linux-kernel - Re: [PATCH v4 2/2] media: rc: ir-spi: constrain carrier frequency

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Message-ID: <aEsycgtDxrypTU0v@gofer.mess.org>
Date: Thu, 12 Jun 2025 21:02:58 +0100
From: Sean Young <sean@...s.org>
To: Cosmin Tanislav <demonsingur@...il.com>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] media: rc: ir-spi: constrain carrier frequency

On Wed, Jun 11, 2025 at 11:35:21PM +0300, Cosmin Tanislav wrote:
> On 6/11/25 11:09 PM, Sean Young wrote:
> > On Wed, Jun 11, 2025 at 02:23:44PM +0300, Cosmin Tanislav wrote:
> > > Carrier frequency is currently unconstrained, allowing the SPI transfer
> > > to be allocated and filled only for it to be later rejected by the SPI
> > > controller since the frequency is too large.
> > > 
> > > Add a check to constrain the carrier frequency inside
> > > ir_spi_set_tx_carrier().
> > > 
> > > Also, move the number of bits per pulse to a macro since it is not used
> > > in multiple places.
> > > 
> > > Signed-off-by: Cosmin Tanislav <demonsingur@...il.com>
> > > ---
> > >   drivers/media/rc/ir-spi.c | 6 +++++-
> > >   1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/media/rc/ir-spi.c b/drivers/media/rc/ir-spi.c
> > > index 50e30e2fae22..bf731204c81e 100644
> > > --- a/drivers/media/rc/ir-spi.c
> > > +++ b/drivers/media/rc/ir-spi.c
> > > @@ -21,6 +21,7 @@
> > >   #define IR_SPI_DRIVER_NAME		"ir-spi"
> > >   #define IR_SPI_DEFAULT_FREQUENCY	38000
> > > +#define IR_SPI_BITS_PER_PULSE		16
> > >   struct ir_spi_data {
> > >   	u32 freq;
> > > @@ -70,7 +71,7 @@ static int ir_spi_tx(struct rc_dev *dev, unsigned int *buffer, unsigned int coun
> > >   	memset(&xfer, 0, sizeof(xfer));
> > > -	xfer.speed_hz = idata->freq * 16;
> > > +	xfer.speed_hz = idata->freq * IR_SPI_BITS_PER_PULSE;
> > >   	xfer.len = len * sizeof(*tx_buf);
> > >   	xfer.tx_buf = tx_buf;
> > > @@ -98,6 +99,9 @@ static int ir_spi_set_tx_carrier(struct rc_dev *dev, u32 carrier)
> > >   	if (!carrier)
> > >   		return -EINVAL;
> > > +	if (carrier * IR_SPI_BITS_PER_PULSE > idata->spi->max_speed_hz)
> > > +		return -EINVAL;
> > 
> > Just a nitpick.
> > 
> > I think carrier * IR_SPI_BITS_PER_PULSE could overflow, and then the check
> > wouldn't work. It might be better to do:
> > 
> > 	if (carrier > idata->spi->max_speed_hz / IR_SPI_BITS_PER_PULSE)
> > 
> > However since IR_SPI_BITS_PER_PULSE is 16, which is just a shift left by 4,
> > I don't think this can be abused in any useful way.
> > 
> 
> I have another concern regarding overflow, inside ir_spi_tx().
> 
> DIV_ROUND_CLOSEST() is called with buffer[i] * idata->freq and 1000000.
> buffer[i] comes from userspace, it's the number of microseconds for this
> pulse. It's unsigned int. lirc core already checks that each element
> is not bigger than 500000 microseconds. Issue is, at 500000, it would
> take a carrier frequency as low as 8590 to overflow the unsigned int.

Interesting, you are right.

> Maybe it would make sense to switch this one to mult_frac()? But we
> would lose rounding.
> 
> mult_frac(buffer[i], idata->freq, 1000000)
> 
> Optionally, we could cast buffer[i] to u64/unsigned long long, and use
> DIV_ROUND_CLOSEST_ULL.
> 
> DIV_ROUND_CLOSEST_ULL((u64)buffer[i] * idata->freq, 1000000)
> 
> Let me know what you think.

I've given it some thought and I'm not sure there is a better solution. It's
an edge case of course, but we should deal with it correctly.

Nice catch, solution looks good.


Sean