dmesg: make time stamps to be printed consistently

Earlier uptime determination, which was done with sysinfo(2), had one
second resolution, which made time stamps to be rounded unstable way
depending on when a dmesg command was executed.  In practical terms; the
command below was supposed not to differ but it did.

$ diff -q <(dmesg --ctime) <(sleep 0.5 ; dmesg --ctime)

[kzak@redhat.com: - add -lrt to Makemodule.am,
                  - fallback to sysinfo() based boot time
                  - use #ifdef]

CC: Kay Sievers <kay@vrfy.org>
References: https://lkml.org/lkml/2013/6/30/37
Buglink: https://github.com/karelzak/util-linux/issues/24
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/sys-utils/Makemodule.am b/sys-utils/Makemodule.am
index 977be5aff73055cc99caab97cd51c772e8b0eb59..658b203c6cc18b11e3607fdf6eda88f56a3e7a62 100644 (file)
--- a/sys-utils/Makemodule.am
+++ b/sys-utils/Makemodule.am
@@ -55,7 +55,7 @@ if LINUX
 bin_PROGRAMS += dmesg
 dist_man_MANS += sys-utils/dmesg.1
 dmesg_SOURCES = sys-utils/dmesg.c
-dmesg_LDADD = $(LDADD) libcommon.la
+dmesg_LDADD = $(LDADD) libcommon.la -lrt
 
 sbin_PROGRAMS += ctrlaltdel
 dist_man_MANS += sys-utils/ctrlaltdel.8

diff --git a/sys-utils/dmesg.c b/sys-utils/dmesg.c
index c098323c22b58bd178ce20ed1cda68a5f8ba54d9..e6819308cf3fcd143c03259742dbb2229d356a04 100644 (file)
--- a/sys-utils/dmesg.c
+++ b/sys-utils/dmesg.c
@@ -145,7 +145,7 @@ struct dmesg_control {
 
        struct timeval  lasttime;       /* last printed timestamp */
        struct tm       lasttm;         /* last localtime */
-       time_t          boot_time;      /* system boot time */
+       struct timeval  boot_time;      /* system boot time */
 
        int             action;         /* SYSLOG_ACTION_* */
        int             method;         /* DMESG_METHOD_* */
@@ -472,17 +472,30 @@ static int get_syslog_buffer_size(void)
        return n > 0 ? n : 0;
 }
 
-static time_t get_boot_time(void)
+static int get_boot_time(struct timeval *boot_time)
 {
+       struct timespec hires_uptime;
+       struct timeval lores_uptime, now;
        struct sysinfo info;
-       struct timeval tv;
 
+       if (gettimeofday(&now, NULL) != 0) {
+               warn(_("gettimeofday failed"));
+               return -errno;
+       }
+
+#ifdef CLOCK_BOOTTIME
+       if (clock_gettime(CLOCK_BOOTTIME, &hires_uptime) == 0) {
+               TIMESPEC_TO_TIMEVAL(&lores_uptime, &hires_uptime);
+               timersub(&now, &lores_uptime, boot_time);
+               return 0;
+       }
+#endif
+       /* fallback */
        if (sysinfo(&info) != 0)
                warn(_("sysinfo failed"));
-       else if (gettimeofday(&tv, NULL) != 0)
-               warn(_("gettimeofday failed"));
-       else
-               return tv.tv_sec -= info.uptime;
+
+       boot_time->tv_sec = now.tv_sec - info.uptime;
+       boot_time->tv_usec = 0;
        return 0;
 }
 
@@ -771,7 +784,7 @@ static struct tm *record_localtime(struct dmesg_control *ctl,
                                   struct dmesg_record *rec,
                                   struct tm *tm)
 {
-       time_t t = ctl->boot_time + rec->tv.tv_sec;
+       time_t t = ctl->boot_time.tv_sec + rec->tv.tv_sec;
        return localtime_r(&t, tm);
 }
 
@@ -1339,8 +1352,7 @@ int main(int argc, char *argv[])
            is_timefmt(&ctl, CTIME) ||
            is_timefmt(&ctl, ISO8601)) {
 
-               ctl.boot_time = get_boot_time();
-               if (!ctl.boot_time)
+               if (get_boot_time(&ctl.boot_time) != 0)
                        ctl.time_fmt = DMESG_TIMEFTM_NONE;
        }