[PATCH v2 1/2][next] overflow: Add STACK_FLEX_ARRAY_SIZE() helper

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

Add new STACK_FLEX_ARRAY_SIZE() helper to get the size of a
flexible-array member defined using DEFINE_FLEX()/DEFINE_RAW_FLEX()
at compile time.

This is essentially the same as ARRAY_SIZE() but for on-stack
flexible-array members.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Use "number of elements" instead of "size of" in kernel-doc blocks.

v1:
 - Link: https://lore.kernel.org/linux-hardening/8f9ab8fcd26ce59c0e0e25e095b446a77849c08e.1745342381.git.gustavoars@kernel.org/

 include/linux/overflow.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 6ee67c20b575..f33d74dac06f 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -420,6 +420,8 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * flexible array member.
  * Use __struct_size(@name) to get compile-time size of it afterwards.
  * Use __member_size(@name->member) to get compile-time size of @name members.
+ * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of
+ * elements in array @member.
  */
 #define DEFINE_RAW_FLEX(type, name, member, count)	\
 	_DEFINE_FLEX(type, name, member, count, = {})
@@ -438,8 +440,21 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * flexible array member.
  * Use __struct_size(@NAME) to get compile-time size of it afterwards.
  * Use __member_size(@NAME->member) to get compile-time size of @NAME members.
+ * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of
+ * elements in array @member.
  */
 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)	\
 	_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
 
+/**
+ * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.
+ * Returns the number of elements in @array.
+ *
+ * @name: Name for a variable defined in DEFINE_RAW_FLEX()/DEFINE_FLEX().
+ * @array: Name of the array member.
+ */
+#define STACK_FLEX_ARRAY_SIZE(name, array)						\
+	(__member_size((name)->array) / sizeof(*(name)->array) +			\
+						__must_be_array((name)->array))
+
 #endif /* __LINUX_OVERFLOW_H */
-- 
2.43.0