linux-kernel - [PATCH 1/2] stddef: Add a comment about why TRAILING

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Message-ID:  <7ac4049bf1b8fc31490e2f54a93abeb198112d3b.1757672423.git.u.kleine-koenig@baylibre.com>
Date: Fri, 12 Sep 2025 12:28:01 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Kees Cook <kees@...nel.org>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] stddef: Add a comment about why TRAILING_OVERLAP() exists

Just from reading the definition it's not obvious what the macro does
and why it's needed and sensible to be used.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...libre.com>
---
 include/linux/stddef.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index dab49e2ec8c0..a748efcd626f 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -97,7 +97,17 @@ enum {
  * TRAILING_OVERLAP() - Overlap a flexible-array member with trailing members.
  *
  * Creates a union between a flexible-array member (FAM) in a struct and a set
- * of additional members that would otherwise follow it.
+ * of additional members that would otherwise follow it. This is needed because
+ * the traditional
+ *
+ * 	struct {
+ * 		struct some_struct_with_FAM mystruct;
+ * 		struct some_other_struct fill_FAM;
+ * 	}
+ *
+ * with the purpose that fill_FAM overlaps the flexible-array in mystruct
+ * triggers a compiler warning about the flexible array member not being at the
+ * end of the structure.
  *
  * @TYPE: Flexible structure type name, including "struct" keyword.
  * @NAME: Name for a variable to define.
-- 
2.50.1