On Wed, Apr 30, 2025 at 05:46:21PM -0600, Gustavo A. R. Silva wrote:
> Currently, to statically initialize the struct members of the `type`
> object created by _DEFINE_FLEX(), the internal `obj` member must be
> explicitly referenced at the call site. See:
>
> struct flex {
> int a;
> int b;
> struct foo flex_array[];
> };
>
> _DEFINE_FLEX(struct flex, instance, flex_array,
> FIXED_SIZE, = {
> .obj = {
> .a = 0,
> .b = 1,
> },
> });
>
> This leaks _DEFINE_FLEX() internal implementation details and make
> the helper harder to use and read.
>
> Fix this and allow for a more natural and intuitive C99 init-style:
>
> _DEFINE_FLEX(struct flex, instance, flex_array,
> FIXED_SIZE, = {
> .a = 0,
> .b = 1,
> });
>
> Also, update "counter" member initialization in DEFINE_FLEX().
>
> Fixes: 26dd68d293fd ("overflow: add DEFINE_FLEX() for on-stack allocs")
> Link:
> https://lore.kernel.org/linux-hardening/[email protected]/
> Signed-off-by: Gustavo A. R. Silva <[email protected]>
> ---
> include/linux/overflow.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 69533e703be5..170d3cfe7ecc 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -404,7 +404,7 @@ static inline size_t __must_check size_sub(size_t
> minuend, size_t subtrahend)
Earlier up in the file:
...
* @initializer: initializer expression (could be empty for no init).
^^^^^^^^^
*/
#define _DEFINE_FLEX(type, name, member, count, initializer...)
This argument now becomes required, which is fine, but we should keep
the docs updated and double check any existing "_DEFINE_FLEX" users that
may have an empty final argument (I don't see any, so that's nice).
> union {
> \
> u8 bytes[struct_size_t(type, member, count)];
> \
> type obj;
> \
> - } name##_u initializer;
> \
> + } name##_u = { .obj initializer };
> \
> type *name = (type *)&name##_u
>
> /**
> @@ -444,7 +444,7 @@ static inline size_t __must_check size_sub(size_t
> minuend, size_t subtrahend)
> * elements in array @member.
> */
> #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
> - _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
> + _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
>
> /**
> * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.
But otherwise, yes, let's do this!
--
Kees Cook
