[webkit-dev] Implementing OffscreenCanvas

John Wilander wilander at apple.com
Thu Oct 10 08:32:00 PDT 2019

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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