Re: Fix overflow in DecodeInterval

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Joseph Koshakow <koshy44(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Fix overflow in DecodeInterval
Date: 2022-04-02 19:08:24
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Joseph Koshakow <koshy44(at)gmail(dot)com> writes:
> Ok I actually remember now, the issue is with the rounding
> code in AdjustFractMicroseconds.
> ...
> I believe it's possible for `frac -= usec;` to result in a value greater
> than 1 or less than -1 due to the lossiness of int64 to double
> conversions.

I think it's not, at least not for the interesting range of possible
values in this code. Given that abs(frac) < 1 to start with, the
abs value of usec can't exceed the value of scale, which is at most
USECS_PER_DAY so it's at most 37 or so bits, which is well within
the exact range for any sane implementation of double. It would
take a very poor floating-point implementation to not get the right
answer here. (And we're largely assuming IEEE-compliant floats these

Anyway, the other issue indeed turns out to be easy to fix.
Attached is a v11 that deals with that. If the cfbot doesn't
complain about it, I'll push this later today.

regards, tom lane

Attachment Content-Type Size
v11-0001-Check-for-overflow-when-decoding-an-interval.patch text/x-diff 96.0 KB

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