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Date: Mon, 25 Mar 2024 15:47:25 -0400
From: Rich Felker <dalias@...c.org>
To: Alexander Weps <exander77@...me>
Cc: musl@...ts.openwall.com
Subject: Re: Broken mktime calculations when crossing DST boundary
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
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