A kernel panic is what Mac OS X does when it encounters an error from which it cannot safely recover. The kernel, which is the essentially the core part of the operating system, shuts itself down to prevent data loss and/or corruption. The outward result is the a screen you may have seen that tells you that you in several languages that you must restart your computer; there is often some user panic as well since most of us have never even heard of or seen a kernel panic.
Wee see them somewhat regularly in the repair department, and the causes and solutions are usually straightforward. When a MacBook Pro was checked in the other day with reports of kernel panics, I figured it would be a pretty simple diagnosis and resolution.
The first step in troubleshooting is to confirm the reported symptom. The owners said that the computer would kernel panic while viewing webpages with Flash content, so I figured up a few videos on Hulu. Sure enough, the problem was confirmed after a few minutes. I was able to recreate this a number of times, but only while viewing content on Hulu; a quick call to the customer confirmed that the issue only arose while streaming video on this site. There were no problems reported while viewing other Flash-heavy sites like YouTube.
I felt confident at this point that I was dealing with a software issue. A hardware issue would likely have had a broader impact on the system that causing a kernel panic while on one specific website. As a quick check to see if the issue was systemic or user-specific, I logged into another user on the system and fired up Hulu. I could stream any amount of content without issue so the problem was likely to be found in the affected user’s account.
Earlier, I mentioned that the causes of kernel panics are pretty straightforward, and in most cases that’s true. But, sometimes, the cause is buried in the software. This can lead to a lot of time spent checking extensions and preferences and updates. At the extreme, it could involve not only reinstalling the operating system but also rebuilding the user’s account one step at a time to find the offending files.
I got off easy and in the process, came across something I hadn’t seen before. The account in question had parental controls enabled. Parental controls let an administrator restrict many aspects of a standard user’s account (such as which applications they can use, which web sites they can visit, and who they can chat with in iChat).
It turns out that there were a bunch of web sites in both the “always restrict” and the “always allow” columns. Right away I saw that Hulu.com was listed in both categories. The kernel panics were resulting from the conflict of hulu being listed on the whitelist and the blacklist. I know the operating system should be able to handle a basic conflict like this. If it were that easy to crash and operating system, we’d be restarting our computers all day long. So, I suspected a corrupted property list (.plist, or preference) file which would be cured by deleting the old and allowing the operating system to re-generate a clean one.
First I tried just cleaning the list in the Parental Controls preference pane to see if that would resolve the issue. Sure enough, the problem disappeared. When I added Hulu.com back to both lists, the problem recurred.
So, for parents noticing odd behaviors on the computers, it is well worth your time to check your parental controls settings every now and again!