[PATCH] docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY() usage

[Kees Cook]

There wasn't any mention of when/where DECLARE_FLEX_ARRAY() should be
used, so add the rationale and an example to the deprecation docs.

Suggested-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 Documentation/process/deprecated.rst | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index c8fd53a11a20..fcac0a929efa 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -346,3 +346,29 @@ struct_size() and flex_array_size() helpers::
         instance->count = count;
 
         memcpy(instance->items, source, flex_array_size(instance, items, instance->count));
+
+There are two special cases of replacement where the DECLARE_FLEX_ARRAY()
+helper needs to be used. (Note that it is named __DECLARE_FLEX_ARRAY() for
+use in UAPI headers.) When the flexible array is either alone in a struct,
+or is part of a union. These are disallowed by the C99 specification,
+but for no technical reason (as can be seen by both the existing use of
+such arrays in those places and the work-around that DECLARE_FLEX_ARRAY()
+uses). For example, to convert this::
+
+	struct something {
+		...
+		union {
+			struct type1 one[0];
+			struct type2 two[0];
+		};
+	};
+
+The helper must be used::
+
+	struct something {
+		...
+		union {
+			DECLARE_FLEX_ARRAY(struct type1, one);
+			DECLARE_FLEX_ARRAY(struct type2, two);
+		};
+	};
-- 
2.34.1