linux-kernel - Re: [PATCH v3 10/15] iio: resolver: ad2s1210: use bitmap

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Message-ID: <4052fd7b-dd8c-4a5a-8f82-dd515de14d20@baylibre.com>
Date: Thu, 20 Feb 2025 14:54:53 -0600
From: David Lechner <dlechner@...libre.com>
To: Linus Walleij <linus.walleij@...aro.org>,
 Bartosz Golaszewski <brgl@...ev.pl>, Andy Shevchenko <andy@...nel.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>,
 Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Jonathan Cameron <jic23@...nel.org>, Ulf Hansson <ulf.hansson@...aro.org>,
 Peter Rosin <peda@...ntia.se>, Andrew Lunn <andrew@...n.ch>,
 Heiner Kallweit <hkallweit1@...il.com>, Russell King
 <linux@...linux.org.uk>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Vinod Koul <vkoul@...nel.org>,
 Kishon Vijay Abraham I <kishon@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>, Liam Girdwood <lgirdwood@...il.com>,
 Mark Brown <broonie@...nel.org>, Jaroslav Kysela <perex@...ex.cz>,
 Takashi Iwai <tiwai@...e.com>
Cc: linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-iio@...r.kernel.org, linux-mmc@...r.kernel.org,
 netdev@...r.kernel.org, linux-phy@...ts.infradead.org,
 linux-sound@...r.kernel.org
Subject: Re: [PATCH v3 10/15] iio: resolver: ad2s1210: use bitmap_write

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@...aro.org>
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---
>  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.

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 */