<thread>: Avoid overflow in _To_absolute_time()#5237
Merged
StephanTLavavej merged 1 commit intomicrosoft:mainfrom Jan 24, 2025
Merged
<thread>: Avoid overflow in _To_absolute_time()#5237StephanTLavavej merged 1 commit intomicrosoft:mainfrom
<thread>: Avoid overflow in _To_absolute_time()#5237StephanTLavavej merged 1 commit intomicrosoft:mainfrom
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CaseyCarter
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Jan 15, 2025
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I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
zacklj89
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Jan 24, 2025
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Fixes #5234.
We were carefully forming
decltype(_Now + _Rel_time) _Abs_time, the common type of the given_Rel_time(which could be coarse or fine) andchrono::steady_clock::now()(which is nanoseconds).In this bug scenario,
_Rel_timeand therefore the common type are picoseconds.The problem was that we weren't careful about our
_Foreverconstant. We were using(chrono::steady_clock::time_point::max)(), but then converting it to the common type (implicitly in the subtraction_Forever - _Rel_time, and in the assignment_Abs_time = _Forever). When the common type is finer-grained, this attempts to multiply the stored value, but it's already the maximum, so we overflow.The fix is to use the common type for
_Forever. This stores the maximum rep with the proper period of the common type, so we won't attempt to multiply it further. (When_Rel_timeis coarse, the common type is nanoseconds, so there's no change. When_Rel_timeis fine, this means that_Foreveris effectively "earlier", becauseINT64_MAXpicoseconds from the epoch happens earlier thanINT64_MAXnanoseconds. This is exactly what we want, since we're performing our overflow check and clamping with the common type that we're about to return.)