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LaurieWired Profile picture
@lauriewired
Dec 26, 2024 • 5 tweets • 2 min read • Read on X
“My wife complains that open office will never print on Tuesdays”

A bizarre sentence; which kicked off one of the most interesting bug hunts in Ubuntu’s history.

It all starts with some goofy pattern matching. Image
It’s not a bug with the printer, or OpenOffice, or the printer driver.

It’s a mistake in the way the “file” utility parses file signatures.

When printing from OpenOffice, a PostScript file is created with the creation date. Image
CUPS, the Common Unix Printing System, then uses the file utility as part of its pipeline to determine the type.

But if "Tue" appears at byte 4 (Tuesday's creation date), it's mistakenly identified as an Erlang JAM file, causing the print job to fail.
Essentially, the logic in the file utility for recognizing Erlang JAM files were too broad.

Looking for a single static string position is an extremely fragile detection method.

Simply changing the creation date to “XTue” completely solves the problem! Image
After some back and forth, the logic in the file utility was improved, and the bug was fixed.

It’s a good lesson for us programmers to not blindly dismiss “implausible” sounding issues.

Next time someone makes an outlandish claim; take a closer look. You might just be onto a bizarre edge case!

Here’s the original bug report:
bugs.launchpad.net/ubuntu/+source…

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More from @lauriewired

LaurieWired Profile picture
Sep 11
How do you program an unknown CPU?

The original specs are gone; no compilers exist, and the ISA is completely unrecognized.

It happens more often than you think, behind very closed doors.

It's almost always military hardware. Image
Image
There is *one* glimpse of this that I know of in the wild.

In 2012, two Russian PhDs researchers gave a presentation at RECon.

Titled quite innocuously, they were tasked with reversing a single, unknown binary.

No hardware. No datasheet. No Documentation. Image
Details about *why* they were doing this are sparse.

No, they aren't reversing Alien CPUs.

If I had to guess, they were likely reversing a Soviet, cold-war era clone of the PDP-11.

Smells like an Avionics / Defense MCU. Image
Image
Read 4 tweets
LaurieWired Profile picture
Sep 3
It’s time to get rid of frame rates.

In weird corners of the internet, researchers and standards committees discuss frameless video containers.

Sensor data as a continuous function, down-sampled to any frame rate you want.

Here's what it'll look like in 10 years: Image
Image
There’s two schools of thought, depending on the crowd you hang around.

NeurIPS folks tend to like continuous-time fields (software).

Hardcore EE types discuss event-based sensing (hardware, timestamps).

Bear with me, it's easier than it sounds: Image
Image
Consumers are gonna see continuous-time fields first.

It doesn’t require new cameras or hardware.

Take a video, re-encode frames into “extracted features”, decode with a ML model, refit to any framerate.

Popular topic at NeurIPS, MPEG Standard is already discussing it. Image
Read 5 tweets
LaurieWired Profile picture
Sep 2
Much like humans, CPUs heal in their sleep.

CPUs are *technically* replaceable / wear items. They don’t last forever.

Yet, the moment stress is removed, transistor degradation (partially) reverses.

It's called Bias Temperature Instability (BTI) recovery: Image
Image
Transistors are little switches.

When you hold a switch on, especially when it’s hot, a bit of charge gets stuck where it shouldn’t.

Every time that happens, it gets a little bit harder to switch.

In other words, the transistor gets a little “lazier”. Image
Over 10 years, in a modern processor, the ALU can slow down 6%!

FPGAs get hit even harder. Run it hard (slightly over-volted), and you’re looking at a few % a year of slowdown.

Not something the average user would notice, but definitely has to be accounted for. Image
Image
Read 5 tweets
LaurieWired Profile picture
Aug 29
I’ve been on a filesystem kick, and it’s interesting to see the DNA of older ideas pop up in modern designs.

BeFS was crazy in the 90s; the whole architecture was basically a searchable database.

Skimming through their book…it sounds a lot like current Apple FS design. Image
Image
Turns out there’s a good reason for that.

“Practical File System Design”, was written by Dominic Giampaolo in 1999, for BeOS.

Giampaolo joined Apple in 2002, where he became lead architect for…APFS.

Released in 2016, it's funny to see the same ideas 17 years later. Image
Image
In some ways, BeFS is still the more “modern” filesystem!

BeFS embedded search into the FS itself, Apple keeps the indexing+search layer separate.

Both love B-trees, per-file metadata, and 64-bit structures.

It's really not *that* different. Image
Read 4 tweets
LaurieWired Profile picture
Aug 21
Why do most uninterruptible power supplies still use old, lead-acid battery tech?

Nearly every battery in your house (phone, watch, even electric car) is lithium based...except UPSs.

It all has to do with battery chemistry. Lead-Acid has some unique advantages: Image
Image
Contrary to what you might think; lithium batteries are not a “straight upgrade”.

Lead-Acid handles being “floated” at near ~100% capacity for years.

Considering UPS’s spend 99.9% of their life sitting at full charge…waiting for an outage, it's an ideal use-case. Image
Lithium-based cells *hate* sitting at 100%.

The thermal management, per-cell protection, and balancing electronics are significantly more complex than lead-acid’s simple float charge.

Lithium UPSs do exist, but they are pricey and make some huge compromises. Image
Read 4 tweets
LaurieWired Profile picture
Aug 19
The West has a blindspot when it comes to alternative CPU designs.

We’re so entrenched in the usual x86, ARM, RISC-V world, that most people have no idea what’s happening over in China.

LoongArch is a fully independent ISA that’s sorta MIPS…sorta RISC-V…and sorta x87! Image
Image
Of course, Loongson (the company) realizes that most software is compiled for x86 and ARM.

Thus, they decided to add some hefty translation layers (LBT) built into the hardware.

LBT gives you for extra scratch registers, x86+ARM eflags, and an x87(!) stack pointer. Image
LoongArch is a *hefty* ISA; about ~2,000 instructions.
To put it in perspective, base RISC-V is like 50.

That said, it’s pretty clean to read. All instructions are 32 bits, and there are only 9 possible formats.

Certainly easier to decipher than modern x86. Image
Read 4 tweets

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