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

David Herrmann dh.herrmann at gmail.com
Fri Aug 29 05:57:12 PDT 2014


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.


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