[clang analysis][thread-safety] Handle return-by-reference... (llvm#67776)

...of guarded variables, when the function is not marked as requiring
locks:

```
class Return {
  Mutex mu;
  Foo foo GUARDED_BY(mu);

  Foo &returns_ref_locked() {
    MutexLock lock(&mu);
    return foo;  // BAD
  }

  Foo &returns_ref_locks_required() SHARED_LOCKS_REQUIRED(mu) {
    return foo;  // OK
  }
};
```

Review on Phabricator: https://reviews.llvm.org/D153131

Author: legrosbuffle

clang/include/clang/Analysis/Analyses/ThreadSafety.h

@@ -47,7 +47,13 @@ enum ProtectedOperationKind {
   POK_PassByRef,
 
   /// Passing a pt-guarded variable by reference.
-  POK_PtPassByRef
+  POK_PtPassByRef,
+
+  /// Returning a guarded variable by reference.
+  POK_ReturnByRef,
+
+  /// Returning a pt-guarded variable by reference.
+  POK_PtReturnByRef,
 };
 
 /// This enum distinguishes between different kinds of lock actions. For

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3858,7 +3858,7 @@ def warn_fun_requires_negative_cap : Warning<
   "calling function %0 requires negative capability '%1'">,
   InGroup<ThreadSafetyAnalysis>, DefaultIgnore;
 
-// Thread safety warnings on pass by reference
+// Thread safety warnings on pass/return by reference
 def warn_guarded_pass_by_reference : Warning<
   "passing variable %1 by reference requires holding %0 "
   "%select{'%2'|'%2' exclusively}3">,
@@ -3867,6 +3867,14 @@ def warn_pt_guarded_pass_by_reference : Warning<
   "passing the value that %1 points to by reference requires holding %0 "
   "%select{'%2'|'%2' exclusively}3">,
   InGroup<ThreadSafetyReference>, DefaultIgnore;
+def warn_guarded_return_by_reference : Warning<
+  "returning variable %1 by reference requires holding %0 "
+  "%select{'%2'|'%2' exclusively}3">,
+  InGroup<ThreadSafetyReference>, DefaultIgnore;
+def warn_pt_guarded_return_by_reference : Warning<
+  "returning the value that %1 points to by reference requires holding %0 "
+  "%select{'%2'|'%2' exclusively}3">,
+  InGroup<ThreadSafetyReference>, DefaultIgnore;
 
 // Imprecise thread safety warnings
 def warn_variable_requires_lock : Warning<

clang/lib/Analysis/ThreadSafety.cpp

@@ -1008,7 +1008,7 @@ class ThreadSafetyAnalyzer {
   threadSafety::SExprBuilder SxBuilder;
 
   ThreadSafetyHandler &Handler;
-  const CXXMethodDecl *CurrentMethod = nullptr;
+  const FunctionDecl *CurrentFunction;
   LocalVariableMap LocalVarMap;
   FactManager FactMan;
   std::vector<CFGBlockInfo> BlockInfo;
@@ -1243,10 +1243,10 @@ bool ThreadSafetyAnalyzer::inCurrentScope(const CapabilityExpr &CapE) {
 
   // Members are in scope from methods of the same class.
   if (const auto *P = dyn_cast<til::Project>(SExp)) {
-    if (!CurrentMethod)
+    if (!isa_and_nonnull<CXXMethodDecl>(CurrentFunction))
       return false;
     const ValueDecl *VD = P->clangDecl();
-    return VD->getDeclContext() == CurrentMethod->getDeclContext();
+    return VD->getDeclContext() == CurrentFunction->getDeclContext();
   }
 
   return false;
@@ -1541,6 +1541,8 @@ class BuildLockset : public ConstStmtVisitor<BuildLockset> {
 
   ThreadSafetyAnalyzer *Analyzer;
   FactSet FSet;
+  // The fact set for the function on exit.
+  const FactSet &FunctionExitFSet;
   /// Maps constructed objects to `this` placeholder prior to initialization.
   llvm::SmallDenseMap<const Expr *, til::LiteralPtr *> ConstructedObjects;
   LocalVariableMap::Context LVarCtx;
@@ -1566,9 +1568,11 @@ class BuildLockset : public ConstStmtVisitor<BuildLockset> {
                         bool SkipFirstParam = false);
 
 public:
-  BuildLockset(ThreadSafetyAnalyzer *Anlzr, CFGBlockInfo &Info)
+  BuildLockset(ThreadSafetyAnalyzer *Anlzr, CFGBlockInfo &Info,
+               const FactSet &FunctionExitFSet)
       : ConstStmtVisitor<BuildLockset>(), Analyzer(Anlzr), FSet(Info.EntrySet),
-        LVarCtx(Info.EntryContext), CtxIndex(Info.EntryIndex) {}
+        FunctionExitFSet(FunctionExitFSet), LVarCtx(Info.EntryContext),
+        CtxIndex(Info.EntryIndex) {}
 
   void VisitUnaryOperator(const UnaryOperator *UO);
   void VisitBinaryOperator(const BinaryOperator *BO);
@@ -1577,6 +1581,7 @@ class BuildLockset : public ConstStmtVisitor<BuildLockset> {
   void VisitCXXConstructExpr(const CXXConstructExpr *Exp);
   void VisitDeclStmt(const DeclStmt *S);
   void VisitMaterializeTemporaryExpr(const MaterializeTemporaryExpr *Exp);
+  void VisitReturnStmt(const ReturnStmt *S);
 };
 
 } // namespace
@@ -1758,6 +1763,8 @@ void ThreadSafetyAnalyzer::checkPtAccess(const FactSet &FSet, const Expr *Exp,
   // Pass by reference warnings are under a different flag.
   ProtectedOperationKind PtPOK = POK_VarDereference;
   if (POK == POK_PassByRef) PtPOK = POK_PtPassByRef;
+  if (POK == POK_ReturnByRef)
+    PtPOK = POK_PtReturnByRef;
 
   const ValueDecl *D = getValueDecl(Exp);
   if (!D || !D->hasAttrs())
@@ -2142,6 +2149,25 @@ void BuildLockset::VisitMaterializeTemporaryExpr(
   }
 }
 
+void BuildLockset::VisitReturnStmt(const ReturnStmt *S) {
+  if (Analyzer->CurrentFunction == nullptr)
+    return;
+  const Expr *RetVal = S->getRetValue();
+  if (!RetVal)
+    return;
+
+  // If returning by reference, check that the function requires the appropriate
+  // capabilities.
+  const QualType ReturnType =
+      Analyzer->CurrentFunction->getReturnType().getCanonicalType();
+  if (ReturnType->isLValueReferenceType()) {
+    Analyzer->checkAccess(
+        FunctionExitFSet, RetVal,
+        ReturnType->getPointeeType().isConstQualified() ? AK_Read : AK_Written,
+        POK_ReturnByRef);
+  }
+}
+
 /// Given two facts merging on a join point, possibly warn and decide whether to
 /// keep or replace.
 ///
@@ -2251,8 +2277,7 @@ void ThreadSafetyAnalyzer::runAnalysis(AnalysisDeclContext &AC) {
 
   CFG *CFGraph = walker.getGraph();
   const NamedDecl *D = walker.getDecl();
-  const auto *CurrentFunction = dyn_cast<FunctionDecl>(D);
-  CurrentMethod = dyn_cast<CXXMethodDecl>(D);
+  CurrentFunction = dyn_cast<FunctionDecl>(D);
 
   if (D->hasAttr<NoThreadSafetyAnalysisAttr>())
     return;
@@ -2278,7 +2303,7 @@ void ThreadSafetyAnalyzer::runAnalysis(AnalysisDeclContext &AC) {
   PostOrderCFGView::CFGBlockSet VisitedBlocks(CFGraph);
 
   CFGBlockInfo &Initial = BlockInfo[CFGraph->getEntry().getBlockID()];
-  CFGBlockInfo &Final   = BlockInfo[CFGraph->getExit().getBlockID()];
+  CFGBlockInfo &Final = BlockInfo[CFGraph->getExit().getBlockID()];
 
   // Mark entry block as reachable
   Initial.Reachable = true;
@@ -2348,6 +2373,25 @@ void ThreadSafetyAnalyzer::runAnalysis(AnalysisDeclContext &AC) {
     }
   }
 
+  // Compute the expected exit set.
+  // By default, we expect all locks held on entry to be held on exit.
+  FactSet ExpectedFunctionExitSet = Initial.EntrySet;
+
+  // Adjust the expected exit set by adding or removing locks, as declared
+  // by *-LOCK_FUNCTION and UNLOCK_FUNCTION.  The intersect below will then
+  // issue the appropriate warning.
+  // FIXME: the location here is not quite right.
+  for (const auto &Lock : ExclusiveLocksAcquired)
+    ExpectedFunctionExitSet.addLock(
+        FactMan, std::make_unique<LockableFactEntry>(Lock, LK_Exclusive,
+                                                     D->getLocation()));
+  for (const auto &Lock : SharedLocksAcquired)
+    ExpectedFunctionExitSet.addLock(
+        FactMan,
+        std::make_unique<LockableFactEntry>(Lock, LK_Shared, D->getLocation()));
+  for (const auto &Lock : LocksReleased)
+    ExpectedFunctionExitSet.removeLock(FactMan, Lock);
+
   for (const auto *CurrBlock : *SortedGraph) {
     unsigned CurrBlockID = CurrBlock->getBlockID();
     CFGBlockInfo *CurrBlockInfo = &BlockInfo[CurrBlockID];
@@ -2407,7 +2451,7 @@ void ThreadSafetyAnalyzer::runAnalysis(AnalysisDeclContext &AC) {
     if (!CurrBlockInfo->Reachable)
       continue;
 
-    BuildLockset LocksetBuilder(this, *CurrBlockInfo);
+    BuildLockset LocksetBuilder(this, *CurrBlockInfo, ExpectedFunctionExitSet);
 
     // Visit all the statements in the basic block.
     for (const auto &BI : *CurrBlock) {
@@ -2483,24 +2527,8 @@ void ThreadSafetyAnalyzer::runAnalysis(AnalysisDeclContext &AC) {
   if (!Final.Reachable)
     return;
 
-  // By default, we expect all locks held on entry to be held on exit.
-  FactSet ExpectedExitSet = Initial.EntrySet;
-
-  // Adjust the expected exit set by adding or removing locks, as declared
-  // by *-LOCK_FUNCTION and UNLOCK_FUNCTION.  The intersect below will then
-  // issue the appropriate warning.
-  // FIXME: the location here is not quite right.
-  for (const auto &Lock : ExclusiveLocksAcquired)
-    ExpectedExitSet.addLock(FactMan, std::make_unique<LockableFactEntry>(
-                                         Lock, LK_Exclusive, D->getLocation()));
-  for (const auto &Lock : SharedLocksAcquired)
-    ExpectedExitSet.addLock(FactMan, std::make_unique<LockableFactEntry>(
-                                         Lock, LK_Shared, D->getLocation()));
-  for (const auto &Lock : LocksReleased)
-    ExpectedExitSet.removeLock(FactMan, Lock);
-
   // FIXME: Should we call this function for all blocks which exit the function?
-  intersectAndWarn(ExpectedExitSet, Final.ExitSet, Final.ExitLoc,
+  intersectAndWarn(ExpectedFunctionExitSet, Final.ExitSet, Final.ExitLoc,
                    LEK_LockedAtEndOfFunction, LEK_NotLockedAtEndOfFunction);
 
   Handler.leaveFunction(CurrentFunction);

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -1983,6 +1983,12 @@ class ThreadSafetyReporter : public clang::threadSafety::ThreadSafetyHandler {
         case POK_PtPassByRef:
           DiagID = diag::warn_pt_guarded_pass_by_reference;
           break;
+        case POK_ReturnByRef:
+          DiagID = diag::warn_guarded_return_by_reference;
+          break;
+        case POK_PtReturnByRef:
+          DiagID = diag::warn_pt_guarded_return_by_reference;
+          break;
       }
       PartialDiagnosticAt Warning(Loc, S.PDiag(DiagID) << Kind
                                                        << D
@@ -2013,6 +2019,12 @@ class ThreadSafetyReporter : public clang::threadSafety::ThreadSafetyHandler {
         case POK_PtPassByRef:
           DiagID = diag::warn_pt_guarded_pass_by_reference;
           break;
+        case POK_ReturnByRef:
+          DiagID = diag::warn_guarded_return_by_reference;
+          break;
+        case POK_PtReturnByRef:
+          DiagID = diag::warn_pt_guarded_return_by_reference;
+          break;
       }
       PartialDiagnosticAt Warning(Loc, S.PDiag(DiagID) << Kind
                                                        << D

clang/test/SemaCXX/warn-thread-safety-analysis.cpp

@@ -5580,6 +5580,85 @@ class Bar {
   }
 };
 
+class Return {
+  Mutex mu;
+  Foo foo GUARDED_BY(mu);
+  Foo* foo_ptr PT_GUARDED_BY(mu);
+
+  Foo returns_value_locked() {
+    MutexLock lock(&mu);
+    return foo;
+  }
+
+  Foo returns_value_locks_required() EXCLUSIVE_LOCKS_REQUIRED(mu) {
+    return foo;
+  }
+
+  Foo returns_value_releases_lock_after_return() UNLOCK_FUNCTION(mu) {
+    MutexLock lock(&mu, true);
+    return foo;
+  }
+
+  Foo returns_value_aquires_lock() EXCLUSIVE_LOCK_FUNCTION(mu) {
+    mu.Lock();
+    return foo;
+  }
+  
+  Foo returns_value_not_locked() {
+    return foo;               // expected-warning {{reading variable 'foo' requires holding mutex 'mu'}}
+  }
+  
+  Foo returns_value_releases_lock_before_return() UNLOCK_FUNCTION(mu) {
+    mu.Unlock();
+    return foo;               // expected-warning {{reading variable 'foo' requires holding mutex 'mu'}}
+  }
+
+  Foo &returns_ref_not_locked() {
+    return foo;               // expected-warning {{returning variable 'foo' by reference requires holding mutex 'mu'}}
+  }
+
+  Foo &returns_ref_locked() {
+    MutexLock lock(&mu);
+    return foo;               // expected-warning {{returning variable 'foo' by reference requires holding mutex 'mu'}}
+  }
+
+  Foo &returns_ref_shared_locks_required() SHARED_LOCKS_REQUIRED(mu) {
+    return foo;               // expected-warning {{returning variable 'foo' by reference requires holding mutex 'mu' exclusively}}
+  }
+
+  Foo &returns_ref_exclusive_locks_required() EXCLUSIVE_LOCKS_REQUIRED(mu) {
+    return foo;
+  }
+
+  Foo &returns_ref_releases_lock_after_return() UNLOCK_FUNCTION(mu) {
+    MutexLock lock(&mu, true);
+    return foo;               // expected-warning {{returning variable 'foo' by reference requires holding mutex 'mu' exclusively}}
+  }
+
+  Foo& returns_ref_releases_lock_before_return() UNLOCK_FUNCTION(mu) {
+    mu.Unlock();
+    return foo;               // // expected-warning {{returning variable 'foo' by reference requires holding mutex 'mu' exclusively}}
+  }
+  
+  Foo &returns_ref_aquires_lock() EXCLUSIVE_LOCK_FUNCTION(mu) {
+    mu.Lock();
+    return foo;
+  }
+  
+  const Foo &returns_constref_shared_locks_required() SHARED_LOCKS_REQUIRED(mu) {
+    return foo;
+  }
+  
+  Foo *returns_ptr() {
+    return &foo;              // FIXME -- Do we want to warn on this ?
+  }
+
+  Foo &returns_ref2() {
+    return *foo_ptr;          // expected-warning {{returning the value that 'foo_ptr' points to by reference requires holding mutex 'mu' exclusively}}
+  }
+
+};
+
 
 }  // end namespace PassByRefTest