linux-kernel - Re: New kernel interface for sys

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Message-ID: <87tvakuak6.fsf@mid.deneb.enyo.de>
Date:   Tue, 13 Aug 2019 21:31:21 +0200
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Paul Eggert <eggert@...ucla.edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        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?

* 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 settings for of this, as 'struct timezone' is obsolete/vestigial and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.

Maybe it is possible to replace these things with other mechanisms
which exist today.  For example, the mount program could read a
configuration file to determine the system default for the time_offset
mount option and apply the value automatically.  The real-time clock
offset could be maintained by whatever mechanism hwclock uses.

I think whatever we end up doing, we should maintain consistency after
a system upgrade after architectures.  It does not make sense to keep
using the settimeofday hack on amd64 indefinitely, and switch to a
different mechanism on RV32, so that the two ways of supplying the
offset are never reconciled across architectures.