fidzu : debian

I was getting "<XF86AudioPlay> is undefined" in the status bar of Emacs displayed every 2-3 seconds. Nowhere else I noticed any misbehavior or problems, and also couldn't find any related log entries. It didn't stop, though didn't want to reboot my system to see whether that would fix the problem, but it was driving me nuts.

Now, as a starting point I adjusted my sway configuration, to react to the XF86AudioPlay key press event:

bindsym XF86AudioPlay exec playerctl play-pause

After reloading sway, my music player started to play for 2-3 seconds, stopped playing, started again, etc. It wasn't a Emacs bug, but something indeed seemed to send the XF86AudioPlay key event every 2-3 seconds. It wasn't my USB keyboard or any stuck key on it, as verified also by unplugging it. So which device was causing this?

libinput from libinput-tools to the rescue:

% sudo libinput debug-events
[...]
-event12  KEYBOARD_KEY                 +0.000s  KEY_PLAYPAUSE (164) pressed
 event12  KEYBOARD_KEY                 +0.000s  KEY_PLAYPAUSE (164) released
 event12  KEYBOARD_KEY                 +2.887s  KEY_PLAYPAUSE (164) pressed
 event12  KEYBOARD_KEY                 +2.887s  KEY_PLAYPAUSE (164) released
 event12  KEYBOARD_KEY                 +5.773s  KEY_PLAYPAUSE (164) pressed
 event12  KEYBOARD_KEY                 +5.774s  KEY_PLAYPAUSE (164) released
[...]

The `event12` device was sending this event, what's behind this?

% sudo udevadm info /dev/input/event12
P: /devices/pci0000:00/0000:00:1f.3/skl_hda_dsp_generic/sound/card0/input17/event12
M: event12
R: 12
J: c13:76
U: input
D: c 13:76
N: input/event12
L: 0
S: input/by-path/pci-0000:00:1f.3-platform-skl_hda_dsp_generic-event
E: DEVPATH=/devices/pci0000:00/0000:00:1f.3/skl_hda_dsp_generic/sound/card0/input17/event12
E: DEVNAME=/dev/input/event12
E: MAJOR=13
E: MINOR=76
E: SUBSYSTEM=input
E: USEC_INITIALIZED=12468722
E: ID_INPUT=1
E: ID_INPUT_KEY=1
E: ID_INPUT_SWITCH=1
E: ID_PATH=pci-0000:00:1f.3-platform-skl_hda_dsp_generic
E: ID_PATH_TAG=pci-0000_00_1f_3-platform-skl_hda_dsp_generic
E: XKBMODEL=pc105
E: XKBLAYOUT=us
E: XKBOPTIONS=lv3:ralt_switch,compose:rctrl
E: BACKSPACE=guess
E: LIBINPUT_DEVICE_GROUP=0/0/0:ALSA
E: DEVLINKS=/dev/input/by-path/pci-0000:00:1f.3-platform-skl_hda_dsp_generic-event
E: TAGS=:power-switch:
E: CURRENT_TAGS=:power-switch:

% sudo udevadm info -a /dev/input/event12 | grep -iE 'kernels|drivers|name'
    KERNELS=="input17"
    DRIVERS==""
    ATTRS{name}=="sof-hda-dsp Headphone"
    KERNELS=="card0"
    DRIVERS==""
    KERNELS=="skl_hda_dsp_generic"
    DRIVERS=="skl_hda_dsp_generic"
    KERNELS=="0000:00:1f.3"
    DRIVERS=="sof-audio-pci-intel-tgl"
    KERNELS=="pci0000:00"
    DRIVERS==""

Behind this event12 is sof-hda-dsp Headphone, and evtest confirms that:

% sudo evtest
No device specified, trying to scan all of /dev/input/event*
Available devices:
/dev/input/event0:      AT Translated Set 2 keyboard
/dev/input/event1:      Sleep Button
/dev/input/event10:     ThinkPad Extra Buttons
/dev/input/event11:     sof-hda-dsp Mic
/dev/input/event12:     sof-hda-dsp Headphone
/dev/input/event13:     sof-hda-dsp HDMI/DP,pcm=3
/dev/input/event14:     sof-hda-dsp HDMI/DP,pcm=4
/dev/input/event15:     sof-hda-dsp HDMI/DP,pcm=5
/dev/input/event16:     Yubico YubiKey OTP+FIDO+CCID
/dev/input/event17:     Apple Inc. Magic Keyboard with Numeric Keypad
/dev/input/event18:     Apple Inc. Magic Keyboard with Numeric Keypad
[...]
Select the device event number [0-24]: ^C

We can even get further information:

% sudo evtest /dev/input/event12
Input driver version is 1.0.1
Input device ID: bus 0x0 vendor 0x0 product 0x0 version 0x0
Input device name: "sof-hda-dsp Headphone"
Supported events:
  Event type 0 (EV_SYN)
  Event type 1 (EV_KEY)
    Event code 114 (KEY_VOLUMEDOWN)
    Event code 115 (KEY_VOLUMEUP)
    Event code 164 (KEY_PLAYPAUSE)
    Event code 582 (KEY_VOICECOMMAND)
  Event type 5 (EV_SW)
    Event code 2 (SW_HEADPHONE_INSERT) state 0
Properties:
Testing ... (interrupt to exit)
Event: time 1779295060.175766, type 5 (EV_SW), code 2 (SW_HEADPHONE_INSERT), value 1
Event: time 1779295060.175766, -------------- SYN_REPORT ------------
Event: time 1779295061.951168, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295061.951168, -------------- SYN_REPORT ------------
Event: time 1779295061.951194, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295061.951194, -------------- SYN_REPORT ------------
Event: time 1779295064.548671, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295064.548671, -------------- SYN_REPORT ------------
Event: time 1779295064.548689, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295064.548689, -------------- SYN_REPORT ------------
Event: time 1779295067.437172, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295067.437172, -------------- SYN_REPORT ------------
Event: time 1779295067.437187, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295067.437187, -------------- SYN_REPORT ------------
Event: time 1779295070.323775, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295070.323775, -------------- SYN_REPORT ------------
Event: time 1779295070.323790, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295070.323790, -------------- SYN_REPORT ------------
Event: time 1779295073.200350, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295073.200350, -------------- SYN_REPORT ------------
Event: time 1779295073.200373, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295073.200373, -------------- SYN_REPORT ------------
Event: time 1779295076.076228, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295076.076228, -------------- SYN_REPORT ------------
Event: time 1779295076.076250, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295076.076250, -------------- SYN_REPORT ------------
Event: time 1779295078.961740, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295078.961740, -------------- SYN_REPORT ------------
Event: time 1779295078.961754, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295078.961754, -------------- SYN_REPORT ------------
Event: time 1779295081.850156, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 1
Event: time 1779295081.850156, -------------- SYN_REPORT ------------
Event: time 1779295081.850175, type 1 (EV_KEY), code 164 (KEY_PLAYPAUSE), value 0
Event: time 1779295081.850175, -------------- SYN_REPORT ------------
Event: time 1779295083.306612, type 5 (EV_SW), code 2 (SW_HEADPHONE_INSERT), value 0
Event: time 1779295083.306612, -------------- SYN_REPORT ------------

So when I plug in my headphone (see the `SW_HEADPHONE_INSERT` event), the unexpected behavior starts, unplugging stops the problem.
Good! But what was totally unexpected for me: my headphone, being a Beyerdynamic DT-990 Pro, does not have any keys. 8-)

As it turned out, the headphone jack seemed to have been not entirely clean. The analog side of the jack triggers a behavior within the audio codec, where it seems to interpret the fluctuating impedance as a play button of the headset, being pressed, again and again.

I cleaned the jack of my headphone and my XF86AudioPlay problem is gone, case closed.

20 May 2026 5:19pm GMT

20 May 2026 2:27pm GMT

I've heard "containers are not a security boundary" enough times that it's started to feel like received wisdom, and my honest read (after 13+ years) is that it's technically defensible but practically sloppy - and the sloppiness matters.

The part that's true: containers share a kernel, and a kernel exploit crosses the container boundary where a VM would not. That difference is real and non-trivial, and the CVE history backs it up - CVE-2019-5736, CVE-2022-0492, and CVE-2024-21626 all happened in "correctly configured" production containers.

The part I'd push back on is that the comparison point is almost never stated. "Containers aren't a security boundary" is being used as shorthand for "containers aren't a VM boundary" - but the conclusion people seem to draw from that is "therefore don't bother", which doesn't actually follow. The more honest version is that default Docker doesn't provide strong isolation between mutually untrusting parties, but a hardened configuration does.

What ships by default in Moby is actually a pretty reasonable foundation: seccomp is enabled (with a builtin profile blocking ~50 syscalls - credit where it's due: this is mostly @jessfraz's work; she even ran contained.af as a public CTF for years daring people to escape a container under her seccomp profile, and to my knowledge it was never claimed), AppArmor is enabled (the docker-default profile), and several sensitive /proc paths are masked. What's not on by default: no-new-privileges (setuid binaries inside can escalate), CAP_NET_RAW is still granted to every container (even though the kernel has supported unprivileged ICMP sockets for over a decade, meaning most modern distributions no longer need CAP_NET_RAW for ping), and user namespace remapping - though user namespaces aren't quite the silver bullet they might sound like; Debian left them disabled by default for years because the kernel attack surface they exposed hadn't been hardened against unprivileged callers.

The boundary isn't absent - it doesn't come completely pre-assembled. With VMs, the hypervisor is there whether you asked for it or not; with containers, assembling the boundary is left as an exercise for the operator. That's a much more solvable problem than "the technology is incapable", but it does mean the work falls to whoever's running the containers.

So, some things you can do today without waiting for defaults to change:

--user (or USER in your Dockerfile) is worth calling out specifically, because I think it's arguably stronger than user namespace remapping in one important way - and partly for the same reason Debian was hesitant about user namespaces in the first place. User namespace remapping protects the host from a root-in-container escape: if you do escape, you land as an unprivileged user on the host. But you were still root inside the container the whole time. Running as a non-root user means you were never root anywhere. The blast radius of a compromised process is limited whether or not it escapes, including for things like reading secrets, modifying container contents, or lateral movement within the container itself. Most application containers have no legitimate reason to be root.

Beyond that, a short list of things that are easy to enable and hard to justify leaving off:

• --security-opt no-new-privileges - prevents setuid binaries from escalating; can also be set daemon-wide in daemon.json with "no-new-privileges": true
• --read-only - a read-only root filesystem means a compromised process can't easily persist tooling or modify the container (pair with a writable tmpfs mount for /tmp etc as needed)
• --cap-drop NET_RAW - or --cap-drop ALL and add back only what you actually need; CAP_NET_RAW is almost never legitimately needed by application containers
• never --privileged - if something seems to require it, the right answer is almost always a more targeted capability grant or bind mount, not the nuclear option

docker run \
  --user 1234:5678 \
  --security-opt no-new-privileges \
  --read-only \
  --tmpfs /tmp \
  --cap-drop ALL \
  acme/untrusted-workload:latest

None of these require a daemon restart or infrastructure changes, and stacked together they go a long way toward actually building the boundary that the defaults leave unbuilt.

(this post was written with the assistance of "claude my eyes right out" but all thoughts and understanding are Tianon's)

20 May 2026 7:00am GMT