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Message-Id: <20250530191312.work.661-kees@kernel.org>
Date: Fri, 30 May 2025 12:13:16 -0700
From: Kees Cook <kees@...nel.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
Alexander Lobakin <aleksander.lobakin@...el.com>,
linux-hardening@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH] overflow: Introduce __DEFINE_FLEX for having no initializer
While not yet in the tree, there is a proposed patch[1] that was
depending on the prior behavior of _DEFINE_FLEX, which did not have an
explicit initializer. Provide this via __DEFINE_FLEX now, which can also
have attributes applied (e.g. __uninitialized).
Examples of the resulting initializer behaviors can be seen here:
https://godbolt.org/z/P7Go8Tr33
Link: https://lore.kernel.org/netdev/20250520205920.2134829-9-anthony.l.nguyen@intel.com [1]
Fixes: 47e36ed78406 ("overflow: Fix direct struct member initialization in _DEFINE_FLEX()")
Signed-off-by: Kees Cook <kees@...nel.org>
---
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: <linux-hardening@...r.kernel.org>
---
include/linux/overflow.h | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 7b7be27ca113..154ed0dbb43f 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -389,24 +389,37 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
struct_size((type *)NULL, member, count)
/**
- * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
- * Enables caller macro to pass (different) initializer.
+ * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
+ * Enables caller macro to pass arbitrary trailing expressions
*
* @type: structure type name, including "struct" keyword.
* @name: Name for a variable to define.
* @member: Name of the array member.
* @count: Number of elements in the array; must be compile-time const.
- * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
+ * @trailer: Trailing expressions for attributes and/or initializers.
*/
-#define _DEFINE_FLEX(type, name, member, count, initializer...) \
+#define __DEFINE_FLEX(type, name, member, count, trailer...) \
_Static_assert(__builtin_constant_p(count), \
"onstack flex array members require compile-time const count"); \
union { \
u8 bytes[struct_size_t(type, member, count)]; \
type obj; \
- } name##_u = { .obj initializer }; \
+ } name##_u trailer; \
type *name = (type *)&name##_u
+/**
+ * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
+ * Enables caller macro to pass (different) initializer.
+ *
+ * @type: structure type name, including "struct" keyword.
+ * @name: Name for a variable to define.
+ * @member: Name of the array member.
+ * @count: Number of elements in the array; must be compile-time const.
+ * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
+ */
+#define _DEFINE_FLEX(type, name, member, count, initializer...) \
+ __DEFINE_FLEX(type, name, member, count, = { .obj initializer })
+
/**
* DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
* flexible array member, when it does not have a __counted_by annotation.
@@ -424,7 +437,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
* elements in array @member.
*/
#define DEFINE_RAW_FLEX(type, name, member, count) \
- _DEFINE_FLEX(type, name, member, count, = {})
+ __DEFINE_FLEX(type, name, member, count, = { })
/**
* DEFINE_FLEX() - Define an on-stack instance of structure with a trailing
--
2.34.1
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