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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Nov 28, 2019 at 01:36:58PM +0100, Cornelia Huck wrote:
> 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.
> 
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
> 
> Not sure if that is the correct approach, but the current error
> message is really quite confusing.

I guess the alternative is to just #ifndef __s390x__ the whole
impl of the qmp_guest_set_time  method, but I don't have a
strong opinion on which is best.

> 
> Gave it a quick test with an s390x and an x86_64 guest; invoking
> 'virsh domtime <value>' now fails with 'not currently supported'
> on s390x and continues to work as before on x86_64.
> 
> ---
>  qga/commands-posix.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> index 1c1a165daed8..bd298a38b716 100644
> --- a/qga/commands-posix.c
> +++ b/qga/commands-posix.c
> @@ -149,6 +149,13 @@ int64_t qmp_guest_get_time(Error **errp)
>     return tq.tv_sec * 1000000000LL + tq.tv_usec * 1000;
>  }
>  
> +static int check_hwclock_available(const char *path)
> +{
> +    struct stat st;
> +
> +    return (stat(path, &st) < 0) ? 0 : 1;
> +}
> +
>  void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
>  {
>      int ret;
> @@ -156,6 +163,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
>      pid_t pid;
>      Error *local_err = NULL;
>      struct timeval tv;
> +    const char *hwclock_path = "/sbin/hwclock";
> +    static int hwclock_available = -1;
> +
> +    if (hwclock_available < 0) {
> +        hwclock_available = check_hwclock_available(hwclock_path);

Could do this inline with:

    hwclock_available = (access(hwclock_available, X_OK) == 0);

getting a slightly better result as this check for it being
executable as well as existing.

> +    }
> +
> +    if (!hwclock_available) {
> +        error_setg(errp, QERR_UNSUPPORTED);
> +        return;
> +    }
>  
>      /* If user has passed a time, validate and set it. */
>      if (has_time) {
> @@ -195,7 +213,7 @@ 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
> 
> 

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|