filesystem

9 Articles

BCacheFS Is Now A DKMS Module After Exile From The Linux Kernel

September 19, 2025 by 46 Comments

It’s been a tense few months for users of the BCacheFS filesystem, as amidst the occasional terse arguments and flowery self-praise on the Linux Kernel mailing list the future of this filesystem within the Linux kernel hung very much in the balance. After some initial confusion about what ‘externally maintained’ means in Linux parlance, it’s now clear that this means that BCacheFS has effectively been kicked out of the kernel as [Linus] promised and will ship as a DKMS module instead. The gory details of this change are discussed in a recent video by [Brodie Robertson].

We covered the BCacheFS controversy in the Linux world a few months ago, amidst reports of data loss and filesystem corruption among its users. Its lead developer, [Kent Overstreet], came to blows with [Linus Torvalds] on the LKML after [Kent] insisted on repeatedly pushing new features into kernel release candidate branches along with rather haughty statements on why he should be able to do this.

To make a long story short, [Linus] didn’t like this and froze BCacheFS support in the current kernel release with all future in-kernel development ceased. Distributions like SuSE have initially said that will disable BCacheFS starting in kernel version 6.17, meaning that users of BCacheFS may now have to install the DKMS module themselves. Some distributions like Arch are likely to include this DKMS module by default, which is something you want to check if you use this filesystem.

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The Ongoing BcacheFS Filesystem Stability Controversy

June 10, 2025 by 58 Comments

In a saga that brings to mind the hype and incidents with ReiserFS, [SavvyNik] takes us through the latest data corruption bug report and developer updates regarding the BcacheFS filesystem in the Linux kernel. Based on the bcache (block cache) cache mechanism in the Linux kernel, its author [Kent Overstreet] developed it into what is now known as BcacheFS, with it being announced in 2015 and subsequently merged into the Linux kernel (6.7) in early 2024. As a modern copy-on-write (COW) filesystem along the lines of ZFS and btfs, it was supposed to compete directly with these filesystems.

Despite this, it has become clear that BcacheFS is rather unstable, with frequent and extensive patches being submitted to the point where [Linus Torvalds] in August of last year pushed back against it, as well as expressing regret for merging BcacheFS into mainline Linux. As covered in the video, [Kent] has pushed users reporting issues to upgrade to the latest Linux kernel to get critical fixes, which really reinforces the notion that BcacheFS is at best an experimental Alpha-level filesystem implementation and should probably not be used with important data or systems.

Although one can speculate on the reasons for BcacheFS spiraling out of control like this, ultimately if you want a reliable COW filesystem in Linux, you are best off using btrfs or ZFS. Of course, regardless of which filesystem you use, always make multiple backups, test them regularly and stay away from shiny new things on production systems.

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A Potential Exploit With The Ext Filesystem

December 10, 2024 by 58 Comments

The extended filesystem, otherwise known as ext, has been a fundamental part of Linux since before the 1.0 release in 1994. Currently the filesystem is on its fourth major revision, in use since its release in 2008 thanks to its stability, reliability, and backwards compatibility with the other ext filesystem versions. But with that much history there are bound to be a few issues cropping up here and there. [Will] recently found an exploit with this filesystem that can cause a Linux kernel to immediately panic when a manipulated USB drive is inserted into a computer.

[Will] discovered this quirk when investigating the intricacies of the filesystem for problems and other vulnerabilities. A tool called tune2fs, used for administering and modifying ext filesystems, includes the ability to pass certain commands to the Linux kernel when certain situations arise with the filesystem itself, including that the kernel should panic. One situation is that the ext filesystem itself becomes corrupted, which can then cause the kernel panic. Armed with this knowledge, a USB drive can be purposefully given a corrupted ext filesystem which, when plugged into a Linux machine, can cause the computer to shut down.

The post linked above goes into some discussion about how this exploit could be used maliciously to gain access to a Linux system, including rebooting computers where no access to a power button is otherwise enabled or making other changes to the system before needing a reboot to apply the changes. In general, though, it’s good to assume an attacker could take any route to gain access to a machine. This exploit from a few years ago, for example, allowed another Linux tool to be used to gain root access.

Thanks to [Timothy] for the tip!

An Old Netbook Spills Its Secrets

March 27, 2023 by 35 Comments

For a brief moment in the late ’00s, netbooks dominated the low-cost mobile computing market. These were small, low-cost, low-power laptops, some tiny enough to only have a seven-inch display, and usually with extremely limiting hardware even for the time. There aren’t very many reasons to own a machine of this era today, since even the cheapest of tablets or Chromebooks are typically far more capable than the Atom-based devices from over a decade ago. There is one set of these netbooks from that time with a secret up its sleeve, though: Phoenix Hyperspace.

Hyperspace was envisioned as a way for these slow, low-power computers to instantly boot or switch between operating systems. [cathoderaydude] wanted to figure out what made this piece of software tick, so he grabbed one of the only netbooks that it was ever installed on, a Samsung N210. The machine has both Windows 7 and a custom Linux distribution installed on it, and with Hyperspace it’s possible to switch almost seamlessly between them in about six seconds; effectively instantly for the time. Continue reading “An Old Netbook Spills Its Secrets” →

File Systems For Tiny Devices

December 7, 2019 by 11 Comments

Sometimes you build a computer and use it every day. Sometimes you build a different type of computer and it sits alone on a mountaintop for years. The design considerations for these two setups are remarkably different, right down to the type of file system used. For small computers like [Jo] is using, and for the amount of time they sit alone in remote locations, he decided to build his own file system for them.

Known as JesFs ([Jo]’s embedded serial File system), the file system is for SPI Flash and intended for use in scientific data logging. It can be used on the chip-scale processors found in many development boards, and is robust enough to use in applications where remoteness is a concern. It has a small RAM footprint, is completely open source, includes wear leveling, and has a number of security features built-in as well.

Some of the benefits of using a file system on such a tiny chip aren’t immediately obvious unless you’re doing a lot of data logging, but it does allow you to change virtually any aspect of the firmware much more easily if everything is accessible as a file, and not something you would have to change by reflashing the whole chip, for example. There are also a number of traps that you can easily fall into when working with file systems for tiny devices.

Analysing National Budgets With File System Tools

April 15, 2019 by 20 Comments

Understanding a national budget can be difficult, with political interests and distorted talking points obscuring the truth. There’s no substitute for diving into the hard data yourself, but it can be difficult to know where to start. [D. Scott Williamson] wanted to do just that, and took an unconventional path – using file system tools to analyse the 2019 US budget.

The basic idea is simple. A file is created for every line item on the budget, and this file is filled with a $ for every million dollars that item costs. These files are sorted into folders for the relevant bureaus and agencies responsible, creating a Fiscal File System representation of the entire budget. The file system can then be analysed with standard tools to get a look at the overall make up of the budget – in this case, WinDirStat does a particularly good job.

It’s a novel way to go about budget analysis, and fills a gap in the market now that Time Plots aren’t producing their Death and Taxes infographics anymore. Pre-cooked file systems covering budgets back to 1977 are available for your perusal, should you need to investigate how things have changed over time.

Unsurprisingly, we haven’t covered the national budget before, but we’re always down to talk file systems.

Cool Tools: A Little Filesystem That Keeps Your Bits On Lock

January 24, 2019 by 52 Comments

Filesystems for computers are not the best bet for embedded systems. Even those who know this fragment of truth still fall into the trap and pay for it later on while surrounded by the rubble that once was a functioning project. Here’s how it happens.

The project starts small, with modest storage needs. It’s just a temperature logger and you want to store that data, so you stick on a little EEPROM. That works pretty well! But you need to store a little more data so the EEPROM gets paired with a small blob of NOR flash which is much larger but still pretty easy to work with. Device settings go to EEPROM, data logs go to NOR. That works for a time but then you remember that people on the Internet are all about the Internet of Things so it’s time to add WiFi. You start serving a few static pages with that surprisingly capable processor and bump into storage problems again so the NOR flash gets replaced with an SD card and now the logs go there too. Suddenly you’re dealing with multiple files and want access on a computer so a real filesystem is in order. FAT is easy, so the card grows a FAT filesystem. Everything is great, but you start to notice patches missing from the logs. Then the SD card gets totally corrupted. What’s going on? Let’s take a look at the problem, and how to reach embedded file nirvana.

Continue reading “Cool Tools: A Little Filesystem That Keeps Your Bits On Lock” →