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

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

[Colin King]

From: Colin Ian King <colin.king@canonical.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@canonical.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;
 
 			mutex_lock(&chip->reg_lock);
 			ns = timecounter_cyc2time(&chip->tstamp_tc, ns);
-- 
2.15.1

Re: [PATCH][V2] net: dsa: mv88e6xxx: avoid unintended sign extension on a 16 bit shift

[David Miller]

From: Colin King <colin.king@canonical.com>
Date: Fri, 16 Feb 2018 16:55:05 +0000

> From: Colin Ian King <colin.king@canonical.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@canonical.com>

Please indicate the appropriate target tree in your Subject lines
in the future, for this it should be "[PATCH net-next]".

Applied, thanks.

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

[David Laight]

From: Colin King
> Sent: 16 February 2018 16:55
> 
> From: Colin Ian King <colin.king@canonical.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@canonical.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

Re: [PATCH][V2] net: dsa: mv88e6xxx: avoid unintended sign extension on a 16 bit shift

[Richard Cochran]

On Mon, Feb 19, 2018 at 12:19:16PM +0000, David Laight wrote:
> This seems to be somewhat excessive 64bit maths on a 32bit system.
> It is more than enough to make timelo/timehi 'unsigned int'.

Do you see a difference in the generated code?

Thanks,
Richard