Reading from /dev/serial0 changes group from dialout to tty and read permisson is gone

[RDPUser]

RPI4 latest packages, just upgraded

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 uname -r
5.4.72-v7l+

Boot RPi4

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ls -l /dev/serial0
lrwxrwxrwx 1 root root 5 Nov 22 00:07 /dev/serial0 -> ttyS0
pi@raspberrypi:~ $ ls -l /dev/ttyS0
crw-rw---- 1 root dialout 4, 64 Nov 22 00:07 /dev/ttyS0

Now I'm running the script from here https://pypi.org/project/mh-z19/, note as user pi

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pi@raspberrypi:~ $ python3 -m mh_z19
{"co2": 395}

Now group of serial port changed

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ls -l /dev/ttyS0
crw--w---- 1 root tty 4, 64 Nov 22 00:09 /dev/ttyS0

Subsquent calls of reading the sensor fails

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 python3 -m mh_z19
Traceback (most recent call last):
  File "/home/pi/.local/lib/python3.7/site-packages/serial/serialposix.py", line 265, in open
    self.fd = os.open(self.portstr, os.O_RDWR | os.O_NOCTTY | os.O_NONBLOCK)
PermissionError: [Errno 13] Permission denied: '/dev/serial0'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/pi/.local/lib/python3.7/site-packages/mh_z19/__init__.py", line 60, in mh_z19
    ser = connect_serial()
  File "/home/pi/.local/lib/python3.7/site-packages/mh_z19/__init__.py", line 56, in connect_serial
    timeout=1.0)
  File "/home/pi/.local/lib/python3.7/site-packages/serial/serialutil.py", line 240, in __init__
    self.open()
  File "/home/pi/.local/lib/python3.7/site-packages/serial/serialposix.py", line 268, in open
    raise SerialException(msg.errno, "could not open port {}: {}".format(self._port, msg))
serial.serialutil.SerialException: [Errno 13] could not open port /dev/serial0: [Errno 13] Permission denied: '/dev/serial0'
null

Pleae note not only group changes, but also the read permission of group is gone.

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sudo chmod g+r  /dev/ttyS0

Works only once. After executing

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python3 -m mh_z19

I get a value. But then read permission is gone again.

Whats wrong here? Since I'm not root the script shouldn't be able to change group and permission of /dev/ttyS0
Trying it manually fails, as expected.

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pi@raspberrypi:~ $ chmod g-r  /dev/ttyS0
chmod: changing permissions of '/dev/ttyS0': Operation not permitted

[cleverca22]

is there a login console running on that serial port?
check the output of "ps aux | grep getty" ?

[RDPUser]

Indeed it seems running.

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ps aux | grep getty
root       560  0.0  0.0   4308  1300 tty1     Ss+  00:19   0:00 /sbin/agetty -o -p -- \u --noclear tty1 linux
pi        6132  0.0  0.0   7348   564 pts/0    S+   08:59   0:00 grep --color=auto getty

However raspi-config tells me:

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The serial login shell is disabled                       │
 The serial interface is enabled    

Yesterday I've enabled serial port and I am 100% sure I've selected no at the question for serial login shell.

How can the login shell cause such a behaviour?
How can I deactivate it?

[SurferTim]

I think this will do it:
Menu -> Preferences -> Raspberry Pi Configuration -> Interfaces.
enable serial port
disable serial console

[RDPUser]

I'm on buster lite only connected via SSH. I habe only raspi-config

vid.gif (157.07 KiB) Viewed 408 times

If I enable serial console, I have another agetty process

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ps aux | grep getty
root       563  0.0  0.0   6616  1552 ttyS0    Ss+  09:46   0:00 /sbin/agetty -o -p -- \u --keep-baud 115200,38400,9600 ttyS0 vt220
root       568  0.0  0.0   4308  1240 tty1     Ss+  09:46   0:00 /sbin/agetty -o -p -- \u --noclear tty1 linux
pi        1001  0.0  0.0   7348   544 pts/0    S+   09:46   0:00 grep --color=auto getty

When I disable again I have again

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pi@raspberrypi:~ $ ps aux | grep getty
root       590  0.0  0.0   4308  1304 tty1     Ss+  09:47   0:00 /sbin/agetty -o -p -- \u --noclear tty1 linux
pi        1010  0.0  0.0   7348   556 pts/0    S+   09:47   0:00 grep --color=auto getty

[cleverca22]

the one on tty1 is for login with a usb keyboard and the hdmi port, so that one is fine

[hippy]

Note the Python code does things with "getty". I didn't analyse it further but there could be something in that.

https://github.com/UedaTakeyuki/mh-z19/ ... /mh_z19.py

[trejan]

Note the Python code does things with "getty". I didn't analyse it further but there could be something in that.

Eww. Why is code for reading a CO2 sensor messing about with this. getty is the process that handles the login prompt. If you don't give mh_z19.py the serial_console_untouched argument then it stops getty, does a reading and then starts getty even if it wasn't running before.

RDPUser: You must run mh_z19.py as root or pass that argument to stop it messing with getty.

[edit]removed comment about code[/edit]

[cleverca22]

Note the Python code does things with "getty". I didn't analyse it further but there could be something in that.

Eww. Why is code for reading a CO2 sensor messing about with this. getty is the process that handles the login prompt. If you don't give mh_z19.py the serial_console_untouched argument then it stops getty, does a reading and then starts getty even if it wasn't running before.

ewww!, that explains everything!

[hippy]

Eww. Why is code for reading a CO2 sensor messing about with this. getty is the process that handles the login prompt. If you don't give mh_z19.py the serial_console_untouched argument then it stops getty, does a reading and then starts getty even if it wasn't running before.

Thanks for the analysis. It appears the author of the code imagined people would be running a login on the serial port using a UART module and would be swapping that for the sensor, wrote the code to disable 'getty' and re-enable it once sensor reading had been taken.

Doing it the way the author did required running with 'sudo' and the 'serial_console_untouched' option was primarily added so 'sudo' wasn't required.

We should however not be too harsh on the author. This is the inevitable consequences of encouraging people to code, them doing that as best they can, then letting their code loose on the world.

[trejan]

We should however not be too harsh on the author. This is the inevitable consequences of encouraging people to code, them doing that as best they can, then letting their code loose on the world.

Yes. That is true. It is a very dubious feature IMO but they must have needed it for themselves. I've removed my comment regarding that as they were good enough to upload this for other people and also take requests for changes.