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s-nail/s-nail-utc-offset.patch
Tomas Korbar 0925197f7f Fix UTC offset calculation in Date headers 
Resolves: RHEL-240793
2026-08-14 13:36:41 +02:00

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From 60ed19eed59a13f494bf0880e132208a9f4c6bc1 Mon Sep 17 00:00:00 2001
From: Steffen Nurpmeso <steffen@sdaoden.eu>
Date: Sun, 7 Nov 2021 02:43:37 +0100
Subject: [PATCH] FIX UTC offset "calculation" (Andrea Biardi)..
Andrea (thanks!) reported to the ML:
Looking at the source of s-nail, I tracked it all down to a
naive/incorrect handling of tm->tm_isdst in mkdate() in sendout.c (the
same logic appears in other places, for example see the comment in
src/mx/header.c that reads "/* TODO simply adding an hour for ISDST
is .. buuh */".)
Yeah the issue was known, but i hoped for SU tools.. whatever.
After a lot of research (and I feel I've become an expert on the
subject by now!) this is what I've found:
In 2018, the tzdata maintainers (IANA) corrected a historical mistake
with the Europe/Dublin timezone. The mistake was rooted in a
misunderstanding of whether IST meant "Irish Summer Time" or "Irish
Standard Time".
The problem was discussed at great length
(http://mm.icann.org/pipermail/tz/2018-January/thread.html) and it was
concluded that IST really meant Irish *Standard* Time (in constrast
with, say, British *Summer* Time), and that this standard time is
defined as UTC+0100.
This corresponds to the article at
https://en.wikipedia.org/wiki/Time_in_the_Republic_of_Ireland and the
notes at https://en.wikipedia.org/wiki/Winter_time_(clock_lag); the
source archive of tzdata has a long section dedicated to this problem
and a large set of official references and links to
www.irishstatutebook.ie.
Once the question was settled, the only possible solution for keeping
the Irish local time in sync with the rest of the world (for example,
Belfast & London) was for IANA to _reverse_ the functioning of the DST
flag for Ireland. The result is that in the current IANA timezone
database (2021e), Europe/Dublin has DST applied in *winter*, with an
adjustment of -1h (that is, negative).
Digging deeper, one uncovers that there are a few other countries that
have (or once had) the same time-switch mechanism as Ireland;
amongst others, https://github.com/MenoData/Time4J/issues/742 also
concedes that negative DST is a reality.
In s-nail, the logic that works out the UTC offset does the right
thing in my testcase (october 2021, Ireland = UTC+0100), but then upon
inspecting tm->tm_isdst it sees that DST is in effect (remember,
flag has been reversed, so DST in Ireland is on in winter time) it adds
one hour (it should subtract one, because the adjustment is negative).
That's why I get a +0200 instead of +0000 out of s-nail.
You may wonder why this problem hasn't been noticed by Irish people in
the past three years (hey, there's quite an IT industry over here!).
It turns out that the introduction of a negative DST adjustment caused
all sorts of bugs back in 2018; in the source distribution of IANA's
tzdata, one can spot this inside ./europe:
# In January 2018 we discovered that the negative SAVE values in the
# Eire rules cause problems with tests for ICU [...] and with tests
# for OpenJDK [...]
# To work around this problem, the build procedure can translate the
# following data into two forms, one with negative SAVE values and the
# other form with a traditional approximation for Irish timestamps
# after 1971-10-31 02:00 UTC; although this approximation has tm_isdst
# flags that are reversed, its UTC offsets are correct and this often
# suffices. This source file currently uses only nonnegative SAVE
# values, but this is intended to change and downstream code should
# not rely on it.
So, a temporary hack was put in place in order to allow distro
maintainers to retain the old broken convention of IST and support
buggy software, but it is clear that the current (and technically, and
politically, correct) implementation of a negative DST adjustment for
Ireland is there to stay.
As a matter of fact, the distro maintainer can choose to compile
tzdata to keep buggy software happy ("make DATAFORM=rearguard"),
which replicates the behaviour of tzdata prior to 2018. Many distros
seem to be doing that for one reason or another, while some have passed
the upstream change down to their users (probably, without knowing).
(also, remember that the IANA tz database is basically the only source
of tz data and is used globally by pretty much anybody who has a system
clock; I can see why embedded platforms may not be easy to patch to
support the negative DST adjustment).
This explains why I see this problem in Slackware, but not on, for
example, CentOS (and also why some tools work correctly, and some, like
s-nail in this case, does not).
I am in the process of evaluating different approaches to solve this
bug in a portable way (e.g. without relying on non-standard extensions
like tm->tm_gmtoff, strftime's "%z" or timelocal()/gmtime()).
He then also mentioned
tm_gmtoff seems like the easiest way. If you're interested in
portability, the only *portable* way of finding the offset from UTC
that I've found is:
struct tm l,g;
g = *gmtime(&t);
l = *localtime(&t);
and work out the difference between g and l by checking the members
(e.g. normalizing to either number of minutes) with something like:
diff = (l->tm_sec-g->tm_sec)/60
+(l->tm_min-g->tm_min)
+(l->tm_hour-g->tm_hour)*60
/* + compensate for day/month/year difference */
And that is an interesting approach that in one way or another
other MUAs have gone. Do so, too!
Many thanks for the effort, Andrea.
---
include/mx/nailfuns.h | 5 +++++
mk/make-config.sh | 10 +++++++++
src/mx/auxlily.c | 47 +++++++++++++++++++++++++++++++++++++++++++
src/mx/header.c | 16 +++++----------
src/mx/imap-search.c | 19 +++++++----------
src/mx/obs-imap.c | 18 ++++++-----------
src/mx/sendout.c | 14 ++++++-------
7 files changed, 87 insertions(+), 42 deletions(-)
diff --git a/include/mx/nailfuns.h b/include/mx/nailfuns.h
index 9c1f2a58..f912944b 100644
--- a/include/mx/nailfuns.h
+++ b/include/mx/nailfuns.h
@@ -294,6 +294,11 @@ FL struct n_timespec const *n_time_now(boole force_update);
FL void time_current_update(struct time_current *tc,
boole full_update);
+/* TZ difference in seconds.
+ * secsepoch is only used if any of the tm's is NIL. */
+FL s32 n_time_tzdiff(s64 secsepoch, struct tm const *utcp_or_nil,
+ struct tm const *localp_or_nil);
+
/* ctime(3), but do ensure 26 byte limit, do not crash XXX static buffer.
* NOTE: no trailing newline */
FL char *n_time_ctime(s64 secsepoch, struct tm const *localtime_or_nil);
diff --git a/mk/make-config.sh b/mk/make-config.sh
index 434a9039..ea822428 100644
--- a/mk/make-config.sh
+++ b/mk/make-config.sh
@@ -2104,6 +2104,16 @@ int main(void){
fi
fi
+link_check tm_gmtoff 'struct tm::tm_gmtoff' '#define mx_HAVE_TM_GMTOFF' << \!
+#include <time.h>
+int main(void){
+ time_t t;
+
+ t = time((void*)0);
+ return gmtime(&t)->tm_gmtoff != 0;
+}
+!
+
##
## optional and selectable
##
diff --git a/src/mx/auxlily.c b/src/mx/auxlily.c
index 54b89d50..bf386815 100644
--- a/src/mx/auxlily.c
+++ b/src/mx/auxlily.c
@@ -743,6 +743,53 @@ jredo:
NYD_OU;
}
+FL s32
+n_time_tzdiff(s64 secsepoch, struct tm const *utcp_or_nil,
+ struct tm const *localp_or_nil){
+ struct tm tmbuf[2], *tmx;
+ time_t t;
+ s32 rv;
+ NYD2_IN;
+ UNUSED(utcp_or_nil);
+
+ rv = 0;
+
+ if(localp_or_nil == NIL){
+ t = S(time_t,secsepoch);
+ if((tmx = localtime(&t)) == NIL)
+ goto jleave;
+ tmbuf[0] = *tmx;
+ localp_or_nil = &tmbuf[0];
+ }
+
+#ifdef mx_HAVE_TM_GMTOFF
+ rv = localp_or_nil->tm_gmtoff;
+
+#else
+ if(utcp_or_nil == NIL){
+ t = S(time_t,secsepoch);
+ if((tmx = gmtime(&t)) == NIL)
+ goto jleave;
+ tmbuf[1] = *tmx;
+ utcp_or_nil = &tmbuf[1];
+ }
+
+ rv = ((((localp_or_nil->tm_hour - utcp_or_nil->tm_hour) * 60) +
+ (localp_or_nil->tm_min - utcp_or_nil->tm_min)) * 60) +
+ (localp_or_nil->tm_sec - utcp_or_nil->tm_sec);
+
+ if((t = (localp_or_nil->tm_yday - utcp_or_nil->tm_yday)) != 0){
+ s32 const ds = 24 * 60 * 60;
+
+ rv += (t == 1) ? ds : -S(s32,ds);
+ }
+#endif
+
+jleave:
+ NYD2_OU;
+ return rv;
+}
+
FL char *
n_time_ctime(s64 secsepoch, struct tm const *localtime_or_nil){/* TODO err*/
/* Problem is that secsepoch may be invalid for representation of ctime(3),
diff --git a/src/mx/header.c b/src/mx/header.c
index b2d727dc..91947107 100644
--- a/src/mx/header.c
+++ b/src/mx/header.c
@@ -2748,9 +2748,8 @@ FL time_t
unixtime(char const *fromline)
{
char const *fp, *xp;
- time_t t, t2;
- s32 i, year, month, day, hour, minute, second, tzdiff;
- struct tm *tmptr;
+ time_t t;
+ s32 i, year, month, day, hour, minute, second;
NYD2_IN;
for (fp = fromline; *fp != '\0' && *fp != '\n'; ++fp)
@@ -2786,14 +2785,9 @@ unixtime(char const *fromline)
goto jinvalid;
if ((t = combinetime(year, month, day, hour, minute, second)) == (time_t)-1)
goto jinvalid;
- if((t2 = mktime(gmtime(&t))) == (time_t)-1)
- goto jinvalid;
- tzdiff = t - t2;
- if((tmptr = localtime(&t)) == NULL)
- goto jinvalid;
- if (tmptr->tm_isdst > 0)
- tzdiff += 3600; /* TODO simply adding an hour for ISDST is .. buuh */
- t -= tzdiff;
+
+ t += n_time_tzdiff(t, NIL, NIL);
+
jleave:
NYD2_OU;
return t;
diff --git a/src/mx/imap-search.c b/src/mx/imap-search.c
index 3b129255..458c6a29 100644
--- a/src/mx/imap-search.c
+++ b/src/mx/imap-search.c
@@ -615,9 +615,8 @@ jleave:
static time_t
_imap_read_date(char const *cp)
{
- time_t t, t2;
- s32 year, month, day, i, tzdiff;
- struct tm *tmptr;
+ time_t t;
+ s32 year, month, day, i;
char const *xp, *yp;
NYD_IN;
@@ -641,16 +640,12 @@ _imap_read_date(char const *cp)
goto jerr;
if (yp[0] != '\0' && (yp[1] != '"' || yp[2] != '\0'))
goto jerr;
- if ((t = combinetime(year, month, day, 0, 0, 0)) == (time_t)-1)
+
+ if((t = combinetime(year, month, day, 0, 0, 0)) == (time_t)-1)
goto jleave/*jerr*/;
- if((t2 = mktime(gmtime(&t))) == (time_t)-1)
- goto jerr;
- tzdiff = t - t2;
- if((tmptr = localtime(&t)) == NULL)
- goto jerr;
- if (tmptr->tm_isdst > 0)
- tzdiff += 3600;
- t -= tzdiff;
+
+ t += n_time_tzdiff(t, NIL, NIL);
+
jleave:
NYD_OU;
return t;
diff --git a/src/mx/obs-imap.c b/src/mx/obs-imap.c
index 095a594a..230599f4 100644
--- a/src/mx/obs-imap.c
+++ b/src/mx/obs-imap.c
@@ -4615,26 +4615,19 @@ imap_make_date_time(time_t t)
char const *mn;
s32 y, md, th, tm, ts;
struct tm *tmp;
- int tzdiff, tzdiff_hour, tzdiff_min;
- time_t t2;
+ int tzdiff_hour, tzdiff_min;
NYD2_IN;
jredo:
- if((t2 = mktime(gmtime(&t))) == (time_t)-1){
- t = 0;
- goto jredo;
- }
- tzdiff = t - t2;
if((tmp = localtime(&t)) == NULL){
t = 0;
goto jredo;
}
- tzdiff_hour = (int)(tzdiff / 60);
- tzdiff_min = tzdiff_hour % 60;
- tzdiff_hour /= 60;
- if (tmp->tm_isdst > 0)
- tzdiff_hour++;
+ tzdiff_min = S(int,n_time_tzdiff(t, NIL, tmp));
+ tzdiff_min /= 60; /* TODO su_TIME_MIN_SECS */
+ tzdiff_hour = tzdiff_min / 60;
+ tzdiff_min %= 60; /* TODO su_TIME_HOUR_MINS */
if(UNLIKELY((y = tmp->tm_year) < 0 || y >= 9999/*S32_MAX*/ - 1900)){
y = 1970;
@@ -4659,6 +4652,7 @@ jredo:
snprintf(s, sizeof s, "\"%02d-%s-%04d %02d:%02d:%02d %+03d%02d\"",
md, mn, y, th, tm, ts, tzdiff_hour, tzdiff_min);
+
NYD2_OU;
return s;
}
diff --git a/src/mx/sendout.c b/src/mx/sendout.c
index 00dab059..0eb734d8 100644
--- a/src/mx/sendout.c
+++ b/src/mx/sendout.c
@@ -2435,17 +2435,17 @@ FL int
mkdate(FILE *fo, char const *field)
{
struct tm tmpgm, *tmptr;
- int tzdiff, tzdiff_hour, tzdiff_min, rv;
+ int tzdiff_hour, tzdiff_min, rv;
NYD_IN;
su_mem_copy(&tmpgm, &time_current.tc_gm, sizeof tmpgm);
- tzdiff = time_current.tc_time - mktime(&tmpgm);
- tzdiff_hour = (int)(tzdiff / 60);
- tzdiff_min = tzdiff_hour % 60;
- tzdiff_hour /= 60;
tmptr = &time_current.tc_local;
- if (tmptr->tm_isdst > 0)
- ++tzdiff_hour;
+
+ tzdiff_min = S(int,n_time_tzdiff(time_current.tc_time, NIL, tmptr));
+ tzdiff_min /= 60; /* TODO su_TIME_MIN_SECS */
+ tzdiff_hour = tzdiff_min / 60;
+ tzdiff_min %= 60; /* TODO su_TIME_HOUR_MINS */
+
rv = fprintf(fo, "%s: %s, %02d %s %04d %02d:%02d:%02d %+05d\n",
field,
n_weekday_names[tmptr->tm_wday],
--
2.54.0
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