1 //=== UndefBranchChecker.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 UndefBranchChecker, which checks for undefined branch
10 // condition.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "clang/AST/StmtObjC.h"
15 #include "clang/AST/Type.h"
16 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.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/CheckerContext.h"
21 #include <utility>
22 
23 using namespace clang;
24 using namespace ento;
25 
26 namespace {
27 
28 class UndefBranchChecker : public Checker<check::BranchCondition> {
29   mutable std::unique_ptr<BuiltinBug> BT;
30 
31   struct FindUndefExpr {
32     ProgramStateRef St;
33     const LocationContext *LCtx;
34 
35     FindUndefExpr(ProgramStateRef S, const LocationContext *L)
36         : St(std::move(S)), LCtx(L) {}
37 
38     const Expr *FindExpr(const Expr *Ex) {
39       if (!MatchesCriteria(Ex))
40         return nullptr;
41 
42       for (const Stmt *SubStmt : Ex->children())
43         if (const Expr *ExI = dyn_cast_or_null<Expr>(SubStmt))
44           if (const Expr *E2 = FindExpr(ExI))
45             return E2;
46 
47       return Ex;
48     }
49 
50     bool MatchesCriteria(const Expr *Ex) {
51       return St->getSVal(Ex, LCtx).isUndef();
52     }
53   };
54 
55 public:
56   void checkBranchCondition(const Stmt *Condition, CheckerContext &Ctx) const;
57 };
58 
59 } // namespace
60 
61 void UndefBranchChecker::checkBranchCondition(const Stmt *Condition,
62                                               CheckerContext &Ctx) const {
63   // ObjCForCollection is a loop, but has no actual condition.
64   if (isa<ObjCForCollectionStmt>(Condition))
65     return;
66   SVal X = Ctx.getSVal(Condition);
67   if (X.isUndef()) {
68     // Generate a sink node, which implicitly marks both outgoing branches as
69     // infeasible.
70     ExplodedNode *N = Ctx.generateErrorNode();
71     if (N) {
72       if (!BT)
73         BT.reset(new BuiltinBug(
74             this, "Branch condition evaluates to a garbage value"));
75 
76       // What's going on here: we want to highlight the subexpression of the
77       // condition that is the most likely source of the "uninitialized
78       // branch condition."  We do a recursive walk of the condition's
79       // subexpressions and roughly look for the most nested subexpression
80       // that binds to Undefined.  We then highlight that expression's range.
81 
82       // Get the predecessor node and check if is a PostStmt with the Stmt
83       // being the terminator condition.  We want to inspect the state
84       // of that node instead because it will contain main information about
85       // the subexpressions.
86 
87       // Note: any predecessor will do.  They should have identical state,
88       // since all the BlockEdge did was act as an error sink since the value
89       // had to already be undefined.
90       assert (!N->pred_empty());
91       const Expr *Ex = cast<Expr>(Condition);
92       ExplodedNode *PrevN = *N->pred_begin();
93       ProgramPoint P = PrevN->getLocation();
94       ProgramStateRef St = N->getState();
95 
96       if (Optional<PostStmt> PS = P.getAs<PostStmt>())
97         if (PS->getStmt() == Ex)
98           St = PrevN->getState();
99 
100       FindUndefExpr FindIt(St, Ctx.getLocationContext());
101       Ex = FindIt.FindExpr(Ex);
102 
103       // Emit the bug report.
104       auto R = std::make_unique<PathSensitiveBugReport>(
105           *BT, BT->getDescription(), N);
106       bugreporter::trackExpressionValue(N, Ex, *R);
107       R->addRange(Ex->getSourceRange());
108 
109       Ctx.emitReport(std::move(R));
110     }
111   }
112 }
113 
114 void ento::registerUndefBranchChecker(CheckerManager &mgr) {
115   mgr.registerChecker<UndefBranchChecker>();
116 }
117 
118 bool ento::shouldRegisterUndefBranchChecker(const CheckerManager &mgr) {
119   return true;
120 }
121