Month: April 2008

Diff bug fixed, hopefully

For a long time we’ve had intermittent problems with diffs displaying incorrectly, with lines on the left side mysteriously repeated:

Reports skyrocketed the other day, when the wikidiff2 extension (our C++ reimplementation of MediaWiki’s diff algorithm, about a billion times faster than the PHP one) was upgraded to match upgrades of PHP on our older, Fedora Core-based servers.

I added in some logging hacks to try to track it down, but didn’t get a lot of data points until I tried the simple expedient of running every diff twice — if the results don’t match, log the error.

With a few hundred instances logged, it became clear that the problem was limited to servers running Fedora 4; even-older Fedora 3 boxes were unaffected, as were all our newer Ubuntu boxes. Mysterious problems caused by C++ run-time library mismatches between different Linux releases are not at all uncommon; it looked like we’d installed an FC3 binary on all the machines, and it was intermittently failing on FC4.

I recompiled the extension, this time with separate builds on FC3 and FC4, and haven’t seen any bad diffs come through my log in the last half hour… so far so good! :)

So what’s in the job queue anyway?

In en.wikipedia.org’s job queue at the moment, breakdown by job type…

job_cmd count(*)
htmlCacheUpdate 31,147
refreshLinks 10,106,739
renameUser 119

Note that the current system allows for duplicate entries to get put in the queue; the dupes are removed as the first one in the stack gets run. This makes the raw number of refreshLinks entries much higher than it “really” is — [[Wikipedia:Talk:Union Station (Louisville)|Talk:Union Station (Louisville)]] is listed 9 times, presumably once for each template edit that triggered an “update me!” job.

Update: Figured out why the queues were growing so big last few days — system clock was 7 seconds slow on the database master. This made the replication lag detection misread a 7-second minimum lag on every slave. The job queue batch runners were all sitting waiting for the lag to resolve. :)

Resynced the clock (presumably drifted during the period when some IPs were broken), things are moving again.

Visual Voicemail fixed

One of the oh-so-cute features of the iPhone is visual voicemail, the “duh” feature of showing you an actual on-screen list of your voicemails instead of making you sit through voice prompts.

Bad: My iPhone mysteriously reverted to the classic “press 7 to delete” system when I changed rate plans a couple weeks ago… with voice mail disabled altogether so callers couldn’t leave messages until I noticed it and set up a new password.

A little Googling indicates this is a fairly common mix-up, and the only way to restore visual voicemail is to call AT&T tech support and have them fiddle with your account settings.

Good: AT&T tech support was able to fix the account settings so it works again… after a half hour on hold… :)

WTF: The AT&T tech swore that visual voicemail doesn’t work if you have a WiFi connection active. He had me disable WiFi while initially testing it, then when I asked him about it he told me outright that Visual Voicemail only works on the EDGE network and therefore you must turn off WiFi to check your voicemail.

This is demonstrably false; just to confirm I hadn’t been crazy for the couple of months my voicemail was working just fine, I turned WiFi back on, left myself a voicemail, and retrieved it just fine in all its visual glory.

It’s entirely possible that the voicemails still download over EDGE, but having the WiFi up doesn’t seem to interfere at all.

Now if they can just add a feature to route phone calls over WiFi, I could actually get calls through from my flat. ;)

Suggestion search drop-down

Another in today’s series of fun feature enablings…

The search boxes on Wikimedia wikis now have an AJAX-powered search suggestion drop-down. This calls our JSON OpenSearch suggestion interface, which has been used for some time by Firefox’s search box and Mac OS X 10.5’s Dictionary application, but is now built-in for your viewing pleasure.

(In MediaWiki 1.13 development trunk, turn on $wgEnableMWSuggest to experience this yourself!)

A similar AJAX-powered search feature has been in MediaWiki for some time, but the user interface for it took over the whole article area, which was a bit distracting, and we never used it ourselves.

Robert Stojnic, the tireless coder who’s put a huge amount of effort into fixing up our Lucene-based search engine over the last months, patched up the front-end to fit more naturally into the existing forms.

The built-in search for suggestions is currently a simple prefix match, so it’ll help you complete words and names, but isn’t smart enough to fill out from a last name or skip “the” etc. Robert’s got a new backend in the works, which will add all those smarts when we’re ready to upgrade the search systems with the new software and a bit beefier hardware.

Prefix matches are a heck of a lot better than nothing, though, and as long as it’s not causing undue server load we’ll keep it on until the new backend’s ready.

(If you don’t like the suggestions widget, you can disable them by checking “Disable AJAX suggestions” in the “Search” tab at Special:Preferences.)

HttpOnly cookies

Thanks to Werdna’s implementation of support, and Tim’s mass upgrade of our older PHP installations, I’ve today enabled the use of HttpOnly cookies on the Wikimedia wikis for our login session data.

“What’s that,” I hear you say, “and why do I want it?”

The HttpOnly marker on cookies tells a supporting browser that the cookie will only be used directly by the web server (sent only with the HTTP requests for each page), so it will hide the cookie from any JavaScript client code which asks for it.

This provides protection against certain kinds of security vulnerabilities — namely, XSS attacks which steal authenticated session and long-term login token cookies.

HttpOnly doesn’t fix XSS, not by a long shot, but it does reduce what an attacker can do; particularly nice when we’re soon going to start using global login cookies which will allow a unified account to continue a login session across multiple wikis on different domains.

The same origin policy prevents JavaScript on one subdomain from directly accessing another domain. Keeping the cross-domain session cookies away from compromised JavaScript will help prevent a hypothetical attack on one domain from jumping to other subdomains without the vulnerability.

Unfortunately, this marker isn’t standard; it’s an extension which Microsoft added for Internet Explorer in 6.0 SP1, but support has been slowly creeping into other browsers, finally hitting Firefox somewhere in the 2.0 patch cycle while nobody was looking.

Browsers I tested that currently support HttpOnly cookies:

  • IE/Win 6 SP1 or 7
  • Firefox 2.0.0.5 or later
  • Opera 9.50 beta
  • Konqueror (3.4?)

Other browsers will still expose the cookies to JavaScript, as they always have:

  • Safari 3.1
  • Opera 9.27 (current non-Beta release)
  • Old scary browsers like IE for Mac and Netscape 4 ;)

There’s a rumor that some versions of WebTV fail altogether when the cookies are marked this way, but I have no way to confirm or deny that yet.

Update 2008-05-01: Mac IE turns out to eat HttpOnly cookies…. sometimes… when the moon is just right. :) Added a browser blacklist, so we feed Mac IE regular cookies. Other browsers are still given the benefit of the doubt.

SUL status update…

Status update…

CentralAuth global logins are still restricted to the sysop beta, but Werdna and Tim have been doing some good work on cleaning things up…

  • Tim’s done a lot of code refactoring to clean up User object behavior
  • Werdna’s added support for global sessions based on Tim’s suggested model. Tim and I have helped with some cleanup on it…
  • I put together a threat assessment of the security impact of global session cookies and some mitigration strategies
  • One of my suggestions was to use HttpOnly mode for session and token cookies, where browsers support them. This will largely block XSS attacks from jumping between subdomains or stealing cookies for reuse by an attacker. Werdna’s added support for HttpOnly cookies under PHP 5.2; currently we can’t deploy this until we finish upgrading some of our machines.
  • I’ve enabled global sessions on secure.wikimedia.org, where there’s a single domain and few other services to increase the attack surface. It _seems_ to mostly work so far. ;)

    Logging out doesn’t quite clear all sessions correctly yet, but so far so good. :)