[RFC] dma-buf: Import/export the implicit fences on the dma-buf

[Chris Wilson]

On Tue, Jul 12, 2016 at 01:53:56PM +0200, Christian König wrote:
> Am 11.07.2016 um 23:59 schrieb Chris Wilson:
> >When dealing with user interfaces that utilize explicit fences, it is
> >convenient to sometimes create those fences after the fact, i.e. to
> >query the dma-buf for the current set of implicit fences, encapsulate
> >those into a sync_file and hand that fd back to userspace.
> >Correspondingly, being able to add an explicit fence back into the mix
> >of fences being tracked by the dma-buf allows that userspace fence to be
> >included in any implicit tracking.
> 
> Well I think that this is a rather questionable interface.
> 
> For example how do you deal with race conditions? E.g. between
> querying the fences from the reservation object and adding a new one
> we could gain new fences because of the kernel swapping things out
> or another thread making some submission with this buffer.
> 
> Additional to that IIRC reservation_object_add_excl_fence()
> currently replaces all shared fences with the exclusive one. A
> malicious application could use this to trick the kernel driver into
> thinking that this buffer object is idle while it is still accessed
> by some operation. At least with GPU operations you can easily take
> over the system when you manage to get access to a page table with
> this.

The only difference here is that we believe the GPU drivers to enforce
the ordering between each other. So we can either insert a wait before
adding the exclusive fence, or we can just not expose an import ioctl.
Extracting the set of fences isn't an issue? (That's the part that has a
more legitimate usecase.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre