iio: buffer: Fix demux update

commit 19ef7b70ca9487773c29b449adf0c70f540a0aab upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index a4f6bb96d4f42505027f98392b208465ed04e9fa..276b609d791748e3eb9fb31bd81b7c9f555c5721 100644 (file)
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -865,12 +865,12 @@ 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);