Category Archives: programming

Random notes on GNOME, GDM3, systemd –user and ephemeral environment variables such as SSH_AUTH_SOCK

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This is totally a thought dump, as I just spent hours (!) figuring out why my environment had been persistently setting SSH_AUTH_SOCK across login sessions. It’s not a solution for readers’ particular issues, nor a tutorial on how to resolve my particular issue either: just a log of surprising things I found out today about a machine I’m, using.

The managed machine I’m using has switched to GDM3 as the display manager, and the default environment is GNOME3. I don’t enjoy GNOME3, and prefer i3 for work uses. I gave it a chance, but after restoring my homedir, I decided to go back to i3.

Note: During the homedir restoration, I had the GNOME3 session running. I moved my homedir away, signed out, and rsynced away. I hope this order of operation got GNOME3 confused and made it forget to clean things up.

Symptom: tools have been complaining they can’t talk to a valid-looking value for SSH_AUTH_SOCK. The socket file and its directory were both missing. ssh-agent was not running in the session.

  • I use ~/.Xsession to configure my graphical session before starting i3. My first suspicion was something ran, set SSH_AUTH_SOCK and the ssh-agent crashed afterwards. This was not the case.
  • I still had the terminal session running. It had an ssh-agent in it. Could that be the cause? No, after nuking the terminal session and signing in and out in the graphical one, the issue was stil present.
  • Was ~/.Xsession supposed to execute /etc/X11/Xsession? No, that happens separately. This is fine.
  • Is /etc/X11/Xsession.d/90x11-common_ssh-agent getting executed? Yes, it is. But $STARTUP is not getting updated. (Oh, right: /etc/X11/Xsession.d scripts on Debian OSes are not executing things directly, but scheduling later execution by updating envvarSTARTUP`.)
  • Is /etc/X11/Xsession getting executed at all? At what point is SSH_AUTH_SOCK set? No, we are not running it at all. lightdm did (I think), but gdm3 has to be a special puppy.
  • What’s executed instead? /etc/gdm3/Xsession which closely resembles /etc/X11/Xsession, but is not exactly the same.
  • Is /etc/gdm3/Xsession executing /etc/X11/Xsession.d scripts? Yes, it is.
  • So, which of the scripts is setting SSH_AUTH_SOCK? Well, in my individual situation, it looks like it’s happening before any of the script executes.
  • Something in ~/.config? No. Envvar name or value not found.
  • Something in ~/.local? No. Envvar name or value not found.
  • Is something else run? Well, gnome-keychain-daemon is running for some reason and gets restarted upon session restart. It’s run by /etc/xdg/autostart/gnome-keyring-secrets.desktop file, and can be disabled by putting 
    [Desktop Entry]
Hidden=true
# maybe other fields are required too?

    into ~/.config/autostart/gnome-keyring-secrets.desktop, blocking system file from starting up. (Remember, this is a managed machine; even if it weren’t, I don’t want to touch distribution-installed files.)

    However, no, blocking gnome-keyring-daemon from starting up doesn’t fix the issue.

So this is very confusing. A bad environment variable is surviving logout and seems set before any Xsession script is run.

Is gdm3 remembering things for us? Where would it be writing them anyway if not into homedir?

Turns out that no. gdm3 isn’t remembering anything.

Here’s what happened.

systemd can run in per-user mode (systemd --user). It keeps the environment in RAM and can also survive logouts. systemd --user is shared between all logged-in sessions of the current users.

The feature that caused trouble is — management of environment for daemons. systemctl --user show-environment shows that something wrote the entire environment of the GNOME3 session into systemd --user‘s environment. From what I can tell, all daemons started after login will inherit the environment from this. And it had rather ephemeral things like SSH_AUTH_SOCK, XAUTHORITY GPG_AGENT_INFO or XDG_SESSION_DESKTOP written into it!

Killing systemd --user process and restarting the session fixed everything. /etc/gdm3/Xsession no longer had SSH_AUTH_SOCK set when it started (in fact, it was not set by the time /etc/X11/Xsession.d/99x11-common_start was starting to read the $STARTUP envvar.

So, something in GNOME3 decided to write very ephemeral environment variables into systemd --user, never cleaned them up, and systemd --user did not get reaped even after I signed out from both the graphical and the terminal sessions! There’s a chance cleanup of systemd --user did not happen because the homedir was moved away at the time, but isn’t this stuff working with environment variables such as DBUS_SESSION_BUS_ADDRESS envvar, cat /run/systemd/user/$(id -u), /run/user/$(id -u) and other files under /run? How would have moving /home/${LOGNAME} prevented reaping of systemd --user?

I can see some value in these things being per-user rather than per-session, but given how systemd has been pushing for per-session stuff too, this is leaving a bad taste in the mouth, and makes me believe even further that systemd should not try to be “the runtime for Linux” (note, not the other OSes), it should not infect user sessions, and it should simply stick to what it does reasonably well: manage service startup. I really only want the ability to mount a mountpoint after a service has started, and to start a service after a mountpoint appeared. And otherwise similar dependencies on devices, perhaps.

I really don’t appreciate systemd getting into the business of managing cross-session environment variables. Is this why modern free software desktops refuse to start more than one session for a single user? I suspect so.

Previously, I didn’t think whatever we gained by giving up multiple-sessions-per-user was worth it, and after today, I’m not quite encouraged to give up on this gut feeling.

How do headsets know they may trigger Google Assistant or Siri?

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I don’t know what the Bose QC35-ii is doing: the Action button refuses to do anything unless it’s sure it’s talking to either Google Assistant or Alexa (no Siri mentioned in the app, interestingly).

I can’t get the 2021 version of the Star Trek TNG Bluetooth Combadge to trigger anything when connected to a Linux machine. The regular press is triggering KEY_PLAYCD and KEY_PAUSECD, thus mapping onto the relevant X events and interacting well with my desktop’s media players (particularly Chrome) — but doublepress, which normally activates Siri on my iPad, sends no input device events on the relevant /dev/input/event* special file. There’s just no traffic.

btmon is an interesting discovery, and it pointed me in the direction of the world of AT commands flowing as ACL Data on my local hci0 device. Many of the ones flowing are documented on Qt Extended’s modem emulator component documentation from 2009: it starts with the combadge sending AT+BRSF and seeing a response, then sending AT+CIDN and getting and response, and so on and on and on.

If I am reading everything right, the values returned are decimal numbers representing a binary mask. btmon output seems to indicate the combadge (‘hands-free’ device) claims it supports 127 (i.e. all 8 functionalities in the Modem Emulator docs), and the desktop (‘audio gateway’) says it supports 1536, which is binary 110 0000 0000, meaning the only bits that are set are in the reserved range from the perspective of the 2009 Modem Emulator documentation.

A list of flags can also be found in 2013 bluez test for HFP. Over there, one of the formerly ‘reserved’ bits is specified as being AG_CODEC_NEGOTIATION, but we can luckily find the other one in ChromiumOS’s source code: inside something called adhd (apparently, ChromiumOS Audio Daemon) and its cras component’s server part, the constants are in cras_hfp_slc.h. So, the other bit the desktop claims to support is AG_HF_INDICATORS, which also has nothing to do with remote control.

That source code also indicates we can read the Hands-Free Profile specification, the latest one being version 1.8 available on Bluetooth.com.

So, if I am interpreting everything correctly, the combadge says it supports “everything”, but the desktop doesn’t tell it back that it knows what voice recognition is. No wonder we’re not seeing any traffic.

So, we don’t quite need to support Apple-specific HFP commands such as AT+XAPL (bluetooth accessory identification), AT+APLSIRI (confirming the device supports specifically Siri) or AT+IPHONEACCEV (sharing battery level), which is nice. Both of the platforms documented by the combadge’s marketing materials and the manual (Google Now i.e. Assistant and Siri) document they support AT+BVRA from the Hands-Free Profile specification; see Google Assistant’s “Voice Activation Optimization” document for Bluetooth devices, as well as the “Accessory Design Guidelines for Apple Devices (release R16 talks about this in section 30.3.1).

Instead, it looks like we mainly need to trick the desktop to respond to combadge’s AT+BRSF request with a bitmask that includes the voice recognition bit, and move on from that, hoping the combadge starts emitting AT+BVRA, and that we can easily programmatically capture that!

But that’s a topic for another post.

Changing the default file manager away from Nautilus

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Nautilus is opening the desktop, making the experience miserable under i3 when all you’re trying to do is ‘view files in directory’.

XFCE’s Thunar is lightweight, and it doesn’t try to create a desktop window.

To get apps such as Chrome to handle “open directory” commands using Thunar, teach xdg-mime about it:

$ sudo apt-get install thunar
$ xdg-mime query default inode/directory
nautilus.desktop
$ xdg-mime default Thunar.desktop inode/directory  # yep, Thunar's desktop file is titlecased on Debian-derived systems
$ xdg-mime query default inode/directory
Thunar.desktop
$ xdg-open .
...thunar opens...

This is sort-of a follow-up to my previous post on this topic. See also, Debian’s article about usage of MIME types.

Ubuntu, touchscreen and dualscreen: HD Touchscreen on the left, laptop on the right

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If you read about calibrating touchscreen with X11 and are capable of simple addition and division, you won’t have a problem building the transformation matrix and applying it. I won’t cover the same details on how to set the matrix using xinput set-prop since I just want to write down the resulting matrix and, perhaps, save people some time if they have a setup similar to mine.

To get the name of the device, open Terminal and punch in:

xinput

In the output I found that my “Acer T232HL”‘s input component is actually called “Advanced Silicon S.A CoolTouch(TM) System”. I can see all of its properties with:

xinput list-props Advanced Silicon S.A CoolTouch(TM) System

Since I’ve set up the screen to be on the left site, that means top-left corner of my touch area is simply offset by 0,0, and bottom-right is 1920,1080. Considering that the total width of the usable X area is 1920+1280, I get 1920/3600=0.6 as the c0 constant on the above link.

My usable area’s height is max(1080, 800) (the respective y component of the resolutions of the two displays). Considering that touch area’s height is also 1080, then the c2 component does not need to be scaled and should be exactly 1.

I also have absolutely no offsets, so c1 and c3 stay 0.

Result:

xinput set-prop "Advanced Silicon S.A CoolTouch(TM) System" --type=float "Coordinate Transformation Matrix" 0.6 0 0 0 1 0 0 0 1
# And if needed, to make the display configured as described above:
xrandr --output DP-0 --auto --left-of LVDS-0

From the times when I was trying to get Linux to run on iPaq H3800 devices using the kernel and distributions published at the now-defunct handhelds.org, I remember the “tslib” which included a touchscreen calibration tool. I wonder if that can and should be used to calibrate the modern multitouch displays.

Update (April 5th 2014): I just dug out package ‘xinput_calibrator’ by random googling, but have not tried it out yet.

On my final project for university

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I’m completing my studies at the Polytechnic of Zagreb this semester, and my final project will be a compositing window manager (as a follow up to my text on the same subject written for the ‘Report’ class).

In discussion with my mentor, I wanted to double check the well-known fact that the university will be the owner of the resulting work. Yes, the university will claim ownership of the resulting work.

The follow up question was whether the resulting project could be GPLed. In that way, I would be able to continue working on the project later on. The answer is — no, I can’t GPL it. I might be able to wrestle with the bureaucracy and get a special exemption, but I’ve decided not to.

To the best of my knowledge, same policies exist at the Faculty of Computing and Electrical Engineering at the University of Zagreb — widely considered one of the best university-level schools in Croatia, if not the best.

I’m highly disappointed by the Croatian universities’ policy of appropriating work I am forced to do for purposes of acquiring a degree.

I have nothing against appropriating the accompanying paper, which is something that will not evolve further once written. I am highly frustrated by the application of same standards on potentially useful, potentially fast changing program code.

As a result, and as a form of protest, the software part of my project will be experimental, proof-of-concept research-quality code, and I will not try too hard to make it maintainable long term. The paper and the project will not be intentionally worse, but they will also not be intentionally better than they could be. This is because I want a clear road without obstacles whenever I decide to create a well-structured compositing window manager; I don’t want any obstacles to being able to modify my own code.

I am certain that numerous student developers in countries with similar practices do the same. This results in enormous waste of time that would, in the academic spirit of information sharing, better serve contributing to free software ecosystem. Instead, who knows how much code is either useless in the real world, or — worse — is actually useful in the real world, but claimed by the universities as their product?

I would suggest the responsible individuals in Croatia to review the history of Google, including the part where BackRub and PageRank Googol Google was a government-funded research project by two PhD students, yet they managed to take it out of the university and start a big company around it.

I would also suggest a review of Ayn Rand’s Atlas Shrugged which, while flawed and too narrow-focused, does offer some food for thought on government appropriations of works. And in cases of academic software projects that would be GPLed anyway, the situation is even worse: we’re seeing an appropriation of a work that I would gladly share with the world and previously intended to do so.