Possible Loophole (was: Proposal: Property fixing)

Tom Van Cutsem tomvc.be at gmail.com
Sun Jun 19 07:53:37 PDT 2011


2011/6/17 David Bruant <david.bruant at labri.fr>

> **
> Le 17/06/2011 22:20, Mark S. Miller a écrit :
>
> Hmmm. This suggests a loophole that might help out on some of the proxy
> cases we're concerned about. I know of no universal constraint that a
> proxy's handler could violate if it could 1) trap attempts to change the
> value of a writable non-configurable property, and 2) could respond by
> either setting the value as it likes or reporting a failed assignment. The
> handler must of course be prevented from changing any other attribute,
> except to change from writable to non-writable. AFAICT, this does not weaken
> any assumptions that are safe to assume regarding ES5.1 non-native objects.
>
> Actually, since in the recent version of the strawman, the defineProperty
> trap is actualy trapped even for fixed properties, I think that what you're
> describing is happening already. Is it Tom?
>

Yes, with one minor difference: as currently specced in the strawman, the
proxy handler can trap attempts to change the value of any fixed
non-configurable property, both writable and non-writable. Still, the
handler will be prevented from changing writable:false to writable:true,
which seems to be the important invariant to uphold.

So, updates to non-writable, non-configurable fixed properties are currently
trapped, but the handler can't actually change the property. Should the
strawman be changed such that defineProperty is only trapped on writable,
non-configurable fixed properties?
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