[PATCH 2/2] drm: don't recycle used modeset IDs

[David Herrmann]

Hi

On Fri, Aug 29, 2014 at 2:57 PM, David Herrmann <dh.herrmann at gmail.com> wrote:
> Hi
>
> On Fri, Aug 29, 2014 at 2:51 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
>> On Fri, Aug 29, 2014 at 02:01:01PM +0200, David Herrmann wrote:
>>> With MST, we now have connector hotplugging, yey! Pretty easy to use in
>>> user-space, but introduces some nasty races:
>>>  * If a connector is removed and added again while a compositor is in
>>>    background, it will get the same ID again. If the compositor wakes up,
>>>    it cannot know whether its the same connector or a new one, thus they
>>>    must re-read EDID information, etc.
>>>  * possible_clones, possible_crtcs, etc. depend on indices, not IDs. So if
>>>    an object is removed and a new one is added, those bitmasks are invalid
>>>    and must be refreshed. This currently does not affect connectors, but
>>>    only crtcs and encoders, but it's only a matter of time when this will
>>>    change.
>>>
>>> The easiest way to protect against this, is to not recylce modeset IDs.
>>> Instead, always allocate a new, higher ID. All ioctls that affect modeset
>>> objects, take IDs. Thus, during hotplug races, those ioctls will simply
>>> fail if invalid IDs are passed. They will no longer silently run on a
>>> newly hotplugged object.
>>>
>>> Furthermore, hotplug-races during state sync can now be easily detected. A
>>> call to GET_RESOURCES returns a list of available IDs atomically.
>>> User-space can now start fetching all those objects via GET_* ioctls. If
>>> any of those fails, they know that the given object was unplugged. Thus,
>>> the "possible_*" bit-fields are invalidated. User-space can now decide
>>> whether to restart the sync all over or wait for the 'change' uevent that
>>> is sent on modeset object modifications (or, well, is supposed to be sent
>>> and probably will be at some point).
>>>
>>> With this in place, modeset object hotplugging should work fine for all
>>> modeset objects in the KMS API.
>>>
>>> CC'ing stable so we can rely on all kernels with MST to not recycle IDs.
>>>
>>> Cc: <stable at vger.kernel.org> # 3.16+
>>> Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
>>
>> So userspace just needs to cycle through piles of framebuffer objects to
>> make bad things happen? Doesn't sound like a terribly solid plan.
>
> IDs will still get recycled, but only once all IDs got used. So this is "safe".
>
> Sure, user-space can create 2 billion framebuffers and destroy them
> again, thus causing the ID range to overflow and recycle old IDs. Not
> sure how fast you can run 2 billion syscalls.. If that's a real issue,
> I'd vote for using the high-range for user-space managed objects,
> low-range for kernel-space managed objects ([1...INT_MAX] and
> [INT_MAX+1...UINT_MAX] or so).
>
>> I guess we could save this by doing normal id allocations for fbs and
>> monotonically increasing allocations for all other objects.
>
> This doesn't work. A connector with ID #n might get unplugged and
> another process created a new FB, which will then get ID #n. Sure, I
> doubt there's a real conflict as ioctls check the type, but it still
> sounds really ugly to me.

On a second thought: maybe your idea isn't as bad as I thought. I
mean, everyone must do type-checking when looking up mode-objects, so
it seems safe to rely on that.

Thanks
David