iio: triggered-buffer: prevent possible freeing of wrong buffer

Commit ee708e6baacd ("iio: buffer: introduce support for attaching more
IIO buffers") introduced support for multiple buffers per indio_dev but
left indio_dev->buffer for a few legacy use cases.

In the case of the triggered buffer, iio_triggered_buffer_cleanup()
still assumes that indio_dev->buffer points to the buffer allocated by
iio_triggered_buffer_setup_ext(). However, since
iio_triggered_buffer_setup_ext() now calls iio_device_attach_buffer()
to attach the buffer, indio_dev->buffer will only point to the buffer
allocated by iio_device_attach_buffer() if it the first buffer attached.

This adds a check to make sure that no other buffer has been attached
yet to ensure that indio_dev->buffer will be assigned when
iio_device_attach_buffer() is called.

As per discussion in the review thread, we may want to deal with multiple
triggers per device, but this is a fix for the issue in the meantime and
any such support would be unlikely to be suitable for a backport.

Fixes: ee708e6baacd ("iio: buffer: introduce support for attaching more IIO buffers")
Signed-off-by: David Lechner <[e-mail]>
Acked-by: Nuno Sa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/buffer/industrialio-triggered-buffer.c

···
 	struct iio_buffer *buffer;
 	int ret;
 
+	/*
+	 * iio_triggered_buffer_cleanup() assumes that the buffer allocated here
+	 * is assigned to indio_dev->buffer but this is only the case if this
+	 * function is the first caller to iio_device_attach_buffer(). If
+	 * indio_dev->buffer is already set then we can't proceed otherwise the
+	 * cleanup function will try to free a buffer that was not allocated here.
+	 */
+	if (indio_dev->buffer)
+		return -EADDRINUSE;
+
 	buffer = iio_kfifo_allocate();
 	if (!buffer) {
 		ret = -ENOMEM;