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

From: Jonathan Cameron <jic23@jic23.retrosnub.co.uk>
To: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <linux-iio@vger.kernel.org>, Lars-Peter Clausen <lars@metafoo.de>
Subject: Re: [PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the stack
Date: Sun, 14 Apr 2019 13:45:46 +0100	[thread overview]
Message-ID: <20190414134546.1e9a3907@archlinux> (raw)
In-Reply-To: <20190409075821.26104-1-alexandru.ardelean@analog.com>

On Tue, 9 Apr 2019 10:58:21 +0300
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> From: Lars-Peter Clausen <lars@metafoo.de>
> 
> Use a heap allocated memory for the SPI transfer buffer. Using stack memory
> will not work on some architectures when using DMA.
> 
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Yikes, surprised this one snuck in here.
> ---
> 
> Note: this doesn't need a stable/fixes tag. It's more of an enhancement.

Talk me through this...  Right now I think this is a bug that is going
to cause random and extremely hard to debug problems.  I've had these
in the past and they are really really nasty as it's random stack corruption.
The nasty is not the fact it's on the stack, but the fact that it isn't
in it's own cacheline.

So my inclination is this definitely needs stable and fixes tag.
What am I missing?

Jonathan

> 
>  drivers/iio/adc/ad_sigma_delta.c       | 22 +++++++++++-----------
>  include/linux/iio/adc/ad_sigma_delta.h |  3 ++-
>  2 files changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index af6cbc683214..02d9049b9218 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(ad_sd_set_comm);
>  int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
>  	unsigned int size, unsigned int val)
>  {
> -	uint8_t *data = sigma_delta->data;
> +	uint8_t *data = sigma_delta->reg_data;
>  	struct spi_transfer t = {
>  		.tx_buf		= data,
>  		.len		= size + 1,
> @@ -102,7 +102,7 @@ EXPORT_SYMBOL_GPL(ad_sd_write_reg);
>  static int ad_sd_read_reg_raw(struct ad_sigma_delta *sigma_delta,
>  	unsigned int reg, unsigned int size, uint8_t *val)
>  {
> -	uint8_t *data = sigma_delta->data;
> +	uint8_t *data = sigma_delta->reg_data;
>  	int ret;
>  	struct spi_transfer t[] = {
>  		{
> @@ -148,24 +148,24 @@ int ad_sd_read_reg(struct ad_sigma_delta *sigma_delta,
>  {
>  	int ret;
>  
> -	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->data);
> +	ret = ad_sd_read_reg_raw(sigma_delta, reg, size, sigma_delta->reg_data);
>  	if (ret < 0)
>  		goto out;
>  
>  	switch (size) {
>  	case 4:
> -		*val = get_unaligned_be32(sigma_delta->data);
> +		*val = get_unaligned_be32(sigma_delta->reg_data);
>  		break;
>  	case 3:
> -		*val = (sigma_delta->data[0] << 16) |
> -			(sigma_delta->data[1] << 8) |
> -			sigma_delta->data[2];
> +		*val = (sigma_delta->reg_data[0] << 16) |
> +			(sigma_delta->reg_data[1] << 8) |
> +			sigma_delta->reg_data[2];
>  		break;
>  	case 2:
> -		*val = get_unaligned_be16(sigma_delta->data);
> +		*val = get_unaligned_be16(sigma_delta->reg_data);
>  		break;
>  	case 1:
> -		*val = sigma_delta->data[0];
> +		*val = sigma_delta->reg_data[0];
>  		break;
>  	default:
>  		ret = -EINVAL;
> @@ -403,12 +403,12 @@ 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->buf_data;
>  	unsigned int reg_size;
>  	unsigned int data_reg;
> -	uint8_t data[16];
>  	int ret;
>  
> -	memset(data, 0x00, 16);
> +	memset(sigma_delta->buf_data, 0x00, sizeof(sigma_delta->buf_data));
>  
>  	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 6e9fb1932dde..fb77b2ebe498 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -79,7 +79,8 @@ 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				reg_data[4] ____cacheline_aligned;
> +	uint8_t				buf_data[16];
>  };
>  
>  static inline int ad_sigma_delta_set_channel(struct ad_sigma_delta *sd,