[PATCH net] net: dsa: felix: fix taprio guard band overflow at 10Mbps with jumbo frames

[Vladimir Oltean]

The DEV_MAC_MAXLEN_CFG register contains a 16-bit value - up to 65535.
Plus 2 * VLAN_HLEN (4), that is up to 65543.

The picos_per_byte variable is the largest when "speed" is lowest -
SPEED_10 = 10. In that case it is (1000000L * 8) / 10 = 800000.

Their product - 52434400000 - exceeds 32 bits, which is a problem,
because apparently, a multiplication between two 32-bit factors is
evaluated as 32-bit before being assigned to a 64-bit variable.
In fact it's a problem for any MTU value larger than 5368.

Cast one of the factors of the multiplication to u64 to force the
multiplication to take place on 64 bits.

Issue found by Coverity.

Fixes: 55a515b1f5a9 ("net: dsa: felix: drop oversized frames with tc-taprio instead of hanging the port")
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
---
 drivers/net/dsa/ocelot/felix_vsc9959.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
index 903532ea9fa4..bb39fedd46c7 100644
--- a/drivers/net/dsa/ocelot/felix_vsc9959.c
+++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
@@ -1251,7 +1251,7 @@ static void vsc9959_tas_guard_bands_update(struct ocelot *ocelot, int port)
 	/* Consider the standard Ethernet overhead of 8 octets preamble+SFD,
 	 * 4 octets FCS, 12 octets IFG.
 	 */
-	needed_bit_time_ps = (maxlen + 24) * picos_per_byte;
+	needed_bit_time_ps = (u64)(maxlen + 24) * picos_per_byte;
 
 	dev_dbg(ocelot->dev,
 		"port %d: max frame size %d needs %llu ps at speed %d\n",
-- 
2.34.1