Re: [PULL v2 16/16] qga: fence guest-set-time if hwclock not available

[Peter Maydell]

On Sat, 14 Dec 2019 at 09:46, Cornelia Huck <address@hidden> 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.
>
> Reviewed-by: Laszlo Ersek <address@hidden>
> Reviewed-by: Daniel P. Berrangé <address@hidden>
> Reviewed-by: Michael Roth <address@hidden>
> Message-Id: <address@hidden>
> Reviewed-by: Philippe Mathieu-Daudé <address@hidden>
> Signed-off-by: Cornelia Huck <address@hidden>
> ---
>  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) {
> --

FWIW, Coverity thought this might be a time-of-check-time-of-use
bug based on our doing an access() check and then later doing
an execle(). This seems like a false positive to me, so I've
marked it as such (CID 1411029).

thanks
-- PMM