linux-kernel - Re: [PATCH v1 2/2] thermal: sysfs: Add sanity checks for trip temperature and hysteresis

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Message-ID: <CAJZ5v0hyYNCiZ5ESsiLHmKzbHypMuqMC3mi-Pnf8Ws6B8k6Gcw@mail.gmail.com>
Date: Fri, 23 Aug 2024 19:04:02 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux PM <linux-pm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Zhang Rui <rui.zhang@...el.com>, 
	Lukasz Luba <lukasz.luba@....com>, Peter Kästle <peter@...e.net>
Subject: Re: [PATCH v1 2/2] thermal: sysfs: Add sanity checks for trip
 temperature and hysteresis

On Fri, Aug 23, 2024 at 6:39 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Fri, Aug 23, 2024 at 5:26 PM Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
> >
> > On 22/08/2024 21:48, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >
> > > Add sanity checks for new trip temperature and hysteresis values to
> > > trip_point_temp_store() and trip_point_hyst_store() to prevent trip
> > > point thresholds from falling below THERMAL_TEMP_INVALID.
> > >
> > > However, still allow user space to pass THERMAL_TEMP_INVALID as the
> > > new trip temperature value to invalidate the trip if necessary.
> > >
> > > Fixes: be0a3600aa1e ("thermal: sysfs: Rework the handling of trip point updates")
> > > Cc: 6.8+ <stable@...r.kernel.org> # 6.8+
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > ---
> > >   drivers/thermal/thermal_sysfs.c |   38 ++++++++++++++++++++++++++------------
> > >   1 file changed, 26 insertions(+), 12 deletions(-)
> > >
> > > Index: linux-pm/drivers/thermal/thermal_sysfs.c
> > > ===================================================================
> > > --- linux-pm.orig/drivers/thermal/thermal_sysfs.c
> > > +++ linux-pm/drivers/thermal/thermal_sysfs.c
> > > @@ -111,18 +111,25 @@ trip_point_temp_store(struct device *dev
> > >
> > >       mutex_lock(&tz->lock);
> > >
> > > -     if (temp != trip->temperature) {
> > > -             if (tz->ops.set_trip_temp) {
> > > -                     ret = tz->ops.set_trip_temp(tz, trip, temp);
> > > -                     if (ret)
> > > -                             goto unlock;
> > > -             }
> > > +     if (temp == trip->temperature)
> > > +             goto unlock;
> > >
> > > -             thermal_zone_set_trip_temp(tz, trip, temp);
> > > +     if (temp != THERMAL_TEMP_INVALID &&
> > > +         temp <= trip->hysteresis + THERMAL_TEMP_INVALID) {
> >
> > It seems to me the condition is hard to understand.
>
> That's not the key consideration here though.
>
> >
> >    temp <= trip->hysteresis + THERMAL_TEMP_INVALID
>
> This cannot overflow because trip->hysteresis is non-negative.
>
> >
> >           ==>
> >
> >    temp - trip->hysteresis <= THERMAL_TEMP_INVALID
>
> But this can.

Well, I think I should add a comment there to point that out or people
will try to "clean it up".

Also note that in the hysteresis case the condition can be

if (trip->temperature - hyst <= THERMAL_TEMP_INVALID) {

because trip->temperature is never below THERMAL_TEMP_INVALID there.

Moreover, setting the hysteresis when the temperature is
THERMAL_TRIP_INVALID does not make much sense.

I'll send a v2.

> >
> >
> > Could be the test below simpler to understand ?
> >
> >         if (trip->hysteresis &&
> >             temp - trip->hysteresis < THERMAL_TEMP_INVALID))
> >
> > I think more sanity check may be needed also.
> >
> >         if (temp < THERMAL_TEMP_INVALID)
>
> With my version of the check above this is not necessary (unless I'm
> missing something}.
>
> > > +             ret = -EINVAL;
> > > +             goto unlock;
> > > +     }