ao_alsa: fill unused ALSA channels with silence

This happens when ALSA gives us more channels than we asked for, for
whatever reasons. It looks like this wasn't handled correctly. The mpv
and ALSA channel counts could mismatch, which would lead to UB.

I couldn't actually trigger this case, though. I'm fairly sure that
drivers or plugins exist that do it anyway. (Inofficial ALSA motto: if
it can be broken, then why not break it?)

1 files changed, 5 insertions, 0 deletions

diff --git a/audio/out/ao_alsa.c b/audio/out/ao_alsa.c
index f80a46a5cc..f30417ff68 100644
--- a/audio/out/ao_alsa.c
+++ b/audio/out/ao_alsa.c
@@ -794,8 +794,13 @@ static int init_device(struct ao *ao, int mode)
     if (num_channels != ao->channels.num) {
         int req = ao->channels.num;
         mp_chmap_from_channels(&ao->channels, MPMIN(2, num_channels));
+        mp_chmap_fill_na(&ao->channels, num_channels);
         MP_ERR(ao, "Asked for %d channels, got %d - fallback to %s.\n", req,
                num_channels, mp_chmap_to_str(&ao->channels));
+        if (num_channels != ao->channels.num) {
+            MP_FATAL(ao, "mismatching channel counts.\n");
+            goto alsa_error;
+        }
     }
 
     err = snd_pcm_hw_params_get_buffer_size(alsa_hwparams, &p->buffersize);