[LifetimeSafety] Add loan expiry analysis (llvm#148712)

This PR adds the `ExpiredLoansAnalysis` class to track which loans have expired. The analysis uses a dataflow lattice (`ExpiredLattice`) to maintain the set of expired loans at each program point.

This is a very light weight dataflow analysis and is expected to reach fixed point in ~2 iterations.
In principle, this does not need a dataflow analysis but is used for convenience in favour of lean code.

Author: usx95

clang/include/clang/Analysis/Analyses/LifetimeSafety.h

@@ -33,6 +33,7 @@ namespace internal {
 class Fact;
 class FactManager;
 class LoanPropagationAnalysis;
+class ExpiredLoansAnalysis;
 struct LifetimeFactory;
 
 /// A generic, type-safe wrapper for an ID, distinguished by its `Tag` type.
@@ -52,6 +53,10 @@ template <typename Tag> struct ID {
     IDBuilder.AddInteger(Value);
   }
 };
+template <typename Tag>
+inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, ID<Tag> ID) {
+  return OS << ID.Value;
+}
 
 using LoanID = ID<struct LoanTag>;
 using OriginID = ID<struct OriginTag>;
@@ -81,6 +86,9 @@ class LifetimeSafetyAnalysis {
   /// Returns the set of loans an origin holds at a specific program point.
   LoanSet getLoansAtPoint(OriginID OID, ProgramPoint PP) const;
 
+  /// Returns the set of loans that have expired at a specific program point.
+  LoanSet getExpiredLoansAtPoint(ProgramPoint PP) const;
+
   /// Finds the OriginID for a given declaration.
   /// Returns a null optional if not found.
   std::optional<OriginID> getOriginIDForDecl(const ValueDecl *D) const;
@@ -105,6 +113,7 @@ class LifetimeSafetyAnalysis {
   std::unique_ptr<LifetimeFactory> Factory;
   std::unique_ptr<FactManager> FactMgr;
   std::unique_ptr<LoanPropagationAnalysis> LoanPropagation;
+  std::unique_ptr<ExpiredLoansAnalysis> ExpiredLoans;
 };
 } // namespace internal
 } // namespace clang::lifetimes

clang/lib/Analysis/LifetimeSafety.cpp

@@ -23,15 +23,10 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/TimeProfiler.h"
 #include <cstdint>
+#include <memory>
 
 namespace clang::lifetimes {
 namespace internal {
-namespace {
-template <typename Tag>
-inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, ID<Tag> ID) {
-  return OS << ID.Value;
-}
-} // namespace
 
 /// Represents the storage location being borrowed, e.g., a specific stack
 /// variable.
@@ -832,6 +827,91 @@ class LoanPropagationAnalysis
   }
 };
 
+// ========================================================================= //
+//                         Expired Loans Analysis
+// ========================================================================= //
+
+/// The dataflow lattice for tracking the set of expired loans.
+struct ExpiredLattice {
+  LoanSet Expired;
+
+  ExpiredLattice() : Expired(nullptr) {};
+  explicit ExpiredLattice(LoanSet S) : Expired(S) {}
+
+  bool operator==(const ExpiredLattice &Other) const {
+    return Expired == Other.Expired;
+  }
+  bool operator!=(const ExpiredLattice &Other) const {
+    return !(*this == Other);
+  }
+
+  void dump(llvm::raw_ostream &OS) const {
+    OS << "ExpiredLattice State:\n";
+    if (Expired.isEmpty())
+      OS << "  <empty>\n";
+    for (const LoanID &LID : Expired)
+      OS << "  Loan " << LID << " is expired\n";
+  }
+};
+
+/// The analysis that tracks which loans have expired.
+class ExpiredLoansAnalysis
+    : public DataflowAnalysis<ExpiredLoansAnalysis, ExpiredLattice,
+                              Direction::Forward> {
+
+  LoanSet::Factory &Factory;
+
+public:
+  ExpiredLoansAnalysis(const CFG &C, AnalysisDeclContext &AC, FactManager &F,
+                       LifetimeFactory &Factory)
+      : DataflowAnalysis(C, AC, F), Factory(Factory.LoanSetFactory) {}
+
+  using Base::transfer;
+
+  StringRef getAnalysisName() const { return "ExpiredLoans"; }
+
+  Lattice getInitialState() { return Lattice(Factory.getEmptySet()); }
+
+  /// Merges two lattices by taking the union of the expired loan sets.
+  Lattice join(Lattice L1, Lattice L2) const {
+    return Lattice(utils::join(L1.Expired, L2.Expired, Factory));
+  }
+
+  Lattice transfer(Lattice In, const ExpireFact &F) {
+    return Lattice(Factory.add(In.Expired, F.getLoanID()));
+  }
+
+  // Removes the loan from the set of expired loans.
+  //
+  // When a loan is re-issued (e.g., in a loop), it is no longer considered
+  // expired. A loan can be in the expired set at the point of issue due to
+  // the dataflow state from a previous loop iteration being propagated along
+  // a backedge in the CFG.
+  //
+  // Note: This has a subtle false-negative though where a loan from previous
+  // iteration is not overwritten by a reissue. This needs careful tracking
+  // of loans "across iterations" which can be considered for future
+  // enhancements.
+  //
+  //    void foo(int safe) {
+  //      int* p = &safe;
+  //      int* q = &safe;
+  //      while (condition()) {
+  //        int x = 1;
+  //        p = &x;    // A loan to 'x' is issued to 'p' in every iteration.
+  //        if (condition()) {
+  //          q = p;
+  //        }
+  //        (void)*p; // OK  — 'p' points to 'x' from new iteration.
+  //        (void)*q; // UaF - 'q' still points to 'x' from previous iteration
+  //                  // which is now destroyed.
+  //      }
+  // }
+  Lattice transfer(Lattice In, const IssueFact &F) {
+    return Lattice(Factory.remove(In.Expired, F.getLoanID()));
+  }
+};
+
 // ========================================================================= //
 //  TODO:
 // - Modify loan expiry analysis to answer `bool isExpired(Loan L, Point P)`
@@ -873,6 +953,10 @@ void LifetimeSafetyAnalysis::run() {
   LoanPropagation =
       std::make_unique<LoanPropagationAnalysis>(Cfg, AC, *FactMgr, *Factory);
   LoanPropagation->run();
+
+  ExpiredLoans =
+      std::make_unique<ExpiredLoansAnalysis>(Cfg, AC, *FactMgr, *Factory);
+  ExpiredLoans->run();
 }
 
 LoanSet LifetimeSafetyAnalysis::getLoansAtPoint(OriginID OID,
@@ -881,6 +965,11 @@ LoanSet LifetimeSafetyAnalysis::getLoansAtPoint(OriginID OID,
   return LoanPropagation->getLoans(OID, PP);
 }
 
+LoanSet LifetimeSafetyAnalysis::getExpiredLoansAtPoint(ProgramPoint PP) const {
+  assert(ExpiredLoans && "ExpiredLoansAnalysis has not been run.");
+  return ExpiredLoans->getState(PP).Expired;
+}
+
 std::optional<OriginID>
 LifetimeSafetyAnalysis::getOriginIDForDecl(const ValueDecl *D) const {
   assert(FactMgr && "FactManager not initialized");