Re: [PATCH v3 8/9] drm/rockchip: gem: Don't alloc/free gem buf when dev_private is invalid

Daniel Vetter
2017-04-07T06:30:54Z

On Thu, Apr 6, 2017 at 1:09 PM, jeffy <jeffy.chen@rock-chips.com> wrote:
>
> On 04/06/2017 04:26 PM, Daniel Vetter wrote:
>>
>> On Wed, Apr 05, 2017 at 12:28:40PM -0400, Sean Paul wrote:
>>>
>>> On Wed, Apr 05, 2017 at 04:29:26PM +0800, Jeffy Chen wrote:
>>>>
>>>> After unbinding drm, the userspace may still has a chance to access
>>>> gem buf.
>>>>
>>>> Add a sanity check for a NULL dev_private to prevent that from
>>>> happening.
>>>
>>>
>>> I still don't understand how this is happening. You're saying that these
>>> hooks
>>> can be called after rockchip_drm_unbind() has finished?
>>
>>
>> Yeah this is supposed to be impossible. If it isn't, we need to debug and
>> fix this properly. This smells like pretty bad duct-tape ...
>
>
> it looks like after unbind, the user space may still own drm dev fd, and
> could be able to call ioctl:
> lrwx------. 1 chronos chronos 64 Mar 15 12:53 28 -> /dev/dri/card1 (deleted)
>
> and the drm_unplug_dev may help it, maybe we should call it in unbind? or
> just break drm_ioctl when drm_dev not registered?

Yes, by default unbind while userspace is running is totally broken in
drm. drm_unplug_dev would be the fix, but it's only used by udl and
not many use that. You might need to fix infrastructure up a bit.

For normal module unload the module reference will prevent unloading.
So why exactly do you care about the unbind use-case?
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch