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 <optional>
22 #include <utility>
23 
24 using namespace clang;
25 using namespace ento;
26 
27 namespace {
28 
29 class UndefBranchChecker : public Checker<check::BranchCondition> {
30   mutable std::unique_ptr<BuiltinBug> BT;
31 
32   struct FindUndefExpr {
33     ProgramStateRef St;
34     const LocationContext *LCtx;
35 
36     FindUndefExpr(ProgramStateRef S, const LocationContext *L)
37         : St(std::move(S)), LCtx(L) {}
38 
39     const Expr *FindExpr(const Expr *Ex) {
40       if (!MatchesCriteria(Ex))
41         return nullptr;
42 
43       for (const Stmt *SubStmt : Ex->children())
44         if (const Expr *ExI = dyn_cast_or_null<Expr>(SubStmt))
45           if (const Expr *E2 = FindExpr(ExI))
46             return E2;
47 
48       return Ex;
49     }
50 
51     bool MatchesCriteria(const Expr *Ex) {
52       return St->getSVal(Ex, LCtx).isUndef();
53     }
54   };
55 
56 public:
57   void checkBranchCondition(const Stmt *Condition, CheckerContext &Ctx) const;
58 };
59 
60 } // namespace
61 
62 void UndefBranchChecker::checkBranchCondition(const Stmt *Condition,
63                                               CheckerContext &Ctx) const {
64   // ObjCForCollection is a loop, but has no actual condition.
65   if (isa<ObjCForCollectionStmt>(Condition))
66     return;
67   SVal X = Ctx.getSVal(Condition);
68   if (X.isUndef()) {
69     // Generate a sink node, which implicitly marks both outgoing branches as
70     // infeasible.
71     ExplodedNode *N = Ctx.generateErrorNode();
72     if (N) {
73       if (!BT)
74         BT.reset(new BuiltinBug(
75             this, "Branch condition evaluates to a garbage value"));
76 
77       // What's going on here: we want to highlight the subexpression of the
78       // condition that is the most likely source of the "uninitialized
79       // branch condition."  We do a recursive walk of the condition's
80       // subexpressions and roughly look for the most nested subexpression
81       // that binds to Undefined.  We then highlight that expression's range.
82 
83       // Get the predecessor node and check if is a PostStmt with the Stmt
84       // being the terminator condition.  We want to inspect the state
85       // of that node instead because it will contain main information about
86       // the subexpressions.
87 
88       // Note: any predecessor will do.  They should have identical state,
89       // since all the BlockEdge did was act as an error sink since the value
90       // had to already be undefined.
91       assert (!N->pred_empty());
92       const Expr *Ex = cast<Expr>(Condition);
93       ExplodedNode *PrevN = *N->pred_begin();
94       ProgramPoint P = PrevN->getLocation();
95       ProgramStateRef St = N->getState();
96 
97       if (std::optional<PostStmt> PS = P.getAs<PostStmt>())
98         if (PS->getStmt() == Ex)
99           St = PrevN->getState();
100 
101       FindUndefExpr FindIt(St, Ctx.getLocationContext());
102       Ex = FindIt.FindExpr(Ex);
103 
104       // Emit the bug report.
105       auto R = std::make_unique<PathSensitiveBugReport>(
106           *BT, BT->getDescription(), N);
107       bugreporter::trackExpressionValue(N, Ex, *R);
108       R->addRange(Ex->getSourceRange());
109 
110       Ctx.emitReport(std::move(R));
111     }
112   }
113 }
114 
115 void ento::registerUndefBranchChecker(CheckerManager &mgr) {
116   mgr.registerChecker<UndefBranchChecker>();
117 }
118 
119 bool ento::shouldRegisterUndefBranchChecker(const CheckerManager &mgr) {
120   return true;
121 }
122