Re: [PATCH v2] iio: light: veml6030: add support for triggered buffer

[Javier Carrasco <javier.carrasco.cruz@gmail.com>]

On 23/11/2024 16:16, Jonathan Cameron wrote:
> On Sun, 10 Nov 2024 18:49:05 +0100
> Javier Carrasco <javier.carrasco.cruz@gmail.com> wrote:
> 
>> All devices supported by this driver (currently veml6030, veml6035
>> and veml7700) have two 16-bit channels, and can profit for the same
>> configuration to support data access via triggered buffers.
>>
>> The measurements are stored in two 16-bit consecutive registers
>> (addresses 0x04 and 0x05) as little endian, unsigned data.
>>
>> Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
> Hi Javier,
> 
> We have to be a little careful with pushing data from the stack.
> Need to makes sure holes are zero filled.
> 
> Jonathan
> 
>> diff --git a/drivers/iio/light/veml6030.c b/drivers/iio/light/veml6030.c
>> index ccb43dfd5cf7..ce9af9a0e933 100644
>> --- a/drivers/iio/light/veml6030.c
>> +++ b/drivers/iio/light/veml6030.c
> 
>>  
>>  static const struct regmap_config veml6030_regmap_config = {
>> @@ -889,6 +928,35 @@ static irqreturn_t veml6030_event_handler(int irq, void *private)
>>  	return IRQ_HANDLED;
>>  }
>>  
>> +static irqreturn_t veml6030_trigger_handler(int irq, void *p)
>> +{
>> +	struct iio_poll_func *pf = p;
>> +	struct iio_dev *iio = pf->indio_dev;
>> +	struct veml6030_data *data = iio_priv(iio);
>> +	unsigned int reg;
>> +	int ch, ret, i = 0;
>> +	struct {
>> +		u16 chans[2];
> There is a hole here... 
>> +		aligned_s64 timestamp;
>> +	} scan;
>> +
>> +	iio_for_each_active_channel(iio, ch) {
>> +		ret = regmap_read(data->regmap, VEML6030_REG_DATA(ch),
>> +				  &reg);
>> +		if (ret)
>> +			goto done;
>> +
>> +		scan.chans[i++] = reg;
> This fills in at least 1 channel, but maybe not the second.
>> +	}
>> +
> So this leaks random stack data I think.
> 
> Upshot, when holes are involved or not all the channels are set, need
> memset(&scan, 0, sizeof(scan));
> for the structure on the stack which will zero the holes as well as
> both channels.
> 
> Ancient article on this: https://lwn.net/Articles/417989/
> 
> We get away with it when they are in the iio_priv space because they are
> kzalloc + if we do leak data due to changes in configured channels it's
> just old sensor data which is (I think) never a security problem!
> 
>> +	iio_push_to_buffers_with_timestamp(iio, &scan, pf->timestamp);
>> +
>> +done:
>> +	iio_trigger_notify_done(iio->trig);
>> +
>> +	return IRQ_HANDLED;
>> +}
>> +
>>  static int veml6030_set_info(struct iio_dev *indio_dev)
>>  {
>>  	struct veml6030_data *data = iio_priv(indio_dev);
>> @@ -1077,6 +1145,12 @@ static int veml6030_probe(struct i2c_client *client)
>>  	if (ret < 0)
>>  		return ret;
>>  
>> +	ret = devm_iio_triggered_buffer_setup(&client->dev, indio_dev, NULL,
>> +					      veml6030_trigger_handler, NULL);
>> +	if (ret)
>> +		return dev_err_probe(&client->dev, ret,
>> +				     "Failed to register triggered buffer");
>> +
>>  	return devm_iio_device_register(&client->dev, indio_dev);
>>  }
>>  
>>
>> ---
>> base-commit: 9dd2270ca0b38ee16094817f4a53e7ba78e31567
>> change-id: 20241106-veml6030_triggered_buffer-a38886ca4cce
>>
>> Best regards,
> 


Hi Jonathan,

thanks a lot for your explanation and the link, it makes perfect sense.
By the way, when I moved this struct from the iio_priv to the function,
I took a look at some existing code, and a couple of them might have the
same issue:

- temperature/tmp006.c: it also has a hole between the two 16-bit
channels and the timestamp (aligned(8)), but it is not set to zero.

- adc/ti-ads1119.c: the scan consists of an unsigned int and the
timestamp (aligned(8)). I believe there is a hole there as well.

I did not go over all drivers (most of them store the scan struct in the
iio_priv space anyway), but at least those two look suspicious.

Should I fix (e.g. memset) those two I mentioned?

Best regards,
Javier Carrasco