linux-kernel - Re: [PATCH][next] net: dsa: fix unintended sign extension on a u16 left shift

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Date:   Mon, 09 Nov 2020 14:27:52 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Colin King <colin.king@...onical.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] net: dsa: fix unintended sign extension on a u16 left shift

On Mon Nov 09 2020, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
>
> The left shift of u16 variable high is promoted to the type int and
> then sign extended to a 64 bit u64 value.  If the top bit of high is
> set then the upper 32 bits of the result end up being set by the
> sign extension. Fix this by explicitly casting the value in high to
> a u64 before left shifting by 16 places.
>
> Also, remove the initialisation of variable value to 0 at the start
> of each loop iteration as the value is never read and hence the
> assignment it is redundant.
>
> Addresses-Coverity: ("Unintended sign extension")
> Fixes: e4b27ebc780f ("net: dsa: Add DSA driver for Hirschmann Hellcreek switches")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>

Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de>

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