Re: [musl] Broken mktime calculations when crossing DST boundary

[Alexander Weps <exander77@pm.me>]

Analyzing this and Glibc behavior is perfectly valid for that case:

+ day, + day, - day, - day

$  gcc foo.c -o foo && TZ=Pacific/Apia ./foo
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
after2: 2011-12-29 00:00:00 -10 1325152800
after2: 2011-12-30 00:00:00 -10 -1
after3: 2011-12-31 00:00:00 +14 1325239200

I can go forward and backward in time.

$  musl-gcc foo.c -o foo && TZ=Pacific/Apia ./foo
before: 2011-12-31 00:00:00  0
after1: 2011-12-31 00:00:00 +14 1325239200
after1: 2011-12-29 00:00:00 -10 1325152800
after2: 2011-12-28 00:00:00 -10 1325066400
after2: 2011-12-29 00:00:00 -10 1325152800
after3: 2011-12-29 00:00:00 -10 1325152800

Musl is off by 2 days and ran into a cycle.

AW


On Monday, March 25th, 2024 at 21:05, Alexander Weps <exander77@pm.me> wrote:

> Sorry my bad, that was isdst = 0;
>
> $ gcc foo.c -o foo && TZ=Pacific/Apia ./foo
> before: 2011-12-31 00:00:00 +14 0
> after1: 2011-12-31 00:00:00 +14 1325239200
> after2: 2011-12-30 00:00:00 +14 -1
> after3: 2011-12-31 00:00:00 +14 1325239200
>
> $ musl-gcc foo.c -o foo && TZ=Pacific/Apia ./foo
> before: 2011-12-31 00:00:00 0
> after1: 2011-12-31 00:00:00 +14 1325239200
> after2: 2011-12-29 00:00:00 -10 1325152800
> after3: 2011-12-29 00:00:00 -10 1325152800
>
> I agree with isdst = 1; reuslts.
>
> AW
>
>
>
>
> On Monday, March 25th, 2024 at 20:47, Rich Felker dalias@libc.org wrote:
>
> > On Mon, Mar 25, 2024 at 03:38:13PM -0400, Rich Felker wrote:
> >
> > > On Mon, Mar 25, 2024 at 06:57:49PM +0000, Alexander Weps wrote:
> > >
> > > > I am not sure which one you mean, all latest codes even includes
> > > > headers and main...
> > >
> > > https://www.openwall.com/lists/musl/2024/03/25/3
> > >
> > > > I have no idea what to tell you.
> > >
> > > The first version I found that's actually compilable is:
> > >
> > > https://www.openwall.com/lists/musl/2024/03/25/11
> > >
> > > It roughly behaves as expected on musl, except possibly not applying
> > > the tm_isdst=0, which is what was making the output confusing on
> > > glibc -- that threw the input back across the rule change cutoff.
> >
> > No, it's deeper than this. glibc is offsetting the input by an entire
> > day when tm_isdst=0, and I don't know why. It looks like a bug in
> > glibc.
> >
> > > With tm_isdst=1 and tm_mday=31, on glibc, I get:
> > >
> > > before: 2011-12-31 00:00:00 WSDT 0
> > > after1: 2011-12-31 00:00:00 WSDT 1325239200
> > > after2: 2011-12-30 00:00:00 WSDT -1
> > > after3: 2011-12-31 00:00:00 WSDT 1325239200
> > >
> > > The -1 in the after2 line indicates that mktime failed with an error
> > > (and should not have modified tm; that's arguably a bug in glibc). The
> > > partial modification that it made reflects the initial normalization
> > > (type 1 in my notation) but not the rule change normalization (type 2
> > > in my notation) since glibc has failed the operation for an input date
> > > that does not exist on the calendar (it does not do type 2
> > > normalization at all; it just rejects it).
> > >
> > > Running this same change on musl, I get:
> > >
> > > before: 2011-12-31 00:00:00 0
> > > after1: 2011-12-31 00:00:00 +14 1325239200
> > > after2: 2011-12-29 00:00:00 -10 1325152800
> > > after3: 2011-12-29 00:00:00 -10 1325152800
> > >
> > > which again is what I expect. From one side, the move-by-1-day changes
> > > the time to the next calendar day in that direction. From the other
> > > side, it's unable to change it.
> > >
> > > I'll look into why the tm_isdst=0 application was not happening.
> >
> > Hmm, I must have misread the output. It seems to be correct with
> > tm_isdst=0 too:
> >
> > before: 2011-12-31 00:00:00 0
> > after1: 2011-12-31 01:00:00 +14 1325242800
> > after2: 2011-12-29 01:00:00 -10 1325156400
> > after3: 2011-12-29 01:00:00 -10 1325156400
> >
> > (If it's 00:00:00 in standard time, it's 01:00:00 in DST, so the
> > initial time seems to have been interpreted correctly.)
> >
> > I also went back and tested both with tm_isdst=-1, and both glibc and
> > musl do the same thing as they do with tm_isdst=1 (which is correct).
> >
> > Rich