If a user does not have CAP_SYS_ADMIN and runs this code:

    if (ioctl(fd, TIOCGSERIAL, &serialUart) == -1)
    {
        printf("ioctl(TIOCGSERIAL) failed: %s\n", strerror(errno));
        close(fd);
        fd = -1;
        return fd;
    }
 
    // Now set the serial configuration with low latency set.
    serialUart.flags |= ASYNC_LOW_LATENCY;
    if (ioctl(fd, TIOCSSERIAL, &serialUart) == -1)
    {
        printf("ioctl(TIOCSSERIAL) failed: %s\n", strerror(errno));
        close(fd);
        fd = -1;
        return fd;
    }

Code inside uart_set_info will temporarily set retval to -EPERM and execute to "goto check_and_exit"

        if (!capable(CAP_SYS_ADMIN)) {
		retval = -EPERM;
		if (change_irq || change_port ||
		    (new_info->baud_base != uport->uartclk / 16) ||
		    (close_delay != port->close_delay) ||
		    (closing_wait != port->closing_wait) ||
		    (new_info->xmit_fifo_size &&
		     new_info->xmit_fifo_size != uport->fifosize) ||
		    (((new_flags ^ old_flags) & ~UPF_USR_MASK) != 0))
			goto exit;
		uport->flags = ((uport->flags & ~UPF_USR_MASK) |
			       (new_flags & UPF_USR_MASK));
		uport->custom_divisor = new_info->custom_divisor;
		goto check_and_exit;
	}

1. Should uport->flags and uport->custom_divisor still get set?

Additionally, first line of code for check_and_exit clears ret_val = -EPERM by setting it back to 0:

    check_and_exit:
	retval = 0;
	if (uport->type == PORT_UNKNOWN)
		goto exit;
	if (tty_port_initialized(port)) {
		if (((old_flags ^ uport->flags) & UPF_SPD_MASK) ||
		    old_custom_divisor != uport->custom_divisor) {
			/*
			 * If they're setting up a custom divisor or speed,
			 * instead of clearing it, then bitch about it.
			 */
			if (uport->flags & UPF_SPD_MASK) {
				dev_notice_ratelimited(uport->dev,
				       "%s sets custom speed on %s. This is deprecated.\n",
				      current->comm,
				      tty_name(port->tty));
			}
			uart_change_speed(tty, state, NULL);
		}
	} else {
		retval = uart_startup(tty, state, 1);
		if (retval == 0)
			tty_port_set_initialized(port, true);
		if (retval > 0)
			retval = 0;
	}
 exit:
	return retval;

Therefore the -EPERM ret_val is lost and the user is unaware of the capabilities rejection.