Checker.cpp

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//===- Checker.cpp - C++ Lifetime Safety Checker ----------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the LifetimeChecker, which detects use-after-free
// errors by checking if live origins hold loans that have expired.
//
//===----------------------------------------------------------------------===//
 
#include "clang/Analysis/Analyses/LifetimeSafety/Checker.h"
#include "clang/AST/Expr.h"
#include "clang/Analysis/Analyses/LifetimeSafety/Facts.h"
#include "clang/Analysis/Analyses/LifetimeSafety/LiveOrigins.h"
#include "clang/Analysis/Analyses/LifetimeSafety/LoanPropagation.h"
#include "clang/Analysis/Analyses/LifetimeSafety/Loans.h"
#include "clang/Analysis/Analyses/PostOrderCFGView.h"
#include "clang/Analysis/AnalysisDeclContext.h"
#include "clang/Basic/SourceLocation.h"
#include "clang/Basic/SourceManager.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/TimeProfiler.h"
 
namespace clang::lifetimes::internal {
 
static Confidence livenessKindToConfidence(LivenessKind K) {
  switch (K) {
  case LivenessKind::Must:
    return Confidence::Definite;
  case LivenessKind::Maybe:
    return Confidence::Maybe;
  case LivenessKind::Dead:
    return Confidence::None;
  }
  llvm_unreachable("unknown liveness kind");
}
 
namespace {
 
/// Struct to store the complete context for a potential lifetime violation.
struct PendingWarning {
  SourceLocation ExpiryLoc; // Where the loan expired.
  llvm::PointerUnion<const UseFact *, const OriginEscapesFact *> CausingFact;
  Confidence ConfidenceLevel;
};
 
class LifetimeChecker {
private:
  llvm::DenseMap<LoanID, PendingWarning> FinalWarningsMap;
  llvm::DenseMap<const ParmVarDecl *, const Expr *> AnnotationWarningsMap;
  const LoanPropagationAnalysis &LoanPropagation;
  const LiveOriginsAnalysis &LiveOrigins;
  const FactManager &FactMgr;
  LifetimeSafetyReporter *Reporter;
  ASTContext &AST;
 
public:
  LifetimeChecker(const LoanPropagationAnalysis &LoanPropagation,
                  const LiveOriginsAnalysis &LiveOrigins, const FactManager &FM,
                  AnalysisDeclContext &ADC, LifetimeSafetyReporter *Reporter)
      : LoanPropagation(LoanPropagation), LiveOrigins(LiveOrigins), FactMgr(FM),
        Reporter(Reporter), AST(ADC.getASTContext()) {
    for (const CFGBlock *B : *ADC.getAnalysis<PostOrderCFGView>())
      for (const Fact *F : FactMgr.getFacts(B))
        if (const auto *EF = F->getAs<ExpireFact>())
          checkExpiry(EF);
        else if (const auto *OEF = F->getAs<OriginEscapesFact>())
          checkAnnotations(OEF);
    issuePendingWarnings();
    suggestAnnotations();
    //  Annotation inference is currently guarded by a frontend flag. In the
    //  future, this might be replaced by a design that differentiates between
    //  explicit and inferred findings with separate warning groups.
    if (AST.getLangOpts().EnableLifetimeSafetyInference)
      inferAnnotations();
  }
 
  /// Checks if an escaping origin holds a placeholder loan, indicating a
  /// missing [[clang::lifetimebound]] annotation.
  void checkAnnotations(const OriginEscapesFact *OEF) {
    OriginID EscapedOID = OEF->getEscapedOriginID();
    LoanSet EscapedLoans = LoanPropagation.getLoans(EscapedOID, OEF);
    for (LoanID LID : EscapedLoans) {
      const Loan *L = FactMgr.getLoanMgr().getLoan(LID);
      if (const auto *PL = dyn_cast<PlaceholderLoan>(L)) {
        const ParmVarDecl *PVD = PL->getParmVarDecl();
        if (PVD->hasAttr<LifetimeBoundAttr>())
          continue;
        AnnotationWarningsMap.try_emplace(PVD, OEF->getEscapeExpr());
      }
    }
  }
 
  /// Checks for use-after-free & use-after-return errors when a loan expires.
  ///
  /// This method examines all live origins at the expiry point and determines
  /// if any of them hold the expiring loan. If so, it creates a pending
  /// warning with the appropriate confidence level based on the liveness
  /// information. The confidence reflects whether the origin is definitely
  /// or maybe live at this point.
  ///
  /// Note: This implementation considers only the confidence of origin
  /// liveness. Future enhancements could also consider the confidence of loan
  /// propagation (e.g., a loan may only be held on some execution paths).
  void checkExpiry(const ExpireFact *EF) {
    LoanID ExpiredLoan = EF->getLoanID();
    LivenessMap Origins = LiveOrigins.getLiveOriginsAt(EF);
    Confidence CurConfidence = Confidence::None;
    // The UseFact or OriginEscapesFact most indicative of a lifetime error,
    // prioritized by earlier source location.
    llvm::PointerUnion<const UseFact *, const OriginEscapesFact *>
        BestCausingFact = nullptr;
 
    for (auto &[OID, LiveInfo] : Origins) {
      LoanSet HeldLoans = LoanPropagation.getLoans(OID, EF);
      if (!HeldLoans.contains(ExpiredLoan))
        continue;
      // Loan is defaulted.
      Confidence NewConfidence = livenessKindToConfidence(LiveInfo.Kind);
      if (CurConfidence < NewConfidence) {
        CurConfidence = NewConfidence;
        BestCausingFact = LiveInfo.CausingFact;
      }
    }
    if (!BestCausingFact)
      return;
    // We have a use-after-free.
    Confidence LastConf = FinalWarningsMap.lookup(ExpiredLoan).ConfidenceLevel;
    if (LastConf >= CurConfidence)
      return;
    FinalWarningsMap[ExpiredLoan] = {/*ExpiryLoc=*/EF->getExpiryLoc(),
                                     /*BestCausingFact=*/BestCausingFact,
                                     /*ConfidenceLevel=*/CurConfidence};
  }
 
  void issuePendingWarnings() {
    if (!Reporter)
      return;
    for (const auto &[LID, Warning] : FinalWarningsMap) {
      const Loan *L = FactMgr.getLoanMgr().getLoan(LID);
      const auto *BL = cast<PathLoan>(L);
      const Expr *IssueExpr = BL->getIssueExpr();
      llvm::PointerUnion<const UseFact *, const OriginEscapesFact *>
          CausingFact = Warning.CausingFact;
      Confidence Confidence = Warning.ConfidenceLevel;
      SourceLocation ExpiryLoc = Warning.ExpiryLoc;
 
      if (const auto *UF = CausingFact.dyn_cast<const UseFact *>())
        Reporter->reportUseAfterFree(IssueExpr, UF->getUseExpr(), ExpiryLoc,
                                     Confidence);
      else if (const auto *OEF =
                   CausingFact.dyn_cast<const OriginEscapesFact *>())
        Reporter->reportUseAfterReturn(IssueExpr, OEF->getEscapeExpr(),
                                       ExpiryLoc, Confidence);
      else
        llvm_unreachable("Unhandled CausingFact type");
    }
  }
 
  /// Returns the declaration of a function that is visible across translation
  /// units, if such a declaration exists and is different from the definition.
  static const FunctionDecl *getCrossTUDecl(const ParmVarDecl &PVD,
                                            SourceManager &SM) {
    const auto *FD = dyn_cast<FunctionDecl>(PVD.getDeclContext());
    if (!FD)
      return nullptr;
    if (!FD->isExternallyVisible())
      return nullptr;
    const FileID DefinitionFile = SM.getFileID(FD->getLocation());
    for (const FunctionDecl *Redecl : FD->redecls())
      if (SM.getFileID(Redecl->getLocation()) != DefinitionFile)
        return Redecl;
 
    return nullptr;
  }
 
  void suggestAnnotations() {
    if (!Reporter)
      return;
    SourceManager &SM = AST.getSourceManager();
    for (const auto &[PVD, EscapeExpr] : AnnotationWarningsMap) {
      if (const FunctionDecl *CrossTUDecl = getCrossTUDecl(*PVD, SM))
        Reporter->suggestAnnotation(
            SuggestionScope::CrossTU,
            CrossTUDecl->getParamDecl(PVD->getFunctionScopeIndex()),
            EscapeExpr);
      else
        Reporter->suggestAnnotation(SuggestionScope::IntraTU, PVD, EscapeExpr);
    }
  }
 
  void inferAnnotations() {
    // FIXME: To maximise inference propagation, functions should be analyzed in
    // post-order of the call graph, allowing inferred annotations to propagate
    // through the call chain
    // FIXME: Add the inferred attribute to all redeclarations of the function,
    // not just the definition being analyzed.
    for (const auto &[ConstPVD, EscapeExpr] : AnnotationWarningsMap) {
      ParmVarDecl *PVD = const_cast<ParmVarDecl *>(ConstPVD);
      if (!PVD->hasAttr<LifetimeBoundAttr>())
        PVD->addAttr(
            LifetimeBoundAttr::CreateImplicit(AST, PVD->getLocation()));
    }
  }
};
} // namespace
 
void runLifetimeChecker(const LoanPropagationAnalysis &LP,
                        const LiveOriginsAnalysis &LO,
                        const FactManager &FactMgr, AnalysisDeclContext &ADC,
                        LifetimeSafetyReporter *Reporter) {
  llvm::TimeTraceScope TimeProfile("LifetimeChecker");
  LifetimeChecker Checker(LP, LO, FactMgr, ADC, Reporter);
}
 
} // namespace clang::lifetimes::internal