Re: [PATCH 10/16] iio: adc: max1027: Prevent single channel accesses during buffer reads

From: Jonathan Cameron <jic23@kernel.org>
To: "Sa, Nuno" <Nuno.Sa@analog.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 10/16] iio: adc: max1027: Prevent single channel accesses during buffer reads
Date: Mon, 30 Aug 2021 11:20:24 +0100	[thread overview]
Message-ID: <20210830112024.4583f3cb@jic23-huawei> (raw)
In-Reply-To: <SJ0PR03MB6359DB255A253092BFDC04F499C19@SJ0PR03MB6359.namprd03.prod.outlook.com>

On Fri, 20 Aug 2021 07:30:07 +0000
"Sa, Nuno" <Nuno.Sa@analog.com> wrote:

> > -----Original Message-----
> > From: Sa, Nuno <Nuno.Sa@analog.com>
> > Sent: Friday, August 20, 2021 9:21 AM
> > To: Miquel Raynal <miquel.raynal@bootlin.com>; Jonathan Cameron
> > <jic23@kernel.org>; Lars-Peter Clausen <lars@metafoo.de>
> > Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>; linux-
> > iio@vger.kernel.org; linux-kernel@vger.kernel.org
> > Subject: RE: [PATCH 10/16] iio: adc: max1027: Prevent single channel
> > accesses during buffer reads
> > 
> > [External]
> > 
> > 
> >   
> > > -----Original Message-----
> > > From: Miquel Raynal <miquel.raynal@bootlin.com>
> > > Sent: Wednesday, August 18, 2021 1:12 PM
> > > To: Jonathan Cameron <jic23@kernel.org>; Lars-Peter Clausen
> > > <lars@metafoo.de>
> > > Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>; linux-
> > > iio@vger.kernel.org; linux-kernel@vger.kernel.org; Miquel Raynal
> > > <miquel.raynal@bootlin.com>
> > > Subject: [PATCH 10/16] iio: adc: max1027: Prevent single channel
> > > accesses during buffer reads
> > >
> > > [External]
> > >
> > > When hardware buffers are enabled (the cnvst pin being the  
> > trigger),  
> > > one
> > > should not mess with the device state by requesting a single channel
> > > read. Prevent it with a iio_buffer_enabled() check.
> > >
> > > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> > > ---
> > >  drivers/iio/adc/max1027.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/iio/adc/max1027.c b/drivers/iio/adc/max1027.c
> > > index 223c9e4abd86..83526f3d7d3a 100644
> > > --- a/drivers/iio/adc/max1027.c
> > > +++ b/drivers/iio/adc/max1027.c
> > > @@ -335,6 +335,8 @@ static int max1027_read_raw(struct iio_dev
> > > *indio_dev,
> > >
> > >  	switch (mask) {
> > >  	case IIO_CHAN_INFO_RAW:
> > > +		if (iio_buffer_enabled(indio_dev))
> > > +			return -EBUSY;  
> > 
> > I guess 'iio_device_claim_direct_mode()' would be a better option
> > here? There's nothing preventing this check to pass and then,
> > concurrently
> > someone enables the buffer...
> >   
> 
> Taking a second look, it seems that this check is already done [1]? Am I missing
> I missing something?
> 
> Also, I think we are returning with the 'st->lock' held...
> 
> [1]: https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/max1027.c#L247
Absolutely agree this should be done with iio_device_claim_direct_mode() to close the
possible races.

I wonder why this one has been missed in all the cleanups of that stuff? Looks like
a simple case, but I guess it wasn't immediately visible in the read_raw() function
so no one noticed.

Jonathan