reject negative fractional_count in FastFixedDtoa - #315
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floitsch merged 1 commit intoSep 10, 2026
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FastFixedDtoa only validated that fractional_count <= 20, but did not reject negative values (fractional_count < 0). While DoubleToStringConverter::ToFixed rejects negative requested_digits before calling FastFixedDtoa (PR google#298), a direct caller of FastFixedDtoa passing a negative fractional_count could succeed and write integer digits into the output buffer, potentially overrunning smaller buffers. Reject fractional_count < 0 by returning false and ensure *length is reset to 0, matching the behavior in FastDtoa (PR google#313). Add a test in test-fixed-dtoa.cc.
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Problem
FastFixedDtoaonly validated thatf ractional_count <= 20, but did not check for negative values (f ractional_count < 0).While
DoubleToStringConverter::ToFixedguards against negativerequested_digits(PR #298), direct callers ofFastFixedDtoapassing a negativefr ractional_countcould succeed and write integer digits into the buffer, potentially overrunning small buffers. In addition, whenFastFixedDtoareturnedfalse,*lengthwas left uninitialized.Solution
*length = 0;is set at the start ofFastFixedDtoa.fractional_count < 0alongsidef ractional_count > 20by returningfalse, matching the guard pattern inFastDtoa(PR guard non-positive requested_digits in DigitGenCounted #313).FastFixedDtoaNegativeFractionalCountintest/cctest/test-fixed-dtoa.ccto verify that negative fractional counts fail and do not touch the buffer.