I’m continuing to reflect on the past four years with Hubris — April Fool’s
Day was, appropriately enough, the fourth anniversary of the first Hubris user
program, and today is the fourth anniversary of the first kernel code. (I wrote
the user program first to help me understand what the kernel’s API wanted to
look like.)
Of all of Hubris’s design decisions, there’s one that gets a “wait what”
response more often than any other. It’s also proving to be a critical part of
the system’s overall robustness. In this post, I’ll take a look at our 13th and
oddest syscall, REPLY_FAULT.
We found a neat bug in Hubris this week. Like many bugs, it wasn’t a bug when
it was originally written — correct code became a bug as other things
changed around it.
I thought the bug itself, and the process of finding and fixing it, provided an
interesting window into our development process around Hubris. It’s very rare
for us to find a bug in the Hubris kernel, mostly because it’s so small. So I
jumped at the opportunity to write this one down.
This is a tale of how two features, each useful on its own, can combine to
become a bug. Read on for details.