[PATCH] net: sxgbe: fix unintended sign extension

[PATCH] net: sxgbe: fix unintended sign extension

[Colin King]

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

Shifting a u8 by 24 will cause the value to be promoted to an integer. If
the top bit of the u8 is set then the following conversion to an unsigned
long will sign extend the value causing the upper 32 bits to be set in
the result.

Fix this by casting the u8 value to an unsigned long before the shift.

Detected by CoverityScan, CID#1195586 ("Unintended sign extension")

Fixes: 1edb9ca69e8a ("net: sxgbe: add basic framework for Samsung 10Gb ethernet driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c b/drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c
index 6d22dd500790..51fe161e5da9 100644
--- a/drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c
+++ b/drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c
@@ -1822,7 +1822,8 @@ static void sxgbe_set_umac_addr(void __iomem *ioaddr, unsigned char *addr,
 	 * is RO.
 	 */
 	writel(data | SXGBE_HI_REG_AE, ioaddr + SXGBE_ADDR_HIGH(reg_n));
-	data = (addr[3] << 24) | (addr[2] << 16) | (addr[1] << 8) | addr[0];
+	data = ((unsigned long)addr[3] << 24) | (addr[2] << 16) |
+		(addr[1] << 8) | addr[0];
 	writel(data, ioaddr + SXGBE_ADDR_LOW(reg_n));
 }
 
-- 
2.20.1

Re: [PATCH] net: sxgbe: fix unintended sign extension

[David Miller]

From: Colin King <colin.king@canonical.com>
Date: Wed,  6 Feb 2019 10:25:03 +0000

> From: Colin Ian King <colin.king@canonical.com>
> 
> Shifting a u8 by 24 will cause the value to be promoted to an integer. If
> the top bit of the u8 is set then the following conversion to an unsigned
> long will sign extend the value causing the upper 32 bits to be set in
> the result.

We feed this into a writel() which truncates to a 32-bit value, so nothing
bad can happen here.

This is a very canonical way to code up something like this, it works
properly in all situations, and therefore I'd rather not add all of
these fat ugly looking casts.

Thank you.