linux-kernel - Re: [PATCH][next] chrome: wilco_ec: event: Replace zero-length array with flexible-array member

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Date:   Mon, 23 Mar 2020 15:01:44 +0100
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Benson Leung <bleung@...omium.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] chrome: wilco_ec: event: Replace zero-length array
 with flexible-array member

Hi Gustavo

On 21/3/20 0:16, 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>


Queued for 5.7. Thanks!

~ Enric

> ---
>  drivers/platform/chrome/wilco_ec/event.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/platform/chrome/wilco_ec/event.c b/drivers/platform/chrome/wilco_ec/event.c
> index dba3d445623f..814518509739 100644
> --- a/drivers/platform/chrome/wilco_ec/event.c
> +++ b/drivers/platform/chrome/wilco_ec/event.c
> @@ -79,7 +79,7 @@ static DEFINE_IDA(event_ida);
>  struct ec_event {
>  	u16 size;
>  	u16 type;
> -	u16 event[0];
> +	u16 event[];
>  } __packed;
>  
>  #define ec_event_num_words(ev) (ev->size - 1)
> @@ -96,7 +96,7 @@ struct ec_event_queue {
>  	int capacity;
>  	int head;
>  	int tail;
> -	struct ec_event *entries[0];
> +	struct ec_event *entries[];
>  };
>  
>  /* Maximum number of events to store in ec_event_queue */
>