iio: addac: ad74413r: Do not reference negative array offsets

Instead of aiming rx_buf at an invalid array-boundary-crossing location,
just skip the first increment. 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];
      |                    ^~~~~~

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Fixes: fea251b6a5db ("iio: addac: add AD74413R driver")
Reviewed-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220112203456.3950884-1-keescook@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

diff --git a/drivers/iio/addac/ad74413r.c b/drivers/iio/addac/ad74413r.c
index 5271073..aba9a64 100644 (file)
--- 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 = -EINVAL;
@@ -894,9 +894,10 @@ static int ad74413r_update_scan_mode(struct iio_dev *indio_dev,
 
                spi_message_add_tail(xfer, &st->adc_samples_msg);
 
-               xfer++;
                tx_buf += AD74413R_FRAME_SIZE;
-               rx_buf += AD74413R_FRAME_SIZE;
+               if (xfer != st->adc_samples_xfer)
+                       rx_buf += AD74413R_FRAME_SIZE;
+               xfer++;
        }
 
        xfer->rx_buf = rx_buf;