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Date: Wed, 27 Mar 2024 00:38:04 +0000
From: Alexander Weps <exander77@...me>
To: musl@...ts.openwall.com
Subject: Re: Broken mktime calculations when crossing DST boundary
Attaching whole:
Minutes of the 27th July 2023 Teleconference Austin-1332 Page 1 of 1
Submitted by Andrew Josey, The Open Group. 28th July 2023
Also go through:
https://www.austingroupbugs.net/view.php?id=1614
Seems most of what We talk here was already addressed.
AW
On Wednesday, March 27th, 2024 at 01:14, Alexander Weps <exander77@...me> wrote:
> See below.
>
> AW
>
>
>
>
> On Tuesday, March 26th, 2024 at 22:59, Thorsten Glaser tg@...bsd.de wrote:
>
> > Alexander Weps dixit:
> >
> > > > Not at all, glibc’s mktime just throws the towel with
> > > > EOVERFLOW saying that the requested time does not exist.
> > >
> > > How is this not compliant with POSIX?
> >
> > POSIX indicates that this is valid only if the date is not
> > representable in time_t, and it has different handling for
> > dates that fall into gaps, see the other mails from me, as
> > well as below.
>
>
> I think you confuse two things here:
>
> 1. mktime returning -1
> 2. mktime setting errno to EOVERFLOW
>
> The 2. maybe debatable in obscure circles, but that's entirely unrelated.
>
> But mktime can return -1 if and when it deems that struct tm cannot be represented in epoch seconds.
>
> That is entirely compliant with POSIX and I would need to see some hard evidence for it not being the case.
>
> From Issue 7:
>
> > RETURN VALUE
> >
> > The mktime() function shall return the specified time since the Epoch encoded as a value of type time_t. If the time since the Epoch cannot be represented, the function shall return the value (time_t)-1 [CX] [Option Start] and set errno to indicate the error. [Option End]
> >
> > ERRORS
> >
> > The mktime() function shall fail if:
> >
> > [EOVERFLOW]
> > [CX] [Option Start] The result cannot be represented. [Option End]
>
>
>
> First and foremost mktime may fail and when it fails it returns -1.
>
> Issue 6: If the time since the Epoch cannot be represented, the function shall return the value (time_t)-1 and may set errno to indicate the error.
> Isuse 7: If the time since the Epoch cannot be represented, the function shall return the value (time_t)-1 and set errno to indicate the error.
>
> Only errno offered is EOVERFLOW.
> So I interpret it that since Issue 7, the correct behavior is to set errno to EOVERFLOW.
>
> Also from:
> Minutes of the 27th July 2023 Teleconference Austin-1332 Page 1 of 1
> Submitted by Andrew Josey, The Open Group. 28th July 2023
>
> > Bug 1614: XSH 3/mktime does not specify EINVAL and should Accepted as Marked
> > https://austingroupbugs.net/view.php?id=1614
> >
> > An interpretation is required.
> >
> > Interpretation response:
> > The standard clearly states that when an unsuccessful call to
> > mktime() returns (time_t)-1 it sets errno to [EOVERFLOW], and
> > conforming implementations must conform to this.
> >
> > Rationale:
> >
> > The RETURN VALUE section on the mktime() page states:
> > If the time since the Epoch cannot be represented, the function
> > shall return the value (time_t)-1 [CX]and set errno to indicate
> > the error[/CX].
> >
> > This requires that errno is set to indicate "the error", and the
> > beginning of the sentence states the nature of the error condition
> > to which "the error" refers: the time since the Epoch (i.e. the
> > integer value to be returned) cannot be represented. The ERRORS
> > section requires that the error number [EOVERFLOW] is used for this
> > condition.
> >
> > Thus the standard requires that errno is set to [EOVERFLOW] when
> > an unsuccessful call to mktime() returns (time_t)-1 and an
> > implementation that sets it to [EINVAL] does not conform.
> >
> > The mktime() function does not have any way to indicate to the
> > caller that an error other than [EOVERFLOW] occurred.
>
> > > This is perfectly valid behavior, that is both expected and can be
> > > handled in code easily.
> >
> > No, it’s a bug in glibc.
> >
> > > I have to ask, but have you actually used mktime from the application end?
> >
> > Of course.
> >
> > > I am not sure what you mean by correct. Struct tm is neither correct
> > > nor incorrect. It can be in three states:
> > > 1) Set by user.
> > > 2) Normalized by mktime.
> > > 3) Not fully normalized by mktime.
> >
> > Huh? No.
>
>
> Then explain what is incorrect struct tm.
>
> > > If I get -1, I know the struct tm does not represent valid time_t and I
> > > handle it and move on.
> >
> > Define “move on”. With POSIX’ mktime interface, if you get -1 and
> > EOVERFLOW, then moving further into the same direction will never
> > give you not -1 again, because -1 is what you get when your tm_year
> > was too far out of the representation (e.g. 2039 on a system with
> > a 32-bit time_t).
>
>
> Not true as shown above.
>
> > EOVERFLOW means that the time cannot be represented in time_t, not
> > that the time cannot be represented in struct tm. And for these
> > gaps, the time_t values are consecutive (1325239199/1325239200).
>
>
> No, return value -1 means that the time cannot represented as epoch seconds (for any number of reasons).
>
> Required errno is Issue 7 change that should be used to indicate type of error, but only EOVERFLOW is listed.
>
> > > This is perfect example (TZ=Pacific/Apia):
> > >
> > > before: 2011-12-31 00:00:00 +14 0
> > > after1: 2011-12-31 00:00:00 +14 1325239200
> > > after1: 2011-12-30 00:00:00 +14 -1
> >
> > No, this cannot give -1 per POSIX.
>
>
> Not true as shown above.
>
> > > Musl instead of giving sane results starts running in the circle at some point:
> > > after2: 2011-12-29 00:00:00 -10 1325152800
> > > after3: 2011-12-29 00:00:00 -10 1325152800
> >
> > That’s because it does this correctly.
> >
> > > Doesn't work, this will not give the same time next day, this fails on
> > > STD/DST changes.
> > >
> > > Because same time next day is not always 86400 apart.
> >
> > I know. But the basic assumption that there even is such a
> > thing as “same time next day” made by you is invalid. POSIX
> > listed several examples (29ᵗʰ February next year as well as
> > gaps in timezone offsets).
> >
> > One thing you can do is to add 86400, localtime(), then check
> > that at least tm_mday, tm_hour and tm_min (Issue 8d4, line
> > 48052) are what you expect, and handle cases where they aren’t
> > manually. But having added 86400, you have two starting points
> > from which to manually approach this (the original value and
> > the newer value). (Perhaps a location could even skip more than
> > 24 hours in a discontinuity.)
> >
> > bye,
> > //mirabilos
> > --
> > “Having a smoking section in a restaurant is like having
> > a peeing section in a swimming pool.”
> > -- Edward Burr
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