linux-kernel - Re: [PATCH] sched/fair: Replace zero-length array with flexible-array member

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Date:   Thu, 13 Feb 2020 17:45:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Replace zero-length array with
 flexible-array member

On Thu, Feb 13, 2020 at 09:19:51AM -0600, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f38ff5a335d3..12a424878b23 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1081,7 +1081,7 @@ struct numa_group {
>  	 * more by CPU use than by memory faults.
>  	 */
>  	unsigned long *faults_cpu;
> -	unsigned long faults[0];
> +	unsigned long faults[];
>  };

Hurmph, and where are all the other similar changes for kernel/sched/ ?
Because this really isn't the only such usage and I really don't see the
point in having a separate patch for every single one of them.

Also; couldn't you've taught the compiler to also warn about [0] ?
There's really no other purpose to having a zero length array.