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

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Date:	Tue, 2 Feb 2016 11:04:59 -0200
From:	Gustavo Padovan <gustavo@...ovan.org>
To:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	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 06/10] staging/android: turn fence_info into a __64
 pointer

2016-02-01 Gustavo Padovan <gustavo@...ovan.org>:

> Hi Maarten,
> 
> 2016-02-01 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>:
> 
> > 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.
> 
> Oh, I made a mistake. I'll fix this.
> 
> 
> > 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.
> 
> That is not possible because we are not allocating only 64bits there but
> a array of struct fence_info, so it needs to be the last one. Maybe we
> can add some sort of padding?

Actually it is 64bits aligned:

struct sync_file_info {                                                         
        char    name[32];                                                       
        __s32   status;                                                         
        __u32   flags;                                                          
        __u32   num_fences;                                                     
        __u32   len;                                                            
                                                                                
        __u64   fence_info;                                                     
};

So nothing to worry here.

	Gustavo