Author Archives: Mark

About Mark

I'm a systems programmer and jack of a few other trades, sharing what I learn along the way. What would like to read about? Please let me know on Twitter @offlinemark 😊.

To Cage a Dragon: An obscure quirk of /proc

Introduction

An obscure quirk of the /proc/*/mem pseudofile is its “punch through” semantics. Writes performed through this file will succeed even if the destination virtual memory is marked unwritable. In fact, this behavior is intentional and actively used by projects such as the Julia JIT compiler and rr debugger.

This behavior raises some questions: Is privileged code subject to virtual memory permissions? In general, to what degree can the hardware inhibit kernel memory access?

By exploring these questions1, this article will shed light on the constraints the CPU can impose on the kernel, and how the kernel can bypass these constraints. To begin, we must understand how the hardware enforces memory permissions.

Continue reading

How to pick a market that will make you money

As a founder, picking your market is the most important decision you’ll make. It will impact every aspect of your journey, from product development to sales, and ultimately determine how profitable you’ll be. A good market compensates for poor execution on your part, while even the best execution will struggle with a bad one.

So what goes into a good market?

The key attributes are:

Continue reading

Open source licensing for supervillains

This post covers my research into open source software licensing and my analysis of real-world open source projects that profit off of open source code via proprietary licenses.

Keep reading and you’ll learn:

  • What the difference between a restrictive and permissive license is
  • What dual licensing is and how you can use it make money off of open source code
  • What CLAs are and the specific clause your CLA needs for use with dual licensing
  • Examples of companies that implement dual licensing and how they do it

And of course: I am not a lawyer and none of this is legal advice.


Let’s talk evil. And by evil, I mean money.

Continue reading

Evergreen tweets

Twitter’s length limit is deceptive. At a glance, it suggests that writing tweets should be easy and quick. This is true for superficial tweets, but does not mean all tweets are written quickly and with little effort.

Twitter is actually a platform for concise writing, and writing concisely is harder than writing verbosely. There are certain tweets I spend a lot of time on and it’s shame to have them get lost in my feed. So I’m storing them here.

Continue reading

Double fetches, scheduling algorithms, and onion rings

Most people thought I was crazy for doing this, but I spent the last few months of my gap year working as a short order cook at a family-owned fast-food restaurant. (More on this here.) I’m a programmer by trade, so I enjoyed thinking about the restaurant’s systems from a programmer’s point of view. Here’s some thoughts about two such systems.

Continue reading