[Bounds-Safety] Temporarily relax a counted_by attribute restriction on flexible array members

In 0ec3b972e5 an additional restriction
was added when applying the `counted_by` attribute to flexible array
members in structs. The restriction prevented the element type being
a struct that itself had a flexible array member. E.g.:

```
struct has_unannotated_VLA {
  int count;
  char buffer[];
};

struct buffer_of_structs_with_unnannotated_vla {
  int count;
  struct has_unannotated_VLA Arr[] __counted_by(count);
};
```

In this example assuming the size of `Arr` is `sizeof(struct
has_unannotated_VLA)*count` (which is what the attribute says) is wrong
because it doesn't account for the size of
`has_unannotated_VLA::buffer`. This is why this kind of code construct
was treated as an error.

However, it turns out existing Linux kernel code used the attribute
on a flexible array member in this way
(https://github.com/llvm/llvm-project/pull/90786#issuecomment-2118416515).

To unbreak the build this restriction is downgraded to a warning with
the plan to make it an error again once the errornous use of the
attribute in the Linux kernel is resolved.

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1447,6 +1447,10 @@ def FunctionMultiVersioning
 
 def NoDeref : DiagGroup<"noderef">;
 
+// -fbounds-safety and bounds annotation related warnings
+def BoundsSafetyCountedByEltTyUnknownSize :
+  DiagGroup<"bounds-safety-counted-by-elt-type-unknown-size">;
+
 // A group for cross translation unit static analysis related warnings.
 def CrossTU : DiagGroup<"ctu">;
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6552,7 +6552,7 @@ def err_counted_by_attr_refer_to_union : Error<
 def note_flexible_array_counted_by_attr_field : Note<
   "field %0 declared here">;
 def err_counted_by_attr_pointee_unknown_size : Error<
-  "'counted_by' cannot be applied to %select{"
+  "'counted_by' %select{cannot|should not}3 be applied to %select{"
     "a pointer with pointee|" // pointer
     "an array with element}0" // array
   " of unknown size because %1 is %select{"
@@ -6561,8 +6561,14 @@ def err_counted_by_attr_pointee_unknown_size : Error<
     "a function type|"     // CountedByInvalidPointeeTypeKind::FUNCTION
     // CountedByInvalidPointeeTypeKind::FLEXIBLE_ARRAY_MEMBER
     "a struct type with a flexible array member"
+    "%select{|. This will be an error in a future compiler version}3"
+    ""
   "}2">;
 
+def warn_counted_by_attr_elt_type_unknown_size :
+  Warning<err_counted_by_attr_pointee_unknown_size.Summary>,
+  InGroup<BoundsSafetyCountedByEltTyUnknownSize>;
+
 let CategoryName = "ARC Semantic Issue" in {
 
 // ARC-mode diagnostics.

clang/lib/Sema/SemaDeclAttr.cpp

@@ -8687,6 +8687,7 @@ static bool CheckCountedByAttrOnField(
   // Note: The `Decl::isFlexibleArrayMemberLike` check earlier on means
   // only `PointeeTy->isStructureTypeWithFlexibleArrayMember()` is reachable
   // when `FieldTy->isArrayType()`.
+  bool ShouldWarn = false;
   if (PointeeTy->isIncompleteType()) {
     InvalidTypeKind = CountedByInvalidPointeeTypeKind::INCOMPLETE;
   } else if (PointeeTy->isSizelessType()) {
@@ -8694,13 +8695,25 @@ static bool CheckCountedByAttrOnField(
   } else if (PointeeTy->isFunctionType()) {
     InvalidTypeKind = CountedByInvalidPointeeTypeKind::FUNCTION;
   } else if (PointeeTy->isStructureTypeWithFlexibleArrayMember()) {
+    if (FieldTy->isArrayType()) {
+      // This is a workaround for the Linux kernel that has already adopted
+      // `counted_by` on a FAM where the pointee is a struct with a FAM. This
+      // should be an error because computing the bounds of the array cannot be
+      // done correctly without manually traversing every struct object in the
+      // array at runtime. To allow the code to be built this error is
+      // downgraded to a warning.
+      ShouldWarn = true;
+    }
     InvalidTypeKind = CountedByInvalidPointeeTypeKind::FLEXIBLE_ARRAY_MEMBER;
   }
 
   if (InvalidTypeKind != CountedByInvalidPointeeTypeKind::VALID) {
-    S.Diag(FD->getBeginLoc(), diag::err_counted_by_attr_pointee_unknown_size)
+    unsigned DiagID = ShouldWarn
+                          ? diag::warn_counted_by_attr_elt_type_unknown_size
+                          : diag::err_counted_by_attr_pointee_unknown_size;
+    S.Diag(FD->getBeginLoc(), DiagID)
         << SelectPtrOrArr << PointeeTy << (int)InvalidTypeKind
-        << FD->getSourceRange();
+        << (ShouldWarn ? 1 : 0) << FD->getSourceRange();
     return true;
   }
 

clang/test/Sema/attr-counted-by-vla.c

@@ -173,21 +173,24 @@ struct has_annotated_VLA {
 
 struct buffer_of_structs_with_unnannotated_vla {
   int count;
-  // expected-error@+1{{'counted_by' cannot be applied to an array with element of unknown size because 'struct has_unannotated_VLA' is a struct type with a flexible array member}}
+  // Treating this as a warning is a temporary fix for existing attribute adopters. It **SHOULD BE AN ERROR**.
+  // expected-warning@+1{{'counted_by' should not be applied to an array with element of unknown size because 'struct has_unannotated_VLA' is a struct type with a flexible array member. This will be an error in a future compiler version}}
   struct has_unannotated_VLA Arr[] __counted_by(count);
 };
 
 
 struct buffer_of_structs_with_annotated_vla {
   int count;
-  // expected-error@+1{{'counted_by' cannot be applied to an array with element of unknown size because 'struct has_annotated_VLA' is a struct type with a flexible array member}}
+  // Treating this as a warning is a temporary fix for existing attribute adopters. It **SHOULD BE AN ERROR**.
+  // expected-warning@+1{{'counted_by' should not be applied to an array with element of unknown size because 'struct has_annotated_VLA' is a struct type with a flexible array member. This will be an error in a future compiler version}}
   struct has_annotated_VLA Arr[] __counted_by(count);
 };
 
 struct buffer_of_const_structs_with_annotated_vla {
   int count;
+  // Treating this as a warning is a temporary fix for existing attribute adopters. It **SHOULD BE AN ERROR**.
   // Make sure the `const` qualifier is printed when printing the element type.
-  // expected-error@+1{{'counted_by' cannot be applied to an array with element of unknown size because 'const struct has_annotated_VLA' is a struct type with a flexible array member}}
+  // expected-warning@+1{{'counted_by' should not be applied to an array with element of unknown size because 'const struct has_annotated_VLA' is a struct type with a flexible array member. This will be an error in a future compiler version}}
   const struct has_annotated_VLA Arr[] __counted_by(count);
 };