linux-kernel - RE: [PATCH][V2] net: dsa: mv88e6xxx: avoid unintended sign extension on a 16 bit shift

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Message-ID: <a97bab4dc79c4b7da76826843909cae9@AcuMS.aculab.com>
Date:   Mon, 19 Feb 2018 12:19:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Colin King' <colin.king@...onical.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH][V2] net: dsa: mv88e6xxx: avoid unintended sign extension
 on a 16 bit shift

From: Colin King
> Sent: 16 February 2018 16:55
> 
> From: Colin Ian King <colin.king@...onical.com>
> 
> The shifting of timehi by 16 bits to the left will be promoted to
> a 32 bit signed int and then sign-extended to an u64. If the top bit
> of timehi is set then all then all the upper bits of ns end up as also
> being set because of the sign-extension. Fix this by making timehi and
> timelo u64.  Also move the declaration of ns.
> 
> Detected by CoverityScan, CID#1465288 ("Unintended sign extension")
> 
> Fixes: c6fe0ad2c349 ("net: dsa: mv88e6xxx: add rx/tx timestamping support")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/net/dsa/mv88e6xxx/hwtstamp.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/dsa/mv88e6xxx/hwtstamp.c b/drivers/net/dsa/mv88e6xxx/hwtstamp.c
> index b251d534b70d..2149d332dea0 100644
> --- a/drivers/net/dsa/mv88e6xxx/hwtstamp.c
> +++ b/drivers/net/dsa/mv88e6xxx/hwtstamp.c
> @@ -293,7 +293,8 @@ static void mv88e6xxx_get_rxts(struct mv88e6xxx_chip *chip,
>  			       struct sk_buff *skb, u16 reg,
>  			       struct sk_buff_head *rxq)
>  {
> -	u16 buf[4] = { 0 }, status, timelo, timehi, seq_id;
> +	u16 buf[4] = { 0 }, status, seq_id;
> +	u64 ns, timelo, timehi;
>  	struct skb_shared_hwtstamps *shwt;
>  	int err;
> 
> @@ -321,7 +322,7 @@ static void mv88e6xxx_get_rxts(struct mv88e6xxx_chip *chip,
>  	 */
>  	for ( ; skb; skb = skb_dequeue(rxq)) {
>  		if (mv88e6xxx_ts_valid(status) && seq_match(skb, seq_id)) {
> -			u64 ns = timehi << 16 | timelo;
> +			ns = timehi << 16 | timelo;

This seems to be somewhat excessive 64bit maths on a 32bit system.
It is more than enough to make timelo/timehi 'unsigned int'.

	David