linux-kernel - Re: [PATCH] powercap: idle_inject: Replace zero-length array with flexible-array member

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Date:   Wed, 04 Mar 2020 12:06:06 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powercap: idle_inject: Replace zero-length array with flexible-array member

On Thursday, February 27, 2020 8:07:21 PM CET 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]
> 
> Lastly, fix the following checkpatch warning:
> WARNING: Prefer 'unsigned long' over 'unsigned long int' as the int is unnecessary
> +	unsigned long int cpumask[];
> 
> 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>
> ---
>  drivers/powercap/idle_inject.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/powercap/idle_inject.c b/drivers/powercap/idle_inject.c
> index cd1270614cc6..e9bbd3c42eef 100644
> --- a/drivers/powercap/idle_inject.c
> +++ b/drivers/powercap/idle_inject.c
> @@ -67,7 +67,7 @@ struct idle_inject_device {
>  	struct hrtimer timer;
>  	unsigned int idle_duration_us;
>  	unsigned int run_duration_us;
> -	unsigned long int cpumask[0];
> +	unsigned long cpumask[];
>  };
>  
>  static DEFINE_PER_CPU(struct idle_inject_thread, idle_inject_thread);
> 

Applied as 5.7 material, thanks!