ALSA: pcm: Drop async signal support

The async signal (SIGIO) support for ALSA PCM API has been never used
by real applications, but yet it can be a cause of various potential
deadlocks, as spotted by syzkaller.  Let's drop the feature as the
simplest solution.

Reported-by: syzbot+8285e973a41b5aa68902@syzkaller.appspotmail.com
Reported-by: syzbot+669c9abf11a6a011dd09@syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -399,7 +399,6 @@ struct snd_pcm_runtime {
 	snd_pcm_uframes_t twake; 	/* do transfer (!poll) wakeup if non-zero */
 	wait_queue_head_t sleep;	/* poll sleep */
 	wait_queue_head_t tsleep;	/* transfer sleep */
-	struct fasync_struct *fasync;
 	bool stop_operating;		/* sync_stop will be called */
 	struct mutex buffer_mutex;	/* protect for buffer changes */
 	atomic_t buffer_accessing;	/* >0: in r/w operation, <0: blocked */

sound/core/pcm_lib.c

@@ -1815,14 +1815,12 @@ void snd_pcm_period_elapsed_under_stream_lock(struct snd_pcm_substream *substrea
 
 	if (!snd_pcm_running(substream) ||
 	    snd_pcm_update_hw_ptr0(substream, 1) < 0)
-		goto _end;
+		return;
 
 #ifdef CONFIG_SND_PCM_TIMER
 	if (substream->timer_running)
 		snd_timer_interrupt(substream->timer, 1);
 #endif
- _end:
-	kill_fasync(&runtime->fasync, SIGIO, POLL_IN);
 }
 EXPORT_SYMBOL(snd_pcm_period_elapsed_under_stream_lock);
 

sound/core/pcm_native.c

@@ -3938,22 +3938,6 @@ static int snd_pcm_mmap(struct file *file, struct vm_area_struct *area)
 	return 0;
 }
 
-static int snd_pcm_fasync(int fd, struct file * file, int on)
-{
-	struct snd_pcm_file * pcm_file;
-	struct snd_pcm_substream *substream;
-	struct snd_pcm_runtime *runtime;
-
-	pcm_file = file->private_data;
-	substream = pcm_file->substream;
-	if (PCM_RUNTIME_CHECK(substream))
-		return -ENXIO;
-	runtime = substream->runtime;
-	if (runtime->status->state == SNDRV_PCM_STATE_DISCONNECTED)
-		return -EBADFD;
-	return fasync_helper(fd, file, on, &runtime->fasync);
-}
-
 /*
  * ioctl32 compat
  */
@@ -4118,7 +4102,6 @@ const struct file_operations snd_pcm_f_ops[2] = {
 		.unlocked_ioctl =	snd_pcm_ioctl,
 		.compat_ioctl = 	snd_pcm_ioctl_compat,
 		.mmap =			snd_pcm_mmap,
-		.fasync =		snd_pcm_fasync,
 		.get_unmapped_area =	snd_pcm_get_unmapped_area,
 	},
 	{
@@ -4132,7 +4115,6 @@ const struct file_operations snd_pcm_f_ops[2] = {
 		.unlocked_ioctl =	snd_pcm_ioctl,
 		.compat_ioctl = 	snd_pcm_ioctl_compat,
 		.mmap =			snd_pcm_mmap,
-		.fasync =		snd_pcm_fasync,
 		.get_unmapped_area =	snd_pcm_get_unmapped_area,
 	}
 };