linux-kernel - Re: [PATCH v2] pwm: improve args checking in pwm_apply

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Date:	Tue, 21 Jun 2016 11:37:31 -0700
From:	Brian Norris <briannorris@...omium.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Linux PWM List <linux-pwm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Doug Anderson <dianders@...omium.org>,
	linux-renesas-soc@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2] pwm: improve args checking in pwm_apply_state()

Hi Geert,

On Tue, Jun 21, 2016 at 04:42:04PM +0200, Geert Uytterhoeven wrote:
> On Fri, May 27, 2016 at 6:45 PM, Brian Norris <briannorris@...omium.org> wrote:
> > It seems like in the process of refactoring pwm_config() to utilize the
> > newly-introduced pwm_apply_state() API, some args/bounds checking was
> > dropped.
> >
> > In particular, I noted that we are now allowing invalid period
> > selections. e.g.:
> >
> >   # echo 1 > /sys/class/pwm/pwmchip0/export
> >   # cat /sys/class/pwm/pwmchip0/pwm1/period
> >   100
> >   # echo 101 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
> >   [... driver may or may not reject the value, or trigger some logic bug ...]
> >
> > It's better to see:
> >
> >   # echo 1 > /sys/class/pwm/pwmchip0/export
> >   # cat /sys/class/pwm/pwmchip0/pwm1/period
> >   100
> >   # echo 101 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
> >   -bash: echo: write error: Invalid argument
> >
> > This patch reintroduces some bounds checks in both pwm_config() (for its
> > signed parameters; we don't want to convert negative values into large
> > unsigned values) and in pwm_apply_state() (which fix the above described
> > behavior, as well as other potential API misuses).
> >
> > Fixes: 5ec803edcb70 ("pwm: Add core infrastructure to allow atomic updates")
> > Signed-off-by: Brian Norris <briannorris@...omium.org>
> > ---
> > v2:
> >  * changed subject, as this covers more scope now
> >  * add Fixes tag, as this is a v4.7-rc regression
> >  * add more bounds/args checks in pwm_apply_state() and pwm_config()
> >
> >  drivers/pwm/core.c  | 3 ++-
> >  include/linux/pwm.h | 3 +++
> >  2 files changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > index dba3843c53b8..ed337a8c34ab 100644
> > --- a/drivers/pwm/core.c
> > +++ b/drivers/pwm/core.c
> > @@ -457,7 +457,8 @@ int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state)
> >  {
> >         int err;
> >
> > -       if (!pwm)
> > +       if (!pwm || !state || !state->period ||
> > +           state->duty_cycle > state->period)
> >                 return -EINVAL;
> 
> This check breaks the LCD backlight on r8a7740/armadillo.
> Apparently both period and duty_cycle are zero during the first invocation.
> Later, these are initialized from DT, cfr.
> 
>         pwms = <&tpu 2 33333 PWM_POLARITY_INVERTED>;
> 
> in arch/arm/boot/dts/r8a7740-armadillo800eva.dts.

Hmm, this isn't super obvious how to best fix. On one hand, the
pwm_config() API used to reject period<=0, but on the other hand, I
think its replacement (pwm_apply_state()) is getting used in more places
than it used to be, and not all of them are really handling the "atomic
update" concept yet. Seems like a product of Boris's multi-phase attempt
to convert the PWM APIs to support atomic updates -- and many users
haven't really converted yet.

> With added debug printing, the difference between failure and success is:
> 
>  renesas-tpu-pwm e6600000.pwm: TPU PWM -1 registered
>  tpu_pwm_request:223
>  pwm_apply_state:460: pwm backlight/2: period 0, duty_cycle 0
> +Ignoring failure
> +pwm_apply_state:479: polarity 0 -> 1
> +tpu_pwm_set_polarity:343
> +pwm_apply_state:502: period 0 -> 0
> +pwm_apply_state:503: duty_cycle 0 -> 0
> +pwm_apply_state:516: enabled 0 -> 0
>  pwm_config:238: pwm backlight/2: duty_ns 33333, period_ns 33333
>  pwm_apply_state:460: pwm backlight/2: period 33333, duty_cycle 33333
> -pwm_apply_state:479: polarity 0 -> 0
> +pwm_apply_state:479: polarity 1 -> 1
>  pwm_apply_state:502: period 0 -> 33333
>  pwm_apply_state:503: duty_cycle 0 -> 33333
>  tpu_pwm_config:267
>  pwm_apply_state:516: enabled 0 -> 0
>  pwm_apply_state:460: pwm backlight/2: period 33333, duty_cycle 33333
> -pwm_apply_state:479: polarity 0 -> 0
> +pwm_apply_state:479: polarity 1 -> 1
>  pwm_apply_state:502: period 33333 -> 33333
>  pwm_apply_state:503: duty_cycle 33333 -> 33333
>  pwm_apply_state:516: enabled 0 -> 1
>  tpu_pwm_enable:354

I'm not sure I 100% understand this debug log, but I think maybe the
problem is in pwm_apply_args(), which calls pwm_disable() and
pwm_set_polarity() sequentially, without ever configuring a period? What
if pwm_apply_args() were to configure the period for us?

Boris, any thoughts?

> Sorry for not noticing last week, before it hit mainline.

Sorry for the regression :(

Brian