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From: Florian Weimer <fweimer@redhat.com>
To: libc-alpha@sourceware.org
Subject: [PATCH v2 4/4] time: Implement the glibc.time.mktime_dst_adjustment
 tunable
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 <89b8df3c0dc8a504e5bd805c70c698ebd1539a27.1735839459.git.fweimer@redhat.com>
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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 */