[PATCH/RFC 2/2] ALSA: firewire-tascam: Fix integer overflow in midi_port_work()

[Geert Uytterhoeven]

As snd_fw_async_midi_port.consume_bytes is unsigned int, and
NSEC_PER_SEC is 1000000000L, the second multiplication in

    port->consume_bytes * 8 * NSEC_PER_SEC / 31250

always overflows on 32-bit platforms, truncating the result.  Fix this
by precalculating "NSEC_PER_SEC / 31250", which is an integer constant.

Note that this assumes port->consume_bytes <= 16777.

Fixes: 531f471834227d03 ("ALSA: firewire-lib/firewire-tascam: localize async midi port")
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
---
Compile-tested only.

I don't know the maximum transfer length of MIDI, but given it's an old
standard, I guess it's rather small.  If it is larger than 16777, the
constant "8" should be replaced by "8ULL", to force 64-bit arithmetic.
---
 sound/firewire/tascam/tascam-transaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/firewire/tascam/tascam-transaction.c b/sound/firewire/tascam/tascam-transaction.c
index 90288b4b46379526..a073cece4a7d5e3a 100644
--- a/sound/firewire/tascam/tascam-transaction.c
+++ b/sound/firewire/tascam/tascam-transaction.c
@@ -209,7 +209,7 @@ static void midi_port_work(struct work_struct *work)
 
 	/* Set interval to next transaction. */
 	port->next_ktime = ktime_add_ns(ktime_get(),
-				port->consume_bytes * 8 * NSEC_PER_SEC / 31250);
+			port->consume_bytes * 8 * (NSEC_PER_SEC / 31250));
 
 	/* Start this transaction. */
 	port->idling = false;
-- 
2.25.1