[PATCH] iio: addac: Do not reference negative array offsets

[Kees Cook]

Instead of aiming rx_buf at an invalid array-boundary-crossing location,
just skip the first assignment. Seen when building with -Warray-bounds:

drivers/iio/addac/ad74413r.c: In function 'ad74413r_update_scan_mode':
drivers/iio/addac/ad74413r.c:843:22: warning: array subscript -4 is below array bounds of 'u8[16]' { aka 'unsigned char[16]'} [-Warray-bounds]
  843 |         u8 *rx_buf = &st->adc_samples_buf.rx_buf[-1 * AD74413R_FRAME_SIZE];
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/iio/addac/ad74413r.c:84:20: note: while referencing 'rx_buf'
   84 |                 u8 rx_buf[AD74413R_FRAME_SIZE * AD74413R_CHANNEL_MAX];
      |                    ^~~~~~

Fixes: fea251b6a5db ("iio: addac: add AD74413R driver")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/iio/addac/ad74413r.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/addac/ad74413r.c b/drivers/iio/addac/ad74413r.c
index cbd9aa9b399a..b0a6d8ee5133 100644
--- a/drivers/iio/addac/ad74413r.c
+++ b/drivers/iio/addac/ad74413r.c
@@ -840,7 +840,7 @@ static int ad74413r_update_scan_mode(struct iio_dev *indio_dev,
 {
 	struct ad74413r_state *st = iio_priv(indio_dev);
 	struct spi_transfer *xfer = st->adc_samples_xfer;
-	u8 *rx_buf = &st->adc_samples_buf.rx_buf[-1 * AD74413R_FRAME_SIZE];
+	u8 *rx_buf = st->adc_samples_buf.rx_buf;
 	u8 *tx_buf = st->adc_samples_tx_buf;
 	unsigned int channel;
 	int ret;
@@ -877,9 +877,8 @@ static int ad74413r_update_scan_mode(struct iio_dev *indio_dev,
 		if (ret)
 			goto out;
 
-		st->adc_active_channels++;
 
-		if (xfer == st->adc_samples_xfer)
+		if (xfer == st->adc_samples_xfer || st->adc_active_channels == 0)
 			xfer->rx_buf = NULL;
 		else
 			xfer->rx_buf = rx_buf;
@@ -896,7 +895,10 @@ static int ad74413r_update_scan_mode(struct iio_dev *indio_dev,
 
 		xfer++;
 		tx_buf += AD74413R_FRAME_SIZE;
-		rx_buf += AD74413R_FRAME_SIZE;
+		if (st->adc_active_channels)
+			rx_buf += AD74413R_FRAME_SIZE;
+
+		st->adc_active_channels++;
 	}
 
 	xfer->rx_buf = rx_buf;
-- 
2.30.2