linux-kernel - Re: [PATCH 06/10] staging/android: turn fence_info into a _

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Message-ID: <56AF1A43.5010900@linux.intel.com>
Date:	Mon, 1 Feb 2016 09:41:39 +0100
From:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:	Gustavo Padovan <gustavo@...ovan.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	dri-devel@...ts.freedesktop.org,
	Daniel Stone <daniels@...labora.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Riley Andrews <riandrews@...roid.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	John Harrison <John.C.Harrison@...el.com>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>
Subject: Re: [PATCH 06/10] staging/android: turn fence_info into a __64
 pointer

Op 29-01-16 om 22:20 schreef Gustavo Padovan:
> From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
>
> Making fence_info a pointer enables us to extend the struct in the future
> without breaking the ABI.
>
> Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.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..51d4f47 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..9f07aa7 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;
>  };
>
Pointers are awful, it should be a __u64 since it's a pointer type. Userspace should cast it to a uintptr_t in userspace.
This structure also won't work on 64-bits systems, there may be a hole between fence_info and status (or num_fences in next patch).

It's probably best to move it to the top and ensure the struct is 64-bits aligned.