[PATCH v2] iio: dac: mcp47feb02: reject out-of-range raw values

[David Carlier <devnexen@gmail.com>]

mcp47feb02_write_raw() writes the user supplied raw value to the DAC
register without checking it against the chip resolution. The parts
are 8, 10 or 12 bit, and the bits above the resolution are not
implemented, so writing 300 to out_voltage0_raw on an 8-bit part sets
the output to code 44 instead of failing, and -1 is written as 0xffff.

The out-of-range value is also cached in dac_data, so it is written
back on resume and stored to EEPROM by store_eeprom.

Return -EINVAL for values outside [0, 2^resolution - 1].

Fixes: bf394cc80369 ("iio: dac: adding support for Microchip MCP47FEB02")
Cc: stable@vger.kernel.org
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
---
Found by LLM-assisted code inspection, reviewed and verified by hand.
Build tested with W=1 (=m and =y) only; no hardware was available for
runtime testing.

v2:
- Use GENMASK() for the upper bound so it is safe up to 32 bits (Andy)
- Drop the Assisted-by tag, removed from the docs in 1e035297ffda (Joshua)
- Move the testing note below the --- (Joshua)

v1: https://lore.kernel.org/all/20261024214423.865635-1-devnexen@gmail.com/
---
 drivers/iio/dac/mcp47feb02-core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/iio/dac/mcp47feb02-core.c b/drivers/iio/dac/mcp47feb02-core.c
index 88bffbab0854..b5a2ac4a2e67 100644
--- a/drivers/iio/dac/mcp47feb02-core.c
+++ b/drivers/iio/dac/mcp47feb02-core.c
@@ -696,6 +696,9 @@ static int mcp47feb02_write_raw(struct iio_dev *indio_dev, struct iio_chan_spec
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
+		if (val < 0 || val > GENMASK(data->chip_features->resolution - 1, 0))
+			return -EINVAL;
+
 		ret = regmap_write(data->regmap, REG_ADDR(ch->address), val);
 		if (ret)
 			return ret;
-- 
2.55.0