How can I prevent Raspbian from resetting user privileges on /dev/tttyS0?

Question

I've got a GPS module hanging on the TTL UART wires of my Pi 3B. I don't access the module directly, but via gpsd. Now I've got the following issue: At every reboot, Raspbian (Buster) seems to reset the group and the privilege mask of the UART:

pi@autoradio:/import/valen/autoradio $ ls -al /dev/ttyS0
crw--w---- 1 root tty 4, 64 Sep 11 00:06 /dev/ttyS0

Consequence: gpsd can't access the module and quits:

pi@autoradio:/import/valen/autoradio $ sudo systemctl status gpsd
● gpsd.service - GPS (Global Positioning System) Daemon
   Loaded: loaded (/lib/systemd/system/gpsd.service; disabled; vendor preset: enabled)
   Active: active (running) since Sat 2021-09-11 00:05:55 CEST; 17min ago
 Main PID: 608 (gpsd)
    Tasks: 1 (limit: 2059)
   CGroup: /system.slice/gpsd.service
           └─608 /usr/sbin/gpsd /dev/ttyS0

Sep 11 00:05:55 autoradio systemd[1]: Starting GPS (Global Positioning System) Daemon...
Sep 11 00:05:55 autoradio systemd[1]: Started GPS (Global Positioning System) Daemon.
Sep 11 00:13:18 autoradio gpsd[608]: gpsd:ERROR: SER: device open of /dev/ttyS0 failed: Permission denied - retrying read-only
Sep 11 00:13:18 autoradio gpsd[608]: gpsd:ERROR: SER: read-only device open of /dev/ttyS0 failed: Permission denied
Sep 11 00:13:18 autoradio gpsd[608]: gpsd:ERROR: /dev/ttyS0: device activation failed.
Sep 11 00:13:18 autoradio gpsd[608]: gpsd:ERROR: /dev/ttyS0: activation failed, freeing device

As far as I can remember, I had a privilege structure like this before making a new SD card with a fresh Raspbian:

crw-rw---- 1 root dialout

Setting privileges by hand only works until the next shutdown. What's going on here? How can I tell Raspbian to get the hand off the privileges of /dev/ttyS0? Thank you.

Answer 1

Setting privileges by hand only works until the next shutdown. What's going on here?

As per a comment on the question, files in /dev (and various other places such as /sys and /proc) aren't stored on disk anywhere; if you find stuff there on the card when you put it in another machine (ie., not using it as a root fs), it's there by mistake and in any case will be not be used by anything during runtime as a devfs filesystem is mounted there by the kernel. This filesystem exists only in RAM and is used as an interface with the kernel.

To change how /dev/ttyS0 is configured at boot, create a file in /etc/udev/rules.d owned root and prefixed with a number,¹ eg. 00-ttyS0.rules, and the following contents:

KERNEL=="ttyS0", GROUP="dialout", MODE="0660"

You can then test this:

> sudo udevadm control --reload-rules
> sudo udevadm trigger /dev/ttyS0
> stat /dev/ttyS0

You should get the settings you asked for. This will then persist across boots.

Since gpsd is a stock service, I imagine the reason this isn't already done is you could be using any one of a number of serial dev nodes. Udev is a bit awkward to use (IMO) but reasonably well documented; see man udev for a start (and if you are searching online remember, it isn't specific to the Pi or Raspbian/RpiOS, but works the same everywhere).

---

1. It doesn't actually have to have a number prefixed, but this is the convention as the files are processed in lexicographical order, earlier takes precedence and there are a great many udev rule files loaded from other places; but an 00- prefix should trump any of them. Doesn't matter much as I think you will have no conflicts for this one.

Answer 2

After an outside hint, I re-ran raspi-config. At first glance, the serial port settings were correct, so that I hadn't to change them:

The serial login shell is disabled
The serial interface is enabled

…but there must have been an issue (maybe even with udev), which disappeared when raspi-config saved its settings for a second time. Now the rights on the serial port are correct and remain after reboot:

pi@autoradio:/import/valen/autoradio $ ls -al /dev/ttyS*
crw-rw---- 1 root dialout 4, 64 Sep 11 23:24 /dev/ttyS0