Re: [PATCH][next] KEYS: Avoid -Wflex-array-member-not-at-end warning

[Ignat Korchagin]

On Mon, Nov 10, 2025 at 10:46 AM Gustavo A. R. Silva
<gustavoars@xxxxxxxxxx> wrote:
>
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
>
> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>
> crypto/asymmetric_keys/restrict.c:20:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>
> This helper creates a union between a flexible-array member (FAM) and a
> set of MEMBERS that would otherwise follow it.
>
> This overlays the trailing MEMBER unsigned char data[10]; onto the FAM
> struct asymmetric_key_id::data[], while keeping the FAM and the start
> of MEMBER aligned.
>
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after the corresponding structures --no
> blank line in between.
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Reviewed-by: Ignat Korchagin <ignat@xxxxxxxxxxxxxx>

> ---
>  crypto/asymmetric_keys/restrict.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
> index afcd4d101ac5..86292965f493 100644
> --- a/crypto/asymmetric_keys/restrict.c
> +++ b/crypto/asymmetric_keys/restrict.c
> @@ -17,9 +17,12 @@ static struct asymmetric_key_id *ca_keyid;
>
>  #ifndef MODULE
>  static struct {
> -       struct asymmetric_key_id id;
> -       unsigned char data[10];
> +       /* Must be last as it ends in a flexible-array member. */
> +       TRAILING_OVERLAP(struct asymmetric_key_id, id, data,
> +               unsigned char data[10];
> +       );
>  } cakey;
> +static_assert(offsetof(typeof(cakey), id.data) == offsetof(typeof(cakey), data));

The whole thing looks a bit convoluted to me just to declare a
fixed-sized static buffer and call sizeof() in one place in the code.
But I couldn't come up with a better refactor not introducing
potential alignment side-effects. So LGTM.

>  static int __init ca_keys_setup(char *str)
>  {
> --
> 2.43.0
>