ALSA: seq: oss: Send fragmented SysEx messages immediately

The recent bug report spotted on the old OSS sequencer code that tries
to combine incoming SysEx messages to a single sequencer event.  This
is good, per se, but it has more demerits:

- The sysex message delivery is delayed until the very last event
- The use of internal buffer forced the serialization

The recent fix in commit 0179488 ("ALSA: seq: oss: Fix races at
processing SysEx messages") addressed the latter, but a better fix is
to handle the sysex messages immediately, i.e. just send each incoming
fragmented sysex message as is.  And this patch implements that.
This resulted in a significant cleanup as well.

Note that the only caller of snd_seq_oss_synth_sysex() is
snd_seq_oss_process_event(), and all its callers dispatch the event
immediately, so we can just put the passed buffer pointer to the event
record to be handled.

Reported-and-tested-by: Kun Hu <huk23@m.fudan.edu.cn>
Link: https://lore.kernel.org/2B7E93E4-B13A-4AE4-8E87-306A8EE9BBB7@m.fudan.edu.cn
Link: https://patch.msgid.link/20241231115523.15796-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: tiwai

sound/core/seq/oss/seq_oss_device.h

@@ -55,7 +55,6 @@ struct seq_oss_chinfo {
 struct seq_oss_synthinfo {
 	struct snd_seq_oss_arg arg;
 	struct seq_oss_chinfo *ch;
-	struct seq_oss_synth_sysex *sysex;
 	int nr_voices;
 	int opened;
 	int is_midi;

sound/core/seq/oss/seq_oss_synth.c

@@ -26,13 +26,6 @@
  * definition of synth info records
  */
 
-/* sysex buffer */
-struct seq_oss_synth_sysex {
-	int len;
-	int skip;
-	unsigned char buf[MAX_SYSEX_BUFLEN];
-};
-
 /* synth info */
 struct seq_oss_synth {
 	int seq_device;
@@ -66,7 +59,6 @@ static struct seq_oss_synth midi_synth_dev = {
 };
 
 static DEFINE_SPINLOCK(register_lock);
-static DEFINE_MUTEX(sysex_mutex);
 
 /*
  * prototypes
@@ -319,8 +311,6 @@ snd_seq_oss_synth_cleanup(struct seq_oss_devinfo *dp)
 			}
 			snd_use_lock_free(&rec->use_lock);
 		}
-		kfree(info->sysex);
-		info->sysex = NULL;
 		kfree(info->ch);
 		info->ch = NULL;
 	}
@@ -396,8 +386,6 @@ snd_seq_oss_synth_reset(struct seq_oss_devinfo *dp, int dev)
 	info = get_synthinfo_nospec(dp, dev);
 	if (!info || !info->opened)
 		return;
-	if (info->sysex)
-		info->sysex->len = 0; /* reset sysex */
 	reset_channels(info);
 	if (info->is_midi) {
 		if (midi_synth_dev.opened <= 0)
@@ -409,8 +397,6 @@ snd_seq_oss_synth_reset(struct seq_oss_devinfo *dp, int dev)
 					  dp->file_mode) < 0) {
 			midi_synth_dev.opened--;
 			info->opened = 0;
-			kfree(info->sysex);
-			info->sysex = NULL;
 			kfree(info->ch);
 			info->ch = NULL;
 		}
@@ -483,64 +469,26 @@ snd_seq_oss_synth_info(struct seq_oss_devinfo *dp, int dev)
 
 /*
  * receive OSS 6 byte sysex packet:
- * the full sysex message will be sent if it reaches to the end of data
- * (0xff).
+ * the event is filled and prepared for sending immediately
+ * (i.e. sysex messages are fragmented)
  */
 int
 snd_seq_oss_synth_sysex(struct seq_oss_devinfo *dp, int dev, unsigned char *buf, struct snd_seq_event *ev)
 {
-	int i, send;
-	unsigned char *dest;
-	struct seq_oss_synth_sysex *sysex;
-	struct seq_oss_synthinfo *info;
+	unsigned char *p;
+	int len = 6;
 
-	info = snd_seq_oss_synth_info(dp, dev);
-	if (!info)
-		return -ENXIO;
+	p = memchr(buf, 0xff, 6);
+	if (p)
+		len = p - buf + 1;
 
-	guard(mutex)(&sysex_mutex);
-	sysex = info->sysex;
-	if (sysex == NULL) {
-		sysex = kzalloc(sizeof(*sysex), GFP_KERNEL);
-		if (sysex == NULL)
-			return -ENOMEM;
-		info->sysex = sysex;
-	}
-
-	send = 0;
-	dest = sysex->buf + sysex->len;
-	/* copy 6 byte packet to the buffer */
-	for (i = 0; i < 6; i++) {
-		if (buf[i] == 0xff) {
-			send = 1;
-			break;
-		}
-		dest[i] = buf[i];
-		sysex->len++;
-		if (sysex->len >= MAX_SYSEX_BUFLEN) {
-			sysex->len = 0;
-			sysex->skip = 1;
-			break;
-		}
-	}
-
-	if (sysex->len && send) {
-		if (sysex->skip) {
-			sysex->skip = 0;
-			sysex->len = 0;
-			return -EINVAL; /* skip */
-		}
-		/* copy the data to event record and send it */
-		ev->flags = SNDRV_SEQ_EVENT_LENGTH_VARIABLE;
-		if (snd_seq_oss_synth_addr(dp, dev, ev))
-			return -EINVAL;
-		ev->data.ext.len = sysex->len;
-		ev->data.ext.ptr = sysex->buf;
-		sysex->len = 0;
-		return 0;
-	}
-
-	return -EINVAL; /* skip */
+	/* copy the data to event record and send it */
+	if (snd_seq_oss_synth_addr(dp, dev, ev))
+		return -EINVAL;
+	ev->flags = SNDRV_SEQ_EVENT_LENGTH_VARIABLE;
+	ev->data.ext.len = len;
+	ev->data.ext.ptr = buf;
+	return 0;
 }
 
 /*