Archive for 2014

An Invitation

Sunday, November 30th, 2014

I’d like to invite my blog readers and Mozilla coworkers to Jesus Christ.

For most Christians, today marks the beginning of Advent, the season of preparation before Christmas. Not only is this a time for personal preparation and prayer while remembering the first coming of Christ as a child, but also a time to prepare the entire world for Christ’s second coming. Christians invite their friends, coworkers, and neighbors to experience Christ’s love and saving power.

I began my journey to Christ through music and choirs. Through these I discovered beauty in the teachings of Christ. There is a unique beauty that comes from combining faith and reason: belief in Christ does not require superstition nor ignorance of history or science. Rather, belief in Christ’s teachings brought me to a wholeness of understanding the truth in all it’s forms, and our own place within it.

Although Jesus is known to Christians as priest, prophet, and king, I have a special and personal devotion to Jesus as king of heaven and earth. The feast of Christ the King at the end of the church year is my personal favorite, and it is a particular focus when I perform and composing music for the Church. I discovered this passion during college; every time I tried to plan my own life, I ended up in confusion or failure, while every time I handed my life over to Christ, I ended up being successful. My friends even got me a rubber stamp which said “How to make God laugh: tell him your plans!” This understanding of Jesus as ruler of my life has led to a profound trust in divine providence and personal guidance in my life. It even led to my becoming involved with Mozilla and eventually becoming a Mozilla employee: I was a church organist and switching careers to become a computer programmer was a leap of faith, given my lack of education.

Making a religious invitation to coworkers and friends at Mozilla is difficult. We spend our time and build our deepest relationships in a setting of on email, video, and online chat, where off-topic discussions are typically out of place. I want to share my experience of Christ with those who may be interested, but I don’t want to offend or upset those who aren’t.

This year, however, presents me with a unique opportunity. Most Mozilla employees will be together for a shared planning week. If you will be there, please feel free to find me during our down time and ask me about my experience of Christ. If you aren’t at the work week, but you still want to talk, I will try to make that work as well! Email me.

1. On Jordan’s bank, the Baptist’s cry
Announces that the Lord is nigh;
Awake, and hearken, for he brings
Glad tidings of the King of kings!

2. Then cleansed be every breast from sin;
Make straight the way for God within;
Prepare we in our hearts a home
Where such a mighty Guest may come.

3. For Thou art our Salvation, Lord,
Our Refuge, and our great Reward.
Without Thy grace we waste away,
Like flowers that wither and decay.

4. To heal the sick stretch out Thine hand,
And bid the fallen sinner stand;
Shine forth, and let Thy light restore
Earth’s own true lovliness once more.

5. Stretch forth thine hand, to heal our sore,
And make us rise to fall no more;
Once more upon thy people shine,
And fill the world with love divine.3

6. All praise, eternal Son, to Thee
Whose advent sets Thy people free,
Whom, with the Father, we adore,
And Holy Ghost, forevermore.

—Charles Coffin, Jordanis oras prævia (1736), Translated from Latin to English by John Chandler, 1837

How I Do Code Reviews at Mozilla

Wednesday, October 22nd, 2014

Since I received some good feedback about my prior post, How I Hire at Mozilla, I thought I’d try to continue this is a mini-series about how I do other things at Mozilla. Next up is code review.

Even though I have found new module owners for some of the code I own, I still end up doing 8-12 review/feedback cycles per week. Reviews are only as good as the time you spend on them: I approach reviews in a fairly systematic way.

When I load a patch for review, I don’t read it top-to-bottom. I also try to avoid reading the bug report: a code change should be able to explain itself either directly in the code or in the code commit message which is part of the patch. If bugzilla comments are required to understand a patch, those comments should probably be part of the commit message itself. Instead, I try to understand the patch by unwrapping it from the big picture into the small details:

The Commit Message

  • Does the commit message describe accurately what the patch does?
  • Am I the right person to make a decision about this change?
  • Is this the right change to be making?
  • Are there any external specifications for this change? This could include a UX design, or a DOM/HTML specification, or something else.
  • Should there be an external specification for this change?
  • Are there other experts who should be involved in this change? Does this change require a UX design/review, or a security, privacy, legal, localization, accessibility, or addon-compatibility review or notification?

Read the Specification

If there is an external specification that this change should conform to, I will read it or the appropriate sections of it. In the following steps of the review, I try to relate the changes to the specification.


If there is in-tree documentation for a feature, it should be kept up to date by patches. Some changes, such as Firefox data collection, must be documented. I encourage anyone writing Mozilla-specific features and APIs to document them primarily with in-tree docs, and not on In-tree docs are much more likely to remain correct and be updated over time.

API Review

APIs define the interaction between units of Mozilla code. A well-designed API that strikes the right balance between simplicity and power is a key component of software engineering.

In Mozilla code, APIs can come in many forms: IDL, IPDL, .webidl, C++ headers, XBL bindings, and JS can all contain APIs. Sometimes even C++ files can contain an API; for example Mozilla has an mostly-unfortunate pattern of using the global observer service as an API surface between disconnected code.

In the first pass I try to avoid reviewing the implementation of an API. I’m focused on the API itself and its associated doccomments. The design of the system and the interaction between systems should be clear from the API docs. Error handling should be clear. If it’s not perfectly obvious, the threading, asynchronous behavior, or other state-machine aspects of an API should be carefully documented.

During this phase, it is often necessary to read the surrounding code to understand the system. None of our existing tools are very good at this, so I often have several MXR tabs open while reading a patch. Hopefully future review-board integration will make this better!

Brainstorm Design Issues

In my experience, the design review is the hardest phase of a review, the part which requires the most experience and creativity, and provides the most value.

  • How will this change interact with other code in the tree?
  • Are there edge cases or failure cases that need to be addressed in the design?
  • Is it likely that this change will affect performance or security in unexpected ways?

Testing Review

I try to review the tests before I review the implementation.

  • Are there automated unit tests?
  • Are there automated performance tests?
  • Is there appropriate telemetry/field measurement, especially for error conditions?
  • Do the tests cover the specification, if there is one?
  • Do the tests cover error conditions?
  • Do the tests strike the right balance between “unit” testing of small pieces of code versus “integration” tests that ensure a feature works properly end-to-end?

Code Review

The code review is the least interesting part of the review. At this point I’m going through the patch line by line.

  • Make sure that the code uses standard Mozilla coding style. I really desperately want somebody to automated lots of this as part of the patch-submission process. It’s tedious both for me and for the patch author if there are style issues that delay a patch and require another review cycle.
  • Make sure that the number of comments is appropriate for the code in question. New coders/contributors sometimes have a tendency to over-comment things that are obvious just by reading the code. Experienced contributors sometimes make assumptions that are correct based on experience but should be called out more explicitly in the code.
  • Look for appropriate assertions. Assertions are a great form of documentation and runtime checking.
  • Look for appropriate logging. In Mozilla we tend to under-log, and I’d like to push especially new code toward more aggressive logging.
  • Mostly this is scanning the implementation. If there is complexity (such as threading!), I’ll have to slow down a lot and make sure that each access and state change is properly synchronized.

Re-read the Specification

If there is a specification, I’ll briefly re-read it to make sure that it was covered by the code I just finished reading.


Currently, I primarily do reviews in the bugzilla “edit” interface, with the “edit attachment as comment” option. Splinter is confusing and useless to me, and review-board doesn’t seem to be ready for prime-time.

For long or complex reviews, I will sometimes copy and quote the patch in emacs and paste or attach it to bugzilla when I’m finished.

In some cases I will cut off a review after one of the earlier phases: if I have questions about the general approach, the design, or the API surface, I will often try to clarify those questions before proceeding with the rest of the review.

There’s an interesting thread in about whether it is discouraging to new contributors to mark “review-” on a patch, and whether there are less-painful ways of indicating that a patch needs work without making them feel discouraged. My current practice is to mark r- in all cases where a patch needs to be revised, but to thank contributors for their effort so that they are still appreciated and to be as specific as possible about required changes while avoiding any words that could be perceived as an insult.

If I haven’t worked with a coder (paid or volunteer) in the past, I will typically always ask them to submit an updated patch with any changes for re-review. This allows me to make sure that the changes were completed properly and didn’t introduce any new problems. After I gain some experience, I will often trust people to make necessary changes and simply mark “r+ with review comments fixed”.

How I Hire at Mozilla

Thursday, October 2nd, 2014

As a manager, one of my most important responsibilities is hiring. While I’m hiring, I spend a lot of time sifting through resumés, screening candidates, and eventually doing interviews. This is both fun and depressing at the same time, because you get to meet and learn a lot about some interesting people, but you also have to wade through a lot of terrible resumés and phone screens. I want to share some things that I look for during the hiring process, and some tips for potential job-seekers about how to deal with recruiting:

Read the Job Description

Please read the job description before applying for a job! I put a lot of effort into writing a job description, and I try very hard to describe both the job responsibilities and the necessary and desirable skills. Your cover letter should show some evidence that you’ve thought about applying for this particular job.

Write a Good Cover Letter

Periodically, I see articles advising job-seekers to ditch the cover letter. This is terrible advice. I read cover letters carefully. In fact, the cover letter sometimes gives me a better sense for your skill level and ability to do the job than your resumé.

Grammar and Spelling Matter

Every job I’ve ever posted has required good communication skills, and your cover letter is the first evidence of your communication skills. It doesn’t need to be more than a paragraph or two; I’d love to know why you think that this job is a good fit, and some evidence that you read the job description. Spelling and grammar are important.

I’m Picky

The last time I posted a job position, I screened more than 800 resumés, did almost 100 phone screens, and did five interviews before I found the right person. It took six months. It’s better for Mozilla if I’m too picky and reject a qualified candidate than if I’m not picky enough and accept a bad candidate. Please don’t take rejection as a comment on your personal worth. I’m going to reject many people during this process.

Smart and Gets Things Done

Joel Spolsky is right: I’m primarily looking for somebody who is smart and gets things done. When I’m scanning through resumés, I’m looking for evidence that you’ve gotten things done in the past. If you’re just coming out of school, internship and open-source project experience helps. If you’ve been working, I want to see some description of the things you’ve gotten done. Be specific: “Led multiple projects to completion” is meaningless. You can expect phone-screen questions about your prior projects. Be prepared to talk about what worked, what didn’t, and how you solved problems.

No Assholes

It may seem obvious, but assholes need not apply. I will reject a technically-qualified candidate for even a whiff of assholery. I use reference checks to try and guard against assholes.

Passion Isn’t Sufficient

At Mozilla we get a lot of applicants who are passionate, either about the open web in general or about Firefox. Passion is great, perhaps even necessary, but it’s not sufficient.

Interview Questions Are Based on Your Resumé

In phone screens and interview panels, I try very hard to base my questions on the things that you already know. If your resumé says that you are a master of C++, you should expect that there is at least one person on the interview panel who will grill you on C++ in excruciating detail. If you say that you know statistics, you had better be able to answer detailed questions about statistics.

Don’t overstate your skills. If you have written a couple scripts in Python, you are familiar with Python, not proficient. More than once I’ve rejected people because they claimed to be a master of C++ debugging but didn’t know how a vtable is structured in memory. Knowing vtable layout isn’t usually a job prerequisite, but if you are a master of C++ debugging you’d better know how that works in detail.

If you claim to be a master of both Python and JavaScript, expect me to ask you detailed questions about how Python and JS closures and methods work, how they are different in JS and python, how JS this-binding is different from Python bound methods, and other details of the language. I will be impressed if you can discuss these questions intelligently, and reject your application if you can’t.

I Value Code Reading

I value people who can learn new code quickly. You’re going to need to be comfortable learning new systems, libraries, and languages. My team in particular often touches large swaths of the Mozilla codebase; you will be expected to be able to read and understand new code quickly. You can expect an interview session entirely dedicated to reading and explaining a piece of code that you’ve never seen before. Perhaps it will be reviewing a patch, or trying to find a bug in a piece of code. I’ll try to find a problem in a language that you are already comfortable with, so see above about keeping your resumé honest!

Do You Love Your Tools?

Every good programmer has a toolbox. I don’t care whether you use vim or emacs or Visual Studio or Eclipse or XCode, but I do care that you have an editor and use it productively. I also care that you are proficient using a debugger (or two, or three) and hopefully a profiler (or two, or three). If you can’t tell me why you love your editor or your debugger, it’s likely that you won’t be a successful software engineer.

You need to be proficient in a scripting language. I expect you to be able to use at least one scripting language to process text data, read CSV files, write JSON, and that kind of thing. Mozilla has gravitated toward Python for scripting, and you’ll probably be expected to learn Python if you don’t know it already.

Also, can you type? I’m constantly surprised by applicants who are unable to type quickly and accurately. A significant portion of your job is going to be writing code, and not having mastered the act of typing is probably not a good sign.

Phone Screens

When I conduct a phone-screen, it is usually over Skype video. Please make sure that you have a decent headset and that I will be able to hear you. During a phone screen, I try to include at least one coding question which is typically conducted via etherpad. You should be at a computer with access to the web in order to do this successfully.

By the way, did I mention I’m hiring?

Using Software Copyright To Benefit the Public

Friday, March 21st, 2014

Imagine a world where copyright on a piece of software benefits the world even after it expires. A world where eventually all software becomes Free Software.

The purpose of copyright is “To promote the Progress of Science and useful Arts”. The law gives a person the right to profit from their creation for a while, after which everyone gets to profit from it freely. In general, this works for books, music, and other creative works. The current term of copyright is far too long, but at least once the term is up, the whole world gets to read and love Shakespeare or Walter de la Mare equally.

The same is not true of software. In order to be useful, software has to run. Imagine the great commercial software of the past decade: Excel, Photoshop, Pagemaker. Even after copyright expires on Microsoft Excel 95 (in 2090!), nobody will be able to run it! Hardware that can run Windows 95 will not be available, and our only hope of running the software is to emulate the machines and operating systems of a century ago. There will be no opportunity to fix or improve the software.

What should we reasonably require from commercial software producers in exchange for giving them copyright protection?

The code.

In order to get any copyright protection at all, publishers should be required to make the source code available. This can either happen immediately at release, or by putting the code into escrow until copyright expires. This needs to include everything required to build the program and make it run, but since the same copyright rules would apply to operating systems and compilers, it ought to all just work.

The copyright term for software also needs to be rethought. The goal when setting a copyright term should be to balance the competing desires of giving a software author time to make money by selling software, with the natural rights of people to share ideas and use and modify their own tools.

With a term of 14 years, the following software would be leaving copyright protection around now:

  • Windows 95
  • Excel 95
  • Photoshop 6.0
  • Adobe InDesign 1.0

A short copyright term is an incentive to software developers to constantly improve their software, and make the new versions of their software more valuable than older versions which are entering the public domain. It also opens the possibility for other companies to support old software even after the original author has decided that it isn’t worthwhile.

The European Union is currently holding a public consultation to review their copyright laws, and I’ve encouraged Mozilla to propose source availability and a shorter copyright term for software in our official contribution/proposal to that process. Maybe eventually the U.S. Congress could be persuaded to make such significant changes to copyright law, although recent history and powerful money and lobbyists make that difficult to imagine.

Commercial copyrighted software has done great things, and there will continue to be an important place in the world for it. Instead of treating the four freedoms as ethical absolutes and treating non-Free software as a “social problem”, let’s use copyright law to, after a period of time, make all software Free Software.

Use -debugexe to debug apps in Visual Studio

Monday, March 10th, 2014

Many people don’t know about how awesome the windows debuggers are. I recently got a question from a volunteer mentee: he was experiencing a startup crash in Firefox and he wanted to know how to get the debugger attached to Firefox before the crash.

On other systems, I’d say to use mach debug, but that currently doesn’t do useful things on Windows. But it’s still pretty simple. You have two options:

Debug Using Your IDE

Both Visual Studio and Visual C++ Express have a command-line option for launching the IDE ready for debugging.

devenv.exe -debugexe obj-ff-debug/dist/bin/firefox.exe -profile /c/builds/test-profile -no-remote

The -debugexe flag informs the IDE to load your Firefox build with the command lines you specify. Firefox will launch with the “Go” command (F5).

For Visual C++ express edition, run WDExpress.exe instead of devenv.exe.

Debug Using Windbg

windbg is a the Windows command-line debugger. As with any command-line debugger it has an arcane debugging syntax, but it is very powerful.

Launching Firefox with windbg doesn’t require any flags at all:

windbg.exe obj-ff-debug/dist/bin/firefox.exe -profile /c/builds/test-profile -no-remote

Debugging Firefox Release Builds

You can also debug Firefox release builds on Windows! Mozilla runs a symbol server that allows you to automatically download the debugging symbols for recent prerelease builds (I think we keep 30 days of nightly/aurora symbols) and all release builds. See the Mozilla Developer Network article for detailed instructions.

Debugging official builds can be a bit confusing due to inlining, reordering, and other compiler optimizations. I often find myself looking at the disassembly view of a function rather than the source view in order to understand what exactly is going on. Also note that if you are planning on debugging a release build, you probably want to disable automatic crash reporting by setting MOZ_CRASHREPORTER_DISABLE=1 in your environment.

Don’t Use Mozilla Persona to Secure High-Value Data

Tuesday, February 11th, 2014

Mozilla Persona (formerly called Browser ID) is a login system that Mozilla has developed to make it better for users to sign in at sites without having to remember passwords. But I have seen a trend recently of people within Mozilla insisting that we should use Persona for all logins. This is a mistake: the security properties of Persona are simply not good enough to secure high-value data such as the Mozilla security bug database, user crash dumps, or other high-value information.

The chain of trust in Persona has several attack points:

The Public Key: HTTPS Fetch

When the user submits a login “assertion”, the website (Relying Party or RP) fetches the public key of the email provider (Identity Provider or IdP) using HTTPS. For instance, when I log in as, the site I’m logging into will fetch This relies on the public key and CA infrastructure of the internet. Attacking this part of the chain is hard because it’s the network connection between two servers. This doesn’t appear to be a significant risk factor to me except for perhaps some state actors.

The Public Key: Attacking the IdP HTTPS Server

Attacking the email provider’s web server, on the other hand, becomes a very high value proposition. If an attacker can replace the .well-known/browserid file on a major email provider (gmail, yahoo, etc) they have the ability to impersonate every user of that service. This puts a huge responsibility on email providers to monitor and secure their HTTPS site, which may not typically be part of their email system at all. It is likely that this kind of intrusion will cause signin problems across multiple users and will be detected, but there is no guarantee that individual users will be aware of the compromise of their accounts.

Signing: Accessing the IdP Signing System

Persona email providers can silently impersonate any of their users just by the nature of the protocol. This opens the door to silent identity attacks by anyone who can access the private key of the identity/email provider. This can either be subverting the signing server, or by using legal means such as subpoenas or national security letters. In these cases, the account compromise is almost completely undetectable by either the user or the RP.

What About Password-Reset Emails?

One common defense of Persona is that email providers already have access to users account via password-reset emails. This is partly true, but it ignores an essential property of these emails: when a password is reset, a user will be aware of the attack then next time they try to login. Being unable to login will likely trigger a cautious user to review the details of their account or ask for an audit. Attacks against the IdP, on the other hand, are silent and are not as likely to trigger alarm bells.

Who Should Use Persona?

Persona is a great system for the multitude of lower-value accounts people keep on the internet. Persona is the perfect solution for the Mozilla Status Board. I wish the UI were better and built into the browser: the current UI that requires JS, shim libraries, and popup windows; it is not a great experience. But the tradeoff for not having to store and handle passwords on the server is worth that small amount of pain.

For any site with high-value data, Persona is not a good choice. On, we disabled password reset emails for users with access to security bugs. This decision indicates that persona should also be considered an unacceptable security risk for these users. Persona as a protocol doesn’t have the right security properties.

It would be very interesting to combine Persona with some other authentication system such as client certificates or a two-factor system. This would allow most users to use the simple login system, while providing extra security properties when users start to access high-value resources.

In the meantime, Mozilla should be careful how it promotes and uses Persona; it’s not a universal solution and we should be careful not to bill it as one.