linux-kernel - Re: [PATCH][next] drm: etnaviv_gem.h: Replace zero-length array with flexible-array member

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Date:   Thu, 05 Mar 2020 12:17:48 +0100
From:   Lucas Stach <l.stach@...gutronix.de>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] drm: etnaviv_gem.h: Replace zero-length array
 with flexible-array member

Hi Gustavo,

I've adjusted the subject a bit to match the style of the other etnaviv
driver commits and applied this to the etnaviv/next branch.

Regards,
Lucas

On Do, 2020-03-05 at 04:51 -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>
> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.h b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> index 6b68fe16041b..98e60df882b6 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
> @@ -105,7 +105,7 @@ struct etnaviv_gem_submit {
>  	unsigned int nr_pmrs;
>  	struct etnaviv_perfmon_request *pmrs;
>  	unsigned int nr_bos;
> -	struct etnaviv_gem_submit_bo bos[0];
> +	struct etnaviv_gem_submit_bo bos[];
>  	/* No new members here, the previous one is variable-length! */
>  };
>