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Subject: Re: [PATCH v2 06/11] staging/android: turn fence_info into a __u64
 pointer
To: Gustavo Padovan <gustavo@padovan.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
References: <1454419402-10769-1-git-send-email-gustavo@padovan.org>
 <1454419402-10769-7-git-send-email-gustavo@padovan.org>
Cc: linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org,
        dri-devel@lists.freedesktop.org, Daniel Stone <daniels@collabora.com>,
        =?UTF-8?Q?Arve_Hj=c3=b8nnev=c3=a5g?= <arve@android.com>,
        Riley Andrews <riandrews@android.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Rob Clark <robdclark@gmail.com>, Greg Hackmann <ghackmann@google.com>,
        John Harrison <John.C.Harrison@Intel.com>,
        Gustavo Padovan <gustavo.padovan@collabora.co.uk>
From: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Message-ID: <56B0BA29.1040003@linux.intel.com>
Date: Tue, 2 Feb 2016 15:16:09 +0100
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Op 02-02-16 om 14:23 schreef Gustavo Padovan:
> From: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
>
> Making fence_info a pointer enables us to extend the struct in the future
> without breaking the ABI.
>
> v2: use type __u64 for fence_info
> Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
> ---
>  drivers/staging/android/sync.c      | 2 +-
>  drivers/staging/android/uapi/sync.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
> index f7530f0..03b1214 100644
> --- a/drivers/staging/android/sync.c
> +++ b/drivers/staging/android/sync.c
> @@ -525,7 +525,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
>  	if (info->status >= 0)
>  		info->status = !info->status;
>  
> -	len = sizeof(struct sync_file_info);
> +	len = sizeof(struct sync_file_info) - sizeof(__u64);
>  
>  	for (i = 0; i < sync_file->num_fences; ++i) {
>  		struct fence *fence = sync_file->cbs[i].fence;
> diff --git a/drivers/staging/android/uapi/sync.h b/drivers/staging/android/uapi/sync.h
> index ed281fc..8e2ed32 100644
> --- a/drivers/staging/android/uapi/sync.h
> +++ b/drivers/staging/android/uapi/sync.h
> @@ -54,7 +54,7 @@ struct sync_file_info {
>  	char	name[32];
>  	__s32	status;
>  
> -	__u8	fence_info[0];
> +	__u64	fence_info;
>
With the information from the commit I would expect fence_info would be a true pointer.

so if (sync_file_info->num_fences == 0) not copying fence_info, but set all members including this one.
else for_all_fences() copy_to_user(fence_info struct)

Otherwise you still can't extend the struct without breaking the abi.

~Maarten