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

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Date:   Thu, 5 Mar 2020 15:22:38 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] drm/vboxvideo/vboxvideo.h: Replace zero-length
 array with flexible-array member

Hi,

On 3/5/20 11:55 AM, 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>

Patch looks good to me:

Reviewed-by: Hans de Goede <hdegoede@...hat.com>

Regards,

Hans


> ---
>   drivers/gpu/drm/vboxvideo/vboxvideo.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/vboxvideo/vboxvideo.h b/drivers/gpu/drm/vboxvideo/vboxvideo.h
> index 0592004f71aa..a5de40fe1a76 100644
> --- a/drivers/gpu/drm/vboxvideo/vboxvideo.h
> +++ b/drivers/gpu/drm/vboxvideo/vboxvideo.h
> @@ -138,7 +138,7 @@ struct vbva_buffer {
>   
>   	u32 data_len;
>   	/* variable size for the rest of the vbva_buffer area in VRAM. */
> -	u8 data[0];
> +	u8 data[];
>   } __packed;
>   
>   #define VBVA_MAX_RECORD_SIZE (128 * 1024 * 1024)
>