udplite: Protection against coverage value wrap-around

This patch clamps the cscov setsockopt values to a maximum of 0xFFFF.

Setsockopt values greater than 0xffff can cause an unwanted
wrap-around.  Further, IPv6 jumbograms are not supported (RFC 3838,
3.5), so that values greater than 0xffff are not even useful.

Further changes: fixed a typo in the documentation.
Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/udplite.txt

@@ -148,7 +148,7 @@
         getsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, &value, ...);
 
   is meaningless (as in TCP). Packets with a zero checksum field are
-  illegal (cf. RFC 3828, sec. 3.1) will be silently discarded.
+  illegal (cf. RFC 3828, sec. 3.1) and will be silently discarded.
 
   4) Fragmentation
 

net/ipv4/udp.c

@@ -1325,6 +1325,8 @@ int udp_lib_setsockopt(struct sock *sk, int level, int optname,
 			return -ENOPROTOOPT;
 		if (val != 0 && val < 8) /* Illegal coverage: use default (8) */
 			val = 8;
+		else if (val > USHORT_MAX)
+			val = USHORT_MAX;
 		up->pcslen = val;
 		up->pcflag |= UDPLITE_SEND_CC;
 		break;
@@ -1337,6 +1339,8 @@ int udp_lib_setsockopt(struct sock *sk, int level, int optname,
 			return -ENOPROTOOPT;
 		if (val != 0 && val < 8) /* Avoid silly minimal values.       */
 			val = 8;
+		else if (val > USHORT_MAX)
+			val = USHORT_MAX;
 		up->pcrlen = val;
 		up->pcflag |= UDPLITE_RECV_CC;
 		break;