[webkit-dev] "Magic Iframe" removal proposed

Maciej Stachowiak mjs at apple.com
Tue Mar 20 01:31:19 PDT 2012

I'm ok with removing this feature for the reasons you described. I concur with others who think we should update the spec. I am also skeptical of state sharing features that work via newer, less tested API surface instead of latching onto existing features. That seems like a more risky strategy since it may be harder to remove such a feature without compat breakage, and it's not clear how it makes the security problems even easier.

As a side note, this probably should have had more discussion time before being actually committed. If a change is worthy of webkit-dev discussion in the first place, then 5 hours between initial webkit-dev post and committing the patch is cutting it a bit short. Especially when it is outside normal working hours in the time zones where most WebKit contributors live. I don't want to harp on this too much, since I don't personally disagree with the change, but if anyone does, then they may feel that they didn't really get a fair chance to comment.


On Mar 19, 2012, at 5:51 PM, Dmitry Titov wrote:

> Hi,
> There is a patch posted for removal of the 'magic iframe' feature. This is the ability to move 'live' iframe from one page to another w/o unloading it.
> If you have interest or ideas about this feature, please reply.
> This feature was added 2 years ago (bug here). It was intended as a shared app context for complex apps that span several pages. In case when random set of pages is closed, the surviving iframe could be passed between remaining pages and serve as 'app state'.
> This behavior is somewhat described in HTML5 spec: "Removing an iframe from a Document does not cause its browsing context to be discarded. Indeed, an iframe's browsing context can survive its original parent Document if its iframe is moved to another Document."
> All non-WebKit browsers don't have this and always unload the iframe when it is disconnected form the document.
> We did have quite a few issues with this mechanism. The root of the problem seems to be that traditionally, multiple 'permissions' and 'live objects' are associated with a top-level page, or a top frame of some kind, instead of being associated with each Frame. Even HTML specifications often formulate the scope of things like permissions in terms of a page - for example, geo permissions are computed based on the origin of the page. When an iframe is transferred from one page to another, it may enter a different set of permissions while already performing operations authorized before. Association with the top-level page is also used to determine which one should show modal UI for HTTP Auth, per-origin permissions, or certificate issues for example.
> As a result, we had quite a few bugs popping up (and fixed).
> This is somewhat obscure feature of the platform. Only a few apps that we knew used the feature. Most developers, both app and webkit kind, don't even know about it. When new mechanisms/APIs are implemented, a lot of objects get associated with Page (WebView) level and they are almost 'automatically' broken in case of live iframe transfer because once old page closes, it destroys the objects with lifetimes scoped by it. This makes it somewhat dangerous and difficult to support. The benefits that it gives to the big multi-page applications do not seem to warrant the actual complexity of maintaining this feature.
> Other browsers never implemented the feature, siting difficult design due to various thorny security issues as well.
> This is potentially a compatibility issue for sites that use document.adoptNode(iframe) to ensure live transfer of an iframe from one page to another.
> In the future, if there will be sufficient need, it is possible to design a 'shared module' feature that would explicitly deal with various security/lifetime boundaries.
> Please let us know what you think.
> Thanks,
> Dmitry
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