I did something recently I haven’t done in a long time: I recompiled the Linux kernel. There was a time when this was a common occurrence. You might want a feature that the default kernel didn’t support, or you might have an odd piece of hardware. But these days, in almost all the cases where you need something like this, you’ll use loadable kernel modules (LKM) instead. These are modules that the kernel can load and unload at run time, which means you can add that new device or strange file system without having to rebuild or even restart the kernel.
Normally, when you write programs for Linux, they don’t have any special permissions. You typically can’t do direct port I/O, for example, or arbitrarily access memory. The kernel, however, including modules, has no such restriction. That can make debugging modules tricky because you can easily bring the system to its knees. If possible, you might think about developing on a virtual machine until you have what you want. That way, an errant module just brings down your virtual machine. Continue reading “Linux Fu: Kernel Modules Have Privileges”→
You want to pass TCP traffic from one computer to another, but there’s a doggone firewall in the way. Can they both see a shared file? Turns out, that’s all you need. Well, that and some software from [fiddyschmitt].
If you think about it, it makes sense. Unix treats most things as a file, so it is pretty easy to listen on a local TCP port and dump the data into a shared file. The other side reads the file and dumps the same data to the desired TCP port on its side. Another file handles data in the other direction. Of course, the details are a bit more than that, but that’s the basic idea.
Performance isn’t going to be wonderful, and the files keep growing until the program detects that they are bigger than 10 megabytes. When that happens, the program purges the file.
The code is written in C# and there are binaries for Windows and Linux on the release page. The examples show using shared files via Windows share and RDP, but we imagine any sort of filesystem that both computers can see would work. Having your traffic stuffed into a shared file is probably not great for security but, you know, you are already jumping a firewall, so…
Of course, no firewall can beat an air gap. Unless you can control the fans or an LED.
If you haven’t heard by now, Spotify is shutting down support for their “Car Thing” on December 9th of this year. Once that happens the automotive media player will officially be useless, with users being advised to literally throw them in the trash come December 10th. Call it an early Christmas present from your friends at the multi-billion dollar streaming company.
Surely the hardware hacking community can do a bit better than that. As it turns out, there’s actually been a fair amount of hacking and research done on the Car Thing, it’s just that most of it happened a couple years back when the device first hit the market. Things stagnated a bit in the intervening years, but now that the clock is ticking, there’s far more interest in cracking open the gadget and seeing what else we can do with it.
[lmore377]’s Car Thing macropad hack from 2022.The car-thing-reverse-engineering repository on GitHub has a wealth of hardware and software information, and has been something of a rallying point for others who have been poking around inside the device. Unsurprisingly, the Car Thing runs Linux, and with relatively minor work you can gain U-Boot and UART access. With just 512 MB of RAM and a Amlogic S905D2 chip that’s similar to what powers the Radxa Zero, it’s not exactly a powerhouse. Then again, we’ve seen plenty of awesome projects done with less.
If you’re more into the step-by-step approach, security researcher [Nolen Johnson] did a write-up about getting access to the Car Thing’s internal Linux system back in 2022 that’s certainly worth a look. As you’d imagine, there’s also a few YouTube videos out there that walk the viewer through gaining access to the hardware. This one from [Dinosaur Talks Tech] not only provides a good overview of how to get into the system, but covers flashing modified versions of the stock firmware to unlock various features and tweaking the internal Linux OS.
Interestingly enough, while we’ve seen plenty of homebrew hardware players for Spotify over the years, this is the first time the Car Thing has ever crossed our path. Something tells us though that this isn’t the last time we’ll hear about this forlorn Linux gadget.
It certainly isn’t a new idea to compile a language into an intermediate language. The original C++ compiler outputs C code, for example. Enhanced versions of Fortran were often just conversions of new syntax to old syntax. Of course, it makes sense to output to some language that can run on lots of different platforms. So, using that logic, Amber makes perfect sense. It targets — no kidding — bash. You write with nice modern syntax and compile-time checks. The output is a bash script. Admittedly, sometimes a hard-to-read bash script, but still.
If you want to see the source code, it is available on GitHub. Since Windows doesn’t really support bash — if you don’t count things like Cygwin and WSL — Amber only officially supports Linux and MacOS. In addition to compiling files, Amber can also execute scripts directly which can be useful for a quick one-liner. If you use Visual Studio Code, you can find a syntax highlighter extension for Amber.
There was a time when real system administrators just logged into Unix systems as root. But as we all know — with great power comes great responsibility. It’s too easy to do terrible things when you are really just trying to do normal work, and, on top of that, malicious software or scripts can do naughty things without you noticing. So common practice quickly changed to where an administrator had a personal account but then had a way to run certain programs “as root” which means you had to deliberately decide to wield your power.
Before long, people realized you don’t even need a root login account. That way, an attacker can’t try to log into root at all. Sure, they could still compromise your account, but a random hacker knows you might have a root user, but it is harder to guess that your login ID is JTKirkJr or whatever.
There are other ways to control what users can do, but many Linux and Unix installations still use this model. The root can do everything but login, and specific users get the privilege to do certain things.
It may be a very long time since some readers have installed a copy of Windows, but it appears at one point during the installation there’s a step that asks you which OS version you would like to install. Normally this is populated by whichever Windows flavours come on the install medium, but [Naman Sood] has other ideas. How about a Windows installer with Alpine Linux as one of the choices? Sounds good to us.
You can see it in action in the video below the break. Indeed Alpine Linux appears as one of the choices, followed by the normal Windows licence accept screen featuring the GPL instead of any MS text. The rest of the installer talks about installing Windows, but we can forgive it not expecting a Linux install instead.
So, the question we’re all asking is: how is it done? The answer lies in a WIM file, a stock Windows image which the installer unpacks onto your hard drive. The Linux distro needs to be installable onto an NTFS root partition, and to make it installable there’s a trick involving the Windows pre-installation environment.
This is an amusing hack, but the guide admits it’s fragile and perhaps not the most useful. Even so, the sight of Linux in a Windows installer has to be worth it.
Last time, I gave a whirlwind introduction to a very small slice of systemd. If you aren’t comfortable with systemd services, timers, and mounts, you might want to read that now. Otherwise, press on to see a few interesting uses for custom systemd units, including running a few things on a schedule and automatically mounting a Raspberry Pi Zero.
Can you do every one of these things in a different way? Of course you can. I’m not debating the relative merits of using or not using systemd. However, unless you totally control your own environment, good chance you are going to have to interact with systemd at some point.
Stupid Trick #1: Update Your IP Address
A few years ago, I talked about updating your remote DNS server with your public IP address. This lets you refer to a hostname like snoopy.hackaday.com and get back to your computer that often changes IP addresses. Sure, you can get services to do that for you, but you must either pay or agree to read ads on their site to keep your hostname going. This is all under your control. In the original post, I suggested using cron or NetworkManager to run the update script. I also hinted you could do it with systemd, but I didn’t tell you how. Let’s fix that.
