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

[Kurt Kanzenbach <kurt@linutronix.de>]

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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 switches")
> 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/hirschmann/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_switch *ds, int port,
>  		const struct hellcreek_counter *counter = &hellcreek_counter[i];
>  		u8 offset = counter->offset + port * 64;
>  		u16 high, low;
> -		u64 value = 0;
> +		u64 value;
>  
>  		mutex_lock(&hellcreek->reg_lock);
>  
> @@ -320,7 +320,7 @@ static void hellcreek_get_ethtool_stats(struct dsa_switch *ds, int port,
>  		 */
>  		high  = hellcreek_read(hellcreek, HR_CRDH);
>  		low   = hellcreek_read(hellcreek, HR_CRDL);
> -		value = (high << 16) | low;
> +		value = ((u64)high << 16) | low;

Looks good to me. Thank you.

Thanks,
Kurt

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