dnsmasq_time: avoid signed integer overflow when HAVE_BROKEN_RTC

The dnsmasq_time() function, in the case of HAVE_BROKEN_RTC, was calling
times() to read the number of ticks "elapsed since an arbitrary point in
the past" and then dividing that by sysconf(_SC_CLK_TCK) to compute the
number of seconds elapsed since that arbitrary instant. This works fine
until the number of ticks exceeds 2^31, beyond which time the function
would begin erroneously returning negative times. On my system this
happens after approximately 248 days of uptime. A symptom is that
dnsmasq no longer populates the resolver cache with DHCP-derived names
at startup, as the inserted cache entries immediately expire due to
having negative expiration times that cause is_expired() to return true
when called with now==0.

This commit replaces the archaic implementation of dnsmasq_time() with a
call to the POSIX-standardized clock_gettime(CLOCK_MONOTONIC), thereby
eliminating the need to convert manually from ticks to seconds. The new
implementation will yield correct results until the system uptime
exceeds approximately 68 years.

Signed-off-by: Matt Whitlock <dnsmasq@mattwhitlock.name>

src/util.c

@@ -430,13 +430,12 @@ int hostname_issubdomain(char *a, char *b)
 time_t dnsmasq_time(void)
 {
 #ifdef HAVE_BROKEN_RTC
-  struct tms dummy;
-  static long tps = 0;
+  struct timespec ts;
 
-  if (tps == 0)
-    tps = sysconf(_SC_CLK_TCK);
+  if (clock_gettime(CLOCK_MONOTONIC, &ts) < 0)
+    die(_("cannot read monotonic clock: %s"), NULL, EC_MISC);
 
-  return (time_t)(times(&dummy)/tps);
+  return ts.tv_sec;
 #else
   return time(NULL);
 #endif