[PATCH net] net/ncsi: handle overflow when incrementing mac address

[Tao Ren]

Previously BMC's MAC address is calculated by simply adding 1 to the
last byte of network controller's MAC address, and it produces incorrect
result when network controller's MAC address ends with 0xFF.
The problem is fixed by detecting integer overflow when incrementing MAC
address and adding the carry bit (if any) to the next/left bytes of the
MAC address.

Signed-off-by: Tao Ren <taoren at fb.com>
---
 net/ncsi/ncsi-rsp.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/ncsi/ncsi-rsp.c b/net/ncsi/ncsi-rsp.c
index dc07fcc7938e..eb42bbdb7501 100644
--- a/net/ncsi/ncsi-rsp.c
+++ b/net/ncsi/ncsi-rsp.c
@@ -658,7 +658,8 @@ static int ncsi_rsp_handler_oem_bcm_gma(struct ncsi_request *nr)
 	const struct net_device_ops *ops = ndev->netdev_ops;
 	struct ncsi_rsp_oem_pkt *rsp;
 	struct sockaddr saddr;
-	int ret = 0;
+	int ret, offset;
+	u16 carry = 1;
 
 	/* Get the response header */
 	rsp = (struct ncsi_rsp_oem_pkt *)skb_network_header(nr->rsp);
@@ -667,7 +668,12 @@ static int ncsi_rsp_handler_oem_bcm_gma(struct ncsi_request *nr)
 	ndev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
 	memcpy(saddr.sa_data, &rsp->data[BCM_MAC_ADDR_OFFSET], ETH_ALEN);
 	/* Increase mac address by 1 for BMC's address */
-	saddr.sa_data[ETH_ALEN - 1]++;
+	offset = ETH_ALEN - 1;
+	do {
+		carry += (u8)saddr.sa_data[offset];
+		saddr.sa_data[offset] = (char)carry;
+		carry = carry >> 8;
+	} while (carry != 0 && --offset >= 0);
 	ret = ops->ndo_set_mac_address(ndev, &saddr);
 	if (ret < 0)
 		netdev_warn(ndev, "NCSI: 'Writing mac address to device failed\n");
-- 
2.17.1