linux-kernel - Re: [PATCH v2 06/11] staging/android: turn fence_info into a _

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Date:	Wed, 3 Feb 2016 09:32:20 +0100
From:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:	Gustavo Padovan <gustavo@...ovan.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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 v2 06/11] staging/android: turn fence_info into a __u64
 pointer

Op 02-02-16 om 21:28 schreef Gustavo Padovan:
> Hi Maarten,
>
> 2016-02-02 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>:
>
>> Op 02-02-16 om 14:23 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.
>>>
>>> v2: use type __u64 for fence_info
>>> 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..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.
> I missed that. It is fixed for v3 now.
>
>> 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.
> all sync_files have at least 1 fence so we are left to the else case,
> coupy all fences to the user. So we always have variable length at the
> end of the struct. But I still don't see how this will prevent us from
> breaking the API.
Ok this was with fence_info not being a pointer. :)