ModemManager does not honor blacklisted ttys (ID_MM_DEVICE_IGNORE)

[Tommy Vestermark]

In Ubuntu 19.04 (and likely earlier) Modem Manager is installed with filter policy 'strict' as opposed to 'default', which actually makes Modem Manager _less_ strict regarding explicitly blacklisted ttys (see man modemmanager).

This means that ttys, that are explicitly blacklisted from Modem Manager by using ENV{ID_MM_DEVICE_IGNORE}="1" in its udev rule is still opened and probed by Modem Manager although it should not.

This is a common cause of problems and frustrations for people working with actual serial devices, e.g. within embedded development (e.g. Arduino and the like). See https://nick.zoic.org/art/failed-to-set-dtr-rts-systemd-modemmanager/ for an example.

A proposed solution is to change the following line in: /lib/systemd/system/ModemManager.service
from:
ExecStart=/usr/sbin/ModemManager --filter-policy=strict
to:
ExecStart=/usr/sbin/ModemManager --filter-policy=paranoid

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: modemmanager 1.10.0-1 [modified: lib/systemd/system/ModemManager.service]
ProcVersionSignature: Ubuntu 5.0.0-13.14-generic 5.0.6
Uname: Linux 5.0.0-13-generic x86_64
ApportVersion: 2.20.10-0ubuntu27
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu May 2 15:35:58 2019
EcryptfsInUse: Yes
InstallationDate: Installed on 2018-05-26 (340 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: modemmanager
UpgradeStatus: Upgraded to disco on 2019-04-18 (13 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Lionel Penaud (lionep)]

Also observed on Ubuntu 18.04.3, with kernel 5.0.0-37-generic