iio: buffer: Fix demux update

When updating the buffer demux, we will skip a scan element from the
device in the case `in_ind != out_ind` and we enter the while loop.
in_ind should only be refreshed with `find_next_bit()` in the end of the
loop.

Note, to cause problems we need a situation where we are skippig over
an element (channel not enabled) that happens to not have the same size
as the next element.   Whilst this is a possible situation we haven't
actually identified any cases in mainline where it happens as most drivers
have consistent channel storage sizes with the exception of the timestamp
which is the last element and hence never skipped over.

Fixes: 5ada4ea9be16 ("staging:iio: add demux optionally to path from device to buffer")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20201112144323.28887-1-nuno.sa@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index 9663dec3dcf3275f123a7ab21c247386574e21a3..2f7426a2f47ca571a73851a51ba0c22bb6334996 100644 (file)
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -853,12 +853,12 @@ static int iio_buffer_update_demux(struct iio_dev *indio_dev,
                                       indio_dev->masklength,
                                       in_ind + 1);
                while (in_ind != out_ind) {
-                       in_ind = find_next_bit(indio_dev->active_scan_mask,
-                                              indio_dev->masklength,
-                                              in_ind + 1);
                        length = iio_storage_bytes_for_si(indio_dev, in_ind);
                        /* Make sure we are aligned */
                        in_loc = roundup(in_loc, length) + length;
+                       in_ind = find_next_bit(indio_dev->active_scan_mask,
+                                              indio_dev->masklength,
+                                              in_ind + 1);
                }
                length = iio_storage_bytes_for_si(indio_dev, in_ind);
                out_loc = roundup(out_loc, length);