[PATCH v2] iio: adc: ad4030: fix invalid oversampling_ratio validation

[Salah Triki <salah.triki@gmail.com>]

ad4030_set_avg_frame_len() computes avg_log2 = ilog2(avg_val) before
validating avg_val, and the subsequent range check only rejects
negative values or values above the maximum supported OSR. It does
not reject avg_val == 0, nor values that are not exact powers of 2.

- avg_val == 0 passes the check (0 is not < 0 and not > max), so
  ilog2(0) is called with an undefined/garbage result.

- Non-power-of-2 values (e.g. avg_val == 3) also pass the check and
  silently get rounded down by ilog2() to the nearest lower power of
  2, so userspace can write a value to the oversampling_ratio sysfs
  attribute that does not match what actually gets programmed into
  hardware, without any error being reported.

Only powers of 2 in [1, 65536] are valid OSR values, as listed in
ad4030_average_modes[]. Validate avg_val fully before computing its
log2, using is_power_of_2() and requiring avg_val > 0.

This issue was identified with assistance from Claude AI and manually
verified against the code.

Fixes: 949abd1ca5a4 ("iio: adc: ad4030: add averaging support")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
---
Changes since v1:
   - Added note stating the issue was identified with assistance from
   Claude AI and verified manually.
   - Removed initialization of avg_log2 at declaration.

 drivers/iio/adc/ad4030.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/ad4030.c b/drivers/iio/adc/ad4030.c
index 9c5f19321e3b..20911cc55873 100644
--- a/drivers/iio/adc/ad4030.c
+++ b/drivers/iio/adc/ad4030.c
@@ -746,14 +746,16 @@ static int ad4030_set_chan_calibbias(struct iio_dev *indio_dev,
 static int ad4030_set_avg_frame_len(struct iio_dev *dev, int avg_val)
 {
 	struct ad4030_state *st = iio_priv(dev);
-	unsigned int avg_log2 = ilog2(avg_val);
+	unsigned int avg_log2;
 	unsigned int last_avg_idx = ARRAY_SIZE(ad4030_average_modes) - 1;
 	int freq_hz;
 	int ret;
 
-	if (avg_val < 0 || avg_val > ad4030_average_modes[last_avg_idx])
+	if (avg_val <= 0 || avg_val > ad4030_average_modes[last_avg_idx] || !is_power_of_2(avg_val))
 		return -EINVAL;
 
+	avg_log2 = ilog2(avg_val);
+
 	if (st->offload_trigger) {
 		/*
 		 * The sample averaging and sampling frequency configurations
-- 
2.43.0