qga: fence guest-set-time if hwclock not available

The Posix implementation of guest-set-time invokes hwclock to
set/retrieve the time to/from the hardware clock. If hwclock
is not available, the user is currently informed that "hwclock
failed to set hardware clock to system time", which is quite
misleading. This may happen e.g. on s390x, which has a different
timekeeping concept anyway.

Let's check for the availability of the hwclock command and
return QERR_UNSUPPORTED for guest-set-time if it is not available.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-Id: <20191205115350.18713-1-cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

qga/commands-posix.c

@@ -156,6 +156,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
     pid_t pid;
     Error *local_err = NULL;
     struct timeval tv;
+    static const char hwclock_path[] = "/sbin/hwclock";
+    static int hwclock_available = -1;
+
+    if (hwclock_available < 0) {
+        hwclock_available = (access(hwclock_path, X_OK) == 0);
+    }
+
+    if (!hwclock_available) {
+        error_setg(errp, QERR_UNSUPPORTED);
+        return;
+    }
 
     /* If user has passed a time, validate and set it. */
     if (has_time) {
@@ -195,7 +206,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 
         /* Use '/sbin/hwclock -w' to set RTC from the system time,
          * or '/sbin/hwclock -s' to set the system time from RTC. */
-        execle("/sbin/hwclock", "hwclock", has_time ? "-w" : "-s",
+        execle(hwclock_path, "hwclock", has_time ? "-w" : "-s",
                NULL, environ);
         _exit(EXIT_FAILURE);
     } else if (pid < 0) {