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From: Florian Weimer <fw@deneb.enyo.de>
To: Paul Eggert <eggert@cs.ucla.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	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>
Subject: Re: New kernel interface for sys_tz and timewarp?
Date: Tue, 13 Aug 2019 21:31:21 +0200	[thread overview]
Message-ID: <87tvakuak6.fsf@mid.deneb.enyo.de> (raw)
In-Reply-To: <ecf2742a-6cab-cc00-16ab-589fad07b8db@cs.ucla.edu> (Paul Eggert's message of "Tue, 13 Aug 2019 10:49:03 -0700")

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 settings for of this, as 'struct timezone' is obsolete/vestigial and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

  reply	other threads:[~2019-08-13 19:31 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-13  9:05 New kernel interface for sys_tz and timewarp? Arnd Bergmann
2019-08-13 17:10 ` John Stultz
2019-08-13 21:45   ` Alexandre Belloni
2019-08-13 17:30 ` Linus Torvalds
2019-08-13 17:49   ` Paul Eggert
2019-08-13 19:31     ` Florian Weimer [this message]
2019-08-13 20:04       ` Arnd Bergmann
2019-08-13 21:56   ` Alexandre Belloni
2019-08-14  0:06   ` Theodore Y. Ts'o
2019-08-14  8:31     ` Arnd Bergmann
2019-08-14  9:09       ` Lennart Poettering
2019-08-14  9:32         ` Alexandre Belloni
2019-08-14 12:15           ` Lennart Poettering
2019-08-19 11:09           ` Karel Zak
2019-08-19 13:43             ` Thomas Gleixner
2019-08-19 13:49               ` Thomas Gleixner
2019-08-20 18:45                 ` Alexandre Belloni
2019-08-20 18:47               ` Alexandre Belloni
2019-08-20 18:58             ` Alexandre Belloni
2019-08-27 16:27               ` Arnd Bergmann
2019-08-27 16:31                 ` Alexandre Belloni
2019-08-14 16:26     ` David Laight
2019-08-14 16:47       ` hpa
2019-08-15 13:22         ` Arnd Bergmann
2019-08-15 15:05           ` Theodore Y. Ts'o
2019-08-15 15:24             ` hpa

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