[ISSUE] gpsd: Permission issues, does not work on USB serial GPS sources without manual chmod

[ISSUE] gpsd: Permission issues, does not work on USB serial GPS sources without manual chmod

[SorrelArticulata]

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New issue by SorrelArticulata on void-packages repository

https://github.com/void-linux/void-packages/issues/40858

Description:
### Is this a new report?

Yes

### System Info

Void 5.19.17_1 x86_64 GenuineIntel uptodate rFFFFFFFFFFF

### Package(s) Affected

gpsd-3.24_2

### Does a report exist for this bug with the project's home (upstream) and/or another distro?

_No response_

### Expected behaviour

1. Plug in USB GPS source (mine is a u-blox 7)
2. Verify it works and has a GPS lock using gpsmon as root.  This tool directly talks to the GPS device rather than needing gpsd to be working.

``` # gpsmon /dev/ttyACM0 ```

3. Stop gpsmon

4. Start gpsd daemon in one terminal (as root, with verbose debugging flags like this)

```# gpsd -N -F /run/gpsd.sock -D5 /dev/ttyACM0```

5. Run cgps (or any other gps client) in another terminal.  This will talk to gpsd and (if everything is working) give you similar output to what gpsmon did.

``` $ cgps ```

### Actual behaviour

cgps fails to see any data and gpsd spits the following errors:

```
gpsd:PROG: CORE: gpsd_open(/dev/ttyACM0) fd -1
gpsd:PROG: SER: gpsd_serial_open(/dev/ttyACM0) sourcetype 12 fd -1
gpsd:INFO: SER: opening GPS data source type 12 at '/dev/ttyACM0'
gpsd:ERROR: SER: device open of /dev/ttyACM0 failed: Permission denied(13) - retrying read-only
gpsd:ERROR: SER: read-only device open of /dev/ttyACM0 failed: Permission denied(13)
gpsd:ERROR: /dev/ttyACM0: device activation failed, freeing device.
```

Looking at the running process list reveals that the gpsd process has changed to the 'gpsd' user. 

Permissions for the serial device:
```
$ ls -lh /dev/ttyACM0 
crw-rw---- 1 root dialout 166, 0 Nov 30 22:38 /dev/ttyACM0
```

Adding the gpsd user to the dialout group does NOT fix the problem.

Allowing anyone rw access to the serial device DOES fix the problem temporarily
```
# chmod a+x /dev/ttyACM0
```

According to the gpsd manpage:

>       Before doing any processing of GPS data, it tries to drop root
>       privileges by setting its UID to "nobody" (or another configured
>       userid) and its group ID to the group of the initial GPS passed on the
>       command line — or, if that device doesn’t exist, to the group of
>       /dev/ttyS0.

I do not understand how processes can have group ids, but regardless this feature does not seem to be working as intended.

The srcpkg template has this line in it, not sure if it is related to this problem or not:

```make_build_args="dbus_export=0 gpsd_user=gpsd gpsd_group=gpsd```



### Steps to reproduce

See above.

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[SorrelArticulata]

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New comment by SorrelArticulata on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-1332053131

Comment:
It looks like gpsd is changing to user 'gpsd' and group 'gpsd'; instead of user 'gpsd' and group 'dialout' (group of provided special serial file).  This does not match what the manpage says and is probably the cause of the issue here.

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[SorrelArticulata]

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New comment by SorrelArticulata on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-1332053131

Comment:
It looks like gpsd is changing to user 'gpsd' and group 'gpsd'; instead of user 'gpsd' and group 'dialout' (group of provided special serial file).  This does not match what the manpage says and is probably the cause of the issue here.

You can confirm this by (1) looking at the output of gpsd with verbose flags and then (2) checking the user/groupids with getend:
```
...
gpsd:INFO: running with effective group ID 977
gpsd:INFO: running with effective user ID 981
...
```
```
$ getent group 977
gpsd:x:977:
$ getent passwd 981
gpsd:x:981:977:gpsd unprivileged user:/var/empty:/sbin/nologin
```

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[SorrelArticulata]

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New comment by SorrelArticulata on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-1332053131

Comment:
It looks like gpsd is changing to user 'gpsd' and group 'gpsd'; instead of user 'gpsd' and group 'dialout' (group of provided special serial file).  This does not match what the manpage says and is probably the cause of the issue here.

You can confirm this by (1) looking at the output of gpsd with verbose flags and then (2) checking the user/groupids with getent:
```
...
gpsd:INFO: running with effective group ID 977
gpsd:INFO: running with effective user ID 981
...
```
```
$ getent group 977
gpsd:x:977:
$ getent passwd 981
gpsd:x:981:977:gpsd unprivileged user:/var/empty:/sbin/nologin
```

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[sgn]

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New comment by sgn on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-1332306349

Comment:
Switch `gpsd` to run with group `dialout` instead of `gpsd` would work, but I don't think it's a correct thing to do.
I **think** you should either `chown gpsd /dev/ttyACM0` or `chgrp gpsd /dev/ttyACM0` before running gpsd.
I don't really know about `gpsd`, just a pure guess.

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[SorrelArticulata]

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New comment by SorrelArticulata on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-1332699928

Comment:
According to the gpsd manpage: it should be automatically setting its own group to dialout when you provide it a device node (eg /dev/ttyACM0) that has the group dialout.  Something in the default application behavior is being overridden.

N.B. the manpage makes no mention of the group 'gpsd'.  That might be unique to the packaging here.

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[CtrlC-Root]

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New comment by CtrlC-Root on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-2564608365

Comment:
> According to the gpsd manpage: it should be automatically setting its own group to dialout when you provide it a device node (eg /dev/ttyACM0) that has the group dialout. Something in the default application behavior is being overridden.
> 
> N.B. the manpage makes no mention of the group 'gpsd'. That might be unique to the packaging here.

Here's the code that switches from the root user/group to a less privileged user/group:
https://gitlab.com/gpsd/gpsd/-/blob/master/gpsd/gpsd.c?ref_type=heads#L2690

Notice that if `GPSD_GROUP` is set it uses that unconditionally otherwise it uses the group of the first device passed on the command line as far as I can tell.

Here's the SCons build arguments used in the Void Linux `gpsd` package:
https://github.com/void-linux/void-packages/blob/master/srcpkgs/gpsd/template#L6

Notice that `gpsd_user` and `gpsd_group` are both set to `gpsd`. I am assuming these flags are interpreted by the project Scons build system into the `GPSD_GROUP` define referenced above. So if the daemon is started as `root` then it will always attempt to switch to `gpsd:gpsd`.

This is somewhat incovnenient since most serial GPS devices are going to be owned by `root:dialout`. There's also no udev rules, hotplug script, or hook script provided with this package.

Re: gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

[CtrlC-Root]

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New comment by CtrlC-Root on void-packages repository

https://github.com/void-linux/void-packages/issues/40858#issuecomment-2564608365

Comment:
> According to the gpsd manpage: it should be automatically setting its own group to dialout when you provide it a device node (eg /dev/ttyACM0) that has the group dialout. Something in the default application behavior is being overridden.
> 
> N.B. the manpage makes no mention of the group 'gpsd'. That might be unique to the packaging here.

Here's the code that switches from the root user/group to a less privileged user/group:
https://gitlab.com/gpsd/gpsd/-/blob/master/gpsd/gpsd.c?ref_type=heads#L2690

Notice that if `GPSD_GROUP` is set it uses that unconditionally otherwise it uses the group of the first device passed on the command line as far as I can tell.

Here's the SCons build arguments used in the Void Linux `gpsd` package:
https://github.com/void-linux/void-packages/blob/master/srcpkgs/gpsd/template#L6

Notice that `gpsd_user` and `gpsd_group` are both set to `gpsd`. I am assuming these flags are interpreted by the project Scons build system and used to set `GPSD_USER` and `GPSD_GROUP` defines referenced above. So if the daemon is started as `root` then it will always attempt to switch to `gpsd:gpsd`.

This is somewhat incovnenient since most serial GPS devices are going to be owned by `root:dialout`. There's also no udev rules, hotplug script, or hook script provided with this package.