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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
Aug 6
Ring 0 is a highly-privileged state on CPUs.

Negative Ring Levels have even *higher* privilege. You just haven’t heard of them.

For X86, Ring -1 is Hardware Virtualization, Ring -2 is System Management Mode, Ring -3 is Intel ME / AMD PSP.

Arm get's even weirder: Image
Image
Negative rings are mostly due to X86 being really old; as the ISA got more complex, we got "above 0" states.

Armv8 moves in a positive direction; higher numbers have more privilege. From EL0 (user space) to EL3 (State-Switching).

Apple does something extra funky: Image
Image
For Apple’s M-chips, two substates were added to EL1+EL2, called GL1 and GL2.

During their reverse engineering efforts, Asahi Linux confirmed that the EL3 state doesn’t exist on the M1.

GL1 is a sandbox that the XNU kernel “jumps” into every time it edits a page table. Image
Read 4 tweets
LaurieWired Profile picture
Aug 2
An early rule you learn in computer science is:

“Never store currency as floats”

Nearly every popular language has special, built-in types for money. But why?

The *majority* of money-like numbers have no float representation, accumulating to massive errors over time: Image
Image
Go ahead and try this. Let’s add three dimes. Open up a python terminal, and type in:

0.10 + 0.10 + 0.10

Uh oh. See that little remainder?

It may seem trivial, but this mistake happens more often than you’d expect! Image
iBit, a Bitcoin exchange, encountered a “free money” glitch for this reason.

Orders were parsed as floats; you could create an order smaller than a single satoshi (minimum unit of Bitcoin).

Source wallet would round down (no change), target wallet rounded up (+1 satoshi). Image
Image
Read 4 tweets
LaurieWired Profile picture
Jul 30
Programming Languages used to be designed as powerful as possible.

Maximum possible utility given hardware constraints.

The pro move is to choose the *least* powerful, non-Turing-complete solution.

The entire web exists because of the Principle of Least Power: Image
Image
Don’t take my word for it. Tim Berners-Lee (inventor of HTML, HTTP, etc) had this to say:

“the less powerful the language, the more you can do with the data...”

HTML is purposefully *not* a real programming language.

The constraint pushed innovation to data processing. Image
Imagine an alternate-reality Web, where HTML didn’t exist.

Java applets would have been a serious contender; they certainly allowed for rich interactivity.

Yet, without a way to freely scrape simply formatted data, search engines would be a non-starter. Image
Read 4 tweets
LaurieWired Profile picture
Jul 29
When you make a Bank ACH transaction, it’s literally just an SFTP upload.

Sent as a NACHA file, it's 940 bytes of ASCII text.

Bank-to-Bank transactions cost ~0.2 cents. As long as it travels via encrypted tunnel; it’s compliant!

Here’s how the quirky system works: Image
Image
Chase offers a sample NACHA file to look at.

Notice the rows padded with 9s. It’s an artifact of a 1970s rule about magnetic tape, "always fill the block".

To this day, total line count *must* be a multiple of ten; otherwise the bank will drop the transaction. Image
Image
Of course, larger Fintech firms (think Stripe) wrap it up with modern APIs, but SFTP is the default for most US Banks.

Hilariously, NACHA rules don’t clarify *how* transactions should be encrypted.

Only that “commercially reasonable” cryptography should be used. Image
Read 4 tweets
LaurieWired Profile picture
Jul 28
Intel’s not doing so hot lately. Meanwhile vendors are killing it at the RISC-V Summit in China.

One CPU got a specint2006/GHz rating of 10.4/GHz!

To put it in perspective, a i7-4790k (Haswell) scores 8.1/GHz.

RISC-V is hitting high-end desktop territory FAST: Image
Image
No one realizes how quickly a CPU supply chain; completely independent of western IP is progressing:

Not long ago RISC-V performance was a joke.

Now it’s trading blows with x86 and high end ARM!

In another generation or two it’s going to be a *serious* contender. Image
UltraRISC (the vendor with the high specint2006 score) is fabbing on a pretty old node; TSMC 12nm.

They don’t even have full vector support yet!

Imagine in a few years, a ~7nm chip /w full vector extensions.

China will have *fast* CPUs with no license chokepoints! Image
Read 4 tweets
LaurieWired Profile picture
Jul 22
Fading out audio is one of the most CPU-intensive tasks you can possibly do!

Values that get close (but not quite) zero, hit an underflow gap known as "Subnormal" range.

It’s a mathematical conundrum so tricky, both x86 and ARM made special CPU instructions just to handle it! Image
Image
In computer science, mathematical real numbers are approximated by floating-point representations.

For a single 32-bit float, the smallest “normal” positive we can hold is 1.17 × 10^-38

Tiny differences could get rounded off, leading to later Divide by Zero errors! Image
In the 80s, companies were fighting over the IEEE 754 (floating point) standard.

Intel wanted “good arithmetic”. DEC wanted to round off to zero.

As the most contested proposal, Intel suggested a “subnormal” routine, so that 32-bit floats gradually underflow. Image
Image
Read 5 tweets

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