[PATCH 5/5] drm: allocate kernel mode-object IDs in cyclic fashion

[David Herrmann]

Hi

On Wed, Sep 9, 2015 at 3:03 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Wed, Sep 09, 2015 at 02:21:33PM +0200, David Herrmann wrote:
>> Now that we support connector hotplugging, user-space might see
>> mode-object IDs coming and going asynchronously. Therefore, we must make
>> sure to not re-use object IDs, so to not confuse user-space and introduce
>> races. Therefore, all kernel-allocated objects will no longer recycle
>> IDs. Instead, we use the cyclic idr allocator (which performs still fine
>> for reasonable allocation schemes).
>>
>> However, for user-space allocated objects like framebuffers, we don't
>> want to risk allowing malicious users to screw with the ID space.
>> Furthermore, those objects happen to not be subject to ID hotplug races,
>> as they're allocated and freed explicitly. Hence, we still recycle IDs
>> for these objects (which are just framebuffers so far).
>
> Since atomic also blob properties can be created by userspace. And there
> has actually been trouble once with some attempt in SNA to cache the edid
> blob without realizing that they get reused. So working userspace _always_
> has to re-read the blob prop to make sure it has the current one anyway.

I see. So lets include the blobs in recycle, too? This makes the patch
a bit more clunky as we need to distinguish kernel-generated blobs and
user-space generated blobs. I'll see how that works out.

Thanks
David