Re: WARNING in __snd_rawmidi_transmit_ack (2)

[Takashi Iwai]

On Tue, 14 Aug 2018 16:20:02 +0200,
syzbot wrote:
> 
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    4e8b38549b50 Add linux-next specific files for 20180814
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=11f5e78c400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=18edf0289d1b5ab
> dashboard link: https://syzkaller.appspot.com/bug?extid=ec5f605c91812d200367
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=12f8c6d2400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16976b72400000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+ec5f605c91812d200367@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> WARNING: CPU: 1 PID: 1691 at sound/core/rawmidi.c:1170
> __snd_rawmidi_transmit_ack+0x263/0x2f0 sound/core/rawmidi.c:1170

This seems to be a bad side-effect by the code change along with the
offloading the virmidi work.

I'm going to queue the fix below.


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@xxxxxxx>
Subject: [PATCH] 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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/core/seq/seq_virmidi.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index a2f1c6b58693..cb988efd1ed0 100644
--- 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;
 	}
 
-- 
2.18.0