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

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Date:   Wed, 14 Jun 2023 14:05:15 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        UNGLinuxDriver@...rochip.com,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: dsa: felix: fix taprio guard band overflow at
 10Mbps with jumbo frames

On Tue, Jun 13, 2023 at 08:09:07PM +0300, Vladimir Oltean wrote:
> 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@....com>

Reviewed-by: Simon Horman <simon.horman@...igine.com>

In a similar vein, you may want to consider the following suggestion from
Coccinelle.

  .../felix_vsc9959.c:1382:2-8: WARNING: do_div() does a 64-by-32 division, please consider using div64_u64 instead.