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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 13 Aug 2019 22:04:05 +0200
Message-ID: <CAK8P3a13f7an3xkQcP7qcSR-yhp=wYMT_cJ9h7gEdB3Qk8chBw@mail.gmail.com>
Subject: Re: New kernel interface for sys_tz and timewarp?
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     Paul Eggert <eggert@cs.ucla.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        "Theodore Ts'o" <tytso@mit.edu>, "H. Peter Anvin" <hpa@zytor.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Alistair Francis <alistair.francis@wdc.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        Karel Zak <kzak@redhat.com>,
        Lennart Poettering <lennart@poettering.net>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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On Tue, Aug 13, 2019 at 9:31 PM Florian Weimer <fw@deneb.enyo.de> wrote:
> * Paul Eggert:
> > Linus Torvalds wrote:
> >> I assume/think that glibc uses (a) environment
> >> variables and (b) a filesystem-set default (per-user file with a
> >> system-wide default? I don't know what people do).
>
> > glibc relies on the TZ environment variable, with a system-wide
> > default specified in /etc/localtime or suchlike (there is no
> > per-user default). glibc ignores the kernel's 'struct timezone'
> > settings for of this, as 'struct timezone' is obsolete/vestigial and
> > doesn't contain enough info to do proper conversions anyway.
>
> I think the configuration value that settimeofday changes is not
> actually a time zone, but an time offset used to interpret various
> things, mostly in a dual-boot environment with Windows, apparently
> (Like the default time offset for extracting timetamps from FAT
> volumes.)
>
> This data has to come from *somewhere*.  The TZ variable and
> /etc/localtime cover something else entirely.

systemd and hwclock call localtime_r() when setting initial
system time an tz information at boot time, so that information
comes from /etc/localtime.

/etc/adjtime is used to determine whether to set warp the
time at boot and rtc update or not warp it.

      Arnd