login: Use unsigned 32-bit types for seconds-since-epoch

These fields store timestamps when the system was running.  No Linux
systems existed before 1970, so these values are unused.  Switching
to unsigned types allows continued use of the existing struct layouts
beyond the year 2038.

The intent is to give distributions more time to switch to improved
interfaces that also avoid locking/data corruption issues.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

NEWS

@@ -28,7 +28,14 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
-  [Add deprecations, removals and changes affecting compatibility here]
+* Architectures which use a 32-bit seconds-since-epoch field in struct
+  lastlog, struct utmp, struct utmpx (such as i386, powerpc64le, rv32,
+  rv64, x86-64) switched from a signed to an unsigned type for that
+  field.  This allows these fields to store timestamps beyond the year
+  2038, until the year 2106.  Please note that applications are still
+  expected to migrate off the interfaces declared in <utmp.h> and
+  <utmpx.h> (except for login_tty) due to locking and session management
+  problems.
 
 Changes to build and runtime requirements:
 

bits/utmp.h

@@ -36,7 +36,7 @@
 struct lastlog
   {
 #if __WORDSIZE_TIME64_COMPAT32
-    int32_t ll_time;
+    __uint32_t ll_time;
 #else
     __time_t ll_time;
 #endif
@@ -76,7 +76,7 @@ struct utmp
   int32_t ut_session;		/* Session ID, used for windowing.  */
   struct
   {
-    int32_t tv_sec;		/* Seconds.  */
+    __uint32_t tv_sec;		/* Seconds.  */
     int32_t tv_usec;		/* Microseconds.  */
   } ut_tv;			/* Time entry was made.  */
 #else

login/Makefile

@@ -44,9 +44,11 @@ subdir-dirs = programs
 vpath %.c programs
 
 tests := tst-utmp tst-utmpx tst-grantpt tst-ptsname tst-getlogin tst-updwtmpx \
-  tst-pututxline-lockfail tst-pututxline-cache tst-utmp-size tst-utmp-size-64
+  tst-pututxline-lockfail tst-pututxline-cache tst-utmp-size tst-utmp-size-64 \
+  tst-utmp-unsigned tst-utmp-unsigned-64
 
 CFLAGS-tst-utmp-size-64.c += -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64
+CFLAGS-tst-utmp-unsigned-64.c += -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64
 
 # Empty compatibility library for old binaries.
 extra-libs      := libutil

login/tst-utmp-unsigned-64.c

@@ -0,0 +1 @@
+#include "tst-utmp-unsigned.c"

login/tst-utmp-unsigned.c

@@ -0,0 +1,40 @@
+/* Check that struct utmp, struct utmpx, struct lastlog use unsigned epoch.
+   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 <utmp.h>
+#include <utmpx.h>
+#include <utmp-size.h>
+
+/* Undefined.  Used to check that the conditions below are optimized away.  */
+void link_failure_utmp (void);
+void link_failure_utmpx (void);
+void link_failure_lastlog (void);
+
+static int
+do_test (void)
+{
+  if ((struct utmp) { .ut_tv = { 0x80000000U, }, }.ut_tv.tv_sec <= 0)
+    link_failure_utmp ();
+  if ((struct utmpx) { .ut_tv = { 0x80000000U, }, }.ut_tv.tv_sec <= 0)
+    link_failure_utmpx ();
+  if ((struct lastlog) { .ll_time = 0x80000000U, }.ll_time <= 0)
+    link_failure_lastlog ();
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/gnu/bits/utmpx.h

@@ -74,7 +74,7 @@ struct utmpx
   __int32_t ut_session;		/* Session ID, used for windowing.  */
   struct
   {
-    __int32_t tv_sec;		/* Seconds.  */
+    __uint32_t tv_sec;		/* Seconds.  */
     __int32_t tv_usec;		/* Microseconds.  */
   } ut_tv;			/* Time entry was made.  */
 #else