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overflow: Introduce __DEFINE_FLEX for having no initializer

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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: 47e36ed784 ("overflow: Fix direct struct member initialization in _DEFINE_FLEX()")
Signed-off-by: Kees Cook <kees@kernel.org>
This commit is contained in:
Kees Cook
2025-05-30 12:06:47 -07:00
parent d6a0e0bfec
commit 5c78e793f7
1 changed files with 21 additions and 8 deletions
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29
include/linux/overflow.h
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@@ -388,6 +388,25 @@ 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() - 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.
* @trailer: Trailing expressions for attributes and/or initializers.
*/
#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 trailer; \
type *name = (type *)&name##_u
/**
* _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
* Enables caller macro to pass (different) initializer.
@@ -399,13 +418,7 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
* @initializer: Initializer expression (e.g., pass `= { }` at minimum).
*/
#define _DEFINE_FLEX(type, name, member, count, initializer...) \
_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 }; \
type *name = (type *)&name##_u
__DEFINE_FLEX(type, name, member, count, = { .obj initializer })
/**
* DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
@@ -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