Re: [PATCH v3 10/15] iio: resolver: ad2s1210: use bitmap_write

[Jonathan Cameron <jic23@xxxxxxxxxx>]

On Thu, 20 Feb 2025 14:54:53 -0600
David Lechner <dlechner@xxxxxxxxxxxx> wrote:

> On 2/10/25 4:33 PM, David Lechner wrote:
> > Replace bitmap array access with bitmap_write.
> > 
> > Accessing the bitmap array directly is not recommended and now there is
> > a helper function that can be used.
> > 
> > Reviewed-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> > Signed-off-by: David Lechner <dlechner@xxxxxxxxxxxx>
> > ---
> >  drivers/iio/resolver/ad2s1210.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iio/resolver/ad2s1210.c b/drivers/iio/resolver/ad2s1210.c
> > index 7f18df790157f1e411fb70de193a49f0677c999f..04879e6d538bce664469c5f6759d8b1cedea16e9 100644
> > --- a/drivers/iio/resolver/ad2s1210.c
> > +++ b/drivers/iio/resolver/ad2s1210.c
> > @@ -46,6 +46,7 @@
> >   */
> >  
> >  #include <linux/bitfield.h>
> > +#include <linux/bitmap.h>
> >  #include <linux/bits.h>
> >  #include <linux/cleanup.h>
> >  #include <linux/clk.h>
> > @@ -180,7 +181,7 @@ static int ad2s1210_set_mode(struct ad2s1210_state *st, enum ad2s1210_mode mode)
> >  	if (!gpios)
> >  		return mode == st->fixed_mode ? 0 : -EOPNOTSUPP;
> >  
> > -	bitmap[0] = mode;
> > +	bitmap_write(bitmap, mode, 0, 2);
> >  
> >  	return gpiod_multi_set_value_cansleep(gpios, bitmap);
> >  }
> > @@ -1470,7 +1471,7 @@ static int ad2s1210_setup_gpios(struct ad2s1210_state *st)
> >  			return dev_err_probe(dev, -EINVAL,
> >  				      "requires exactly 2 resolution-gpios\n");
> >  
> > -		bitmap[0] = st->resolution;
> > +		bitmap_write(bitmap, st->resolution, 0, 2);
> >  
> >  		ret = gpiod_multi_set_value_cansleep(resolution_gpios, bitmap);
> >  		if (ret < 0)
> >   
> 
> There is actually a bug here pointed out in a similar patch. bitmap_write()
> only modifies the bitmap, so this introduces an unintialized use bug. [1]
> Here, we only use the bits that we set, so runtime behavior would not actually
> be buggy but still best to fully initialize the memory.
> 
> I'm a bit surprised that my local compiler and iio/testing both didn't catch that
> since GCC 14 caught it in the other driver.
> 
> [1]: https://lore.kernel.org/linux-gpio/20250217132152.29d86d6c@jic23-huawei/T/#m3163d2c5db5b7376504d8ad6f23716f1119de761
> 
> The fix is simple, we can zero-initialize the bitmap.

Please send this as a fix patch on top as I'd rather not unwind my tree
for just this and the patch is already pushed out on what is mostly a non
rebasing branch.

> 
> diff --git a/drivers/iio/resolver/ad2s1210.c b/drivers/iio/resolver/ad2s1210.c
> index 04879e6d538b..ab860cedecd1 100644
> --- a/drivers/iio/resolver/ad2s1210.c
> +++ b/drivers/iio/resolver/ad2s1210.c
> @@ -176,7 +176,7 @@ struct ad2s1210_state {
>  static int ad2s1210_set_mode(struct ad2s1210_state *st, enum ad2s1210_mode mode)
>  {
>  	struct gpio_descs *gpios = st->mode_gpios;
> -	DECLARE_BITMAP(bitmap, 2);
> +	DECLARE_BITMAP(bitmap, 2) = { };
>  
>  	if (!gpios)
>  		return mode == st->fixed_mode ? 0 : -EOPNOTSUPP;
> @@ -1427,7 +1427,7 @@ static int ad2s1210_setup_gpios(struct ad2s1210_state *st)
>  	struct device *dev = &st->sdev->dev;
>  	struct gpio_descs *resolution_gpios;
>  	struct gpio_desc *reset_gpio;
> -	DECLARE_BITMAP(bitmap, 2);
> +	DECLARE_BITMAP(bitmap, 2) = { };
>  	int ret;
>  
>  	/* should not be sampling on startup */
>