Kernel: LWN Articles (Paywall Expired), Alexander Popov, and Phoronix on Bcachefs and VPU

• A realtime developer's checklist

  We want all applications to be correct and bug-free, Ogness began; in the realtime domain, correctness "means running at the correct time". The application must wake up within a bounded time limit when there is time-critical work to do. Ogness highlighted that, in realtime systems, the right timing of tasks is a requirement; things will go wrong if the constraints are not met. Developers need to define which tasks and applications are time-critical; he noted that a lot of people mistakenly think that all tasks in a realtime system are realtime, while most of them are not. The good news for developers is that, under Linux, they can write realtime applications using only the POSIX API with the realtime extensions. The code will look familiar, and only three additional header files are required: sched.h (a member of the audience noted that the musl C library does not implement this one), time.h, and pthread.h. There are three properties that a realtime operating system must have: deterministic scheduling behavior, interruptibility (the CPU is always running something, so there should be a way to interrupt a task), and a way to avoid priority inversion, which happens when a high-priority task must wait for a lower-priority one. The third property, which might be less familiar to non-realtime developers, was described with an example.

• iproute2 and libbpf: vendoring on the small scale

  LWN's recent article on Kubernetes in Debian discussed the challenges of packaging a massive project with hundreds of dependencies. Many of the issues that arose there, however, are not limited to such projects, as can be seen in the ongoing discussion about whether a copy of the relatively small libbpf library should be shipped with the iproute2 collection of networking tools. Fast-moving projects, it would seem, continue to feel limited by the restrictions imposed by the Linux distribution model. Iproute2 is a collection of network-configuration tools found on almost any Linux system; it includes utilities like arpd, ip (the command old-timers guiltily think they should be using when they type ifconfig), ss, and tc. That last command, in particular, is used to configure the traffic-control subsystem, which allows administrators to manage and prioritize the flow of network traffic through their systems. This subsystem has, for some years, had the capability to load and run BPF programs to both classify packets and make decisions on how to queue them. This mechanism gives administrators a great deal of flexibility in the management of network traffic. The code for handling BPF programs within iproute2 is old, though, and lacks support for many of the features that have been added to BPF in the last few years. Since that code was written, the BPF community has developed libbpf (which lives in the kernel source tree) as the preferred way to work with BPF programs and load them into the kernel. This is not a small task; libbpf must interpret the instructions encoded as special ELF sections in an executable BPF program and make the necessary calls to the sprawling bpf() system call. This work can include creating maps, "relocating" structure offsets to match the configuration of the running kernel, loading programs, and attaching those programs to the appropriate hooks within the kernel. Libbpf has grown quickly, along with the rest of the BPF ecosystem.

• Systemd catches up with bind events

  The kernel project has a strong focus on not breaking user-space applications; if something works with a given kernel release, it should continue to work with subsequent releases. So it may be discouraging to read the lengthy exposition on an apparent user-space API break in the announcement for the systemd 247-rc2 release. Changes to udev configuration files will be needed to keep systems working, but the systemd project claims that it "is not [the] fault of systemd or udev, but caused by an incompatible kernel change that happened back in Linux 4.12". It seems like an appropriate time to look at what happened, how administrators need to respond, and whether anything can be done to avoid this kind of thing from happening again. Modern computers tend to be highly dynamic, with devices (of both the physical and virtual variety) appearing and disappearing while the system is running. The kernel handles the low-level details with regard to these device events, but it is up to user space to take care of the rest. For that to happen, user space needs to know when something has changed with the system's configuration. To that end, events are emitted to user space from deep within the kernel's driver-core subsystem whenever something changes; for example, plugging in a USB device will result in the creation of one or more ADD events to tell user space that the new device is available. The udev daemon is charged with responding to these events according to a set of rules; it can create device nodes, set permissions, notify other user-space components, and more, all in response to properties attached to events by matching rules. The set of possible events is relatively small and does not change often.

• Changed-block tracking and differential backups in QEMU

  The block layer of QEMU, the open-source machine emulator and virtualizer, forms the backbone of many storage virtualization features: the QEMU Copy-On-Write (QCOW2) disk-image file format, disk image chains, point-in-time snapshots, backups, and more. At the recently concluded 2020 KVM Forum virtual event, Eric Blake gave a talk on the current work in QEMU and libvirt to make differential backups more powerful. As the name implies, "differential backups" address the efficiency problems of full disk backups: space usage and speed of backup creation. There's also the similar-sounding term, "incremental backups". The difference is that differential tracks what has changed since any given backup, while incremental tracks changes only since the last backup. Incremental backups are a subset of differential backups, but both are often lumped under the "incremental backups" term. This article will stick to "differential" as the broader term. With differential backups, one of the two endpoints when creating backups is always the current point in time. In other words, it is not like Git, where, if the latest version of a file is, say, v4, you can still diff between v2 and v3 — with differential backups, one of the two diff points is always v4, the current point in time. QEMU has had block-layer primitives to support full backups for some time; these were most commonly used for live-migrating the entire storage of a virtual machine, or for point-in-time snapshots. But over the past couple of years, QEMU and libvirt have picked up steam toward the goal of making differential backups a first-class feature that is enabled by default.

• Linux kernel heap quarantine versus use-after-free exploits | Alexander Popov

  It's 2020. Quarantines are everywhere – and here I'm writing about one, too. But this quarantine is of a different kind. In this article I'll describe the Linux Kernel Heap Quarantine that I developed for mitigating kernel use-after-free exploitation. I will also summarize the discussion about the prototype of this security feature on the Linux Kernel Mailing List (LKML).

• Bcachefs Going Through Period Of More Performance Optimizations - Phoronix

  Bcachefs was sent out for another round of review at the end of October. While it doesn't look like this file-system born out of Linux's block cache code will be mainlined in the immediate near future, it's still on a nice trajectory. The October post to the Linux kernel mailing list outlined all of the current features and those recently completed like erasure coding, inline data extents, and more -- plus many bug fixes.

• Intel Begins Upstreaming Work For Their Vision Processing Unit On Linux

  While Intel engineers over the course of the year began upstreaming various elements of the Keem Bay SoC support, the actual Vision Processing Unit (VPU) enabling hasn't been sent out for review until now. Intel has sent out their initial patches for bringing up the Vision Processing Unit on the open-source Linux kernel. Intel's Visual Processing Unit that is new to the Keem Bay SoC can be used as a standalone SoC or as a PCI Express vision processing accelerator The Linux kernel work is ultimately about fulfilling both possible uses.

Software: Bolt, GMusicBrowser and Shortwave

• bolt 0.9.1 with fixes for integrated thunderbolt controller

  A new release of bolt is out: 0.9.1 - Unstable icy waters. This is a bug-fix release that addresses some issue on integrated Thunderbolt controller. Intel's Ice Lake is the first architecture where the Thunderbolt controller is part of the CPU die. This is quite a big difference. There is a good article on wikichip called "A Look At The Ice Lake Thunderbolt 3 Integration" for those that are curious about the technical details. What matters for bolt is that there is no DROM, which means that the udev device representing the host switch does not have the usual name and id attributes for the device and vendor. Additionally, the unique_id attribute has a different UUID every boot. This breaks one of the fundamental assumptions for boltd, which used the unique_id of the host to uniquely identify the corresponding Thunderbolt domain. This is important because we store host devices and domains in the store. Now, with the uuid changing this means that 1) we can not match the previously stored domains and hosts to the ones after a reboot and thus will accumulate "stale" domains in the store. Ironically, the fact that the host device also does not have any name and id information means that boltd would refuse to create the BoltDevice for those which meant we did at least not accumulate the stale host devices in the store. It did break the detection of the generation, i.e. if it is Thunderbolt 3 or USB 4.

• GMusicBrowser Back From the Dead with GTK3 Port - OMG! Ubuntu!

  It’s been a long time since I last typed the name ‘Gmusicbrowser’ in a post but what d’ya know: the venerable music player recently issued a pair of new releases. Gmusicbrowser was a fairly popular audio app back in the day due to its customisable nature, and its ability to handle large (and I mean large) music collections. The open source app supports all major music files like .mp3, .ogg, and .flac — with the new release .opus joins the club. It’s possible to tweak, tune, and tailor the look of Gmusicbrowser in a giddying number of ways using its ‘layouts’ feature. You can craft these by hand, or use a built-in preset (e.g., ‘iTunes’, ‘Rhythmbox’, ‘Quod Libet’, etc).

• Repo Review: Shortwave

  A few years ago, I had already reviewed quite a few of the internet radio players available in the repository, but I thought I'd take the time now to have a look at a new radio player available called Shortwave. Shortwave uses the very large, community driven radio-browser.info database for its lists of internet radio stations, giving you access to over 28,000 radio streams. Shortwave has a nicer and more modern user interface than a lot of the other internet radio players I've previously tested. The simplicity of Shortwave is good, making it quick and easy to find the radio stations that you're looking for. The main screen that you're presented with when opening Shortwave is the Library page. This is where you can keep all your favorite radio stations for easy access. Clicking on the + button at the top left of the window will take you to the Discover page, where you can find new radio stations to listen to. [...] Shortwave, in my opinion, is one of the best internet radio players that I've used. It may not have quite as many advanced features as some of the other radio players available, but it makes up for that with its great ease of use. And now would also be the perfect time to use Shortwave for listening to some of your favorite Christmas carols!

Security Leftovers

• Short Topix: 200 Most Common Passwords Of 2020

  Despite the "annual warnings" (which are most likely much, much more frequent), users are still using easy-to-hack, easy-to-crack passwords. I know that, as this magazine's Chief Editor for over 11 years, we have run password articles so many times in the past that it's now impossible to count them all. I know this definitely isn't my first password article of 2020, either. Well, NordPass has released its list of the 200 most common passwords, along with their frequency of use, how many users were found using them, the length of time it took to crack the passwords, and how many times it was exposed. Without much surprise, the top "password" used was 123456. It rose to the number one spot, after occupying the number two spot in 2019. Now, you've got to promise me that you won't LAUGH at some of these passwords! Below is a reprint of the top 20 worst passwords of 2020. [...] Specifically, the issue goes something like this. If a Go SMS Pro user sends photos, videos or files to a user who does NOT have the app installed, the files are uploaded to a server, and the non-user of the app was sent a message via SMS text with a link to the files. This way, non-users did not have to install Go SMS Pro just to receive or view the files. At least, that's how it was supposed to work. However, researchers discovered that even when files were shared between users who both had the app installed, files were still uploaded to the servers. What's worse, the server was unsecured, and could be accessed by anyone with the link. Researchers also discovered that the web addresses were sequential, which means that anyone who could figure out the predictable web address could potentially gain access to millions of private files.

• Certificates from Let's Encrypt (R3 active)

  Let's Encrypt has announced that, as of today, the TLS certificates issued by the Let's Encrypt certificate authority are using a new intermediate certificate. "While LE will start using their new _roots_ next year, the change today is using a _variant_ of their "R3" certificate which is cross-signed from IdenTrust, rather than chaining back to their "ISRG Root X1". This will affect you if you're using DANE, TLSA records in DNS, signed by DNSSEC, to advertise properties of the certificate chain which remote systems should expect to see."

• Josh Bressers: Episode 228 – Door 03: Do all vulnerabilities matter equally?

  Josh and Kurt talk about how many security vulnerabilities matter enough to fix?