linux-hardening - [PATCH 1/2][next] overflow: Add STACK_FLEX_ARRAY

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Message-ID: <8f9ab8fcd26ce59c0e0e25e095b446a77849c08e.1745342381.git.gustavoars@kernel.org>
Date: Tue, 22 Apr 2025 11:21:36 -0600
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2][next] overflow: Add STACK_FLEX_ARRAY_SIZE() helper

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@...nel.org>
---
 include/linux/overflow.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 6ee67c20b575..5b2e0cc9aba2 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -420,6 +420,7 @@ 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 size of array @member.
  */
 #define DEFINE_RAW_FLEX(type, name, member, count)	\
 	_DEFINE_FLEX(type, name, member, count, = {})
@@ -438,8 +439,20 @@ 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 size of 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