The bug that hides from breakpoints

This is the story of the most difficult bug I ever had to solve. See if you can figure it out before the conclusion.


For some years now, I’ve worked on a kernel for Texas Instruments calculators called KnightOS. This kernel is written entirely in assembly, and targets the old-school z80 processor from back in 1976. This classic processor was built without any concept of protection rings. It’s an 8-bit processor, with 150-some instructions and (in this application) 32K of RAM and 32K of Flash. This stuff is so old, I ended up writing most of the KnightOS toolchain from scratch rather than try to get archaic assemblers and compilers running on modern systems.

When you’re working in an enviornment like this, there’s no seperation between kernel and userland. All “userspace” programs run as root, and crashing the entire system is a simple task. All the memory my kernel sets aside for the process table, or memory ownership, file handles, stacks, any other executing process - any program can modify this freely. Of course, we have to rely on the userland to play nice, and it usually does. But when there are bugs, they can be a real pain in the ass to hunt down.

The elusive bug

The original bug report: When running the counting demo and switching between applications, the thread list graphics become corrupted.

I can reproduce this problem, so I settle into my development enviornment and I set a breakpoint near the thread list’s graphical code. I fire up the emulator and repeat the steps… but it doesn’t happen. This happened consistently: the bug was not reproduceable when a breakpoint was set. Keep in mind, I’m running this in a z80 emulator, so the enviornment is supposedly no different. There’s no debugger attached here.

Though this is quite strange, I don’t immediately despair. I try instead setting a “breakpoint” by dropping an infinite loop in the code, instead of a formal breakpoint. I figure that I can halt the program flow manually and open the debugger to inspect the problem. However, the bug wouldn’t be tamed quite so easily. The bug was unreproducable when I had this psuedo-breakpoint in place, too.

At this point, I started to get a little frustrated. How do I debug a problem that disappears when you debug it? I decided to try and find out what caused it after it had taken place, by setting the breakpoint to be hit only after the graphical corruption happened. Here, I gained some ground. I was able to reproduce it, and then halt the machine, and I could examine memory and such after the bug was given a chance to have its way over the system.

I discovered the reason the graphics were being corrupted. The kernel kept the length of the process table at a fixed address. The thread list, in order to draw the list of active threads, looks to this value to determine how many threads it should draw. Well, when the bug occured, the value was too high! The thread list was drawing threads that did not exist, and the text rendering puked non-ASCII characters all over the display. But why was that value being corrupted?

It was an oddly specific address to change. None of the surrounding memory was touched. Making it even more odd was the very specific conditions this happened under - only when the counting demo was running. I asked myself, “what makes the counting demo unique?” It hit me after a moment of thought. The counting demo existed to demonstrate non-supsendable threads. The kernel would stop executing threads (or “suspend” them) when they lost focus, in an attempt to keep the system’s very limited resources available. The counting demo was marked as non-suspendable, a feature that had been implemented a few months prior. It showed a number on the screen that counted up forever, and the idea was that you could go give some other application focus, come back, and the number would have been counting up while you were away. A background task, if you will.

A more accurate description of the bug emerged: “the length of the kernel process table gets corrupted when launching the thread list when a non-suspendable thread is running”. What followed was hours and hours of crawling through the hundreds of lines of assembly between summoning the thread list, and actually seeing it. I’ll spare you the details, because they are very boring. We’ll pick the story back up at the point where I had isolated the area in which it occured: applib.

The KnightOS userland offered “applib”, a library of common functions applications would need to get the general UX of the system. Among these was the function applibGetKey, which was a wrapper around the kernel’s getKey function. The idea was that it would work the same way (return the last key pressed), but for special keys, it would do the appropriate action for you. For example, if you pressed the F5 key, it would suspend the current thread and launch the thread list. This is the mechanism with which most applications transfer control out of their own thread and into the thread list.

Eager that I had found the source of the issue, I placed a breakpoint nearby. That same issue from before struck again - the bug vanished when the breakpoint was set. I tried a more creative approach: instead of using a proper breakpoint, I asked the emulator to halt whenever that address was written to. Even still - the bug hid itself whenever this happened.

I decided to dive into the kernel’s getKey function. Here’s the start of the function, as it appeared at the time:

    call hasKeypadLock
    jr _
    xor a
_:  push bc
; ...

I started going through this code line-by-line, trying to see if there was anything here that could concievably touch the thread table. I noticed a minor error here, and corrected it without thinking:

    call hasKeypadLock
    jr z, _
    xor a
_:  push bc
; ...

The simple error I had corrected: getKey was pressing forward, even when the current thread didn’t have control of the keyboard hardware. This was a silly error - only two characters were omitted.

A moment after I fixed that issue, the answer set in - this was the source of the entire problem. Confirming it, I booted up the emulator with this change applied and the bug was indeed resolved.

Can you guess what happened here? Here’s the other piece of the puzzle to help you out, translated more or less into C for readability:

int applibGetKey() {
    int key = getKey();
    if (key == KEY_F5) {
    return key;

Two more details you might not have picked up on:

  • applibGetKey is non-blocking
  • suspend_thread suspends the current thread immediately, so it doesn’t return until the thread resumes.

The bug, uncovered

Here’s what actually happened. For most threads (the suspendable kind), that thread stops processing when suspend_thread() is called. The usually non-blocking applibGetKey function blocks until the thread is resumed in this scenario. However, the counting demo was non-suspendable. The suspend_thread function has no effect, by design. So, suspend_thread did not block, and the keypress was returned straight away. By this point, the thread list had launched properly and it was given control of the keyboard.

However, the counting demo went back into its main loop, and started calling applibGetKey again. Since the average user’s finger remained pressed against the button for a few moments more, applibGetKey continued to launch the thread list, over and over. The thread list itself is a special thread, and it doesn’t actually have a user-friendly name. It was designed to ignore itself when it drew the active threads. However, it was not designed to ignore other instances of itself, the reason being that there would never be two of them running at once. When attempting to draw these other instances, the thread list started rendering text that wasn’t there, causing the corruption.

This bug vanished whenever I set a breakpoint because it would halt the system’s keyboard processing logic. I lifted my finger from the key before allowing it to move on.

The solution was to make the kernel’s getKey function respect hardware locks by fixing that simple, two-character typo. That way, the counting demo, which had no right to know what keys were being pressed, would not know that they key was still being pressed.

The debugging described by this blog post took approximately three weeks.