Bug 142792 – Objects with numeric properties intermittently get a phantom 'length' property

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

There is a timing bug in iOS8 that causes mobile Safari to incorrectly report a 'length' on objects that don't have one.

To the best of my knowledge, this happens on iOS8+, possibly only on 64-bit systems. The bug is triggered for objects that have only numeric properties. For example:

  foo = { 1: 'a', 2: 'b', 3: 'c' } 

In this case, if you query foo.length then mobile Safari will sometimes return 4 (the highest property + 1).

This causes strange failures in functions like jQuery's $.each() or Underscore's _.each() since the appearance of the '.length' property makes them believe the object is an array.

When the bug manifests itself, the following be true:
  a = foo.length;
  b = typeof a;
  c = foo.hasOwnProperty('length')
-->
  a == 4         // should have been undefined
  b == 'number'  // should have been 'undefined'
  c == false     // correct

Both jQuery and Underscore to use the 'typeof' test on the length property, and I have advised them to add an 'hasOwnProperty' check as well.

You can see more background and some repro steps at the following Stack Overflow discussion: http://stackoverflow.com/questions/28155841/misterious-failure-of-jquery-each-and-underscore-each-on-ios

The Underscore team have apparently managed to create an automated test for this: https://github.com/jashkenas/underscore/issues/2081

The jQuery issue is here: https://github.com/jquery/jquery/issues/2145

[Radar WebKit Bug Importer]

<rdar://problem/20307461>

[Yusuke Suzuki]

ossy, now I think it may be related to aarch64 failures with "length" property.

[Michael Saboff]

I can reproduce on iOS ARM64 in ToT (r182023) WebKit.  It doesn't happen in on ARM32 or Mac.  Curious.

[Michael Saboff]

Created attachment 249534 [details]
Converted supplied test into a regression test

[Michael Saboff]

Looks like we have a bad instruction optimization when comparing a literal with a single bit to a register in MacroAssemblerARM64::branchTest32().

Patch in progress.

[Michael Saboff]

Created attachment 249564 [details]
Patch

[Csaba Osztrogonác]

Comment on attachment 249564 [details]
Patch

nice catch, r=me

[Michael Saboff]

Committed r182058: <http://trac.webkit.org/changeset/182058>

[Mark Hahnenberg]

This might be my favorite bug/fix ever.

[Csaba Osztrogonác]

*** Bug 142575 has been marked as a duplicate of this bug. ***

[Michał Gołębiowski-Owczarek]

It would be great to have it backported to iOS 8. If that's not planned (or if it's planned but we're be notified about that) we'll have to publish another patch release of jQuery dedicated to Safari; the first one was:
http://blog.jquery.com/2014/12/18/jquery-1-11-2-and-2-1-3-released-safari-fail-safe-edition/