Query on Real-Time-Clock (RTC)

ajaygarg · 03-07-2012, 08:55 AM

Hi all.

I require disabling all attempts of a Fedora user to modify system time.
I am thinking of disabling writing to RTC after the OS boots up; that way, no separate hacks per application will be needed.

Is that a sensible solution? If yes, what could be the cleanest implementation for this?

ajaygarg · 03-07-2012, 09:05 AM

Also, I get this seemingly strange behaviour of command-sequences ::

Code:

[ajay@ajay ~]$ ls -l /dev/rtc
lrwxrwxrwx. 1 root root 4 Mar  8  2012 /dev/rtc -> rtc0
[ajay@ajay ~]$ ls -l /dev/rtc
lrwxrwxrwx. 1 root root 4 Mar  8  2012 /dev/rtc -> rtc0
[ajay@ajay ~]$ sudo chmod 0555 /dev/rtc
[ajay@ajay ~]$ ls -l /dev/rtc
lrwxrwxrwx. 1 root root 4 Mar  8  2012 /dev/rtc -> rtc0

In other words, though the "chmod" runs (seemingly) without any errors, yet the permissions do not (seem to) take effect.

Why could this be happening?

Regards,
Ajay

ajaygarg · 03-07-2012, 01:57 PM

Well, I am able to change the permissions for "/dev/rtc0" (but not "/dev/rtc"); however, I am still able to change the time via

a. gnome-applet
b. ntpdate

michaelk · 03-08-2012, 07:42 AM

There are two clocks, the system clock and the RTC or hardware clock. The RTC basically keeps time while the computer is off and at start up the OS will read the hardware clock to set the system clock. Both the gnome-applet and ntpdate change the system clock.

ajaygarg · 03-08-2012, 12:31 PM

Thanks michaelk.

That was a lot of help; it helped me point to http://www.linuxsa.org.au/tips/time.html, and my understanding only got better; thanks

Just wanting a piece of guidance; I am actually wanting that no user should be able to change the time. One obvious solution is to prevent giving "sudo" previleges to the user. However, I am wondering, if a lower-level solution is possible?

Doing

Code:

sudo chmod 0555 /dev/rtc0

would prevent writing to the RTC; how can one do the same for system clock?

Thanks anyways. I am grateful for the pointers.

Regards,
Ajay

ajaygarg · 03-08-2012, 01:03 PM

I tried searching code for "hwclock" in "systemd" package (Fedora distro), in order to find the "keeper" of system-time; however, I find nothing there, except ::

Code:

[ajay@ajay systemd-10]$ grep -r -i -s "hwclock" .
./Makefile.in:	units/hwclock-load.service units/hwclock-save.service \
./Makefile.in:	units/hwclock-load.service units/hwclock-save.service \
./Makefile.am:	units/hwclock-load.service \
./Makefile.am:	units/hwclock-save.service \
./units/hwclock-load.service:ExecStart=/sbin/hwclock --systz
./units/hwclock-load.service:# Note the weird semantics if hwclock and the kernel here: the first
./units/hwclock-save.service:ExecStart=/sbin/hwclock --systohc