[PATCH 2/2] iio: proximity: sx9360: reject unsupported hardware gain values

[Arnav Kapoor <kapoorarnav43@gmail.com>]

sx9360_write_gain() passes any value from userspace to ilog2() and then
FIELD_PREP() into the 3-bit PROX_CTRL0 gain field without checking it.
Only gains of 1, 2, 4 and 8 are supported (and listed in
hardwaregain_available), but other values are not rejected:

  - 3, 5, 6, 7 are rounded down to the next power of two;
  - 16 to 128 write gain codes 4 to 7, which the driver does not
    advertise, and the next read reports gains of 16 to 128;
  - 256 and larger wrap around in the field, e.g. 256 sets a gain of 1;
  - 0 and negative values write code 7 (read back as 128), because
    ilog2(0) is -1 and a negative int becomes a large unsigned value.

The write returns success in every case.

Return -EINVAL for anything other than 1, 2, 4 or 8.

Fixes: 1cdb4c47f7f5 ("iio:proximity:sx9360: Add sx9360 support")
Signed-off-by: Arnav Kapoor <kapoorarnav43@gmail.com>
---
 drivers/iio/proximity/sx9360.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/iio/proximity/sx9360.c b/drivers/iio/proximity/sx9360.c
index 2a9b37df8f62..c6e5b3e557bb 100644
--- a/drivers/iio/proximity/sx9360.c
+++ b/drivers/iio/proximity/sx9360.c
@@ -625,6 +625,9 @@ static int sx9360_write_gain(struct sx_common_data *data,
 {
 	unsigned int gain, reg;
 
+	if (val <= 0 || val > 8 || !is_power_of_2(val))
+		return -EINVAL;
+
 	gain = ilog2(val);
 	reg = SX9360_REG_PROX_CTRL0_PHR + chan->channel;
 	gain = FIELD_PREP(SX9360_REG_PROX_CTRL0_GAIN_MASK, gain);
-- 
2.53.0