ALSA: firewire-tascam: Do not drop unread control events

commit 0749daa upstream.

tscm_hwdep_read_queue() copies as many queued control events as fit in
the userspace buffer. When the buffer is smaller than the current
contiguous queue segment, length is rounded down to the number of bytes
that can be copied.

However, after copying that shortened length, the code advances pull_pos
to the original tail_pos, marking the whole contiguous segment as
consumed. Any events between the copied portion and tail_pos are lost.

Limit tail_pos to the position after the entries actually copied before
updating pull_pos. When the whole segment fits, this is equivalent to the
old tail_pos update; when the buffer is smaller, the remaining events
stay queued for the next read.

Fixes: a8c0d13 ("ALSA: firewire-tascam: notify events of change of state for userspace applications")
Cc: stable@vger.kernel.org
Suggested-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Co-developed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260503-alsa-firewire-tascam-read-queue-v2-1-126c6efd7642@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: CassivsGabriellis

sound/firewire/tascam/tascam-hwdep.c

@@ -73,6 +73,7 @@ static long tscm_hwdep_read_queue(struct snd_tscm *tscm, char __user *buf,
 			length = rounddown(remained, sizeof(*entries));
 		if (length == 0)
 			break;
+		tail_pos = head_pos + length / sizeof(*entries);
 
 		spin_unlock_irq(&tscm->lock);
 		if (copy_to_user(pos, &entries[head_pos], length))