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

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Message-ID: <20200306105220.GX2363188@phenom.ffwll.local>
Date:   Fri, 6 Mar 2020 11:52:20 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     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/bridge/mhl.h: Replace zero-length array with
 flexible-array member

On Thu, Mar 05, 2020 at 05:00:11AM -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


> ---
>  include/drm/bridge/mhl.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/drm/bridge/mhl.h b/include/drm/bridge/mhl.h
> index 1cc77bf38324..d96626a0e3fa 100644
> --- a/include/drm/bridge/mhl.h
> +++ b/include/drm/bridge/mhl.h
> @@ -327,13 +327,13 @@ struct mhl_burst_bits_per_pixel_fmt {
>  	struct {
>  		u8 stream_id;
>  		u8 pixel_format;
> -	} __packed desc[0];
> +	} __packed desc[];
>  } __packed;
>  
>  struct mhl_burst_emsc_support {
>  	struct mhl3_burst_header hdr;
>  	u8 num_entries;
> -	__be16 burst_id[0];
> +	__be16 burst_id[];
>  } __packed;
>  
>  struct mhl_burst_audio_descr {
> -- 
> 2.25.0
> 

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