Pwn2Own this Power Mac (plus: IonPower's time is running out)

All of the mighties fell at this year's Pwn2Own, including Firefox, where clever minds find gaping holes for fun and a small amount of profit (but then it also makes a great resume builder, which may be a more reliable paycheque). The holes last year were quickly patched, and after I determined we were also vulnerable we followed suit. As usual, for this year's naughtiness Mozilla already has patched versions available, including ESR 31.5.3.

However, the two holes used for this year's marvelous and somewhat alarming crack are not exploitable in TenFourFox directly: the SVG navigation fault cannot be effectively used to escalate privileges in TenFourFox's default configuration, and we don't even build the code that uses JavaScript bounds checking. The navigation fault may have other weaponizeable vectors and we do want to roll that fix, but the good news is 31.6 will come out this weekend, so no urgent chemspill is necessary unless I discover some odd way of busting through it between now and then.

I lost about a week of hacking time to one of my occasional bouts of bronchitis, which is pushing IonPower's timing very close to the wire. We need two cycles for 38 to allow localizers to catch up and people to test, and of course somewhere in that timeframe we also have to finish the move from Eric Schmidt is a Poopypants Land Google Code. Over the weekend I got IonPower to pass the test suite in Baseline mode, which is very encouraging, but some of the same problems that doomed PPCBC's Ion work are starting to rear up again.

The biggest problem that recurred is an old one: Ion's allocator is not endian-safe. I get bad indicies off it for stack slots and other in-memory boxed values and all but the simplest scripts either assert deep within Ion's bowels (not our new PowerPC backend) or generate code that is verifiably wrong. Unfortunately, Mozilla doesn't really document Ion's guts anywhere, so I don't know where to start with fixing it, and all the extant Ion backends, even MIPS, are little-endian. Maybe some Mozilla JIT folks are reading and can comment? (See also the post in the JavaScript engine internals group.)

One old problem with bad bailout stack frames, however, is partially solved with IonPower. I say partially because even though the stack frame is sane now, it still crashes, but I have a few ideas about that. However, the acid test is getting Ion code to jump to Baseline, run a bit in Baseline, and then jump back to Ion to finish execution. PPCBC could never manage this without crashing. If IonPower can do no better, there is no point in continuing the porting effort.

Even if this effort runs aground again, that doesn't make IonPower useless. PPCBC may pass the test suite, but some reproducible bugs in Tenderapp indicate that it goes awry in certain extremely-tough-to-debug edge cases, and IonPower (in Baseline mode) does pass the test suite as well now. If I can get IonPower to be as fast or faster than PPCBC even if it can't execute Ion code either, we might ship it anyway as "PPCBC II" in Baseline-only mode to see if it fixes those problems -- I have higher confidence that it will, because it generates much more sane and "correct" output and doesn't rely on the hacks and fragile glue code that PPCBC does in 24 and 31. I have to make this decision sometime mid-April, though, because we're fast approaching EOL for 31.

Also, as of Firefox 38 Mozilla no longer supports gcc 4.6, the compiler which we build with. However, I'm not interested in forcing a compiler change so close to the next ESR, and it appears that we should still be able to get it working on 4.6 with some minor adjustments. That won't be the case for Fx39, if we're even going to bother with that, but fortunately there is a gcc 4.8 in MacPorts and we might even use Sevan's gcc from pkgsrc. Again, the decision to continue will be based on feasibility and how close Electrolysis is to becoming mandatory before 45ESR, which is the next jump after that. For now, TenFourFox 38 is the highest priority.