Re: [PATCH v2] qga: fence guest-set-time if hwclock not available

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 28 Nov 2019 19:38:00 +0100
Laszlo Ersek <lersek@redhat.com> wrote:

> Hi Cornelia,
> 
> On 11/28/19 19:11, 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>
> > ---
> > 
> > v1 (RFC) -> v2:
> > - use hwclock_path[]
> > - use access() instead of stat()
> > 
> > ---
> >  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..ffb6420fa9cd 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;
> > +    const char hwclock_path[] = "/sbin/hwclock";  
> 
> Did you drop the "static" storage-class specifier on purpose?

No, I just need to do patches when I'm less tired :/

> 
> > +    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",  
> 
> I think it's somewhat obscure now that arg="hwclock" is supposed to
> match the last pathname component in hwclock_path="/sbin/hwclock".
> 
> There are multiple ways to compute "arg" like that, of course. But I
> think they all look uglier than the above. So I'm fine if we just keep this.

Yes, I was not able to come up with something elegant, either.

[Side note: does really everyone put hwclock under /sbin (i.e., nobody
doing something creative?)]

> 
> 
> If you purposely dropped the "static", then:
> 
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> 
> If you just missed the "static" and now intend to add it back, then for v3:
> 
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> 
> Thanks
> Laszlo

Thanks for looking! There'll be a v3.

> 
> 
> 
> >                 NULL, environ);
> >          _exit(EXIT_FAILURE);
> >      } else if (pid < 0) {
> >   
>