linux-kernel - Re: [PATCH] rtc: Add an option to invalidate dates in 2038

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Message-ID: <2768647.1bFEcFDZRI@wuerfel>
Date:	Mon, 22 Feb 2016 17:18:03 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	rtc-linux@...glegroups.com,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: Add an option to invalidate dates in 2038

On Monday 22 February 2016 16:56:53 Alexandre Belloni wrote:
> On 22/02/2016 at 16:44:32 +0100, Arnd Bergmann wrote :
> > On Monday 22 February 2016 13:43:19 One Thousand Gnomes wrote:
> > > On Mon, 22 Feb 2016 14:00:14 +0100
> > > Alexandre Belloni <alexandre.belloni@...e-electrons.com> wrote:
> > > > I can also agree that systemd could be a bit more robust there but
> > > > you'll have to convince Lennart...
> > > 
> > > That's a systemd problem. If their code isn't robust then the
> > > distributiosn will just have to keep patching it.
> > > 
> > > The only problem that can actually be "fixed" is the case where it isn't
> > > 2038 yet and the user has a scrambled RTC. In that case your init tools
> > > need to be robust enough to handle the problem or use APIs that don't
> > > break. The kernel can't actually "fix" this because it never knows
> > > whether your userspace is sane or not.
> > > 
> > > I'd argue btw that any code using timerfd_create with TFD_TIMER_ABSTIME
> > > and passing it a value that wraps the range permitted by that time
> > > representation *is* buggy. It's the applications responsibility to use
> > > values that are within the defined behavioural range of the function.
> > 
> > IIRC, the problem is that user space passes in TIME_T_MAX and the kernel
> > is considering that to be in the past because the clock is set beyond 2038.
> > 
> > I find it hard to blame user space for that, but I don't have a good
> > idea for solving this either.
> > 
> > In case of systemd, it is literally the first thing that runs on the kernel
> > after booting, so we could fall back to setting the time to some known
> > working state (1970 or 2016 or something), but that would be a rather
> > bad default policy once the system has been running for a while.
> > 
> 
> Also, how would you know that it is an invalid time, some RTC doesn't
> provide that information. 

What I meant was encountering a time past the 2038 date, which is invalid
as seen from current 32-bit user space, but not necessarily from the
kernel.

> One other workaround is to asked distributions
> using systemd to stop using HCTOSYS so userspace would be responsible to
> set the system time and in that case we won't have the 32/64 discrepancy.

I'm missing a bit of background here. This seems like a fairly useful
piece of infrastructure for the majority of the use cases (working RTC)

How would the time get set when this is disabled? Is systemd able
to read the rtc and write it back to the kernel? That could in fact
be a nicer workaround for the problem, if it just does this before
setting up the timerfd.

	Arnd