UndefResultChecker.cpp revision 360784 
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/ExprEngine.h"
20#include "llvm/ADT/SmallString.h"
21#include "llvm/Support/raw_ostream.h"
22
23using namespace clang;
24using namespace ento;
25
26namespace {
27class UndefResultChecker
28  : public Checker< check::PostStmt<BinaryOperator> > {
29
30  mutable std::unique_ptr<BugType> BT;
31
32public:
33  void checkPostStmt(const BinaryOperator *B, CheckerContext &C) const;
34};
35} // end anonymous namespace
36
37static bool isArrayIndexOutOfBounds(CheckerContext &C, const Expr *Ex) {
38  ProgramStateRef state = C.getState();
39
40  if (!isa<ArraySubscriptExpr>(Ex))
41    return false;
42
43  SVal Loc = C.getSVal(Ex);
44  if (!Loc.isValid())
45    return false;
46
47  const MemRegion *MR = Loc.castAs<loc::MemRegionVal>().getRegion();
48  const ElementRegion *ER = dyn_cast<ElementRegion>(MR);
49  if (!ER)
50    return false;
51
52  DefinedOrUnknownSVal Idx = ER->getIndex().castAs<DefinedOrUnknownSVal>();
53  DefinedOrUnknownSVal NumElements = C.getStoreManager().getSizeInElements(
54      state, ER->getSuperRegion(), ER->getValueType());
55  ProgramStateRef StInBound = state->assumeInBound(Idx, NumElements, true);
56  ProgramStateRef StOutBound = state->assumeInBound(Idx, NumElements, false);
57  return StOutBound && !StInBound;
58}
59
60static bool isShiftOverflow(const BinaryOperator *B, CheckerContext &C) {
61  return C.isGreaterOrEqual(
62      B->getRHS(), C.getASTContext().getIntWidth(B->getLHS()->getType()));
63}
64
65static bool isLeftShiftResultUnrepresentable(const BinaryOperator *B,
66                                             CheckerContext &C) {
67  SValBuilder &SB = C.getSValBuilder();
68  ProgramStateRef State = C.getState();
69  const llvm::APSInt *LHS = SB.getKnownValue(State, C.getSVal(B->getLHS()));
70  const llvm::APSInt *RHS = SB.getKnownValue(State, C.getSVal(B->getRHS()));
71  assert(LHS && RHS && "Values unknown, inconsistent state");
72  return (unsigned)RHS->getZExtValue() > LHS->countLeadingZeros();
73}
74
75void UndefResultChecker::checkPostStmt(const BinaryOperator *B,
76                                       CheckerContext &C) const {
77  if (C.getSVal(B).isUndef()) {
78
79    // Do not report assignments of uninitialized values inside swap functions.
80    // This should allow to swap partially uninitialized structs
81    // (radar://14129997)
82    if (const FunctionDecl *EnclosingFunctionDecl =
83        dyn_cast<FunctionDecl>(C.getStackFrame()->getDecl()))
84      if (C.getCalleeName(EnclosingFunctionDecl) == "swap")
85        return;
86
87    // Generate an error node.
88    ExplodedNode *N = C.generateErrorNode();
89    if (!N)
90      return;
91
92    if (!BT)
93      BT.reset(
94          new BuiltinBug(this, "Result of operation is garbage or undefined"));
95
96    SmallString<256> sbuf;
97    llvm::raw_svector_ostream OS(sbuf);
98    const Expr *Ex = nullptr;
99    bool isLeft = true;
100
101    if (C.getSVal(B->getLHS()).isUndef()) {
102      Ex = B->getLHS()->IgnoreParenCasts();
103      isLeft = true;
104    }
105    else if (C.getSVal(B->getRHS()).isUndef()) {
106      Ex = B->getRHS()->IgnoreParenCasts();
107      isLeft = false;
108    }
109
110    if (Ex) {
111      OS << "The " << (isLeft ? "left" : "right") << " operand of '"
112         << BinaryOperator::getOpcodeStr(B->getOpcode())
113         << "' is a garbage value";
114      if (isArrayIndexOutOfBounds(C, Ex))
115        OS << " due to array index out of bounds";
116    } else {
117      // Neither operand was undefined, but the result is undefined.
118      if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
119           B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
120          C.isNegative(B->getRHS())) {
121        OS << "The result of the "
122           << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
123                                                              : "right")
124           << " shift is undefined because the right operand is negative";
125        Ex = B->getRHS();
126      } else if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
127                  B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
128                 isShiftOverflow(B, C)) {
129
130        OS << "The result of the "
131           << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
132                                                              : "right")
133           << " shift is undefined due to shifting by ";
134        Ex = B->getRHS();
135
136        SValBuilder &SB = C.getSValBuilder();
137        const llvm::APSInt *I =
138            SB.getKnownValue(C.getState(), C.getSVal(B->getRHS()));
139        if (!I)
140          OS << "a value that is";
141        else if (I->isUnsigned())
142          OS << '\'' << I->getZExtValue() << "\', which is";
143        else
144          OS << '\'' << I->getSExtValue() << "\', which is";
145
146        OS << " greater or equal to the width of type '"
147           << B->getLHS()->getType().getAsString() << "'.";
148      } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
149                 C.isNegative(B->getLHS())) {
150        OS << "The result of the left shift is undefined because the left "
151              "operand is negative";
152        Ex = B->getLHS();
153      } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
154                 isLeftShiftResultUnrepresentable(B, C)) {
155        ProgramStateRef State = C.getState();
156        SValBuilder &SB = C.getSValBuilder();
157        const llvm::APSInt *LHS =
158            SB.getKnownValue(State, C.getSVal(B->getLHS()));
159        const llvm::APSInt *RHS =
160            SB.getKnownValue(State, C.getSVal(B->getRHS()));
161        OS << "The result of the left shift is undefined due to shifting \'"
162           << LHS->getSExtValue() << "\' by \'" << RHS->getZExtValue()
163           << "\', which is unrepresentable in the unsigned version of "
164           << "the return type \'" << B->getLHS()->getType().getAsString()
165           << "\'";
166        Ex = B->getLHS();
167      } else {
168        OS << "The result of the '"
169           << BinaryOperator::getOpcodeStr(B->getOpcode())
170           << "' expression is undefined";
171      }
172    }
173    auto report = std::make_unique<PathSensitiveBugReport>(*BT, OS.str(), N);
174    if (Ex) {
175      report->addRange(Ex->getSourceRange());
176      bugreporter::trackExpressionValue(N, Ex, *report);
177    }
178    else
179      bugreporter::trackExpressionValue(N, B, *report);
180
181    C.emitReport(std::move(report));
182  }
183}
184
185void ento::registerUndefResultChecker(CheckerManager &mgr) {
186  mgr.registerChecker<UndefResultChecker>();
187}
188
189bool ento::shouldRegisterUndefResultChecker(const LangOptions &LO) {
190  return true;
191}
192