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

[Rich Felker <dalias@libc.org>]

On Mon, Mar 25, 2024 at 08:00:19PM +0000, Alexander Weps wrote:
> On Monday, March 25th, 2024 at 20:38, Rich Felker <dalias@libc.org> 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.
> >
> > 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
> 
> No, -1 means that was not able to make seconds since beginning of epoch.
> It has nothing to do with modifying tm...

That might actually be the correct interpretation. However, in that
case glibc should not be returning -1, since the time is representable
as time_t..

> Also, can you share the whole code, you did some changes and I don't
> reproduce the result. Which sample it is based on?

https://www.openwall.com/lists/musl/2024/03/25/11

The only change I made was tm_isdst=1 (you already had tm_mday=31
there). I'll attach the file anyway though.

> I set tm_isdst=1 and tm_mday=31 in the example above and I get:
> 
> $  gcc foo.c -o foo && TZ=Pacific/Apia ./foo
> before: 2011-12-31 00:00:00 +13 0
> after1: 2012-01-01 01:00:00 +14 1325329200
> after2: 2011-12-31 01:00:00 +14 1325242800
> after3: 2012-01-01 01:00:00 +14 1325329200

That's close to what I'm seeing with tm_isdst=0:

before: 2011-12-31 00:00:00 WSST 0
after1: 2012-01-01 01:00:00 WSDT 1325329200
after2: 2011-12-31 01:00:00 WSDT 1325242800
after3: 2012-01-01 01:00:00 WSDT 1325329200

With tm_isdst=1 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

This is on the very old glibc on a test system I had lying around,
2.28. It probably has different bugs (and seems unable to read the
current zoneinfo file which is a newer version; thus the WSDT zone
name from whatever old one it came with).

> I do not get any -1 results.

That's presumably because the bug that threw it into 2012 made it so
there are no invalid dates appearing.

> $  musl-gcc foo.c -o foo && TZ=Pacific/Apia ./foo
> 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

Are you sure you're not testing with tm_isdst=0? This is the output I
get for tm_isdst=0 too.

> 
> > 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.
> 
> Glibc works without any issues as shown above on my machine. Both
> forward and backward.

No it doesn't. It jumped to completely the wrong day, 2012-01-01. In
the case where it doesn't do that (tm_isdst=1), it emits a completely
nonexistent date in the reverse direction:

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

This is because it *did* error out before finishing updating the tm
fields. It performed normalization type 1, then said "nope!" and left
the partial work behind.

> Your results are off by one hour. And you start with WSDT (+14).
> Without mktime. The initial date produced by glibc should be +13 as
> Pacific/Apia is +13.

I don't follow. It should never produce +13 in output, as non-DST is
not active anywhere near this date.

Rich