This patch implements PR#22821.

Taking the address of a packed member is dangerous since the reduced
alignment of the pointee is lost. This can lead to memory alignment
faults in some architectures if the pointer value is dereferenced.

This change adds a new warning to clang emitted when taking the address
of a packed member. A packed member is either a field/data member
declared as attribute((packed)) or belonging to a struct/class
declared as such. The associated flag is -Waddress-of-packed-member.
Conversions (either implicit or via a valid casting) to pointer types
with lower or equal alignment requirements (e.g. void* or char*)
will silence the warning.

Differential Revision: https://reviews.llvm.org/D20561

llvm-svn: 278483

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e7647f8..bc62814 100644 (file)
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5489,6 +5489,9 @@ def warn_pointer_indirection_from_incompatible_type : Warning<
   "dereference of type %1 that was reinterpret_cast from type %0 has undefined "
   "behavior">,
   InGroup<UndefinedReinterpretCast>, DefaultIgnore;
+def warn_taking_address_of_packed_member : Warning<
+  "taking address of packed member %0 of class or structure %q1 may result in an unaligned pointer value">,
+  InGroup<DiagGroup<"address-of-packed-member">>;
 
 def err_objc_object_assignment : Error<
   "cannot assign to class object (%0 invalid)">;

diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index d94bcfc..1a50030 100644 (file)
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -9570,6 +9570,10 @@ private:
   void CheckArgumentWithTypeTag(const ArgumentWithTypeTagAttr *Attr,
                                 const Expr * const *ExprArgs);
 
+  /// \brief Check if we are taking the address of a packed field
+  /// as this may be a problem if the pointer value is dereferenced.
+  void CheckAddressOfPackedMember(Expr *rhs);
+
   /// \brief The parser's current scope.
   ///
   /// The parser maintains this state here.
@@ -9664,6 +9668,51 @@ public:
   // Emitting members of dllexported classes is delayed until the class
   // (including field initializers) is fully parsed.
   SmallVector<CXXRecordDecl*, 4> DelayedDllExportClasses;
+
+private:
+  /// \brief Helper class that collects misaligned member designations and
+  /// their location info for delayed diagnostics.
+  struct MisalignedMember {
+    Expr *E;
+    RecordDecl *RD;
+    ValueDecl *MD;
+    CharUnits Alignment;
+
+    MisalignedMember() : E(), RD(), MD(), Alignment() {}
+    MisalignedMember(Expr *E, RecordDecl *RD, ValueDecl *MD,
+                     CharUnits Alignment)
+        : E(E), RD(RD), MD(MD), Alignment(Alignment) {}
+    explicit MisalignedMember(Expr *E)
+        : MisalignedMember(E, nullptr, nullptr, CharUnits()) {}
+
+    bool operator==(const MisalignedMember &m) { return this->E == m.E; }
+  };
+  /// \brief Small set of gathered accesses to potentially misaligned members
+  /// due to the packed attribute.
+  SmallVector<MisalignedMember, 4> MisalignedMembers;
+
+  /// \brief Adds an expression to the set of gathered misaligned members.
+  void AddPotentialMisalignedMembers(Expr *E, RecordDecl *RD, ValueDecl *MD,
+                                     CharUnits Alignment);
+
+public:
+  /// \brief Diagnoses the current set of gathered accesses. This typically
+  /// happens at full expression level. The set is cleared after emitting the
+  /// diagnostics.
+  void DiagnoseMisalignedMembers();
+
+  /// \brief This function checks if the expression is in the sef of potentially
+  /// misaligned members and it is converted to some pointer type T with lower
+  /// or equal alignment requirements.  If so it removes it. This is used when
+  /// we do not want to diagnose such misaligned access (e.g. in conversions to void*).
+  void DiscardMisalignedMemberAddress(const Type *T, Expr *E);
+
+  /// \brief This function calls Action when it determines that E designates a
+  /// misaligned member due to the packed attribute. This is used to emit
+  /// local diagnostics like in reference binding.
+  void RefersToMemberWithReducedAlignment(
+      Expr *E,
+      std::function<void(Expr *, RecordDecl *, ValueDecl *, CharUnits)> Action);
 };
 
 /// \brief RAII object that enters a new expression evaluation context.

diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index e83dd07..e19020c 100644 (file)
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -256,6 +256,7 @@ Sema::BuildCXXNamedCast(SourceLocation OpLoc, tok::TokenKind Kind,
       Op.CheckConstCast();
       if (Op.SrcExpr.isInvalid())
         return ExprError();
+      DiscardMisalignedMemberAddress(DestType.getTypePtr(), E);
     }
     return Op.complete(CXXConstCastExpr::Create(Context, Op.ResultType,
                                   Op.ValueKind, Op.SrcExpr.get(), DestTInfo,
@@ -279,6 +280,7 @@ Sema::BuildCXXNamedCast(SourceLocation OpLoc, tok::TokenKind Kind,
       Op.CheckReinterpretCast();
       if (Op.SrcExpr.isInvalid())
         return ExprError();
+      DiscardMisalignedMemberAddress(DestType.getTypePtr(), E);
     }
     return Op.complete(CXXReinterpretCastExpr::Create(Context, Op.ResultType,
                                     Op.ValueKind, Op.Kind, Op.SrcExpr.get(),
@@ -291,6 +293,7 @@ Sema::BuildCXXNamedCast(SourceLocation OpLoc, tok::TokenKind Kind,
       Op.CheckStaticCast();
       if (Op.SrcExpr.isInvalid())
         return ExprError();
+      DiscardMisalignedMemberAddress(DestType.getTypePtr(), E);
     }
     
     return Op.complete(CXXStaticCastExpr::Create(Context, Op.ResultType,

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index c1781c2..6edb251 100644 (file)
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -8383,6 +8383,8 @@ void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
 
   DiagnoseNullConversion(S, E, T, CC);
 
+  S.DiscardMisalignedMemberAddress(Target, E);
+
   if (!Source->isIntegerType() || !Target->isIntegerType())
     return;
 
@@ -9453,6 +9455,7 @@ void Sema::CheckCompletedExpr(Expr *E, SourceLocation CheckLoc,
     CheckUnsequencedOperations(E);
   if (!IsConstexpr && !E->isValueDependent())
     CheckForIntOverflow(E);
+  DiagnoseMisalignedMembers();
 }
 
 void Sema::CheckBitFieldInitialization(SourceLocation InitLoc,
@@ -10998,3 +11001,67 @@ void Sema::CheckArgumentWithTypeTag(const ArgumentWithTypeTagAttr *Attr,
         << ArgumentExpr->getSourceRange()
         << TypeTagExpr->getSourceRange();
 }
+
+void Sema::AddPotentialMisalignedMembers(Expr *E, RecordDecl *RD, ValueDecl *MD,
+                                         CharUnits Alignment) {
+  MisalignedMembers.emplace_back(E, RD, MD, Alignment);
+}
+
+void Sema::DiagnoseMisalignedMembers() {
+  for (MisalignedMember &m : MisalignedMembers) {
+    Diag(m.E->getLocStart(), diag::warn_taking_address_of_packed_member)
+        << m.MD << m.RD << m.E->getSourceRange();
+  }
+  MisalignedMembers.clear();
+}
+
+void Sema::DiscardMisalignedMemberAddress(const Type *T, Expr *E) {
+  if (!T->isPointerType())
+    return;
+  if (isa<UnaryOperator>(E) &&
+      cast<UnaryOperator>(E)->getOpcode() == UO_AddrOf) {
+    auto *Op = cast<UnaryOperator>(E)->getSubExpr()->IgnoreParens();
+    if (isa<MemberExpr>(Op)) {
+      auto MA = std::find(MisalignedMembers.begin(), MisalignedMembers.end(),
+                          MisalignedMember(Op));
+      if (MA != MisalignedMembers.end() &&
+          Context.getTypeAlignInChars(T->getPointeeType()) <= MA->Alignment)
+        MisalignedMembers.erase(MA);
+    }
+  }
+}
+
+void Sema::RefersToMemberWithReducedAlignment(
+    Expr *E,
+    std::function<void(Expr *, RecordDecl *, ValueDecl *, CharUnits)> Action) {
+  const auto *ME = dyn_cast<MemberExpr>(E);
+  while (ME && isa<FieldDecl>(ME->getMemberDecl())) {
+    QualType BaseType = ME->getBase()->getType();
+    if (ME->isArrow())
+      BaseType = BaseType->getPointeeType();
+    RecordDecl *RD = BaseType->getAs<RecordType>()->getDecl();
+
+    ValueDecl *MD = ME->getMemberDecl();
+    bool ByteAligned = Context.getTypeAlignInChars(MD->getType()).isOne();
+    if (ByteAligned) // Attribute packed does not have any effect.
+      break;
+
+    if (!ByteAligned &&
+        (RD->hasAttr<PackedAttr>() || (MD->hasAttr<PackedAttr>()))) {
+      CharUnits Alignment = std::min(Context.getTypeAlignInChars(MD->getType()),
+                                     Context.getTypeAlignInChars(BaseType));
+      // Notify that this expression designates a member with reduced alignment
+      Action(E, RD, MD, Alignment);
+      break;
+    }
+    ME = dyn_cast<MemberExpr>(ME->getBase());
+  }
+}
+
+void Sema::CheckAddressOfPackedMember(Expr *rhs) {
+  using namespace std::placeholders;
+  RefersToMemberWithReducedAlignment(
+      rhs, std::bind(&Sema::AddPotentialMisalignedMembers, std::ref(*this), _1,
+                     _2, _3, _4));
+}
+

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 81b57ca..5a437d2 100644 (file)
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -6022,7 +6022,9 @@ Sema::ActOnCastExpr(Scope *S, SourceLocation LParenLoc,
   CheckTollFreeBridgeCast(castType, CastExpr);
   
   CheckObjCBridgeRelatedCast(castType, CastExpr);
-  
+
+  DiscardMisalignedMemberAddress(castType.getTypePtr(), CastExpr);
+
   return BuildCStyleCastExpr(LParenLoc, castTInfo, RParenLoc, CastExpr);
 }
 
@@ -10614,6 +10616,8 @@ QualType Sema::CheckAddressOfOperand(ExprResult &OrigOp, SourceLocation OpLoc) {
   if (op->getType()->isObjCObjectType())
     return Context.getObjCObjectPointerType(op->getType());
 
+  CheckAddressOfPackedMember(op);
+
   return Context.getPointerType(op->getType());
 }
 

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 35accac..386c7ab 100644 (file)
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -6660,12 +6660,16 @@ InitializationSequence::Perform(Sema &S,
                                     getAssignmentAction(Entity), CCK);
       if (CurInitExprRes.isInvalid())
         return ExprError();
+
+      S.DiscardMisalignedMemberAddress(Step->Type.getTypePtr(), CurInit.get());
+
       CurInit = CurInitExprRes;
 
       if (Step->Kind == SK_ConversionSequenceNoNarrowing &&
           S.getLangOpts().CPlusPlus && !CurInit.get()->isValueDependent())
         DiagnoseNarrowingInInitList(S, *Step->ICS, SourceType, Entity.getType(),
                                     CurInit.get());
+
       break;
     }