Re: [PATCH 2/2] iio: iio-mux: kzalloc instead of devm_kzalloc to ensure page alignment

[Matteo Martelli <matteomartelli3@gmail.com>]

On Sun, 8 Dec 2024 18:15:31 +0000, Jonathan Cameron <jic23@kernel.org> wrote:
> On Mon, 02 Dec 2024 16:11:08 +0100
> Matteo Martelli <matteomartelli3@gmail.com> wrote:
> 
> > During channel configuration, the iio-mux driver allocates a page with
> > devm_kzalloc(PAGE_SIZE) to read channel ext_info. However, the resulting
> > buffer points to an offset of the page due to the devres header sitting
> > at the beginning of the allocated area. This leads to failure in the
> > provider driver when sysfs_emit* helpers are used to format the ext_info
> > attributes.
> > 
> > Switch to plain kzalloc version. The devres version is not strictly
> > necessary as the buffer is only accessed during the channel
> > configuration phase. Rely on __free cleanup to deallocate the buffer.
> > Also, move the ext_info handling into a new function to have the page
> > buffer definition and assignment in one statement as suggested by
> > cleanup documentation.
> > 
> > Signed-off-by: Matteo Martelli <matteomartelli3@gmail.com>
> This seems fine to me, but the diff ended up a bit complex, so I'd like
> Peter to take a look as well before I apply it.

For a simpler diff I could go for devm_get_free_pages()+devm_free_pages(),
but since devres doesn't seem necessary in this case, I think this patch
provides a cleaner solution at the end.

> 
> Do you have a board that is hitting this?  If so, a fixes tag is definitely
> appropriate. I think it is probably appropriate even it not.

I am not sure if any existing board is affected as I encountered this
issue while experimenting with consumer drivers, thus using a custom DT
on top of sun50i-a64-pine64.dts just for testing. The following DT files
might be affected but only if the iio channel controlled by the iio_mux
multiplexer owns an ext_info attribute which is also exposed on sysfs.

$ grep -Rl 'io-channel-mux' arch
arch/arm/boot/dts/aspeed/aspeed-bmc-ampere-mtmitchell.dts
arch/arm/boot/dts/aspeed/aspeed-bmc-ampere-mtjade.dts
arch/arm/boot/dts/microchip/at91-tse850-3.dts
arch/arm/boot/dts/microchip/at91-natte.dtsi
arch/arm64/boot/dts/rockchip/rk3566-powkiddy-rk2023.dtsi
arch/arm64/boot/dts/rockchip/rk3566-anbernic-rg353x.dtsi
arch/arm64/boot/dts/rockchip/rk3566-anbernic-rg503.dts
arch/arm64/boot/dts/rockchip/rk3326-odroid-go3.dts
arch/arm64/boot/dts/allwinner/sun50i-h700-anbernic-rg35xx-h.dtb
arch/arm64/boot/dts/allwinner/sun50i-h700-anbernic-rg35xx-h.dts

I am also not sure what would be the reference commit for the Fixes tag.
The related ext_info attributes handling was introduced in the first
commit of the iio_mux implementation. If that applies, following the
corresponding Fixes tag.

Fixes: 7ba9df54b091 ("iio: multiplexer: new iio category and iio-mux driver")

> 
> Jonathan
> 

Best regards,
Matteo
> > ---
> >  drivers/iio/multiplexer/iio-mux.c | 84 +++++++++++++++++++++------------------
> >  1 file changed, 46 insertions(+), 38 deletions(-)
> > 
> > diff --git a/drivers/iio/multiplexer/iio-mux.c b/drivers/iio/multiplexer/iio-mux.c
> > index 2953403bef53bbe47a97a8ab1c475ed88d7f86d2..c309d991490c63ba4299f1cda7102f10dcf54982 100644
> > --- a/drivers/iio/multiplexer/iio-mux.c
> > +++ b/drivers/iio/multiplexer/iio-mux.c
> > @@ -7,6 +7,7 @@
> >   * Author: Peter Rosin <peda@axentia.se>
> >   */
> >  
> > +#include <linux/cleanup.h>
> >  #include <linux/err.h>
> >  #include <linux/iio/consumer.h>
> >  #include <linux/iio/iio.h>
> > @@ -237,49 +238,18 @@ static ssize_t mux_write_ext_info(struct iio_dev *indio_dev, uintptr_t private,
> >  	return ret;
> >  }
> >  
> > -static int mux_configure_channel(struct device *dev, struct mux *mux,
> > -				 u32 state, const char *label, int idx)
> > +static int mux_configure_chan_ext_info(struct device *dev, struct mux *mux,
> > +				       int idx, int num_ext_info)
> >  {
> >  	struct mux_child *child = &mux->child[idx];
> > -	struct iio_chan_spec *chan = &mux->chan[idx];
> >  	struct iio_chan_spec const *pchan = mux->parent->channel;
> > -	char *page = NULL;
> > -	int num_ext_info;
> >  	int i;
> >  	int ret;
> >  
> > -	chan->indexed = 1;
> > -	chan->output = pchan->output;
> > -	chan->datasheet_name = label;
> > -	chan->ext_info = mux->ext_info;
> > -
> > -	ret = iio_get_channel_type(mux->parent, &chan->type);
> > -	if (ret < 0) {
> > -		dev_err(dev, "failed to get parent channel type\n");
> > -		return ret;
> > -	}
> > -
> > -	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_RAW))
> > -		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_RAW);
> > -	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_SCALE))
> > -		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_SCALE);
> > -
> > -	if (iio_channel_has_available(pchan, IIO_CHAN_INFO_RAW))
> > -		chan->info_mask_separate_available |= BIT(IIO_CHAN_INFO_RAW);
> > -
> > -	if (state >= mux_control_states(mux->control)) {
> > -		dev_err(dev, "too many channels\n");
> > -		return -EINVAL;
> > -	}
> > -
> > -	chan->channel = state;
> > +	char *page __free(kfree) = kzalloc(PAGE_SIZE, GFP_KERNEL);
> > +	if (!page)
> > +		return -ENOMEM;
> >  
> > -	num_ext_info = iio_get_channel_ext_info_count(mux->parent);
> > -	if (num_ext_info) {
> > -		page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
> > -		if (!page)
> > -			return -ENOMEM;
> > -	}
> >  	child->ext_info_cache = devm_kcalloc(dev,
> >  					     num_ext_info,
> >  					     sizeof(*child->ext_info_cache),
> > @@ -318,8 +288,46 @@ static int mux_configure_channel(struct device *dev, struct mux *mux,
> >  		child->ext_info_cache[i].size = ret;
> >  	}
> >  
> > -	if (page)
> > -		devm_kfree(dev, page);
> > +	return 0;
> > +}
> > +
> > +static int mux_configure_channel(struct device *dev, struct mux *mux, u32 state,
> > +				 const char *label, int idx)
> > +{
> > +	struct iio_chan_spec *chan = &mux->chan[idx];
> > +	struct iio_chan_spec const *pchan = mux->parent->channel;
> > +	int num_ext_info;
> > +	int ret;
> > +
> > +	chan->indexed = 1;
> > +	chan->output = pchan->output;
> > +	chan->datasheet_name = label;
> > +	chan->ext_info = mux->ext_info;
> > +
> > +	ret = iio_get_channel_type(mux->parent, &chan->type);
> > +	if (ret < 0) {
> > +		dev_err(dev, "failed to get parent channel type\n");
> > +		return ret;
> > +	}
> > +
> > +	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_RAW))
> > +		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_RAW);
> > +	if (iio_channel_has_info(pchan, IIO_CHAN_INFO_SCALE))
> > +		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_SCALE);
> > +
> > +	if (iio_channel_has_available(pchan, IIO_CHAN_INFO_RAW))
> > +		chan->info_mask_separate_available |= BIT(IIO_CHAN_INFO_RAW);
> > +
> > +	if (state >= mux_control_states(mux->control)) {
> > +		dev_err(dev, "too many channels\n");
> > +		return -EINVAL;
> > +	}
> > +
> > +	chan->channel = state;
> > +
> > +	num_ext_info = iio_get_channel_ext_info_count(mux->parent);
> > +	if (num_ext_info)
> > +		return mux_configure_chan_ext_info(dev, mux, idx, num_ext_info);
> >  
> >  	return 0;
> >  }
> > 
>