The Universe of Discourse

Thu, 16 Nov 2017

Another system software error

[ Warning: This article is meandering and does not end anywhere in particular ]

My recent article about system software errors kinda blew up the Reddit / Hacker News space, and even got listed on Voat, which I understand is the Group W Bench where they send you if you aren't moral enough to be in Reddit. Many people on these fora were eager to tell war stories of times that they had found errors in the compiler or other infrastructural software.

This morning I remembered another example that had happened to me. In the middle 1990s, I was just testing some network program on one of the Sun Solaris machines that belonged to the Computational Linguistics program, when the entire machine locked up. I had to go into the machine room and power-cycle it to get it to come back up.

I returned to my desk to pick up where I had left off, and the machine locked up, again just as I ran my program. I rebooted the machine again, and putting two and two together I tried the next run on a different, less heavily-used machine, maybe my desk workstation or something.

The problem turned out to be a bug in that version of Solaris: if you bound a network socket to some address, and then tried to connect it to the same address, everything got stuck. I wrote a five-line demonstration program and we reported the bug to Sun. I don't know if it was fixed.

My boss had an odd immediate response to this, something along the lines that connecting a socket to itself is not a sanctioned use case, so the failure is excusable. Channeling Richard Stallman, I argued that no user-space system call should ever be able to crash the system, no matter what stupid thing it does. He at once agreed.

I felt I was on safe ground, because I had in mind the GNU GCC bug reporting instructions of the time, which contained the following unequivocal statement:

If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash.

I love this paragraph. So clear, so pithy! And the second sentence! It could have been left off, but it is there to articulate the writer's moral stance. It is a rock-firm committment in a wavering and uncertain world.

Stallman was a major influence on my writing for a long time. I first encountered his work in 1985, when I was browsing in a bookstore and happened to pick up a copy of Dr. Dobb's Journal. That issue contained the very first publication of the GNU Manifesto. I had never heard of Unix before, but I was bowled over by Stallman's vision, and I read the whole thing then and there, standing up.

(It hit the same spot in my heart as Albert Szent-Györgyi's The Crazy Ape, which made a similarly big impression on me at about the same time. I think programmers don't take moral concerns seriously enough, and this is one reason why so many of them find Stallman annoying. But this is what I think makes Stallman so important. Perhaps Dan Bernstein is a similar case.)

I have very vague memories of perhaps finding a bug in gcc, which is perhaps why I was familiar with that particular section of the gcc documentation. But more likely I just read it because I read a lot of stuff. Also Stallman was probably on my “read everything he writes” list.

Why was I trying to connect a socket to itself, anyway? Oh, it was a bug. I meant to connect it somewhere else and used the wrong variable or something. If the operating system crashes when you try, that is a bug. Reliable operating systems never crash.

[ Final note: I looked for my five-line program that connected a socket to itself, but I could not find it. But I found something better instead: an email I sent in April 1993 reporting a program that caused g++ version 2.3.3 to crash with an internal compiler error. And yes, my report does quote the same passage I quoted above. ]

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