ALSA: seq: oss: Reject reads that cannot fit the next event

snd_seq_oss_read() checks whether the next queued OSS sequencer event
fits in the remaining userspace buffer before removing it from the read
queue.

The check is inverted. It currently stops when the event is smaller than
the remaining buffer, so a normal 4-byte event is not copied for an
8-byte read buffer. Conversely, an 8-byte event can be copied for a
smaller read count.

Break only when the remaining userspace buffer is smaller than the next
event, and report -EINVAL if no complete event has been copied. This
prevents an undersized read from looking like end-of-file while leaving
the event queued for a later read with a large enough buffer.

Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260602-alsa-seq-oss-read-size-check-v1-1-10e59b1742e0@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: CassivsGabriellis

sound/core/seq/oss/seq_oss_rw.c

@@ -57,7 +57,8 @@ snd_seq_oss_read(struct seq_oss_devinfo *dp, char __user *buf, int count)
 			break;
 		}
 		ev_len = ev_length(&rec);
-		if (ev_len < count) {
+		if (count < ev_len) {
+			err = -EINVAL;
 			snd_seq_oss_readq_unlock(readq, flags);
 			break;
 		}