Daily Archives: June 20, 2022

Lessons of Social Media from BBSs

In the recent article The Internet Origin Story You Know Is Wrong, I was somewhat surprised to see the argument that BBSs are a part of the Internet origin story that is often omitted. Surprised because I was there for BBSs, and even ran one, and didn’t really consider them part of the Internet story myself. I even recently enjoyed a great BBS documentary and still didn’t think of the connection on this way.

But I think the argument is a compelling one.

In truth, the histories of Arpanet and BBS networks were interwoven—socially and materially—as ideas, technologies, and people flowed between them. The history of the internet could be a thrilling tale inclusive of many thousands of networks, big and small, urban and rural, commercial and voluntary. Instead, it is repeatedly reduced to the story of the singular Arpanet.

Kevin Driscoll goes on to highlight the social aspects of the “modem world”, how BBSs and online services like AOL and CompuServe were ways for people to connect. And yet, AOL members couldn’t easily converse with CompuServe members, and vice-versa. Sound familiar?

Today’s social media ecosystem functions more like the modem world of the late 1980s and early 1990s than like the open social web of the early 21st century. It is an archipelago of proprietary platforms, imperfectly connected at their borders. Any gateways that do exist are subject to change at a moment’s notice. Worse, users have little recourse, the platforms shirk accountability, and states are hesitant to intervene.

Yes, it does. As he adds, “People aren’t the problem. The problem is the platforms.”

A thought-provoking article, and I think I’ll need to buy the book it’s excerpted from!

Pipe Issue Likely a Kernel Bug

Saturday, I wrote in Pipes, deadlocks, and strace annoyingly fixing them about an issue where a certain pipeline seems to have a deadlock. I described tracing it into kernel code. Indeed, it appears to be kernel bug 212295, which has had a patch for over a year that has never been merged.

After continuing to dig into the issue, I eventually reported it as a bug in ZFS. One of the ZFS people connected this to an older issue my searching hadn’t uncovered.

rincebrain summarized:

I believe, if I understand the bug correctly, it only triggers if you F_SETPIPE_SZ when the writer has put nonzero but not a full unit’s worth in yet, which is why the world isn’t on fire screaming about this – you need to either have a very slow but nonzero or otherwise very strange write pattern to hit it, which is why it doesn’t come up in, say, the CI or most of my testbeds, but my poor little SPARC (440 MHz, 1c1t) and Raspberry Pis were not so fortunate.

You might recall in Saturday’s post that I explained that Filespooler reads a few bytes from the gpg/zstdcat pipeline before spawning and connecting it to zfs receive. I think this is the critical piece of the puzzle; it makes it much more likely to encounter the kernel bug. zfs receive is calls F_SETPIPE_SZ when it starts. Let’s look at how this could be triggered:

In the pre-Filespooler days, the gpg|zstdcat|zfs pipeline was all being set up at once. There would be no data sent to zfs receive until gpg had initialized and begun to decrypt the data, and then zstdcat had begun to decompress it. Those things almost certainly took longer than zfs receive’s initialization, meaning that usually F_SETPIPE_SZ would have been invoked before any data entered the pipe.

After switching to Filespooler, the particular situation here has Filespooler reading somewhere around 100 bytes from the gpg|zstdcat part of the pipeline before ever invoking zfs receive. zstdcat generally emits more than 100 bytes at a time. Therefore, when Filespooler invokes zfs receive and hooks the pipeline up to it, it has a very high chance of there already being data in the pipeline when zfs receive uses F_SETPIPE_SZ. This means that the chances of encountering the conditions that trigger the particular kernel bug are also elevated.

ZFS is integrating a patch to no longer use F_SETPIPE_SZ in zfs receive. I have applied that on my local end to see what happens, and hopefully in a day or two will know for sure if it resolves things.

In the meantime, I hope you enjoyed this little exploration. It resulted in a new bug report to Rust as well as digging up an existing kernel bug. And, interestingly, no bugs in filespooler. Sometimes the thing that changed isn’t the source of the bug!