[Webkit-unassigned] [Bug 161534] New: Regression(r197648): JSObject::setPrototypeWithCycleCheck() allows for cycles but the rest of the code base does not

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=161534

            Bug ID: 161534
           Summary: Regression(r197648):
                    JSObject::setPrototypeWithCycleCheck() allows for
                    cycles but the rest of the code base does not
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: cdumez at apple.com
                CC: ggaren at apple.com, mark.lam at apple.com,
                    sbarati at apple.com
            Blocks: 155002

JSObject::setPrototypeWithCycleCheck() allows for cycles but the rest of the code base does not deal properly with cycles.

This is because of the following check that was added:
if (UNLIKELY(asObject(nextPrototype)->methodTable(vm)->getPrototype != defaultGetPrototype)) 
  break;

This was added to match the EcmaScript spec:
- https://tc39.github.io/ecma262/#sec-ordinarysetprototypeof (step 8)

However, if you create a cycles, we end up with an infinite loop later on under:
Structure::anyObjectInChainMayInterceptIndexedAccesses()

This is likely not the only place we traverse the prototype chain and except there is no cycle.

I noticed this when running html/browsers/history/the-location-interface/allow_prototype_cycle_through_location.sub.html with the patch for Bug 161455 applied.

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