linux-kernel - Re: [PATCH][next] drm/vc4/vc4_drv.h: Replace zero-length array with flexible-array member

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Date:   Fri, 6 Mar 2020 11:50:25 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Eric Anholt <eric@...olt.net>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] drm/vc4/vc4_drv.h: Replace zero-length array with
 flexible-array member

On Thu, Mar 05, 2020 at 04:57:07AM -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>
Applied to drm-misc-next, thanks for your patch.
-Daniel

> ---
>  drivers/gpu/drm/vc4/vc4_drv.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h
> index f90c0d08e740..5ecb8b4a48a1 100644
> --- a/drivers/gpu/drm/vc4/vc4_drv.h
> +++ b/drivers/gpu/drm/vc4/vc4_drv.h
> @@ -65,7 +65,7 @@ struct vc4_perfmon {
>  	 * Note that counter values can't be reset, but you can fake a reset by
>  	 * destroying the perfmon and creating a new one.
>  	 */
> -	u64 counters[0];
> +	u64 counters[];
>  };
>  
>  struct vc4_dev {
> -- 
> 2.25.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch