Re: [PATCH v2] xen/public: fix flexible array definitions

[Julien Grall <julien@xxxxxxx>]

Hi Juergen,

On 30/11/2023 09:21, Juergen Gross wrote:
> ---
> V2:
> - bump interface version and make change only for new version
>    (Jan Beulich)
> ---
>   xen/include/public/io/cameraif.h | 2 +-
>   xen/include/public/io/displif.h  | 2 +-
>   xen/include/public/io/fsif.h     | 4 ++--
>   xen/include/public/io/pvcalls.h  | 2 +-
>   xen/include/public/io/ring.h     | 5 +++--
>   xen/include/public/io/sndif.h    | 2 +-
>   xen/include/public/xen-compat.h  | 2 +-
>   xen/include/public/xen.h         | 6 ++++++
>   8 files changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/xen/include/public/io/cameraif.h b/xen/include/public/io/cameraif.h
> index 13763abef9..a389443769 100644
> --- a/xen/include/public/io/cameraif.h
> +++ b/xen/include/public/io/cameraif.h
> @@ -763,7 +763,7 @@ struct xencamera_buf_create_req {
>    */
>   struct xencamera_page_directory {
>       grant_ref_t gref_dir_next_page;
> -    grant_ref_t gref[1]; /* Variable length */
> +    grant_ref_t gref[XENPV_FLEX_ARRAY_DIM];
>   };
>   
>   /*
> diff --git a/xen/include/public/io/displif.h b/xen/include/public/io/displif.h
> index 73d0cbdf15..132c96fa5c 100644
> --- a/xen/include/public/io/displif.h
> +++ b/xen/include/public/io/displif.h
> @@ -537,7 +537,7 @@ struct xendispl_dbuf_create_req {
>   
>   struct xendispl_page_directory {
>       grant_ref_t gref_dir_next_page;
> -    grant_ref_t gref[1]; /* Variable length */
> +    grant_ref_t gref[XENPV_FLEX_ARRAY_DIM];
>   };
>   
>   /*
> diff --git a/xen/include/public/io/fsif.h b/xen/include/public/io/fsif.h
> index ec57850233..dcade1c698 100644
> --- a/xen/include/public/io/fsif.h
> +++ b/xen/include/public/io/fsif.h
> @@ -40,7 +40,7 @@ struct fsif_read_request {
>       int32_t pad;
>       uint64_t len;
>       uint64_t offset;
> -    grant_ref_t grefs[1];  /* Variable length */
> +    grant_ref_t grefs[XENPV_FLEX_ARRAY_DIM];
>   };
>   
>   struct fsif_write_request {
> @@ -48,7 +48,7 @@ struct fsif_write_request {
>       int32_t pad;
>       uint64_t len;
>       uint64_t offset;
> -    grant_ref_t grefs[1];  /* Variable length */
> +    grant_ref_t grefs[XENPV_FLEX_ARRAY_DIM];
>   };
>   
>   struct fsif_stat_request {
> diff --git a/xen/include/public/io/pvcalls.h b/xen/include/public/io/pvcalls.h
> index 230b0719e3..af0e9abd13 100644
> --- a/xen/include/public/io/pvcalls.h
> +++ b/xen/include/public/io/pvcalls.h
> @@ -30,7 +30,7 @@ struct pvcalls_data_intf {
>       uint8_t pad2[52];
>   
>       RING_IDX ring_order;
> -    grant_ref_t ref[];
> +    grant_ref_t ref[XENPV_FLEX_ARRAY_DIM];
I am probably missing something. In the commit message, you suggested 
that XENPV_FLEX_ARRAY_DIM will use 1 for older interface version for 
compatibility reason.
Yet, if I am not mistaken, [] is not equivalent to [1]. So aren't you 
effectively breaking the compatibility?
Cheers,

--
Julien Grall