Anatomy of a Compiler Bug | Hacker News Reader

Anatomy of a Compiler Bug

mikeash.com

112 pointsnealyoung20 comments

I used to work on compilers, C/C++, COBOL, PL/I, Java, a whole bunch of them. This is actually somewhat similar to my favorite bug I encountered while implementing some stack mapping optimizations.

The compiler itself was crashing out while compiling a SPEC2000 test case (perlbmk I think?) with an illegal instruction. This was already quite suspect since it was branching to somewhere WAY outside of where the program usually resides, and the compiler was compiled with a compiler that's known to be nearly rock solid. I got quite lucky in that I managed to find one level of the stack trace, and it pointed me towards sprintf. Using some awesome tools some coworkers and I had developed over the years, I managed to narrow down the test case to about 5 lines of code that involved long doubles. So I grepped the compiler source code for sprintf, set breakpoints on the ones that I thought would get called, and just kept stepping through them until it finally crashed hard. Then I just reran, and stopped at the final breakpoint and started stepping through the assembly. What I saw happen just blew my mind, the code was just a simple:

sprintf(buffer, "fold: %Lf", result);

But what was happening is that the buffer was only 200 characters long, and the long double was roughly 1000 characters long. It was just a buffer overflow, that was so long it ended up overwriting the register save area, and the return address pointer. So the sprintf completed, but when it went to branch back, it loaded some characters instead of the return address. Just hilarious, and good thing I was working on stack mapping and was familiar with the stack layout of this linkage convention.

The solution of course was to just use snprintf instead. No sorry, that's wrong since that platform doesn't have an snprintf (yay mainframes!), and so I had to use %0.6Lg instead of %Lf.

Compilers are fun!

Why doesn't the standard library warn when it knows %Lf might be too long?

sprintf does not know the size of the destination buffer. That's why the newer snprintf, which takes a size as a 2nd parameter, is recommended.

(I say newer, but according to manpages snprintf is defined by SUSv2, from 1997, and C99, from 1999; so they're pretty old by now.)

You could probably have the compiler warn, provided (1) the destination is still an array and not a pointer (2) the format string is known at compile time. But that's a very different thing. (Most compilers these days do warn about any calls to sprintf, recommending snprintf instead.)

> newer snprintf

Also see C11's sprintf_s() and similarly-named friends.

This is one area where C11 seems like it solves pointless non-problems. snprintf() is not all that "unsafe" in that it does do a bounds check, and the C99 version is clear about saying that the null terminator is included in the count. I also like the error conditions of C99 snprintf() better - it's more interesting to know the exact size required than it is to know that my buffer is not big enough.

> that platform doesn't have an snprintf (yay mainframes!)

One trick I did once when I wrote code somewhere that didn't have snprintf: create a pipe, fprintf into it, and only read at most N bytes back.

It worked and was portable but I'm sure the performance was horrible; this wasn't anything professional, I was just messing around as a kid (back when it was more common to come across platforms that hadn't gotten to SUSv2 or C99 yet). Probably a better solution would be to steal an implementation from an open source libc.

Not sure why that was downvoted. I'm sorry I offended you, downvoter, but in my defense I've seen much more inane comments not downvoted here.

Great analysis and tenacity in hunting this one down :)

Letting new compilers loose on existing codebases is always fun and you learn lots of things in the process, I can only recommend it. I debugged a problem once that also had to do with interfacing runtime-generated code with compiletime-generated code. There were differences in the expectations of the ABI, which is described in this bug:

http://llvm.org/bugs/show_bug.cgi?id=12207

It only surfaced when compiling the codebase with clang (previously gcc). Took quite some digging to find the problem.

Great analysis, indeed. I miss one thing though: what was causing the incomplete printing of 'Testing' in the loop?

That's a good question. I just attributed that to general corruption during the run, but never checked it out in detail. Presumably the calculation of the string pointer was sometimes being offset by a few bytes somehow, but I don't know exactly why.

Thanks for sharing -- I love puzzles like that.

Just curious: Were the lldb session snippets taken from the original debugger session? I keep getting weird looks when I use a command-line debugger just to have a transcript afterwards. (The weird looks being from people who'd rather send me a screenshot of their GUI debugger's call stack.)

If you mean the original but from last year or so, no. I didn't even have it on my computer, and was kind of remote-debugging through my friend to figure it out. It only happened on 10.6, and I didn't have that handy, so I couldn't check it out locally.

Nice article. But I wish it had gotten into an explanation of the actual bug in LLVM's code. Anyone have a bug number?

I wanted to get into that as well, but ran out of brainpower. This is supposedly the revision that fixes the bug:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-...

Would love to hear your conclusions about what the ultimate cause was if you get that far.

Haha, it's almost like some detective short story.

Surprisingly it wasn't as hard to follow as I thought. Maybe I'm starting to get good at this Computer Science thing.

Must be my fantastic writing.

Seriously though, assembly isn't all that hard, mentally. It's difficult in the same way that unloading a truck full of sand with your bare hands is difficult. Which is to say, it takes a long time, but all it really needs is time, not deep thought.

People get scared away from it because they don't know where to start with it, or because it looks really hard, or because it just takes too much time to understand what's going on, but it can be really rewarding.

Or because they don't see immediate value in it, as typical programming isn't done in ASM nowadays. I think it's worthwhile just to "demystify" computers, but since embedded programming, demos and retro gaming are part of my interests, I get practical benefits out of it as well.

True, but when it comes in handy it can be immensely so. I was looking at a customer crash dump at work the other day and the callstack shown by two different debuggers didn't make any sense, it was showing calls sequences the code clearly didn't make. Looking at the raw stack in memory and using the disassembly to help see the layout of the frames made it (relatively) easy to see there were a couple of calls in the sequence that both debuggers simply weren't showing. Once I had the real sequence and the dissassembly to see how the frames evolved and recompute call targets for earlier in the frames life it became a lot easier to see what was going wrong. If I hadn't been able to do that (and my ASM skills are definitely not strong) this would have been a No Repro (and hence no fix) bug for sure.

Being old enough have coded full applications in plain Assembly, I think it is a lost that not many developers know Assembly nowadays.

At least to be able to grasp what a piece of code might do, not necessarily to write code.

I think the hard part mentally (that your incredibly great explanation completely alleviated) is trying to guess at the meaning of the interplay of all the instructions, addressing modes, register overlaps, and calling conventions. It seems like the only way to turn that from difficult (impossible) guess-work to mere sand-unloading drudgery is to either have a lot of it memorized already, or to have a very strong command over the various documentation resources.

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