Re: RTC hctosys disabled for 32-bit systems

From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arm-kernel@lists.infradead.org, y2038@lists.linaro.org
Subject: Re: RTC hctosys disabled for 32-bit systems
Date: Thu, 1 Sep 2022 14:49:54 +0200	[thread overview]
Message-ID: <YxCqco98XvABPtaG@mail.local> (raw)
In-Reply-To: <802ca9c8-4b61-4568-a946-8e6330807eb3@www.fastmail.com>

On 01/09/2022 13:55:19+0200, Arnd Bergmann wrote:
> On Thu, Sep 1, 2022, at 1:31 PM, Reinier Kuipers wrote:
> >
> > I'm working to fix the y2038 issue for an existing sama5d3-based
> > product. This involves updating the kernel and glibc to appropriate
> > versions (5.10 and 2.35.1 respectively) and I got things running up to
> > a state where, from userspace, both date and hwclock commands have no
> > issue accepting dates beyond 2038. However, even with the RTC_HCTOSYS
> > and RTC_HCTOSYS_DEVICE options configured correctly, the RTC driver
> > fails to initialize the system clock at bootup.
> >
> > Some digging in rtc/class.c::rtc_hctosys() indicates that
> > do_settimeofday64() is deliberately not executed on systems with
> > BITS_PER_LONG==32 and a second counter higher than INT_MAX. I assumed
> > that the work on 64-bits timestamps was already fully implemented for
> > 32-bit systems as well, so my gut feel is that this
> > BITS_PER_LONG/INT_MAX check has become unnecessary. A test build with
> > these checks disabled results in correct time initialization at bootup
> > with, at a glance, no adverse effects. Does anybody here know whether
> > do_settimeofday64()  is robust on 32-bit systems or that the checks
> > are still required to prevent further breakage?
> 
> Please see commit b3a5ac42ab18 ("rtc: hctosys: Ensure system time
> doesn't overflow time_t") and  https://github.com/systemd/systemd/issues/1143
> for the problem that originally caused this to be added.
> 
> Removing this check would probably break systemd again for machines
> that return a post-y2038 time with systemd built on 32-bit time_t.
> 
> The only reliable fix I can see would be to disable
> CONFIG_RTC_HCTOSYS_DEVICE. I think this is Alexandre's plan
> for the long run anyway, but I don't know if there has been any
> progress in convincing distros to turn it off.
> 

This is still my plan but systemd mandates RTC_HCTOSYS and I couldn't
convince Lennart otherwise.

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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