Using Rust to generate Mercurial short-hash collisions

At Mozilla, we use Mercurial for the main Firefox repository. Mercurial, like Git, uses SHA1 hashes to identify a commit.

Short hashes

SHA1 hashes are fairly long, a string of 40 hex characters (160 bits), so Mercurial and Git allow using a prefix of that, as long as the prefix is unambiguous. Mercurial also typically only shows the first 12 characters (let’s call them short hashes), for instance:

$ hg id
$ hg log -r tip
changeset:   242221:312707328997
tag:         tip

And those are the hashes most Mercurial users use, for instance they are posted in Bugzilla whenever we land a patch etc.

Collisions with short hashes are much more likely than full SHA1 collisions, because the short hashes are only 48 bits long. As the Mercurial FAQ states, such collisions don’t really matter, because Mercurial will check if the hash is unambiguous and if it’s not it will require more than 12 characters.

So, short hash collisions are not the end of the world, but they are inconvenient because the standard 12-chars hg commit ids will become ambiguous and unusable. Fortunately, the mozilla-central repository at this point does not contain any short hash collisions (it has about 242,000 commits).

Finding short-hash collisions

I’ve wondered for a while, can we create a commit that has the same short hash as another commit in the repository?

A brute force attack that works by committing and then reverting changes to the repository should work, but it’d be super slow. I haven’t tried it, but it’d probably take years to find a collision. Fortunately, there’s a much faster way to brute force this. Mercurial computes the commit id/hash like this:

hash = sha1(min(p1, p2) + max(p1, p2) + contents)

Here p1 and p2 are the hashes of the parent commits, or a null hash (all zeroes) if there’s only one parent. To see what contents is, we can use the hg debugdata command:

$ hg debugdata -c 34828fed1639
Carsten "Tomcat" Book <>
1430739274 -7200
...list of changed files...

merge mozilla-inbound to mozilla-central a=merge

Perfect! This contains the commit message, so all we have to do is append some random data to the commit message, compute the (short) hash, check if there’s a collision and repeat until we find a match.

I wrote a small Rust program to brute-force this. You can use it like this (I used the popular mq extension, there are other ways to do it):

$ cd mozilla-central
$ echo "Foo" >> CLOBBER # make a random change
$ hg qnew patch -m "Some message"
$ hgcollision
Got 242223 prefixes
Generated random prefix: 1631965792_
Tried 242483200 hashes
Found collision! Prefix: b991f0726738, hash: b991f072673876a64c7a36f920b2ad2885a84fac
Add this to the end of your commit message: 1631965792_24262171

After about 2 minutes it’s done and tells us we have to append “1631965792_24262171″ to our commit message to get a collision! Let’s try it (we have to be careful to preserve the original date/time, or we’ll get a different hash):

$ hg log -r tip --template "{date|isodatesec}"
2015-05-05 20:21:59 +0200
$ hg qref -m "Some message1631965792_24262171" -d "2015-05-05 20:21:59 +0200"
$ hg id
b991f0726738 patch/qbase/qtip/tip
$ hg log -r b991f0726738
abort: 00changelog.i@b991f0726738: ambiguous identifier!

Voilà! We successfully created a Mercurial short hash collision!

And no, I didn’t use this on any patches I pushed to mozilla-central.. 😉


The Rust source code is available here. It was my first, quick-and-dirty Rust program but writing it was a nice way to get more familiar with the language. I used the rust-crypto crate to calculate SHA1 hashes, installing and using it was much easier than I expected. Pretty nice experience.

The program can check about 100 million hashes in one minute on my laptop. It usually takes about 1-5 minutes to find a collision, this also depends on the size of the repository (mozilla-central has about 242,000 commits). It’d be easy to use multiple threads (you can also just use X processes though) and there are probably a lot of other ways to improve it. For this experiment it was good and fast enough to get the job done :)

Fast arrow functions in Firefox 31

Last week I spent some time optimizing ES6 arrow functions. Arrow functions allow you to write function expressions like this: => s.length);

Instead of the much more verbose:{ return s.length });

Arrow functions are not just syntactic sugar though, they also bind their this-value lexically. This means that, unlike normal functions, arrow functions use the same this-value as the script in which they are defined. See the documentation for more info.

Firefox has had support for arrow functions since Firefox 22, but they used to be slower than normal functions for two reasons:

  1. Bound functions: SpiderMonkey used to do the equivalent of |arrow.bind(this)| whenever it evaluated an arrow expression. This made arrow functions slower than normal functions because calls to bound functions are currently not optimized or inlined in the JITs. It also used more memory because we’d allocate two function objects instead of one for arrow expressions.
    In bug 989204 I changed this so that we treat arrow functions exactly like normal function expressions, except that we also store the lexical this-value in an extended function slot. Then, whenever this is used inside the arrow function, we get it from the function’s extended slot. This means that arrow functions behave a lot more like normal functions now. For instance, the JITs will optimize calls to them and they can be inlined.
  2. Ion compilation: IonMonkey could not compile scripts containing arrow functions. I fixed this in bug 988993.

With these changes, arrow functions are about as fast as normal functions. I verified this with the following micro-benchmark:

function test(arr) {
    var t = new Date;
    arr.reduce((prev, cur) => prev + cur);
    alert(new Date - t);
var arr = [];
for (var i=0; i<10000000; i++) {

I compared a nightly build from April 1st to today’s nightly and got the following results:

We’re 64x faster because Ion is now able to inline the arrow function directly without going through relatively slow bound function code on every call.

Other browsers don’t support arrow functions yet, so they are not used a lot on the web, but it’s important to offer good performance for new features if we want people to start using them. Also, Firefox frontend developers love arrow functions (grepping for “=>” in browser/ shows hundreds of them) so these changes should also help the browser itself :)

Using segfaults to interrupt JIT code

(I just installed my own blog so I decided to try it out by writing a bit about interrupt checks :) )

Most browsers allow the user to interrupt JS code that runs too long, for instance because it’s stuck in an infinite loop. This is especially important for Firefox as it uses a single process for chrome and content (though that’s about to change), so without this dialog a website could hang the browser forever and the user is forced to kill the browser and could lose work. Firefox will show the slow script dialog when a script runs for more than 10 seconds (power users can customize this).


Firefox uses a separate (watchdog) thread to interrupt script execution. It triggers the “operation callback” (by calling JS_TriggerOperationCallback) every second. Whenever this happens, SpiderMonkey promises to call the operation callback as soon as possible. The browser’s operation callback then checks the execution limit, shows the dialog if necessary and returns true to continue execution or false to stop the script.

How this works internally is that JS_TriggerOperationCallback sets a flag on the JSRuntime, and the main thread is responsible for checking this flag every now and then and invoke the operation callback if it’s set. We check this flag for instance on JS function calls and loop headers. We have to do this both for scripts running in the interpreter and the JITs, of course. For example, consider this function:

function f() {
    for (var i=0; i<100000000; i++) {

Until Firefox 26, IonMonkey would emit the following code for this loop:

   0x228a3b9:	cmpl   $0x0,0x2b764ec    // interrupt check
   0x228a3c0:	jne    0x228a490
   0x228a3c6:	cmp    $0x5f5e100,%edx
   0x228a3cc:	jge    0x228a3da
   0x228a3d2:	add    $0x1,%edx
   0x228a3d5:	jmp    0x228a3b9

Note that the loop itself is only 4 instructions, but we need 2 more instructions for the interrupt check. These 2 instructions can measurably slow down tight loops like this one. Can we do better?


OdinMonkey is our ahead-of-time (AOT) compiler for asm.js. When developing Odin, we (well, mostly Luke) tried to shave off as much overhead as possible, for instance we want to get rid of bounds checks and interrupt checks if possible to close the gap with native code. The result is that Odin does not emit loop interrupt checks at all! Instead, it makes clever use of signal handlers.

When the watchdog thread wants to interrupt Odin execution on the main thread, it uses mprotect (Unix) or VirtualProtect (Windows) to clear the executable bit of the asm.js code that’s currently executing. This means any asm.js code running on the main thread will immediately segfault. However, before the kernel terminates the process, it gives us one last chance to interfere: because we installed our own signal handler, we can trap the segfault and, if the address is inside asm.js code, we can make the signal handler return to a little trampoline that calls the operation callback. Then we can either jump back to the faulting pc or stop execution by returning from asm.js code. (Note that handling segfaults is serious business: if the faulting address is not inside asm.js code, we have an unrelated, “real” crash and we must be careful not to interfere in any way, so that we don’t sweep real crashes under the rug.)

This works really well and is pretty cool: asm.js code has no runtime interrupt checks, just like native code, but we can still interrupt it and show our slow script dialog.


A while later, Brian Hackett wanted to see if we could make IonMonkey (our optimizing JIT) as fast as OdinMonkey on asm.js code. This means he also had to eliminate interrupt checks for normal JS code running in Ion (we don’t bother doing this for our Baseline JIT as we’ll spend most time in Ion code anyway).

The first thought is to do exactly what Odin does: mprotect all Ion-code, trigger a segfault and return to some trampoline where we handle the interrupt. It’s not that simple though, because Ion-code can modify the GC heap. For instance, when we store a boxed Value, we emit two machine instructions on 32-bit platforms, to store the type tag and the payload. If we use signal handlers the same way Odin does, it’s possible we store the type tag but are interrupted before we can store the payload. Everything will be fine until the GC traces the heap and crashes horribly. Even worse, an attacker could use this to access arbitrary memory.

There’s another problem: when we call into C++ from Ion code, the register allocator tracks GC pointers stored in registers or on the stack, so that the garbage collector can mark them. If we call the operation callback at arbitrary points though, we don’t have this information. This is a problem because the operation callback is also used to trigger garbage collections, so it has to know where all GC pointers are.

What Brian implemented instead is the following:

  1. The watchdog thread will mprotect all Ion code (we had to use a separate allocator for Ion code so that we can do this efficiently).
  2. The main thread will segfault and call our signal handler.
  3. The signal handler unprotects all Ion-code again and patches all loop backedges (jump instructions) to jump to a slow, out-of-line path instead.
  4. We return from the signal handler and continue execution until we reach the next (patched) loop backedge and call the operation callback, show the slow script dialog, etc.
  5. All loop backedges are patched again to jump to the loop header.

Note that this only applies to the loop interrupt check: there’s another interrupt check when we enter a script, but for JIT code we combine it with the stack overflow check: when we trigger the operation callback, the watchdog thread also sets the stack limit to a very high value so that the stack check always fails and we also end up in the VM where we can handle the operation callback and reset the stack limit :)


Firefox 26 and newer uses signal handlers and segfaults for interrupting Ion code. This was a measurable speedup, especially for tight loops. For example, the empty for-loop I posted earlier runs 33% faster (43 ms to 29 ms). It helps more interesting loops as well, for instance Octane-crypto got ~8% faster.