netdev - [PATCH net-next v1 1/7] overflow: add DEFINE

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Date: Thu, 10 Aug 2023 06:35:03 -0400
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Kees Cook <keescook@...omium.org>,
	netdev@...r.kernel.org
Cc: Jacob Keller <jacob.e.keller@...el.com>,
	intel-wired-lan@...ts.osuosl.org,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	linux-hardening@...r.kernel.org,
	Steven Zou <steven.zou@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>
Subject: [PATCH net-next v1 1/7] overflow: add DEFINE_FLEX() for on-stack allocs

Add DEFINE_FLEX() macro for on-stack allocations of structs with
flexible array member.

Add also const_flex_size() macro, that reads size of structs
allocated by DEFINE_FLEX().

Using underlying array for on-stack storage lets us to declare
known-at-compile-time structures without kzalloc().

Actual usage for ice driver is in following patches of the series.

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
---
v1: change macro name; add macro for size read;
    accept struct type instead of ptr to it; change alignment;
---
 include/linux/overflow.h | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index f9b60313eaea..21a4410799eb 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -309,4 +309,31 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 #define struct_size_t(type, member, count)					\
 	struct_size((type *)NULL, member, count)
 
+/**
+ * DEFINE_FLEX() - Define a zeroed, on-stack, instance of @type structure with
+ * a trailing flexible array member.
+ *
+ * @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.
+ */
+#define DEFINE_FLEX(type, name, member, count)					\
+	union {									\
+		u8 bytes[struct_size_t(type, member, count)];			\
+		type obj;							\
+	} name##_u __aligned(_Alignof(type)) = {};				\
+	type *name = (type *)&name##_u
+
+/**
+ * const_flex_size() - Get size of on-stack instance of structure with
+ * a trailing flexible array member.
+ *
+ * @name: Name of the variable, the one defined by DEFINE_FLEX() macro above.
+ *
+ * Get size of @name, which is equivalent to struct_size(name, array, count),
+ * but does not require (repeating) last two arguments.
+ */
+#define const_flex_size(name)	__builtin_object_size(name, 1)
+
 #endif /* __LINUX_OVERFLOW_H */
-- 
2.40.1