Reject extended zones whose transitions remain in negative unix time#357
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…zones
When a TZif file has all explicit transitions in the negative-epoch
range (<0) and includes a POSIX DST footer string, ExtendTransitions()
sets extended_ = true and generates 401 years of transitions. Load()
previously checked `if (last.unix_time < 0)` unconditionally and
appended a static sentinel transition at unix_time = 2147483647 (2038).
This corrupted the transition table and lookup behavior:
1. Lookups for modern dates (e.g., 1970–2038) in BreakTime() skipped
the `if (extended_)` branch because unix_time < 2147483647, matching
the negative-epoch static offset and returning standard time (EST)
instead of daylight saving time (EDT).
2. Far-future lookups (e.g., time_point<seconds>::max()) caused signed
integer overflow when computing `shift * kSecsPer400Years` and when
subtracting negative transition timestamps from large unix times.
This change:
- Guards the 2038 sentinel addition in TimeZoneInfo::Load() with
`if (!extended_)`, preserving the extended 400-year cycle transitions.
- Uses unsigned 64-bit subtraction in TimeZoneInfo::BreakTime() and
clamps `shift` to `seconds::max().count() / kSecsPer400Years`.
- Uses unsigned arithmetic in TimeZoneInfo::MakeTime() to safely
compute civil year differences when `last_year_` is negative.
- Adds unit test coverage in `time_zone_lookup_test.cc`.
devbww
reviewed
Jul 21, 2026
devbww
reviewed
Jul 22, 2026
Contributor
|
PS: It looks like the "This change" part of the PR description could now do with some re-wording. |
derekmauro
changed the title
devbww
approved these changes
Jul 22, 2026
mkruskal-google
approved these changes
Jul 22, 2026
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When a TZif file has explicit transitions ending before the epoch and
includes a POSIX DST footer string, ExtendTransitions() sets extended_ = true
and generates 401 years of transitions. Load() previously checked
if (last.unix_time < 0)unconditionally and appended a static sentineltransition at unix_time = 2147483647 (2038).
Appending a static transition to an extended zone table is incompatible with
the 400-year cycle shifting used for future lookups.
This change:
extended transitions fail to reach the non-negative unix time space
(
if (extended_) return false;). All valid zoneinfo files easily meetthis requirement.
preserving the generated 400-year cycle transition table.
last.unix_timeand
last_year_are non-negative, ensuring that 400-year cycle calculationsin BreakTime() and MakeTime() operate safely within bounds.
time_zone_lookup_test.ccto verify lookups forvalid zones with transitions before the epoch, and to confirm rejection of
zones whose extended transitions fail to reach non-negative unix time.