (C.getCalleeName(EnclosingFunctionDecl) == "swap")
86         return;
87 
88     // Generate an error node.
89     ExplodedNode *N = C.generateErrorNode();
90     if (!N)
91       return;
92 
93     if (!BT)
94       BT.reset(
95           new BuiltinBug(this, "Result of operation is garbage or undefined"));
96 
97     SmallString<256> sbuf;
98     llvm::raw_svector_ostream OS(sbuf);
99     const Expr *Ex = nullptr;
100     bool isLeft = true;
101 
102     if (C.getSVal(B->getLHS()).isUndef()) {
103       Ex = B->getLHS()->IgnoreParenCasts();
104       isLeft = true;
105     }
106     else if (C.getSVal(B->getRHS()).isUndef()) {
107       Ex = B->getRHS()->IgnoreParenCasts();
108       isLeft = false;
109     }
110 
111     if (Ex) {
112       OS << "The " << (isLeft ? "left" : "right") << " operand of '"
113          << BinaryOperator::getOpcodeStr(B->getOpcode())
114          << "' is a garbage value";
115       if (isArrayIndexOutOfBounds(C, Ex))
116         OS << " due to array index out of bounds";
117     } else {
118       // Neither operand was undefined, but the result is undefined.
119       if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
120            B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
121           C.isNegative(B->getRHS())) {
122         OS << "The result of the "
123            << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
124                                                               : "right")
125            << " shift is undefined because the right operand is negative";
126         Ex = B->getRHS();
127       } else if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
128                   B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
129                  isShiftOverflow(B, C)) {
130 
131         OS << "The result of the "
132            << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
133                                                               : "right")
134            << " shift is undefined due to shifting by ";
135         Ex = B->getRHS();
136 
137         SValBuilder &SB = C.getSValBuilder();
138         const llvm::APSInt *I =
139             SB.getKnownValue(C.getState(), C.getSVal(B->getRHS()));
140         if (!I)
141           OS << "a value that is";
142         else if (I->isUnsigned())
143           OS << '\'' << I->getZExtValue() << "\', which is";
144         else
145           OS << '\'' << I->getSExtValue() << "\', which is";
146 
147         OS << " greater or equal to the width of type '"
148            << B->getLHS()->getType().getAsString() << "'.";
149       } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
150                  C.isNegative(B->getLHS())) {
151         OS << "The result of the left shift is undefined because the left "
152               "operand is negative";
153         Ex = B->getLHS();
154       } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
155                  isLeftShiftResultUnrepresentable(B, C)) {
156         ProgramStateRef State = C.getState();
157         SValBuilder &SB = C.getSValBuilder();
158         const llvm::APSInt *LHS =
159             SB.getKnownValue(State, C.getSVal(B->getLHS()));
160         const llvm::APSInt *RHS =
161             SB.getKnownValue(State, C.getSVal(B->getRHS()));
162         OS << "The result of the left shift is undefined due to shifting \'"
163            << LHS->getSExtValue() << "\' by \'" << RHS->getZExtValue()
164            << "\', which is unrepresentable in the unsigned version of "
165            << "the return type \'" << B->getLHS()->getType().getAsString()
166            << "\'";
167         Ex = B->getLHS();
168       } else {
169         OS << "The result of the '"
170            << BinaryOperator::getOpcodeStr(B->getOpcode())
171            << "' expression is undefined";
172       }
173     }
174     auto report = std::make_unique<PathSensitiveBugReport>(*BT, OS.str(), N);
175     if (Ex) {
176       report->addRange(Ex->getSourceRange());
177       bugreporter::trackExpressionValue(N, Ex, *report);
178     }
179     else
180       bugreporter::trackExpressionValue(N, B, *report);
181 
182     C.emitReport(std::move(report));
183   }
184 }
185 
registerUndefResultChecker(CheckerManager & mgr)186 void ento::registerUndefResultChecker(CheckerManager &mgr) {
187   mgr.registerChecker<UndefResultChecker>();
188 }
189 
shouldRegisterUndefResultChecker(const CheckerManager & mgr)190 bool ento::shouldRegisterUndefResultChecker(const CheckerManager &mgr) {
191   return true;
192 }
193