[chrony-dev] [PATCH] More reliable method of reading rtc for initial trim

["Hattink, Tjalling [FINT]" <T.Hattink@xxxxxxxx>]

When chrony reads in the linux rtc for the first time to trim the system
clock, it only reads it once. As it is possible that the rtc updates
itself
during the read operation, the reported rtc time could be false. To
prevent
this I've added a loop that reads the rtc clock twice, if the seconds do
not
match retry the two read operations. If they match you can assume the
read
operation was successful.

This is based on the hwclock implementation of reading the rtc clock
from the
util-linux package.
---
 rtc_linux.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/rtc_linux.c b/rtc_linux.c
index 91b0cac..add828f 100644
--- a/rtc_linux.c
+++ b/rtc_linux.c
@@ -971,7 +971,7 @@ void
 RTC_Linux_TimePreInit(void)
 {
   int fd, status;
-  struct rtc_time rtc_raw;
+  struct rtc_time rtc_raw, rtc_raw_retry;
   struct tm rtc_tm;
   time_t rtc_t, estimated_correct_rtc_t;
   long interval;
@@ -989,7 +989,15 @@ RTC_Linux_TimePreInit(void)
     return; /* Can't open it, and won't be able to later */
   }
 
-  status = ioctl(fd, RTC_RD_TIME, &rtc_raw);
+  /* Retry reading the rtc until both read attempts give the same sec
value.
+     This way the race condition is prevented that the RTC has updated
itself
+     during the first read operation. */
+  do {
+    status = ioctl(fd, RTC_RD_TIME, &rtc_raw);
+    if (status >= 0) {
+      status = ioctl(fd, RTC_RD_TIME, &rtc_raw_retry);
+    }
+  } while (status >= 0 && rtc_raw.tm_sec != rtc_raw_retry.tm_sec);
 
   if (status >= 0) {
     /* Convert to seconds since 1970 */
-- 
1.9.1


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