ping: use CLOCK_REALTIME for ICMP Originate Timestamp

RFC 792 defines the ICMP Originate Timestamp field as milliseconds
since midnight UTC.  However, ping(8) currently derives this value
from CLOCK_MONOTONIC, which represents time since an unspecified
starting point and is not related to UTC.

The issue was introduced by commit 1ad76f1b60, which replaced
gettimeofday(2) with clock_gettime(CLOCK_MONOTONIC) for timekeeping
in ping(8).

Fix this by using CLOCK_REALTIME when generating the ICMP originate
timestamp.

Before:

$ ping -Mt -c1 127.0.0.1
ICMP_TSTAMP
PING 127.0.0.1 (127.0.0.1): 56 data bytes
<...> time=0.061 ms tso=16:50:31 tsr=17:38:28 tst=17:38:28

(note the tso is off)

After:

$ ping -Mt -c1 127.0.0.1
ICMP_TSTAMP
PING 127.0.0.1 (127.0.0.1): 56 data bytes
<...> time=0.038 ms tso=17:42:09 tsr=17:42:09 tst=17:42:09

Reviewed by:		asomers, glebius
Fixes:			1ad76f1b60
MFC after:		1 month
Differential Revision:	https://reviews.freebsd.org/D56759

sbin/ping/ping.c

@@ -1029,9 +1029,16 @@ pinger(void)
 		 */
 		tv32.tv32_sec = (uint32_t)htonl(now.tv_sec);
 		tv32.tv32_nsec = (uint32_t)htonl(now.tv_nsec);
-		if (options & F_TIME)
+		if (options & F_TIME) {
+			/*
+			 * However, per RFC 792 the Originate Timestamp (otime)
+			 * should be milliseconds since midnight UTC. Something,
+			 * that CLOCK_MONOTONIC does not guarantee.
+			 */
+			(void)clock_gettime(CLOCK_REALTIME, &now);
 			icp.icmp_otime = htonl((now.tv_sec % (24*60*60))
 				* 1000 + now.tv_nsec / 1000000);
+		}
 		if (timing)
 			bcopy((void *)&tv32,
 			    (void *)&outpack[ICMP_MINLEN + phdr_len],

sbin/ping/tests/ping_test.sh

@@ -253,6 +253,40 @@ inject_reply_cleanup()
 	ifconfig `cat tun.txt` destroy
 }
 
+atf_test_case timestamp_origin
+timestamp_origin_head()
+{
+	atf_set "descr" "ICMP Originate Timestamp"
+}
+timestamp_origin_body()
+{
+	require_ipv4
+	# Run ping timestamp
+	out=$(ping -Mt -c1 127.0.0.1)
+
+	# Extract tso and tsr
+	tso=$(echo "$out" | sed -n 's/.*tso=\([0-9:]*\).*/\1/p')
+	tsr=$(echo "$out" | sed -n 's/.*tsr=\([0-9:]*\).*/\1/p')
+
+	atf_check test -n "$tso"
+	atf_check test -n "$tsr"
+
+	# Convert tso and tsr from HH:MM:SS to seconds
+	tso_s=`date -jf %H:%M:%S $tso`
+	tsr_s=`date -jf %H:%M:%S $tsr`
+
+	diff=$((tso_s - tsr_s))
+	# Tolerate negative time difference between the sender and receiver
+	if [ $diff -lt 0 ]; then
+		diff=$(( -diff ))
+	fi
+
+	# Tolerate 2 seconds difference
+	if [ $diff -gt 2 ]; then
+		atf_fail "tso ($tso) differs from tsr ($tsr) by $diff seconds"
+	fi
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case ping_c1_s56_t1
@@ -271,6 +305,7 @@ atf_init_test_cases()
 	atf_add_test_case inject_opts
 	atf_add_test_case inject_pip
 	atf_add_test_case inject_reply
+	atf_add_test_case timestamp_origin
 }
 
 check_ping_statistics()