linux-kernel - Re: [PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack

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Date:   Thu, 12 Nov 2020 10:54:33 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Alexandru Ardelean <alexandru.ardelean@...log.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     jic23@...nel.org
Subject: Re: [PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the
 stack

On 11/12/20 10:10 AM, Alexandru Ardelean wrote:
> From: Lars-Peter Clausen <lars@...afoo.de>
>
> Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> can corrupt stack memory when using DMA on some systems.
>
> This change adds 4 bytes at the end of the current DMA buffer, which will
> be used by the trigger handler.
> This is required because the first 4 bytes are reserved for register data.
>
> Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
> Signed-off-by: Lars-Peter Clausen <lars@...afoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> ---
>   drivers/iio/adc/ad_sigma_delta.c       | 4 ++--
>   include/linux/iio/adc/ad_sigma_delta.h | 2 +-
>   2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index 86039e9ecaca..33297f26508a 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -395,11 +395,11 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
>   	struct iio_poll_func *pf = p;
>   	struct iio_dev *indio_dev = pf->indio_dev;
>   	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
> +	uint8_t *data = &sigma_delta->data[4];
>   	unsigned int reg_size;
>   	unsigned int data_reg;
> -	uint8_t data[16];
>   
> -	memset(data, 0x00, 16);
> +	memset(data, 0x00, 4);

Younger me didn't know what he was doing, this is wrong. We need the 
extra space for the padding and timestamp.

We also can't put the beginning of the buffer at an 4 byte offset since 
it needs to be 8 byte aligned for the timestamp.

>   
>   	reg_size = indio_dev->channels[0].scan_type.realbits +
>   			indio_dev->channels[0].scan_type.shift;
> diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> index a3a838dcf8e4..ac4ac4752c62 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -80,7 +80,7 @@ struct ad_sigma_delta {
>   	 * DMA (thus cache coherency maintenance) requires the
>   	 * transfer buffers to live in their own cache lines.
>   	 */
> -	uint8_t				data[4] ____cacheline_aligned;
> +	uint8_t				data[8] ____cacheline_aligned;
>   };
>   
>   static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,