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

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

[Gustavo A. R. Silva]

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>
---
 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

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

[Daniel Vetter]

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