[PATCH v3] staging: iio: adc: ad7816: Protect sysfs attributes with mutex

[Abdelnasser Hussein <abdelnasserhussein11@gmail.com>]

The sysfs store functions modify driver state that is critical during
SPI transactions. ad7816_store_mode toggles hardware GPIO pins, while
ad7816_store_channel updates the internal software state
(chip->channel_id).

If these are modified concurrently by userspace while an SPI transfer
is in progress, it could corrupt the hardware transaction or cause the
driver to communicate with the wrong channel.

Add the existing chip->lock guard to these functions to serialize
access and prevent these race conditions.

Fixes: 7924425db04a ("staging: iio: adc: new driver for AD7816 devices")
Suggested-by: Joshua Crofts <joshua.crofts1@gmail.com>
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Abdelnasser Hussein <abdelnasserhussein11@gmail.com>
---
Changes in v3:
- Expanded commit message to accurately explain why ad7816_store_channel
  needs locking to protect internal state, as pointed out by Jonathan.

Changes in v2:
- Added a blank line after guard(mutex) statements as requested by Joshua.
- Added Fixes tag as requested by Dan.
- Collected Reviewed-by tag from Joshua.

 drivers/staging/iio/adc/ad7816.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
index f76f0215119a..2a15d7598abb 100644
--- a/drivers/staging/iio/adc/ad7816.c
+++ b/drivers/staging/iio/adc/ad7816.c
@@ -143,6 +143,8 @@ static ssize_t ad7816_store_mode(struct device *dev,
 	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
 	struct ad7816_chip_info *chip = iio_priv(indio_dev);
 
+	guard(mutex)(&chip->lock);
+
 	if (strcmp(buf, "full") == 0) {
 		gpiod_set_value(chip->rdwr_pin, 1);
 		chip->mode = AD7816_FULL;
@@ -207,6 +209,8 @@ static ssize_t ad7816_store_channel(struct device *dev,
 		return -EINVAL;
 	}
 
+	guard(mutex)(&chip->lock);
+
 	chip->channel_id = data;
 
 	return len;
-- 
2.54.0