linux-kernel - Re: [PATCH v3] rtc: wilco-ec: Handle reading invalid times

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Date:   Tue, 1 Oct 2019 13:42:24 -0700
From:   Dmitry Torokhov <dtor@...gle.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Nick Crews <ncrews@...omium.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        Pavel Machek <pavel@....cz>, enric.balletbo@...labora.com,
        Benson Leung <bleung@...omium.org>, dlaurie@...omium.org,
        Daniel Kurtz <djkurtz@...omium.org>
Subject: Re: [PATCH v3] rtc: wilco-ec: Handle reading invalid times

On Tue, Oct 1, 2019 at 12:53 PM Alexandre Belloni
<alexandre.belloni@...tlin.com> wrote:
>
> Hi Nick,
>
> On 25/09/2019 14:32:09-0600, Nick Crews wrote:
> > If the RTC HW returns an invalid time, the rtc_year_days()
> > call would crash. This patch adds error logging in this
> > situation, and removes the tm_yday and tm_wday calculations.
> > These fields should not be relied upon by userspace
> > according to man rtc, and thus we don't need to calculate
> > them.
> >
> > Signed-off-by: Nick Crews <ncrews@...omium.org>
> > ---
> >  drivers/rtc/rtc-wilco-ec.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/rtc/rtc-wilco-ec.c b/drivers/rtc/rtc-wilco-ec.c
> > index 8ad4c4e6d557..53da355d996a 100644
> > --- a/drivers/rtc/rtc-wilco-ec.c
> > +++ b/drivers/rtc/rtc-wilco-ec.c
> > @@ -110,10 +110,15 @@ static int wilco_ec_rtc_read(struct device *dev, struct rtc_time *tm)
> >       tm->tm_mday     = rtc.day;
> >       tm->tm_mon      = rtc.month - 1;
> >       tm->tm_year     = rtc.year + (rtc.century * 100) - 1900;
> > -     tm->tm_yday     = rtc_year_days(tm->tm_mday, tm->tm_mon, tm->tm_year);
> > -
> > -     /* Don't compute day of week, we don't need it. */
> > -     tm->tm_wday = -1;
> > +     /* Ignore other tm fields, man rtc says userspace shouldn't use them. */
> > +
> > +     if (rtc_valid_tm(tm)) {
> > +             dev_err(dev,
> > +                      "Time from RTC is invalid: second=%u, minute=%u, hour=%u, day=%u, month=%u, year=%u, century=%u",
> > +                      rtc.second, rtc.minute, rtc.hour, rtc.day, rtc.month,
> > +                      rtc.year, rtc.century);
>
> Do you mind using %ptR? At this point you already filled the tm struct
> anyway and if you print century separately, you can infer tm_year.

I do not think this is a good idea: we have just established that tm
does not contain valid data. Does %ptR guarantee that it handles junk
better than, let's say, rtc_year_days(), and does not crash when
presented with garbage?

Thanks,
Dmitry