Accessing USB device from within snap?

[hadmut]

Hi,

I was trying to use the snap

which works in general, but can’t access the 3d printer connected to USB.

Aug 17 20:49:37 pc kernel: [ 8721.055150] audit: type=1400 audit(1629226177.364:694): apparmor=“DENIED” operation=“open” profile=“snap.cura-slicer.cura” name="/sys/devices/pnp0/00:04/id" pid=57119 comm=“python3” requested_mask=“r” denied_mask=“r” fsuid=1000 ouid=0
Aug 17 20:49:37 pc kernel: [ 8721.055484] audit: type=1400 audit(1629226177.364:695): apparmor=“DENIED” operation=“open” profile=“snap.cura-slicer.cura” name="/sys/devices/pci0000:00/0000:00:01.3/0000:01:00.0/usb1/1-4/bNumInterfaces" pid=57119 comm=“python3” requested_mask=“r” denied_mask=“r” fsuid=1000 ouid=0

It’s a serial connection, i.e. like most 3d printers, an arduino board with a serial IO, in conjunction with a USB-serial converter chip. Pretty normal and widely used under Linux.

The older (outdated) version that comes as a regular ubuntu package, works (but has other flaws, that’s why I would like to use the significantly newer version in the snap).

There’s plenty of web pages describing how to access USB devices from within a snap, but they all do cover USB sticks only, i.e. they do not explain how to access a USB device itself, just to access a mounted file system from a USB device. But things here are different. I need to access the device itself.

Where would I find some docs about setting the permissions correctly?

thanks and regards

[ogra]

for the snap side of things there are the raw-usb interface and there is

Hotplug support

Hotplug support gives snaps the ability to integrate with Linux’s USB hotplugging and Udev device management subsystem. This enables snaps to recognise specific classes of device and create corresponding system slots when a device is connected. Developers wishing to add hotplug functionality to their own interfaces should see Developing hotplug interfaces. Hotplug support is currently under active development, and will become widely available with a future release of snap…

but that might indeed not change/add the permissions of the device itself, if your app does not run as a daemon (i.e. as root) you might need to cater for that as well to make sure the user is in the correct groups etc …

here is an example snap where i pop up a warning for the user if she is not in the video group … the script is a simple command-chain script using yad to show the popup:

https://github.com/ogra1/vlc-pi-snap/blob/master/scripts/checks.sh