core: fall back to bind-mounts for PrivateDevices= execution environments

[Christian Brauner]

Hey,

Currently any service that has PrivateDevices=true set will fail to start in unprivileged containers since mknod is not possible and in privileged containers that drop CAP_MKNOD. I pushed a patch to systemd upstream that solves this problem and makes PrivateDevices useable in both scenarios. It would be great if this could be backported to Ubuntu 16.04 and 18.04. We already have a lot of users that would like this feature enabled/don't want to edit each service file:

16498617443da94533ef9ae28be0ffaace40c526 : https://github.com/systemd/systemd/commit/af984e137e7f53ca3e2fd885b03a25e17fdd0fad

af984e137e7f53ca3e2fd885b03a25e17fdd0fad : https://github.com/systemd/systemd/commit/16498617443da94533ef9ae28be0ffaace40c526

Thanks!
Christian

[Christian Brauner (cbrauner)]

We just had a short discussion on systemd and for systemd 229 on 16.04 we also need:

9e5f825280192be429cc79153235d12778427fae : https://github.com/systemd/systemd/commit/9e5f825280192be429cc79153235d12778427fae

[Launchpad Janitor (janitor)]

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

[Brian Morton (rokclimb15)]

Confirmed problem on Ubuntu 16.04 host running 16.04 container with a packaged version of Proxysql2. Changing PrivateDevices to "no" allows the service to start.

```
[Unit]
Description=High Performance Advanced Proxy for MySQL
After=network.target

[Service]
Type=forking
RuntimeDirectory=proxysql
ExecStart=/usr/bin/proxysql -c /etc/proxysql.cnf
PIDFile=/var/lib/proxysql/proxysql.pid
SyslogIdentifier=proxysql
Restart=no
User=proxysql
Group=proxysql
PermissionsStartOnly=true
UMask=0007
LimitNOFILE=102400
LimitCORE=1073741824
ProtectHome=yes
NoNewPrivileges=true
CapabilityBoundingSet=CAP_SETGID CAP_SETUID CAP_SYS_RESOURCE
RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX AF_ALG
ProtectSystem=full
PrivateDevices=yes

[Install]
Alias=proxysql
WantedBy=multi-user.target
```

[Dan Streetman (ddstreet)]

please reopen if this is still an issue