Re: [PATCH v3 06/11] staging/android: turn fence_info into a __u64 pointer

[Daniel Vetter]

On Wed, Feb 03, 2016 at 03:39:22PM +0100, Maarten Lankhorst wrote:
> Op 03-02-16 om 14:25 schreef Gustavo Padovan:
> > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> >
> > Turn sync_fence_info into __u64 type enable us to extend the struct in the
> > future without breaking the ABI.
> >
> > v2: use type __u64 for fence_info
> >
> > v3: fix commit message to reflect the v2 change
> >
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> > ---
> >  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 2ab0c20..8425457 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 a0cf357..e649953 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	sync_fence_info[0];
> > +	__u64	sync_fence_info;
> >  };
> >  
> >  #define SYNC_IOC_MAGIC		'>'
> This still doesn't do what you expect it to.
> 
> I think this is what you want is for userspace to do:
> 
> struct sync_file_info info;
> 
> info.flags = info.num_fences = 0;
> ioctl(fd, SYNC_IOC_FENCE_INFO, &info);
> if (info.num_fences) {
> info.sync_fence_info = (uintptr)kcalloc(info.num_fences, sizeof(struct sync_fence_info));
> ioctl(fd, SYNC_IOC_FENCE_INFO, &info);

s/kcalloc/calloc/ since this is userspace, i.e. yes I agree. In general if
you have a variable sized array for your ioctl what you need to do in the
kernel:

- compute how many you need.
- if userspace array was too small, fail with -EINVAL
- but _always_ update the array size to what would be needed.

Then the loop above which Maarten laid out will work nicely. Of course
since it's a separate array then you also need separate copy_to_user calls
(I didn't check whether that would work as-is).
-Daniel

> }
> 
> Maybe userspace could preallocate the max in advance and set num_fences higher,
> 
> kernel would do something like:
> 
> num_fences = min(info.num_fences, sync->num_fences);
> struct sync_fence_info array[num_fences];
> 
> info.num_fences = sync->num_fences;
> if (num_fences &&
>     copy_to_user((void * __user)(unsigned long)info.sync_fence_info, array, num_fences  * sizeof(array)))
>  return -EFAULT;
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch