linux-kernel - Re: [PATCH 3/4] rtc: rtc-hid-sensor-time: add option hctosys to set time at boot

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Date:	Wed, 29 May 2013 06:42:14 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	John Stultz <john.stultz@...aro.org>
CC:	linux-kernel@...r.kernel.org, rtc-linux@...glegroups.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Jonathan Cameron <jic23@....ac.uk>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH 3/4] rtc: rtc-hid-sensor-time: add option hctosys to set
 time at boot

Am 28.05.2013 21:37, schrieb John Stultz:
> On 05/21/2013 04:15 PM, Alexander Holler wrote:
>> Am 22.05.2013 00:02, schrieb John Stultz:
>>
>>>
>>> Like Andrew, I think this feels particularly hacky.
>>>
>>> Why exactly is late_init too early? (I'm unfamiliar with the
>>> rtc-hid-sensor-time driver)
>>
>> Currently it can be an USB device (and maybe Bluetooth or even i2c in
>> the future, depends on hid-sensor-hub). That has some implications:
>>
>> (1) Initialization might need longer (or happens later) than
>> late_init, even if everything is linked into the kernel (same problem
>> as with a boot from USB-storage)
>> (2) It might not even be available at boot, but it should work if a
>> user plugs it in afterwards.
>> (3) To accomplish (2) it should set the system time (by default) IFF
>> nothing else did set the time.
>>
>> That "nothing else" in (3) is for security reasons, because no
>> plugable HID device should be able to change the system time by default.
>>
>> The check if something else did set the system time can't be
>> accomplished only by the RTC subsystem because userspace, network or
>> whatever else is able to set the system time most likely doesn't use
>> the RTC subsystem (or hctosys).
>>
>> E.g. one of those setups could be:
>>
>> boot
>> hctosys (fails because of no RTC)
>> ntpdate/rdate/date < whatever
>> load modules (rtc-hid-sensor-time)
>>
>> If we would use a flag in the hctosys module then rtc-hid-sensor-time
>> would be able to change the time (in the setup above).
>>
>> Using a module option which is by default off doesn't help too. Users
>> (or even distros) which would turn it on, might forget it and systems
>> would be at risk if no HID clock will be found at boot (but later
>> plugged in by some blackhat).
>>
>> A flag in the time subsystem itself would do the trick. Such a flag
>> might help with the problem if the RTC subsystem or the persistent
>> clock code did set the time too. You've mentioned in another thread
>> that you had to solve such a problem, but I'm not aware how you did that.
>>
>> Implementation could be as easy as a bool "time_set_at_least_once" in
>> the timer subsystem itself (e.g. in do_settimeofday() and whatever
>> similiar is available).
>
> If it were to be done this way, it would be good to have the RTC layer
> check when RTC devices are registered and call the internal hctosys
> functionality then (rather then just at late_init). Do you want to try
> to rework the patch in this way?

That sounds like what Andrew Morton wanted to trick me to do. ;)

Of course, if the time subsystem would offer such a flag, it would be 
easy to get rid of the hctosys module/driver. The RTC subsystem just 
could check that flag and could set the system time whenever a (usually 
the first) RTC driver would register (and offers a valid time).

In addition that would offer the possibility to add a kernel parameter 
which describes the driver/module (by name) which should be used to set 
the time. I never really liked that rtcN from hctosys, because it 
doesn't really specify the RTC but depends on the order drivers get loaded.

And I still like the idea to have a timewait or clockwait kernel 
parameter which prevents booting until the system has a valid time. 
Booting without having a valid time, can have serious implications (see 
e.g. https://lkml.org/lkml/2013/5/2/260 for which I never got feedback).

> I'm not totally sure I'd agree that it would be better over leaving it
> to userspace, but if we're going to go with an in-kernel policy for
> this, then it seems like a better approach then the current patch.

The only way I see to do such in userspace would be with that hacky 
looking trick I'm using in this patch, because userspace can't reliable 
manage a flag "time_set_at_least_once".

Regards,

Alexander

PS: I left out what happens on resume, because I just don't know how the 
kernel gets the time after a resume (and never looked at it). That might 
especially be tricky with a RTC which needs some time before it can 
offer the time itself (e.g. gps or radio) or is only available after 
some other subsystem like USB is available.

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