Ubuntu – create a python daemon that accepts keywords and excecutes commands as root

I think this is more of a Linux permissions question than a programming question, but nevertheless:

You could use ls -l to determine the group-owner of the files in /sys/class/pwm/..., for example, this might be group gpio. Then add all users who need to execute those commands to that group with adduser username gpio. Maybe you need some udev rules to set up the group ownership first.

Or you could write a small C program for this, compile it, and then put the compiled binary in /usr/local/bin/pwmcontrol with SETGID to the gpio group, or whichever group owns the control files ¹. Then it will always run with the rights of that group. Note that SETGID (and SETUID) only works with a compiled program, not with a script.

gcc -Wall -Werror -pedantic pwmcontrol.c -o pwmcontrol
sudo cp pwmcontrol /usr/local/bin/
sudo chgrp gpio /usr/local/bin/pwmcontrol
sudo chmod g+s /usr/local/bin/pwmcontrol

Here’s some C code to get you started, including error handling.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int pwm_enable() {
    /// TODO implement
    return 0; // success
}

int pwm_disable() {
    /// TODO implement
    return 0; // success
}


int write_file(const char *path, const char *data) {
    FILE *f = fopen(path, "w");
    if (!f) return 1;
    size_t length = strlen(data);
    size_t written = fwrite(data, 1, length, f);
    fclose(f);
    return (length != written) ? 1 : 0; // 0 == success, 1 == fail
}

int pwm_export() {
    /// TODO implement
    return 0; // success
}

void err_exit(const char *message, int exitcode) {
    perror(message);
    pwm_disable(); // for safety reasons, turn off PWM if something went wrong
    exit(exitcode);
}

int pwm_set(const char *period, const char *duty_cycle) {
    if (0 != write_file("/sys/class/pwm/pwmchip5/pwm0/period", period)) {
        return 1;
    }
    if (0 != write_file("/sys/class/pwm/pwmchip5/pwm0/duty_cycle", duty_cycle)) {
        return 1;
    }
    return 0; // success
}

int main(int argc, char **argv) {
    // Determine what to do according to the first argument
    if (argc == 4 && strcmp(argv[1], "set") == 0) {
        if (0 != pwm_export()) {
            err_exit("Error exporting PWM chip", 1);
        }
        if (0 != pwm_set(argv[2], argv[2])) {
            err_exit("Error setting PWM period and duty cycle", 1);
        }
        if (0 != pwm_enable()) {
            err_exit("Error enabling PWM", 1);
        }
        exit (0); // 0 == success
    }
    else if (argc == 2 && strcmp(argv[1], "start") == 0) {
        // TODO
        exit (0); // 0 == success
    }
    else if (argc == 2 && strcmp(argv[1], "stop") == 0) {
        // TODO
        exit (0); // 0 == success
    }

    printf("Usage:n");
    printf("    %s set PERIOD DUTYCYCLEn", argv[0]);
    printf("    %s startn", argv[0]);
    printf("    %s stopn", argv[0]);
    exit(1);
}

¹ Of course, you could also use SETUID root (chown root + chmod u+s), but running something as root is dangerous. Especially if it is a C program that you wrote yourself without any idea of how C security exploits work.