From: Alan Young <redacted>

commit b6409dd6bdc03aa178bbff0d80db2a30d29b63ac upstream.

When control_compat.c:copy_ctl_value_to_user() is used, by
ctl_elem_read_user() & ctl_elem_write_user(), it must also copy back the
snd_ctl_elem_id value that may have been updated (filled in) by the call
to snd_ctl_elem_read/snd_ctl_elem_write().

This matches the functionality provided by snd_ctl_elem_read_user() and
snd_ctl_elem_write_user(), via snd_ctl_build_ioff().

Without this, and without making additional calls to snd_ctl_info()
which are unnecessary when using the non-compat calls, a userspace
application will not know the numid value for the element and
consequently will not be able to use the poll/read interface on the
control file to determine which elements have updates.

Signed-off-by: Alan Young <redacted>
Cc: <redacted>
Link: https://lore.kernel.org/r/20211202150607.543389-1-consult.awy@gmail.com (local)
Signed-off-by: Takashi Iwai <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 sound/core/control_compat.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/sound/core/control_compat.c
+++ b/sound/core/control_compat.c

@@ -279,6 +279,7 @@ static int copy_ctl_value_to_user(void _
 				  struct snd_ctl_elem_value *data,
 				  int type, int count)
 {
+	struct snd_ctl_elem_value32 __user *data32 = userdata;
 	int i, size;
 
 	if (type == SNDRV_CTL_ELEM_TYPE_BOOLEAN ||

@@ -295,6 +296,8 @@ static int copy_ctl_value_to_user(void _
 		if (copy_to_user(valuep, data->value.bytes.data, size))
 			return -EFAULT;
 	}
+	if (copy_to_user(&data32->id, &data->id, sizeof(data32->id)))
+		return -EFAULT;
 	return 0;
 }