Re: [PATCH][next] net: inet_sock.h: Avoid thousands of -Wflex-array-member-not-at-end warnings

[Gustavo A. R. Silva]

On 10/24/25 01:25, Jakub Kicinski wrote:
> On Tue, 21 Oct 2025 12:43:30 +0100 Gustavo A. R. Silva wrote:
> >   struct ip_options_data {
> > -	struct ip_options_rcu	opt;
> > -	char			data[40];
> > +	TRAILING_OVERLAP(struct ip_options_rcu, opt, opt.__data,
> > +			 char			data[40];
> > +	);
> >   };
>
> Is there a way to reserve space for flexible length array on the stack
> without resorting to any magic macros? This struct has total of 5 users.

Not that I know of. That's the reason why we had to implement macros like
TRAILING_OVERLAP(), DEFINE_FLEX(), DEFINE_RAW_FLEX().

Regarding these three macros, the simplest and least intrusive one to use is
actually TRAILING_OVERLAP(), when the flex-array member is not annotated with
the counted_by attribute (otherwise, DEFINE_FLEX() would be preferred).

Of course, the most straightforward alternative is to use fixed-size arrays
if flex arrays are not actually needed.

Thanks
-Gustavo