iio: backend: fix uninitialized data in debugfs

commit a6e8b14 upstream.

If the *ppos value is non-zero then simple_write_to_buffer() will not
initialize the start of the buf[] buffer.  Non-zero ppos values aren't
going to work at all.  Check for that at the start of the function and
return -ENOSPC.

Fixes: cdf01e0 ("iio: backend: add debugFs interface")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: error27

drivers/iio/industrialio-backend.c

@@ -155,7 +155,7 @@ static ssize_t iio_backend_debugfs_write_reg(struct file *file,
 	ssize_t rc;
 	int ret;
 
-	if (count >= sizeof(buf))
+	if (*ppos != 0 || count >= sizeof(buf))
 		return -ENOSPC;
 
 	rc = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf, count);