Linux Hacks

552 Articles

Linux Fu: The Great Power Of Make

June 22, 2018 by 76 Comments

Over the years, Linux (well, the operating system that is commonly known as Linux which is the Linux kernel and the GNU tools) has become much more complicated than its Unix roots. That’s inevitable, of course. However, it means old-timers get to slowly grow into new features while new people have to learn all in one gulp. A good example of this is how software is typically built on a Linux system. Fundamentally, most projects use make — a program that tries to be smart about running compiles. This was especially important when your 100 MHz CPU connected to a very slow disk drive would take a day to build a significant piece of software. On the face of it, make is pretty simple. But today, looking at a typical makefile will give you a headache, and many projects use an abstraction over make that further obscures things.

In this article, I want to show you how simple a makefile can be. If you can build a simple makefile, you’ll find they have more uses than you might think. I’m going to focus on C, but only because that’s sort of the least common denominator. A make file can build just about anything from a shell prompt.

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Linux Fu: Watch That Filesystem

June 7, 2018 by 31 Comments

The UNIX Way™ is to cobble together different, single-purpose programs to get the effect you want, for instance in a Bash script that you run by typing its name into the command line. But sometimes you want the system to react to changes in the system without your intervention. For example, you might like to watch a directory and kick off some program automatically when a file appears from a completed FTP transaction, without having to sit there and refresh the directory yourself.

The simple but ugly way to do this just scans the directory periodically. Here’s a really dumb shell script:

#!/bin/bash
while true
 do
   for I in `ls`
    do cat $I; rm $I
   done
 sleep 10
done

Just for an example, I dump the file to the console and remove it, but in real life, you’d do something more interesting. This is really not a good script because it executes all the time and it just isn’t a very elegant solution. (If you think I should use for I in *, try doing that in an empty directory and you’ll see why I use the ls command instead.)

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Linux Fu: Counter Rotate Keys!

June 1, 2018 by 22 Comments

If you’ve done anything with a modern Linux system — including most variants for the Raspberry Pi — you probably know about sudo. This typically allows an authorized user to elevate themselves to superuser status to do things.

However, there is a problem. If you have sudo access, you can do anything — at least, anything the sudoers file allows you to do. But what about extremely critical operations? We’ve all seen the movies where launching the nuclear missile requires two keys counter-rotated at the same time and third firing key. Is there an equivalent for Linux systems?

It isn’t exactly a counter-rotating key, but the sudo_pair project — a prelease open-source project from Square — gives you something similar. The project is a plugin for sudo that allows you to have another user authorize a sudo request. Not only do they authorize it, but they get to see what is happening, and even abort it if something bad is happening.

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Windows Notepad Now Supports Unix Line Endings

May 8, 2018 by 141 Comments

In what is probably this century’s greatest advancement in technology, Windows Notepad now supports Unix line endings. This is it, people. Where were you when Kennedy was assassinated? Where were you when Neil Armstrong set foot on the moon? Where were you when Challenger blew up? Where are you now?

Previously, Windows Notepad only supported Windows End of Line Characters — a Carriage Return (CR) and Line Feed (LF). Unix text documents use LF for line endings, and Macs use CR for line endings. The end result of this toppling of the Tower of Babel for End of Line characters is a horrific mess; Windows users can’t read Unix text files in Notepad, and everything is just terrible. Opening a Unix text file in Windows produces a solid block of text without any whitespace. Opening a Windows text file in anything else puts little rectangles at the end of each line.

Starting with the current Window 10 Insider build, Notepad now supports Unix line endings, Macintosh line endings, and Windows line endings. Rejoice, the greatest problem in technology has now been solved.

Harmony Hub Hacked And Patched

May 7, 2018 by 15 Comments

When we say “hack” here we most often mean either modifying something to do something different or building something out of parts. But as we build more Internet-connected things, it is worthwhile to think about the other kind of hack where people gain unauthorized access to a system. For example, you wouldn’t think a remote control would be a big deal for hackers. But the Logitech Harmony Hub connects to the Internet and runs Linux. What’s more is it can control smart devices like door locks and thermostats, so hacking it could cause problems. FireEye’s Mandian Red Team set out to hack the Harmony and found it had a lot of huge security problems.

The remote didn’t check Logitech’s SSL certificate for validity. It didn’t have a secure update process. There were developer tools (an SSH server) left inactive in the production firmware and — surprisingly — the root password was blank! The team shared their findings with Logitech before publishing the report and the latest patch from the company fixes these problems. But it is instructive to think about how your Raspberry Pi project would fare under the same scrutiny.

In fact, that’s the most interesting part of the story is the blow-by-blow description of the attack. We won’t spoil the details, but the approach was to feed the device a fake update package that turned on a dormant ssh server. Although they started by trying to solder wires to a serial port, that wasn’t productive and the final attack didn’t require any of that.

We’ve looked at some ways to harden Linux systems like the Raspberry Pi before, but honestly, it is an ongoing battle. We’ve seen plenty of devices with cybersecurity holes in them — some not found by good guy hackers first.

Linux Fu: File Aliases, Links, And Mappings

March 16, 2018 by 37 Comments

Have you heard it said that everything in Linux is a file? That is largely true, and that’s why the ability to manipulate files is crucial to mastering Linux Fu.

One thing that makes a Linux filesystem so versatile is the ability for a file to be many places at once. It boils down to keeping the file in one place but using it in another. This is handy to keep disk access snappy, to modify a running system, or merely to keep things organized in a way that suits your needs.

There are several key features that lend to this versatility: links, bind mounts, and user space file systems immediately come to mind. Let’s take a look at how these work and how you’ll often see them used.

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Linux Fu: Regular Expressions

March 9, 2018 by 37 Comments

If you consider yourself a good cook, you may or may not know how to make a souffle or baklava. But there are certain things you probably do know how to do that form the basis of many recipes. For example, you can probably boil water, crack an egg, and brown meat. With Linux or Unix systems, you can make the same observation. You might not know how to set up a Wayland server or write a kernel module. But there are certain core skills like file manipulation and editing that will serve you no matter what you do. One of the pervasive skills that often gives people trouble is regular expressions. Many programs use these as a way to specify search patterns, usually in text strings such as files.

If you aren’t comfortable with regular expressions, that’s easy to fix. They aren’t that hard to learn and there are some great tools to help you. Many tools use regular expressions and the core syntax is the same. The source of confusion is that the details beyond core syntax have variations.

Let’s look at the foundation you need to understand regular expression well.

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