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SubjectRe: [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@embeddedor.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

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