Snap hangs when accessing a special /dev/file (/dev/vcio) [ubuntu-core 16 / RaspberryPi2]

[Simon Fels]

On 07.11.2016 08:53, Didier Roche wrote:
> Le 06/11/2016 à 11:50, Jacques Supcik a écrit :
>> Dear community.
>>
>> I have a program <https://gitlab.com/telecom-tower/tower>, written in
>> go, and running on a raspberry Pi 2, which displays messages on a LED
>> panel. The panel <https://www.adafruit.com/product/2294> is made of
>> "neo pixels" and is connected to the GPIO of the raspberry Pi.
>> Internally, it uses the DMA and PWM to generate and accurate signals.
>>
>> The program currently runs well on a Raspbian system and I would like
>> to move to ubuntu-core. I was able to build the snap and to deploy it
>> on a raspberry Pi 2 running Core version 16. I installed the snap with
>> "--dangerous", but as soon as the system configure the DMA/PWM/GPIO,
>> it just freezes.
>>
>> I installed snappy-debug to check the logs and this is what I get:
>>
>> = AppArmor =
>> Time: Nov  6 09:28:45
>> Log: apparmor="DENIED" operation="open"
>> profile="snap.telecom-tower.daemon" name="/dev/vcio" pid=4441
>> comm="tower" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
>> File: /dev/vcio (read)
>>
>> = Seccomp =
>> Time: Nov  6 09:28:45
>> Log: auid=4294967295 uid=0 gid=0 ses=4294967295 pid=4441 comm="tower"
>> exe="/snap/telecom-tower/x1/bin/tower" sig=31 arch=40000028 14(mknod)
>> compat=0 ip=0x76e3aaf6 code=0x0
>> Syscall: mknod
>>
>> Note that if I scp the binary file and run on ubuntu core as root,
>> then the program runs fine. (I don't install the snap, I just run the
>> binary from the home directory.) 
>>
>> Does someone have an idea about what I can do to make my snap run?
>>
>> Thank you very much in advance for your support.
> 
> Hey Jacques,
> 
> I think we did discuss about GPIO support in confined mode, but there is
> no interface for this today AFAIK (CCing Jamie who can confirm and maybe
> give a path/ETA forward).


There is a GPIO interface. The downside of it is that you need a slot
defined on the gadget snap. As long as this isn't done for the official
gadget snaps for Pi 2/3 you can only build your own. See
https://github.com/snapcore/snapd/blob/master/interfaces/builtin/gpio.go
for details about the interface itself.

regards,
Simon