readutmp: Fix the boot time returned on Minix

[Bruno Haible]

On Minix 3.3, the boot time returned by read_utmp is 1970-01-01 00:00:00.
Two workarounds are possible:
  - Take the time of the "system down" or "run-level m" entry instead,
    like the workaround for Raspbian does.
  - Use the time stamp of the file /var/run/syslogd.pid, like the workarounds
    for Alpine Linux, OpenBSD, Windows.
I'm choosing the first one.


2023-08-11  Bruno Haible  <bruno@clisp.org>

        readutmp: Fix the boot time returned on Minix.
        * lib/readutmp.c (read_utmp_from_file): [__minix] When the time of the
        BOOT_TIME entry is very close to the Epoch, replace it with the time
        from the "run-level m" entry.

diff --git a/lib/readutmp.c b/lib/readutmp.c
index c19ec2f6b1..40cee0e58c 100644
--- a/lib/readutmp.c
+++ b/lib/readutmp.c
@@ -373,7 +373,7 @@ read_utmp_from_file (char const *file, idx_t *n_entries, 
STRUCT_UTMP **utmp_buf,
 
   SET_UTMP_ENT ();
 
-#   if defined __linux__ && !defined __ANDROID__
+#   if (defined __linux__ && !defined __ANDROID__) || defined __minix
   bool file_is_utmp = (strcmp (file, UTMP_FILE) == 0);
   /* Timestamp of the "runlevel" entry, if any.  */
   struct timespec runlevel_ts = {0};
@@ -461,6 +461,12 @@ read_utmp_from_file (char const *file, idx_t *n_entries, 
STRUCT_UTMP **utmp_buf,
           && memcmp (UT_USER (ut), "runlevel", strlen ("runlevel") + 1) == 0
           && memcmp (ut->ut_line, "~", strlen ("~") + 1) == 0)
         runlevel_ts = ts;
+#   endif
+#   if defined __minix
+      if (file_is_utmp
+          && UT_USER (ut)[0] == '\0'
+          && memcmp (ut->ut_line, "run-level ", strlen ("run-level ")) == 0)
+        runlevel_ts = ts;
 #   endif
     }
 
@@ -571,6 +577,30 @@ read_utmp_from_file (char const *file, idx_t *n_entries, 
STRUCT_UTMP **utmp_buf,
     }
 #   endif
 
+#   if defined __minix
+  /* On Minix, during boot,
+       1. an entry gets written into /var/run/utmp, with ut_type = BOOT_TIME,
+          ut_user = "", ut_line = "system boot", time = 1970-01-01 00:00:00,
+       2. an entry gets written into /var/run/utmp, with
+          ut_user = "", ut_line = "run-level m", time = correct value.
+     In this case, copy the time from the "run-level m" entry to the
+     "system boot" entry.  */
+  if ((options & (READ_UTMP_USER_PROCESS | READ_UTMP_NO_BOOT_TIME)) == 0
+      && file_is_utmp)
+    {
+      for (idx_t i = 0; i < a.filled; i++)
+        {
+          struct gl_utmp *ut = &a.utmp[i];
+          if (UT_TYPE_BOOT_TIME (ut))
+            {
+              if (ut->ut_ts.tv_sec <= 60 && runlevel_ts.tv_sec != 0)
+                ut->ut_ts = runlevel_ts;
+              break;
+            }
+        }
+    }
+#   endif
+
 #  else /* old FreeBSD, OpenBSD, HP-UX */
 
   FILE *f = fopen (file, "re");