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/DynamicSize.h"
20#include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
21#include "llvm/ADT/SmallString.h"
22#include "llvm/Support/raw_ostream.h"
23
24using namespace clang;
25using namespace ento;
26
27namespace {
28class UndefResultChecker
29  : public Checker< check::PostStmt<BinaryOperator> > {
30
31  mutable std::unique_ptr<BugType> BT;
32
33public:
34  void checkPostStmt(const BinaryOperator *B, CheckerContext &C) const;
35};
36} // end anonymous namespace
37
38static 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
61static bool isShiftOverflow(const BinaryOperator *B, CheckerContext &C) {
62  return C.isGreaterOrEqual(
63      B->getRHS(), C.getASTContext().getIntWidth(B->getLHS()->getType()));
64}
65
66static 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
76void 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
186void ento::registerUndefResultChecker(CheckerManager &mgr) {
187  mgr.registerChecker<UndefResultChecker>();
188}
189
190bool ento::shouldRegisterUndefResultChecker(const CheckerManager &mgr) {
191  return true;
192}
193