iio: dac: ad3552r-hs: fix uninitialized data ni ad3552r_hs_write_data_source()

commit eaaa7ee upstream.

If the *ppos value is non-zero then the simple_write_to_buffer() function
won't initialized the start of the buf[] buffer.  Non-zero values for
*ppos won't work here generally, so just test for them and return -ENOSPC
at the start of the function.

Fixes: b1c5d68 ("iio: dac: ad3552r-hs: add support for internal ramp")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Angelo Dureghello <adureghello@baylibre.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/dac/ad3552r-hs.c

@@ -549,7 +549,7 @@ static ssize_t ad3552r_hs_write_data_source(struct file *f,
 
 	guard(mutex)(&st->lock);
 
-	if (count >= sizeof(buf))
+	if (*ppos != 0 || count >= sizeof(buf))
 		return -ENOSPC;
 
 	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,