ALSA: seq_oss: return full count for successful SEQ_FULLSIZE writes

commit bbc6c0d upstream.

snd_seq_oss_write() currently returns the raw load_patch() callback
result for SEQ_FULLSIZE events.

That callback is documented as returning 0 on success and -errno on
failure, but snd_seq_oss_write() is the file write path and should
report the number of user bytes consumed on success. Some in-tree
backends also return backend-specific positive values, which can still
be shorter than the original write size.

Return the full byte count for successful SEQ_FULLSIZE writes.
Preserve negative errors and convert any nonnegative completion to the
original count.

Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260324-alsa-seq-oss-fullsize-write-return-v1-1-66d448510538@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: CassivsGabriellis

sound/core/seq/oss/seq_oss_rw.c

@@ -101,9 +101,9 @@ snd_seq_oss_write(struct seq_oss_devinfo *dp, const char __user *buf, int count,
 				break;
 			}
 			fmt = (*(unsigned short *)rec.c) & 0xffff;
-			/* FIXME the return value isn't correct */
-			return snd_seq_oss_synth_load_patch(dp, rec.s.dev,
-							    fmt, buf, 0, count);
+			err = snd_seq_oss_synth_load_patch(dp, rec.s.dev,
+							   fmt, buf, 0, count);
+			return err < 0 ? err : count;
 		}
 		if (ev_is_long(&rec)) {
 			/* extended code */