[PATCH v2 4/4] time: Implement the glibc.time.mktime_dst_adjustment tunable
Florian Weimer
fweimer@redhat.com
Thu Jan 2 17:42:52 GMT 2025
It helps with upgrades from older glibc versions (prior to 2.29)
which performed DST adjustments in fewer cases.
---
elf/dl-tunables.list | 9 ++
elf/libc_early_init.c | 3 +
manual/time.texi | 3 +-
manual/tunables.texi | 12 +++
time/Makefile | 7 +-
time/mktime.c | 8 +-
time/time-tunables.c | 29 +++++++
time/tst-mktime-dst-adjust-0.c | 154 +++++++++++++++++++++++++++++++++
time/tst-mktime-dst-adjust-1.c | 2 +
time/tzset.h | 6 ++
10 files changed, 228 insertions(+), 5 deletions(-)
create mode 100644 time/time-tunables.c
create mode 100644 time/tst-mktime-dst-adjust-0.c
create mode 100644 time/tst-mktime-dst-adjust-1.c
diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
index 0b6721bc51..c96ad343ad 100644
--- a/elf/dl-tunables.list
+++ b/elf/dl-tunables.list
@@ -84,6 +84,15 @@ glibc {
}
}
+ time {
+ mktime_dst_adjustment {
+ type: INT_32
+ minval: 0
+ maxval: 1
+ default: 1
+ }
+ }
+
elision {
enable {
type: INT_32
diff --git a/elf/libc_early_init.c b/elf/libc_early_init.c
index 0720231753..43e4fda533 100644
--- a/elf/libc_early_init.c
+++ b/elf/libc_early_init.c
@@ -24,6 +24,7 @@
#include <pthread_early_init.h>
#include <sys/single_threaded.h>
#include <getrandom-internal.h>
+#include <time/tzset.h>
#ifdef SHARED
_Bool __libc_initial;
@@ -49,4 +50,6 @@ __libc_early_init (_Bool initial)
#if ENABLE_ELISION_SUPPORT
__lll_elision_init ();
#endif
+
+ __time_tunable_init ();
}
diff --git a/manual/time.texi b/manual/time.texi
index bc1f472886..ffbebd741a 100644
--- a/manual/time.texi
+++ b/manual/time.texi
@@ -1324,7 +1324,8 @@ positive. This forced adjustment only happens if time zone data
indicates that there is at least one daylight saving transition (at any
time). For zones such as @samp{UTC} which never use daylight saving
time, the adjustment does not happen even if @code{tm_isdst} is
-positive.
+positive. The @code{glibc.time.mktime_dst_adjustment} tunable
+can be used to disable the adjustment heuristic. @xref{Time Tunables}.
The @code{mktime} function ignores the specified contents of the
@code{tm_wday}, @code{tm_yday}, @code{tm_gmtoff}, and @code{tm_zone}
diff --git a/manual/tunables.texi b/manual/tunables.texi
index 1ac58b9ae9..8959ee5183 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -76,6 +76,7 @@ glibc.malloc.check: 0 (min: 0, max: 3)
@menu
* Tunable names:: The structure of a tunable name
* Memory Allocation Tunables:: Tunables in the memory allocation subsystem
+* Time Tunables:: Tunables related to time and time zone processing.
* Dynamic Linking Tunables:: Tunables in the dynamic linking subsystem
* Elision Tunables:: Tunables in elision subsystem
* POSIX Thread Tunables:: Tunables in the POSIX thread subsystem
@@ -294,6 +295,17 @@ supported ones. If provided value is invalid, @code{MAP_HUGETLB} will not
be used.
@end deftp
+@node Time Tunables
+@section Time Tunables
+@cindex time tunables
+
+@deftp Tunable glibc.time.mktime_dst_adjustment
+If this tunable is @code{1}, the @code{mktime} function uses a heuristic
+if the the @code{tm_isdst} input value disagrees with available time
+zone data. If the tunable is @code{0}, this heuristic is disabled.
+@xref{Broken-down Time}.
+@end deftp
+
@node Dynamic Linking Tunables
@section Dynamic Linking Tunables
@cindex dynamic linking tunables
diff --git a/time/Makefile b/time/Makefile
index 029322ebd7..36d3861fcc 100644
--- a/time/Makefile
+++ b/time/Makefile
@@ -30,7 +30,7 @@ headers := time.h sys/time.h sys/timeb.h bits/time.h \
bits/types/time_t.h bits/types/struct_timeb.h
routines := offtime asctime clock ctime ctime_r difftime \
- gmtime localtime mktime time \
+ gmtime localtime mktime time time-tunables \
gettimeofday settimeofday settimezone \
adjtime tzset tzfile getitimer setitimer \
stime dysize timegm ftime \
@@ -67,6 +67,8 @@ tests := \
tst-itimer \
tst-mktime \
tst-mktime-dst-adjust \
+ tst-mktime-dst-adjust-0 \
+ tst-mktime-dst-adjust-1 \
tst-mktime2 \
tst-mktime3 \
tst-mktime4 \
@@ -155,3 +157,6 @@ CPPFLAGS-tst-tzname.c += -DTZDEFRULES='"$(posixrules-file)"'
bug-getdate1-ARGS = ${objpfx}bug-getdate1-fmt
tst-tzfile-fault-ENV = GLIBC_TUNABLES=glibc.rtld.enable_secure=1
+
+tst-mktime-dst-adjust-0-ENV = GLIBC_TUNABLES=glibc.time.mktime_dst_adjustment=0
+tst-mktime-dst-adjust-1-ENV = GLIBC_TUNABLES=glibc.time.mktime_dst_adjustment=1
diff --git a/time/mktime.c b/time/mktime.c
index 92889f3e14..f2d81d9f56 100644
--- a/time/mktime.c
+++ b/time/mktime.c
@@ -424,9 +424,11 @@ __mktime_internal (struct tm *tp, bool use_local, mktime_offset_t *offset)
value, if any. */
if (isdst_differ (isdst, tm.tm_isdst)
#if _LIBC
- /* Only perform the adjustment if the time zone ever uses DST.
- This skips the adjustment for UTC, for example. */
- && __daylight
+ /* The glibc.time.mktime_dst_adjustment tunable disable all such
+ adjustments. Otherwise, only perform the adjustment if the
+ time zone ever uses DST. This skips the adjustment for UTC,
+ for example. */
+ && __mktime_dst_adjustment && __daylight
#endif
)
{
diff --git a/time/time-tunables.c b/time/time-tunables.c
new file mode 100644
index 0000000000..78d28f38e0
--- /dev/null
+++ b/time/time-tunables.c
@@ -0,0 +1,29 @@
+/* Tunables for time-related functions.
+ Copyright 2024 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+ Contributed by Paul Eggert <eggert@cs.ucla.edu>.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#define TUNABLE_NAMESPACE time
+#include <elf/dl-tunables.h>
+
+bool __mktime_dst_adjustment;
+
+void
+__time_tunable_init (void)
+{
+ __mktime_dst_adjustment = TUNABLE_GET (mktime_dst_adjustment, int32_t, NULL);
+}
diff --git a/time/tst-mktime-dst-adjust-0.c b/time/tst-mktime-dst-adjust-0.c
new file mode 100644
index 0000000000..386e69429f
--- /dev/null
+++ b/time/tst-mktime-dst-adjust-0.c
@@ -0,0 +1,154 @@
+/* Test lack of mktime DST adjustment with the adjustment heuristic disabled.
+ Copyright (C) 2024 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#include <time.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+ TEST_COMPARE (setenv ("TZ", "UTC", 1), 0);
+
+ {
+ struct tm t =
+ {
+ .tm_year = 124,
+ .tm_mon = 9,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 1, /* Not actually true. */
+ };
+ TEST_COMPARE (mktime (&t), 1727774453);
+ }
+
+ /* IST used DST at one point, but no longer does. */
+ {
+ char *path = realpath ("../timezone/testdata/IST", NULL);
+ TEST_VERIFY (path != NULL);
+ TEST_COMPARE (setenv ("TZ", path, 1), 0);
+ free (path);
+ }
+
+ {
+ struct tm t =
+ {
+ .tm_year = 124,
+ .tm_mon = 9,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 0, /* Correct value. */
+ };
+ TEST_COMPARE (mktime (&t), 1727774453 - (int) (5.5 * 3600));
+ TEST_COMPARE (t.tm_gmtoff, (int) (5.5 * 3600));
+ TEST_COMPARE (t.tm_isdst, 0);
+ }
+
+ /* No adjustment for historic incorrect value. */
+ {
+ struct tm t =
+ {
+ .tm_year = 124,
+ .tm_mon = 9,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 1, /* Incorrect value. */
+ };
+ TEST_COMPARE (mktime (&t), 1727774453 - (int) (5.5 * 3600));
+ TEST_COMPARE (t.tm_gmtoff, (int) (5.5 * 3600));
+ TEST_COMPARE (t.tm_isdst, 0);
+ }
+
+ /* No adjustment for correct DST. */
+ {
+ struct tm t =
+ {
+ .tm_year = 42,
+ .tm_mon = 9,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 1, /* Correct value, DST was in effect. */
+ };
+ TEST_COMPARE (mktime (&t), -860015347);
+ TEST_COMPARE (t.tm_gmtoff, (int) (6.5 * 3600));
+ TEST_COMPARE (t.tm_isdst, 1);
+ }
+
+ /* No adjustment for mismatch: DST incorrectly claimed not in effect. */
+ {
+ struct tm t =
+ {
+ .tm_year = 42,
+ .tm_mon = 9,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 0, /* Incorrect value. */
+ };
+ TEST_COMPARE (mktime (&t), -860015347);
+ TEST_COMPARE (t.tm_gmtoff, (int) (6.5 * 3600));
+ TEST_COMPARE (t.tm_isdst, 1);
+ }
+
+ /* Test using correct standard time. */
+ {
+ struct tm t =
+ {
+ .tm_year = 42,
+ .tm_mon = 7,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 0, /* Correct value, standard time in effect. */
+ };
+ TEST_COMPARE (mktime (&t), -865282147);
+ TEST_COMPARE (t.tm_gmtoff, (int) (5.5 * 3600));
+ TEST_COMPARE (t.tm_isdst, 0);
+ }
+
+ /* No adjustment: Test using standard time with mismatch. */
+ {
+ struct tm t =
+ {
+ .tm_year = 42,
+ .tm_mon = 7,
+ .tm_mday = 1,
+ .tm_hour = 9,
+ .tm_min = 20,
+ .tm_sec = 53,
+ .tm_isdst = 1, /* Incorrect value. */
+ };
+ TEST_COMPARE (mktime (&t), -865282147);
+ TEST_COMPARE (t.tm_gmtoff, (int) (5.5 * 3600));
+ TEST_COMPARE (t.tm_isdst, 0);
+ }
+
+ return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/time/tst-mktime-dst-adjust-1.c b/time/tst-mktime-dst-adjust-1.c
new file mode 100644
index 0000000000..137d1e7411
--- /dev/null
+++ b/time/tst-mktime-dst-adjust-1.c
@@ -0,0 +1,2 @@
+/* Test with GLIBC_TUNABLES=glibc.time.mktime_dst_adjust=1 (default). */
+#include "tst-mktime-dst-adjust.c"
diff --git a/time/tzset.h b/time/tzset.h
index 254540aa05..3999c71de6 100644
--- a/time/tzset.h
+++ b/time/tzset.h
@@ -31,4 +31,10 @@ extern void __tzset_unlocked (void) attribute_hidden;
extern struct tm *__tz_convert (__time64_t timer, int use_localtime,
struct tm *tp) attribute_hidden;
+/* True if mktime uses the DST adjustment heuristic based on tm_isdst. */
+extern bool __mktime_dst_adjustment attribute_hidden;
+
+/* Initializes tunables. Called from __libc_early_init. */
+void __time_tunable_init (void) attribute_hidden;
+
#endif /* _TZSET_H */
--
2.47.1
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