[patch] ipvs: prevent some underflows

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Quite a few drivers allow very low settings for dev->mtu.  My static
checker complains this could cause some underflow problems when we do
the subtractions in set_sync_mesg_maxlen().

I don't know that it's harmful necessarily, but it seems like an easy
thing to prevent the underflows.

Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
---
Please review this one carefully, because I'm not very sure of myself
here.

diff --git a/net/netfilter/ipvs/ip_vs_sync.c b/net/netfilter/ipvs/ip_vs_sync.c
index b08ba95..b4e148b 100644
--- a/net/netfilter/ipvs/ip_vs_sync.c
+++ b/net/netfilter/ipvs/ip_vs_sync.c
@@ -1352,7 +1352,7 @@ static int set_sync_mesg_maxlen(struct net *net, int 
sync_state)
 {
        struct netns_ipvs *ipvs = net_ipvs(net);
        struct net_device *dev;
-       int num;
+       unsigned int num;
 
        if (sync_state == IP_VS_STATE_MASTER) {
                dev = __dev_get_by_name(net, ipvs->master_mcast_ifn);
@@ -1363,7 +1363,8 @@ static int set_sync_mesg_maxlen(struct net *net, int 
sync_state)
                       sizeof(struct udphdr) -
                       SYNC_MESG_HEADER_LEN - 20) / SIMPLE_CONN_SIZE;
                ipvs->send_mesg_maxlen = SYNC_MESG_HEADER_LEN +
-                       SIMPLE_CONN_SIZE * min(num, MAX_CONNS_PER_SYNCBUFF);
+                       SIMPLE_CONN_SIZE * min_t(uint, num,
+                                                MAX_CONNS_PER_SYNCBUFF);
                IP_VS_DBG(7, "setting the maximum length of sync sending "
                          "message %d.\n", ipvs->send_mesg_maxlen);
        } else if (sync_state == IP_VS_STATE_BACKUP) {
@@ -1371,8 +1372,11 @@ static int set_sync_mesg_maxlen(struct net *net, int 
sync_state)
                if (!dev)
                        return -ENODEV;
 
-               ipvs->recv_mesg_maxlen = dev->mtu -
-                       sizeof(struct iphdr) - sizeof(struct udphdr);
+               if (dev->mtu < sizeof(struct iphdr) + sizeof(struct udphdr))
+                       ipvs->recv_mesg_maxlen = 0;
+               else
+                       ipvs->recv_mesg_maxlen = dev->mtu -
+                               sizeof(struct iphdr) - sizeof(struct udphdr);
                IP_VS_DBG(7, "setting the maximum length of sync receiving "
                          "message %d.\n", ipvs->recv_mesg_maxlen);
        }
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