How can I change Linux system time without root privileges?

Question

My scenario is the following: I've got a C++ app, which accesses a radio receiver to get a time signal from it and then updates system time according to the time from the radio signal. For the sake of security, the app shall not run with root privileges. After having read this advice from a forum thread, I tried out a code snippet like this:

tm              current_time;
struct timeval *epoch         = new timeval;

// current_time is filled with time data from the radio tuner here.

epoch -> tv_sec  = mktime (&current_time);
epoch -> tv_usec = 0;

if (difftime (epoch -> tv_sec, mktime (&this -> last_system_time_update)) > (time_t) receiver::SYSTEM_TIME_UPDATE_INTERVAL) {
  retval += setgid       ((uid_t) 0);
  retval += setuid       ((uid_t) 0);
  retval += prctl        (PR_SET_KEEPCAPS, 1L, 0L, 0L, 0L);
  retval += setgid       (group_id);
  retval += setuid       (user_id);
  retval += settimeofday (epoch, NULL);
}

Contrary to the advice, this snippet won't work when I'm not running it as root. I'd always get errno = 1.

What's wrong here? And: Is there a workaround?

Answer 1

You're still trying to get root privileges. If you have CAP_SYS_TIME capability, all you need is settimeofday().

if (difftime (epoch -> tv_sec, mktime (&this -> last_system_time_update)) > 
(time_t) receiver::SYSTEM_TIME_UPDATE_INTERVAL) {
  retval += settimeofday (epoch, NULL);
}