ALSA: seq: virmidi: Fix discarding the unsubscribed output

The recent change to move the virmidi output processing to a work
slightly modified the code to discard the unsubscribed outputs so that
it works without a temporary buffer.  However, this is actually buggy,
and may spew a kernel warning due to the unexpected call of
snd_rawmidi_transmit_ack(), as triggered by syzbot.

This patch takes back to the original code in that part, use a
temporary buffer and simply repeat snd_rawmidi_transmit(), in order to
address the regression.

Fixes: f7debfe54090 ("ALSA: seq: virmidi: Offload the output event processing")
Reported-by: syzbot+ec5f605c91812d200367@syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index a2f1c6b586930d740cc8b674b0ea9dce4f06bf17..cb988efd1ed0d666494f76f23797fbde6a7e833e 100644 (file)
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -149,8 +149,9 @@ static void snd_vmidi_output_work(struct work_struct *work)
        /* discard the outputs in dispatch mode unless subscribed */
        if (vmidi->seq_mode == SNDRV_VIRMIDI_SEQ_DISPATCH &&
            !(vmidi->rdev->flags & SNDRV_VIRMIDI_SUBSCRIBE)) {
-               while (!snd_rawmidi_transmit_empty(substream))
-                       snd_rawmidi_transmit_ack(substream, 1);
+               char buf[32];
+               while (snd_rawmidi_transmit(substream, buf, sizeof(buf)) > 0)
+                       ; /* ignored */
                return;
        }