dd(1) uses gettimeofday(2) to compute the throughput statistics. However,

gettimeofday returns the system clock, which may jump forward or back,
especially if NTP is in use.  If the time jumps backwards, then dd will see
negative elapsed time, round it up to 1usec, and print an absurdly fast
transfer rate.

The solution is to use clock_gettime(2) with CLOCK_MONOTONIC_PRECISE as the
clock_id.  That clock advances steadily, regardless of changes to the system
clock.

Reviewed by:	delphij
MFC after:	3 days
Sponsored by:	Spectra Logic

2 files changed, 20 insertions, 12 deletions

diff --git a/bin/dd/dd.c b/bin/dd/dd.c
index 7e5bd91..8048b6a 100644
--- a/bin/dd/dd.c
+++ b/bin/dd/dd.c
@@ -50,7 +50,6 @@ __FBSDID("$FreeBSD$");
 #include <sys/conf.h>
 #include <sys/disklabel.h>
 #include <sys/filio.h>
-#include <sys/time.h>
 
 #include <ctype.h>
 #include <err.h>
@@ -61,6 +60,8 @@ __FBSDID("$FreeBSD$");
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sysexits.h>
+#include <time.h>
 #include <unistd.h>
 
 #include "dd.h"
@@ -123,7 +124,7 @@ static void
 setup(void)
 {
 	u_int cnt;
-	struct timeval tv;
+	struct timespec tv;
 
 	if (in.name == NULL) {
 		in.name = "stdin";
@@ -240,8 +241,9 @@ setup(void)
 		ctab = casetab;
 	}
 
-	(void)gettimeofday(&tv, NULL);
-	st.start = tv.tv_sec + tv.tv_usec * 1e-6;
+	if (clock_gettime(CLOCK_MONOTONIC_PRECISE, &tv))
+		err(EX_OSERR, "clock_gettime");
+	st.start = tv.tv_sec + tv.tv_nsec * 1.0e-9;
 }
 
 static void
diff --git a/bin/dd/misc.c b/bin/dd/misc.c
index 61f843b..0d8da70 100644
--- a/bin/dd/misc.c
+++ b/bin/dd/misc.c
@@ -40,14 +40,16 @@ static char sccsid[] = "@(#)misc.c	8.3 (Berkeley) 4/2/94";
 __FBSDID("$FreeBSD$");
 
 #include <sys/types.h>
-#include <sys/time.h>
 
+#include <err.h>
 #include <errno.h>
 #include <inttypes.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sysexits.h>
+#include <time.h>
 #include <unistd.h>
 
 #include "dd.h"
@@ -56,16 +58,20 @@ __FBSDID("$FreeBSD$");
 void
 summary(void)
 {
-	struct timeval tv;
-	double secs;
+	struct timespec tv, tv_res;
+	double secs, res;
 
 	if (ddflags & C_NOINFO)
 		return;
 
-	(void)gettimeofday(&tv, NULL);
-	secs = tv.tv_sec + tv.tv_usec * 1e-6 - st.start;
-	if (secs < 1e-6)
-		secs = 1e-6;
+	if (clock_gettime(CLOCK_MONOTONIC_PRECISE, &tv))
+		err(EX_OSERR, "clock_gettime");
+	if (clock_getres(CLOCK_MONOTONIC_PRECISE, &tv_res))
+		err(EX_OSERR, "clock_getres");
+	secs = tv.tv_sec + tv.tv_nsec * 1.0e-9 - st.start;
+	res = tv_res.tv_sec + tv_res.tv_nsec * 1.0e-9;
+	if (secs < res)
+		secs = res;
 	(void)fprintf(stderr,
 	    "%ju+%ju records in\n%ju+%ju records out\n",
 	    st.in_full, st.in_part, st.out_full, st.out_part);
@@ -77,7 +83,7 @@ summary(void)
 		     st.trunc, (st.trunc == 1) ? "block" : "blocks");
 	if (!(ddflags & C_NOXFER)) {
 		(void)fprintf(stderr,
-		    "%ju bytes transferred in %.6f secs (%.0f bytes/sec)\n",
+		    "%ju bytes transferred in %.9f secs (%.0f bytes/sec)\n",
 		    st.bytes, secs, st.bytes / secs);
 	}
 	need_summary = 0;