[PATCH 4/5] drm/vmwgfx: remove custom ioctl io encoding check

[Thomas Hellstrom]

Hi, Emil

On Fri, 2019-05-24 at 16:26 +0100, Emil Velikov wrote:
> On 2019/05/24, Thomas Hellstrom wrote:
> > On Fri, 2019-05-24 at 13:14 +0100, Emil Velikov wrote:
> > > On 2019/05/23, Thomas Hellstrom wrote:
> > > > Hi, Emil,
> > > > 
> > > > On Wed, 2019-05-22 at 17:41 +0100, Emil Velikov wrote:
> > > > > From: Emil Velikov <emil.velikov at collabora.com>
> > > > > 
> > > > > Drop the custom ioctl io encoding check - core drm does it
> > > > > for
> > > > > us.
> > > > 
> > > > I fail to see where the core does this, or do I miss something?
> > > 
> > > drm_ioctl() allows for the encoding to be changed and attributes
> > > that
> > > only the
> > > appropriate size is copied in/out of the kernel.
> > > 
> > > Technically the function is more relaxed relative to the vmwgfx
> > > check, yet
> > > seems perfectly reasonable.
> > > 
> > > Is there any corner-case that isn't but should be handled in
> > > drm_ioctl()?
> > 
> > I'd like to turn the question around and ask whether there's a
> > reason
> > we should relax the vmwgfx test? In the past it has trapped quite a
> > few
> > user-space errors.
> > 
> The way I see it either:
>  - the check, as-is, is unnessesary, or
>  - it is needed, and we should do something equivalent for all of DRM
> 
> We had a very long brainstorming session with a colleague and we
> could not see
> any cases where this would cause a problem. If you recall anything
> concrete
> please let me know - I would be more than happy to take a closer
> look.

If you have a good reason to drop an ioctl sanity check, I'd be
perfectly happy to do it. To me, a good reason even includes "I have a
non-open-source customer having problems with this check" because of
reason etc. etc. as long as I have a way to evaluate those reasons and
determine if they're valid or not. "No other drm driver nor the core is
doing this" is NOT a valid reason to me. In particular if the check is
not affecting performance. So unless you provide additional reasons to
drop this check, it's a solid NAK from my side.

Thanks,
Thomas


> 
> Thanks
> Emil