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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
Oct 17
You’re (probably) measuring application performance wrong.

Humans have a strong bias for throughput.

"I can handle X requests per second."

Real capacity engineers use response-time curves. Image
Image
It all comes down to queueing theory.

Unfortunately, computers don’t degrade gracefully under load.

70% CPU is smooth sailing. 95% is a nightmare.

Programmers (incorrectly) focus on the absolute value, when really they should be looking at the derivative. Image
Image
Highways are the perfect real life example of this.

Traffic engineers study flow density, not overall vehicle counts.

A road handling 10,000 cars per hour (throughput) means nothing if the average speed drops to 5mph (response time).

Computers are the same. Image
Image
Read 4 tweets
LaurieWired Profile picture
Oct 14
GPU computing before CUDA was *weird*.

Memory primitives were graphics shaped, not computer science shaped.

Want to do math on an array? Store it as an RGBA texture.

Fragment Shader for processing. *Paint* the result in a big rectangle. Image
Image
As you hit the more theoretical sides of Computer Science, you start to realize almost *anything* can produce useful compute.

You just have to get creative with how it’s stored.

The math might be stored in a weird box, but the representation is still valid. Image
BrookGPU (Stanford) is widely considered the birth of a pre-CUDA GPGPU framework.

Virtualizing CPU-style primitives; it hid a lot of graphical “weirdness”.

By extending C with stream, kernel, and reduction constructs, GPUs started to act more like a co-processor. Image
Read 4 tweets
LaurieWired Profile picture
Oct 13
Colleges do a terrible job of teaching C++.

It’s not “C with Classes”. Injected into curriculums as a demonstration of early CS concepts, it leaves many with a sour taste.

Students later immediately fall in love with the first language that *doesn’t* feel that way. Image
Admittedly, professors are in a tough spot.

To teach the concept, you fundamentally have to constrain the scope of the language. Many schools choose C++ out of practicality.

Controversially, I think toy languages that *aren't* industry standards are better suited for this. Image
Imagine learning the fundamentals of carpentry, but for teaching reasons, an otherwise reputable brand is artificially constrained to hand tools.

Of course, the moment a student jumps into the real world, and experiences their first power tool, it will blow their mind! Image
Image
Read 5 tweets
LaurieWired Profile picture
Oct 3
DDR5 is unstable garbage.

Max out your memory channels? Flaky.
Temperature a bit too hot? Silent Throttle with no logs.
Too “Dense” of a stick? Good luck training.

Last gen was rock solid by comparison. Here's what happened. Image
Image
More than ever, manufacturers have been pushing memory to the absolute limits.

JEDEC, the standards committee, is pretty conservative.

Yet the moment DDR5 launched, everyone threw JEDEC out the window.

Intel + AMD's memory controllers were *not* ready to handle it. Image
DDR5-4800 was the baseline.

Day one kits were pushing 6000+. Today, even 8000+.

On-die error correction is masking chips that would have been binned as trash in the DDR4 era.

The gap between JEDEC spec and retail has never been wider. Image
Read 4 tweets
LaurieWired Profile picture
Oct 2
Virtual Machines render fonts. It’s kind of insane.

TrueType has its own instruction set, memory stack, and function calls.

You can debug it like assembly. It’s also exploitable: Image
Image
Anytime you can run code (albeit very limited code), someone will take advantage of it.

TrueType (TT) is unfortunately famous for many Windows Kernel zero days.

TT is memory bound, therefore not Turing-complete…but you can still do crazy things with it. Image
Fontemon is a fun one, a pokemon-style game packaged as a TTF.

llama.ttf is even more insane. A 60MB font that runs a 15M parameter llama model to generate stories.

Seemingly normal at first, when you use excessive exclamation points it starts to generate text!
Read 4 tweets
LaurieWired Profile picture
Oct 1
This processor doesn’t (officially) exist.

Pre-production Engineering Samples sometimes make it into the grey market.

Rarer still are Employee Loaner Chips. Ghosts abandoned before ever becoming products: Image
Image
A few days ago, someone found an Intel Pentium Extreme 980.

No laser etched model number; just some scribbled sharpie.

In 2004, Intel (very publicly) canceled the 4Ghz Pentium 4…yet here it is.

It's a hint at some internal politics. Image
The Pentium group was all-in on single core performance.

In the early 2000s, Intel advertised wild charts expecting to hit 10Ghz.

Meanwhile, the Core2Duo team was the backup plan.

An underdog team in Haifa, focused on laptops. Image
Image
Read 4 tweets

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