[webkit-dev] Implementing OffscreenCanvas

Maciej Stachowiak mjs at apple.com
Thu Oct 10 09:42:06 PDT 2019

For clarity, it’s already possible to render to a regular canvas offscreen. The <canvas> can be hidden using any of the techniques that can make any other canvas invisible. Name notwithstanding, OffscreenCanvas is mainly about being able to render from a Worker, not about enabling rendering offscreen.

Thus, I would not expect it to make it easier to invisibly fingerprint using canvas.

> On Oct 10, 2019, at 9:32 AM, Chris Lord <clord at igalia.com> wrote:
> Hi John,
> I don't know what the current state is of counter-measures for such an
> attack, but I don't immediately imagine OffscreenCanvas would make them
> more effective. The patch series doesn't add any new rendering paths, so
> whatever was possible before will likely still be possible and whatever
> wasn't will hopefully still not be possible. That said, I'll look into
> this and discuss it with some people that will know better than me and
> try to get a better picture.
> Thanks,
> Chris
> On 2019-10-10 17:32, John Wilander wrote:
>> Hi Chris!
>> Canvas is a very popular GPU fingerprinting vector and allowing it
>> offscreen sounds like a more convenient way to perform such an attack
>> on user privacy. Do you know if Blink or Gecko have elaborated on
>> this? What is your assessment?
>> Given the cross-engine effort to fight device fingerprinting and
>> WebKit and Gecko’s recently published tracking prevention policies, we
>> should do a threat analysis of this feature.
>>   Regards, John
>>> On Oct 10, 2019, at 4:24 AM, Chris Lord <clord at igalia.com> wrote:
>>> Hi all,
>>> I've spent the last month or so 'finishing' the implementation of
>>> OffscreenCanvas[1], based on Žan Doberšek's work from a year ago[2].
>>> OffscreenCanvas is an API for being able to use canvas drawing without a
>>> visible canvas, and from within Workers. It's supported by Blink and has
>>> partial support in Gecko.
>>> It's at the point now where I'd consider it a finished draft - it is
>>> almost fully implemented and passes the majority of relevant tests in a
>>> debug build without crashing, but has some areas that need completion on
>>> other platforms (async drawing on non-Linux) and some missing parts (Web
>>> Inspector, ImageBitmapRenderingContext). It almost certainly needs
>>> reworking in places.
>>> My work is on GitHub[3] - I'd like to solicit reviews and comment. Some
>>> of the bugs hanging off [2] have patches that need review and I think
>>> are near ready to being landable as the foundation of this work. It is
>>> broadly split up like so:
>>> - Refactor to move functionality from HTMLCanvasElement to CanvasBase
>>> - Refactor to not unnecessarily require HTMLCanvasElement in places
>>> - Implement OffscreenCanvas functionality
>>> - Make font loading/styling usable from a Worker and without a Document
>>> - Implement AnimationFrameProvider on DedicatedWorkerGlobalScope
>>> - Implement asynchronous drawing updates on placeholder canvases
>>> I expect the font-related stuff to be the most contentious, and my
>>> AnimationFrameProvider implementation may be too trivial (but might be
>>> ok for a first go?)
>>> All feedback appreciated. Best regards,
>>> Chris
>>> [1]
>>> https://html.spec.whatwg.org/multipage/canvas.html#the-offscreencanvas-interface
>>> [2] https://bugs.webkit.org/show_bug.cgi?id=183720
>>> [3] https://github.com/Cwiiis/webkit/tree/offscreen-canvas
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