netdev - Re: [PATCH net-next] ptp: only allow phase values lower than 1 period

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Message-ID: <20200804234052.z54jncpjissagex5@skbuf>
Date:   Wed, 5 Aug 2020 02:40:52 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        jacob.e.keller@...el.com
Subject: Re: [PATCH net-next] ptp: only allow phase values lower than 1 period

On Tue, Aug 04, 2020 at 04:23:35PM -0700, Richard Cochran wrote:
> On Mon, Aug 03, 2020 at 10:49:21PM +0300, Vladimir Oltean wrote:
> > @@ -218,6 +218,19 @@ long ptp_ioctl(struct posix_clock *pc, unsigned int cmd, unsigned long arg)
> >  					break;
> >  				}
> >  			}
> > +			if (perout->flags & PTP_PEROUT_PHASE) {
> > +				/*
> > +				 * The phase should be specified modulo the
> > +				 * period, therefore anything larger than 1
> > +				 * period is invalid.
> > +				 */
> > +				if (perout->phase.sec > perout->period.sec ||
> > +				    (perout->phase.sec == perout->period.sec &&
> > +				     perout->phase.nsec > perout->period.nsec)) {
> > +					err = -ERANGE;
> > +					break;
> > +				}
> 
> So if perout->period={1,0} and perout->phase={1,0} then the phase has
> wrapped 360 degrees back to zero.
> 
> Shouldn't this code catch that case as well?
> 
> So why not test for (perout->phase.nsec >= perout->period.nsec) instead?
> 
> Thanks,
> Richard

Oof, I guess this is what one would call 'brain fart'. In my mind,
checking for equality between period and phase required an extra 'if',
which I was reluctant to add (or to even think about, it seems). I
converted the nsec check to >= and it works as it should (I checked with
a modified ts2phc).

ts2phc[326.764]: config item /dev/ptp1.ts2phc.perout_phase is 1000000000
ts2phc[326.764]: config item /dev/ptp1.ts2phc.pulsewidth is 500000000
ts2phc[326.764]: PTP_PEROUT_REQUEST2 failed: Numerical result out of range

I'm sending a v2 with your change very soon.

Thanks.
-Vladimir