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

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Message-ID: <87y2sfni66.fsf@intel.com>
Date:   Thu, 05 Mar 2020 13:40:49 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH][next] drm/drm_displayid.h: Replace zero-length array with flexible-array member

On Thu, 05 Mar 2020, "Gustavo A. R. Silva" <gustavo@...eddedor.com> 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>

Reviewed-by: Jani Nikula <jani.nikula@...el.com>

> ---
>  include/drm/drm_displayid.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/drm/drm_displayid.h b/include/drm/drm_displayid.h
> index 9d3b745c3107..94b4390bf990 100644
> --- a/include/drm/drm_displayid.h
> +++ b/include/drm/drm_displayid.h
> @@ -89,7 +89,7 @@ struct displayid_detailed_timings_1 {
>  
>  struct displayid_detailed_timing_block {
>  	struct displayid_block base;
> -	struct displayid_detailed_timings_1 timings[0];
> +	struct displayid_detailed_timings_1 timings[];
>  };
>  
>  #define for_each_displayid_db(displayid, block, idx, length) \

-- 
Jani Nikula, Intel Open Source Graphics Center