linux-kernel - Re: [PATCH] stm class: Replace zero-length array with flexible-array member

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Date:   Thu, 27 Feb 2020 07:15:25 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>
Cc:     linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] stm class: Replace zero-length array with flexible-array
 member

Hi all,

Friendly ping: Who can take this?

Thanks
--
Gustavo

On 2/12/20 17:54, 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>
> ---
>  drivers/hwtracing/stm/policy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/stm/policy.c b/drivers/hwtracing/stm/policy.c
> index 4f932a419752..603b4a9969d3 100644
> --- a/drivers/hwtracing/stm/policy.c
> +++ b/drivers/hwtracing/stm/policy.c
> @@ -34,7 +34,7 @@ struct stp_policy_node {
>  	unsigned int		first_channel;
>  	unsigned int		last_channel;
>  	/* this is the one that's exposed to the attributes */
> -	unsigned char		priv[0];
> +	unsigned char		priv[];
>  };
>  
>  void *stp_policy_node_priv(struct stp_policy_node *pn)
>