linux-kernel - Re: New kernel interface for sys

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Date:   Tue, 13 Aug 2019 22:04:05 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Paul Eggert <eggert@...ucla.edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        "Theodore Ts'o" <tytso@....edu>, "H. Peter Anvin" <hpa@...or.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Alistair Francis <alistair.francis@....com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Karel Zak <kzak@...hat.com>,
        Lennart Poettering <lennart@...ttering.net>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: New kernel interface for sys_tz and timewarp?

On Tue, Aug 13, 2019 at 9:31 PM Florian Weimer <fw@...eb.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 settings for of this, as 'struct timezone' is obsolete/vestigial and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