[PATCH v3] qga: fence guest-set-time if hwclock not available

[Cornelia Huck <cohuck@redhat.com>]

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>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---

v2->v3:
  - added 'static' keyword to hwclock_path

Not sure what tree this is going through; if there's no better place,
I can also take this through the s390 tree.

---
 qga/commands-posix.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 1c1a165daed8..0be301a4ea77 100644
--- a/qga/commands-posix.c
+++ b/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) {
-- 
2.21.0