From 8763a30bc15b36c3bd90d6da7272d972b995f318 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Fri, 6 Jan 2023 12:06:04 -0800
Subject: [PATCH] docs: deprecated.rst: Add note about DECLARE_FLEX_ARRAY()
usage
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@wanadoo.fr>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230106200600.never.735-kees@kernel.org
[jc: minor wording tweaks]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
---
Documentation/process/deprecated.rst | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index c8fd53a11a207..f91b8441f2ef7 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.) Those cases are 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);
+ };
+ };
--
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