On 9/17/26 06:50, Dag-Erling Smørgrav wrote:
Paul Eggert via tz <[email protected]> writes:
* localtime.c (localtime): Add comment re difference from FreeBSD 15.1.
[...]
+ /* Omit "tzloadflags &= ~TZLOAD_FROMENV;" here, as that
+ would hurt performance by rereading and reanalyzing the
+ TZDEFAULT file even when it is unchanged. */
This is not accurate.
What wording would you suggest to address inaccuracy in the comment?
TZLOAD_FROMENV was originally used to indicate that the value of `name`
came from the environment and should therefore be subject to additional
scrutiny.
When TZLOAD_FROMENV was originally introduced to tzcode in commit
6d77c92872c0dc5ee0d6189d77337882596768a3 (2024-12-09), it was set
whenever the code used a timezone setting derived either from the TZ
environment variable, or from the default fallback when TZ is unset.
This can be seen by inspecting the source code of that commit, in the
tzset_unlocked function. In contrast, TZLOAD_FROMENV was unset when the
timezone setting came from the argument of the tzalloc function.
Today, TZLOAD_FROMENV has the same meaning that it did back then. This
can be seen by inspecting today's tzset_unlocked function.
At some point between 2025a and 2025c, it was robbed of its original
meaning and turned into a flag that controls change detection.
In tzcode, TZLOAD_FROMENV was always intended to mean the timezone
setting came from the environment or default fallback. What subsequent
code did with that information was up to the subsequent code. Subsequent
parts of 2024 tzcode used TZLOAD_FROMENV only in setuid programs, where
it checked that a file with name derived from the environment (or
default fallback) was a regular file and that the file was readable by
the real user.
However, in later commits tzcode security was tightened, to check that
the file was a regular file regardless of whether the program was setuid
and regardless of whether its name came from the environment (or default
fallback), and to check that the file was under /usr/share/zoneinfo/ in
setuid programs regardless of whether its name came from the environment
(or default fallback). This meant TZLOAD_FROMENV was no longer useful
and indeed it was removed in commit
87abb1135ef7bd5d2e57041868ad9a135b9fa67d (2025-09-25).
However, even though TZLOAD_FROMENV's original meaning was no longer
useful for security, it turned out that the original meaning was useful
for change detection. So TZLOAD_FROMENV was brought back in commit
59ebd6afa671cc218d4198e58269202eff0a838e (2025-10-04), still with its
original meaning, but now localtime.c used it to improve the performance
of change detection for settings derived from the environment (or
default fallback).
No explanation has ever been provided for the change in meaning of the
flag, or the loss of its original function,
I hope the above helps to explain why the flag still has its originally
intended meaning.
The comment does not explain the purpose of the flag,
The TZLOAD_FROMENV flag's purpose is explained in the comment next to
the flag's definition.
it just attacks FreeBSD
The comment does not mention FreeBSD. Although the commit message notes
"Add comment re difference from FreeBSD", this was meant to say only
that the two codebases differ.
you changed the meaning of this flag overnight
without warning or explanation
The flag still means what it originally meant in tzcode.
and then refused to help when I approached you about it
I've tried to explain localtime.c in some detail to be helpful, and I'm
happy to continue clarifying code or comments where needed. If my emails
have sounded sharp or dismissive, I apologize - that was not my intent.
