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      1 //=== UndefBranchChecker.cpp -----------------------------------*- C++ -*--===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file defines UndefBranchChecker, which checks for undefined branch
     11 // condition.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "ClangSACheckers.h"
     16 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
     17 #include "clang/StaticAnalyzer/Core/Checker.h"
     18 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
     19 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
     20 
     21 using namespace clang;
     22 using namespace ento;
     23 
     24 namespace {
     25 
     26 class UndefBranchChecker : public Checker<check::BranchCondition> {
     27   mutable std::unique_ptr<BuiltinBug> BT;
     28 
     29   struct FindUndefExpr {
     30     ProgramStateRef St;
     31     const LocationContext *LCtx;
     32 
     33     FindUndefExpr(ProgramStateRef S, const LocationContext *L)
     34       : St(S), LCtx(L) {}
     35 
     36     const Expr *FindExpr(const Expr *Ex) {
     37       if (!MatchesCriteria(Ex))
     38         return nullptr;
     39 
     40       for (Stmt::const_child_iterator I = Ex->child_begin(),
     41                                       E = Ex->child_end();I!=E;++I)
     42         if (const Expr *ExI = dyn_cast_or_null<Expr>(*I)) {
     43           const Expr *E2 = FindExpr(ExI);
     44           if (E2) return E2;
     45         }
     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 }
     60 
     61 void UndefBranchChecker::checkBranchCondition(const Stmt *Condition,
     62                                               CheckerContext &Ctx) const {
     63   SVal X = Ctx.getState()->getSVal(Condition, Ctx.getLocationContext());
     64   if (X.isUndef()) {
     65     // Generate a sink node, which implicitly marks both outgoing branches as
     66     // infeasible.
     67     ExplodedNode *N = Ctx.generateSink();
     68     if (N) {
     69       if (!BT)
     70         BT.reset(new BuiltinBug(
     71             this, "Branch condition evaluates to a garbage value"));
     72 
     73       // What's going on here: we want to highlight the subexpression of the
     74       // condition that is the most likely source of the "uninitialized
     75       // branch condition."  We do a recursive walk of the condition's
     76       // subexpressions and roughly look for the most nested subexpression
     77       // that binds to Undefined.  We then highlight that expression's range.
     78 
     79       // Get the predecessor node and check if is a PostStmt with the Stmt
     80       // being the terminator condition.  We want to inspect the state
     81       // of that node instead because it will contain main information about
     82       // the subexpressions.
     83 
     84       // Note: any predecessor will do.  They should have identical state,
     85       // since all the BlockEdge did was act as an error sink since the value
     86       // had to already be undefined.
     87       assert (!N->pred_empty());
     88       const Expr *Ex = cast<Expr>(Condition);
     89       ExplodedNode *PrevN = *N->pred_begin();
     90       ProgramPoint P = PrevN->getLocation();
     91       ProgramStateRef St = N->getState();
     92 
     93       if (Optional<PostStmt> PS = P.getAs<PostStmt>())
     94         if (PS->getStmt() == Ex)
     95           St = PrevN->getState();
     96 
     97       FindUndefExpr FindIt(St, Ctx.getLocationContext());
     98       Ex = FindIt.FindExpr(Ex);
     99 
    100       // Emit the bug report.
    101       BugReport *R = new BugReport(*BT, BT->getDescription(), N);
    102       bugreporter::trackNullOrUndefValue(N, Ex, *R);
    103       R->addRange(Ex->getSourceRange());
    104 
    105       Ctx.emitReport(R);
    106     }
    107   }
    108 }
    109 
    110 void ento::registerUndefBranchChecker(CheckerManager &mgr) {
    111   mgr.registerChecker<UndefBranchChecker>();
    112 }
    113