Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Kurt Kanzenbach <kurt@linutronix.de>
To:     Colin King <colin.king@canonical.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Vivien Didelot <vivien.didelot@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Vladimir Oltean <olteanv@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org
Cc:     kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][next] net: dsa: fix unintended sign extension on a u16 left shift
In-Reply-To: <20201109124008.2079873-1-colin.king@canonical.com>
References: <20201109124008.2079873-1-colin.king@canonical.com>
Date:   Mon, 09 Nov 2020 13:51:45 +0100
Message-ID: <877dqusnfi.fsf@kurt>
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On Mon Nov 09 2020, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.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 s=
witches")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  drivers/net/dsa/hirschmann/hellcreek.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/dsa/hirschmann/hellcreek.c b/drivers/net/dsa/hir=
schmann/hellcreek.c
> index dfa66f7260d6..d42f40c76ba5 100644
> --- a/drivers/net/dsa/hirschmann/hellcreek.c
> +++ b/drivers/net/dsa/hirschmann/hellcreek.c
> @@ -308,7 +308,7 @@ static void hellcreek_get_ethtool_stats(struct dsa_sw=
itch *ds, int port,
>  		const struct hellcreek_counter *counter =3D &hellcreek_counter[i];
>  		u8 offset =3D counter->offset + port * 64;
>  		u16 high, low;
> -		u64 value =3D 0;
> +		u64 value;
>=20=20
>  		mutex_lock(&hellcreek->reg_lock);
>=20=20
> @@ -320,7 +320,7 @@ static void hellcreek_get_ethtool_stats(struct dsa_sw=
itch *ds, int port,
>  		 */
>  		high  =3D hellcreek_read(hellcreek, HR_CRDH);
>  		low   =3D hellcreek_read(hellcreek, HR_CRDL);
> -		value =3D (high << 16) | low;
> +		value =3D ((u64)high << 16) | low;

Looks good to me. Thank you.

Thanks,
Kurt

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