[Clang] Add __builtin_counted_by_ref builtin (llvm#114495)

The __builtin_counted_by_ref builtin is used on a flexible array
pointer and returns a pointer to the "counted_by" attribute's COUNT
argument, which is a field in the same non-anonymous struct as the
flexible array member. This is useful for automatically setting the
count field without needing the programmer's intervention. Otherwise
it's possible to get this anti-pattern:
    
      ptr = alloc(<ty>, ..., COUNT);
      ptr->FAM[9] = 42; /* <<< Sanitizer will complain */
      ptr->count = COUNT;
    
To prevent this anti-pattern, the user can create an allocator that
automatically performs the assignment:
    
      #define alloc(TY, FAM, COUNT) ({ \
          TY __p = alloc(get_size(TY, COUNT));             \
          if (__builtin_counted_by_ref(__p->FAM))          \
              *__builtin_counted_by_ref(__p->FAM) = COUNT; \
          __p;                                             \
      })

The builtin's behavior is heavily dependent upon the "counted_by"
attribute existing. It's main utility is during allocation to avoid
the above anti-pattern. If the flexible array member doesn't have that
attribute, the builtin becomes a no-op. Therefore, if the flexible
array member has a "count" field not referenced by "counted_by", it
must be set explicitly after the allocation as this builtin will
return a "nullptr" and the assignment will most likely be elided.

---------

Co-authored-by: Bill Wendling <[email protected]>
Co-authored-by: Aaron Ballman <[email protected]>

Author: 3 people

clang/docs/LanguageExtensions.rst

@@ -3774,6 +3774,74 @@ type-generic alternative to the ``__builtin_clz{,l,ll}`` (respectively
 ``__builtin_ctz{,l,ll}``) builtins, with support for other integer types, such
 as ``unsigned __int128`` and C23 ``unsigned _BitInt(N)``.
 
+``__builtin_counted_by_ref``
+----------------------------
+
+``__builtin_counted_by_ref`` returns a pointer to the count field from the
+``counted_by`` attribute.
+
+The argument must be a flexible array member. If the argument isn't a flexible
+array member or doesn't have the ``counted_by`` attribute, the builtin returns
+``(void *)0``.
+
+**Syntax**:
+
+.. code-block:: c
+
+  T *__builtin_counted_by_ref(void *array)
+
+**Examples**:
+
+.. code-block:: c
+
+  #define alloc(P, FAM, COUNT) ({                                 \
+     size_t __ignored_assignment;                                 \
+     typeof(P) __p = NULL;                                        \
+     __p = malloc(MAX(sizeof(*__p),                               \
+                      sizeof(*__p) + sizeof(*__p->FAM) * COUNT)); \
+                                                                  \
+     *_Generic(                                                   \
+       __builtin_counted_by_ref(__p->FAM),                        \
+         void *: &__ignored_assignment,                           \
+         default: __builtin_counted_by_ref(__p->FAM)) = COUNT;    \
+                                                                  \
+     __p;                                                         \
+  })
+
+**Description**:
+
+The ``__builtin_counted_by_ref`` builtin allows the programmer to prevent a
+common error associated with the ``counted_by`` attribute. When using the
+``counted_by`` attribute, the ``count`` field **must** be set before the
+flexible array member can be accessed. Otherwise, the sanitizers may view such
+accesses as false positives. For instance, it's not uncommon for programmers to
+initialize the flexible array before setting the ``count`` field:
+
+.. code-block:: c
+
+  struct s {
+    int dummy;
+    short count;
+    long array[] __attribute__((counted_by(count)));
+  };
+
+  struct s *ptr = malloc(sizeof(struct s) + sizeof(long) * COUNT);
+
+  for (int i = 0; i < COUNT; ++i)
+    ptr->array[i] = i;
+
+  ptr->count = COUNT;
+
+Enforcing the rule that ``ptr->count = COUNT;`` must occur after every
+allocation of a struct with a flexible array member with the ``counted_by``
+attribute is prone to failure in large code bases. This builtin mitigates this
+for allocators (like in Linux) that are implemented in a way where the counter
+assignment can happen automatically.
+
+**Note:** The value returned by ``__builtin_counted_by_ref`` cannot be assigned
+to a variable, have its address taken, or passed into or returned from a
+function, because doing so violates bounds safety conventions.
+
 Multiprecision Arithmetic Builtins
 ----------------------------------
 

clang/docs/ReleaseNotes.rst

@@ -313,6 +313,29 @@ Non-comprehensive list of changes in this release
   as well as declarations.
 - ``__builtin_abs`` function can now be used in constant expressions.
 
+- The new builtin ``__builtin_counted_by_ref`` was added. In contexts where the
+  programmer needs access to the ``counted_by`` attribute's field, but it's not
+  available --- e.g. in macros. For instace, it can be used to automatically
+  set the counter during allocation in the Linux kernel:
+
+  .. code-block:: c
+
+     /* A simplified version of Linux allocation macros */
+     #define alloc(PTR, FAM, COUNT) ({ \
+         sizeof_t __ignored_assignment;                             \
+         typeof(P) __p;                                             \
+         size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT;      \
+         __p = malloc(__size);                                      \
+         *_Generic(                                                 \
+           __builtin_counted_by_ref(__p->FAM),                      \
+             void *: &__ignored_assignment,                         \
+             default: __builtin_counted_by_ref(__p->FAM)) = COUNT;  \
+         __p;                                                       \
+     })
+
+  The flexible array member (FAM) can now be accessed immediately without causing
+  issues with the sanitizer because the counter is automatically set.
+
 New Compiler Flags
 ------------------
 

clang/include/clang/Basic/Builtins.td

@@ -4932,3 +4932,9 @@ def ArithmeticFence : LangBuiltin<"ALL_LANGUAGES"> {
   let Attributes = [CustomTypeChecking, Constexpr];
   let Prototype = "void(...)";
 }
+
+def CountedByRef : Builtin {
+  let Spellings = ["__builtin_counted_by_ref"];
+  let Attributes = [NoThrow, CustomTypeChecking];
+  let Prototype = "int(...)";
+}

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6652,6 +6652,18 @@ def warn_counted_by_attr_elt_type_unknown_size :
   Warning<err_counted_by_attr_pointee_unknown_size.Summary>,
   InGroup<BoundsSafetyCountedByEltTyUnknownSize>;
 
+// __builtin_counted_by_ref diagnostics:
+def err_builtin_counted_by_ref_must_be_flex_array_member : Error<
+  "'__builtin_counted_by_ref' argument must reference a flexible array member">;
+def err_builtin_counted_by_ref_cannot_leak_reference : Error<
+  "value returned by '__builtin_counted_by_ref' cannot be assigned to a "
+  "variable, have its address taken, or passed into or returned from a function">;
+def err_builtin_counted_by_ref_invalid_lhs_use : Error<
+  "value returned by '__builtin_counted_by_ref' cannot be used in "
+  "%select{an array subscript|a binary}0 expression">;
+def err_builtin_counted_by_ref_has_side_effects : Error<
+  "'__builtin_counted_by_ref' argument cannot have side-effects">;
+
 let CategoryName = "ARC Semantic Issue" in {
 
 // ARC-mode diagnostics.

clang/include/clang/Sema/Sema.h

@@ -2510,6 +2510,8 @@ class Sema final : public SemaBase {
 
   bool BuiltinNonDeterministicValue(CallExpr *TheCall);
 
+  bool BuiltinCountedByRef(CallExpr *TheCall);
+
   // Matrix builtin handling.
   ExprResult BuiltinMatrixTranspose(CallExpr *TheCall, ExprResult CallResult);
   ExprResult BuiltinMatrixColumnMajorLoad(CallExpr *TheCall,

clang/lib/AST/Decl.cpp

@@ -3657,6 +3657,10 @@ unsigned FunctionDecl::getBuiltinID(bool ConsiderWrapperFunctions) const {
       (!hasAttr<ArmBuiltinAliasAttr>() && !hasAttr<BuiltinAliasAttr>()))
     return 0;
 
+  if (getASTContext().getLangOpts().CPlusPlus &&
+      BuiltinID == Builtin::BI__builtin_counted_by_ref)
+    return 0;
+
   const ASTContext &Context = getASTContext();
   if (!Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID))
     return BuiltinID;

clang/lib/CodeGen/CGBuiltin.cpp

@@ -3691,6 +3691,35 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     return RValue::get(emitBuiltinObjectSize(E->getArg(0), Type, ResType,
                                              /*EmittedE=*/nullptr, IsDynamic));
   }
+  case Builtin::BI__builtin_counted_by_ref: {
+    // Default to returning '(void *) 0'.
+    llvm::Value *Result = llvm::ConstantPointerNull::get(
+        llvm::PointerType::getUnqual(getLLVMContext()));
+
+    const Expr *Arg = E->getArg(0)->IgnoreParenImpCasts();
+
+    if (auto *UO = dyn_cast<UnaryOperator>(Arg);
+        UO && UO->getOpcode() == UO_AddrOf) {
+      Arg = UO->getSubExpr()->IgnoreParenImpCasts();
+
+      if (auto *ASE = dyn_cast<ArraySubscriptExpr>(Arg))
+        Arg = ASE->getBase()->IgnoreParenImpCasts();
+    }
+
+    if (const MemberExpr *ME = dyn_cast_if_present<MemberExpr>(Arg)) {
+      if (auto *CATy =
+              ME->getMemberDecl()->getType()->getAs<CountAttributedType>();
+          CATy && CATy->getKind() == CountAttributedType::CountedBy) {
+        const auto *FAMDecl = cast<FieldDecl>(ME->getMemberDecl());
+        if (const FieldDecl *CountFD = FAMDecl->findCountedByField())
+          Result = GetCountedByFieldExprGEP(Arg, FAMDecl, CountFD);
+        else
+          llvm::report_fatal_error("Cannot find the counted_by 'count' field");
+      }
+    }
+
+    return RValue::get(Result);
+  }
   case Builtin::BI__builtin_prefetch: {
     Value *Locality, *RW, *Address = EmitScalarExpr(E->getArg(0));
     // FIXME: Technically these constants should of type 'int', yes?

clang/lib/CodeGen/CGExpr.cpp

@@ -1145,15 +1145,7 @@ static bool getGEPIndicesToField(CodeGenFunction &CGF, const RecordDecl *RD,
   return false;
 }
 
-/// This method is typically called in contexts where we can't generate
-/// side-effects, like in __builtin_dynamic_object_size. When finding
-/// expressions, only choose those that have either already been emitted or can
-/// be loaded without side-effects.
-///
-/// - \p FAMDecl: the \p Decl for the flexible array member. It may not be
-///   within the top-level struct.
-/// - \p CountDecl: must be within the same non-anonymous struct as \p FAMDecl.
-llvm::Value *CodeGenFunction::EmitLoadOfCountedByField(
+llvm::Value *CodeGenFunction::GetCountedByFieldExprGEP(
     const Expr *Base, const FieldDecl *FAMDecl, const FieldDecl *CountDecl) {
   const RecordDecl *RD = CountDecl->getParent()->getOuterLexicalRecordContext();
 
@@ -1182,12 +1174,25 @@ llvm::Value *CodeGenFunction::EmitLoadOfCountedByField(
     return nullptr;
 
   Indices.push_back(Builder.getInt32(0));
-  Res = Builder.CreateInBoundsGEP(
+  return Builder.CreateInBoundsGEP(
       ConvertType(QualType(RD->getTypeForDecl(), 0)), Res,
       RecIndicesTy(llvm::reverse(Indices)), "..counted_by.gep");
+}
 
-  return Builder.CreateAlignedLoad(ConvertType(CountDecl->getType()), Res,
-                                   getIntAlign(), "..counted_by.load");
+/// This method is typically called in contexts where we can't generate
+/// side-effects, like in __builtin_dynamic_object_size. When finding
+/// expressions, only choose those that have either already been emitted or can
+/// be loaded without side-effects.
+///
+/// - \p FAMDecl: the \p Decl for the flexible array member. It may not be
+///   within the top-level struct.
+/// - \p CountDecl: must be within the same non-anonymous struct as \p FAMDecl.
+llvm::Value *CodeGenFunction::EmitLoadOfCountedByField(
+    const Expr *Base, const FieldDecl *FAMDecl, const FieldDecl *CountDecl) {
+  if (llvm::Value *GEP = GetCountedByFieldExprGEP(Base, FAMDecl, CountDecl))
+    return Builder.CreateAlignedLoad(ConvertType(CountDecl->getType()), GEP,
+                                     getIntAlign(), "..counted_by.load");
+  return nullptr;
 }
 
 void CodeGenFunction::EmitBoundsCheck(const Expr *E, const Expr *Base,

clang/lib/CodeGen/CodeGenFunction.h

@@ -3305,6 +3305,10 @@ class CodeGenFunction : public CodeGenTypeCache {
                                         const FieldDecl *FAMDecl,
                                         uint64_t &Offset);
 
+  llvm::Value *GetCountedByFieldExprGEP(const Expr *Base,
+                                        const FieldDecl *FAMDecl,
+                                        const FieldDecl *CountDecl);
+
   /// Build an expression accessing the "counted_by" field.
   llvm::Value *EmitLoadOfCountedByField(const Expr *Base,
                                         const FieldDecl *FAMDecl,

clang/lib/Sema/SemaChecking.cpp

@@ -2973,6 +2973,10 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     }
     break;
   }
+  case Builtin::BI__builtin_counted_by_ref:
+    if (BuiltinCountedByRef(TheCall))
+      return ExprError();
+    break;
   }
 
   if (getLangOpts().HLSL && HLSL().CheckBuiltinFunctionCall(BuiltinID, TheCall))
@@ -5575,6 +5579,55 @@ bool Sema::BuiltinSetjmp(CallExpr *TheCall) {
   return false;
 }
 
+bool Sema::BuiltinCountedByRef(CallExpr *TheCall) {
+  if (checkArgCount(TheCall, 1))
+    return true;
+
+  ExprResult ArgRes = UsualUnaryConversions(TheCall->getArg(0));
+  if (ArgRes.isInvalid())
+    return true;
+
+  // For simplicity, we support only limited expressions for the argument.
+  // Specifically a pointer to a flexible array member:'ptr->array'. This
+  // allows us to reject arguments with complex casting, which really shouldn't
+  // be a huge problem.
+  const Expr *Arg = ArgRes.get()->IgnoreParenImpCasts();
+  if (!isa<PointerType>(Arg->getType()) && !Arg->getType()->isArrayType())
+    return Diag(Arg->getBeginLoc(),
+                diag::err_builtin_counted_by_ref_must_be_flex_array_member)
+           << Arg->getSourceRange();
+
+  if (Arg->HasSideEffects(Context))
+    return Diag(Arg->getBeginLoc(),
+                diag::err_builtin_counted_by_ref_has_side_effects)
+           << Arg->getSourceRange();
+
+  if (const auto *ME = dyn_cast<MemberExpr>(Arg)) {
+    if (!ME->isFlexibleArrayMemberLike(
+            Context, getLangOpts().getStrictFlexArraysLevel()))
+      return Diag(Arg->getBeginLoc(),
+                  diag::err_builtin_counted_by_ref_must_be_flex_array_member)
+             << Arg->getSourceRange();
+
+    if (auto *CATy =
+            ME->getMemberDecl()->getType()->getAs<CountAttributedType>();
+        CATy && CATy->getKind() == CountAttributedType::CountedBy) {
+      const auto *FAMDecl = cast<FieldDecl>(ME->getMemberDecl());
+      if (const FieldDecl *CountFD = FAMDecl->findCountedByField()) {
+        TheCall->setType(Context.getPointerType(CountFD->getType()));
+        return false;
+      }
+    }
+  } else {
+    return Diag(Arg->getBeginLoc(),
+                diag::err_builtin_counted_by_ref_must_be_flex_array_member)
+           << Arg->getSourceRange();
+  }
+
+  TheCall->setType(Context.getPointerType(Context.VoidTy));
+  return false;
+}
+
 namespace {
 
 class UncoveredArgHandler {