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

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Message-ID: <949f472c-baca-4c2f-af23-7ba76fff1ddc@embeddedor.com>
Date: Fri, 24 Oct 2025 12:24:09 +0100
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Jakub Kicinski <kuba@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] net: inet_sock.h: Avoid thousands of
 -Wflex-array-member-not-at-end warnings

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