ALSA: ctl: Fix copy of updated id with element read/write

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 <consult.awy@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20211202150607.543389-1-consult.awy@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/control_compat.c b/sound/core/control_compat.c
index 470dabc..edff063 100644 (file)
--- a/sound/core/control_compat.c
+++ b/sound/core/control_compat.c
@@ -264,6 +264,7 @@ static int copy_ctl_value_to_user(void __user *userdata,
                                  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 ||
@@ -280,6 +281,8 @@ static int copy_ctl_value_to_user(void __user *userdata,
                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;
 }