[NETLINK]: Fix NLMSG_OK/RTA_OK length checking.

Verify at least a full header is there before derefencing
length struct member.

include/linux/netlink.h

@@ -73,7 +73,8 @@ struct nlmsghdr
 #define NLMSG_DATA(nlh)  ((void*)(((char*)nlh) + NLMSG_LENGTH(0)))
 #define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
 				  (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
-#define NLMSG_OK(nlh,len) ((len) > 0 && (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
+#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
+			   (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
 			   (nlh)->nlmsg_len <= (len))
 #define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
 

include/linux/rtnetlink.h

@@ -73,7 +73,8 @@ struct rtattr
 
 #define RTA_ALIGNTO	4
 #define RTA_ALIGN(len) ( ((len)+RTA_ALIGNTO-1) & ~(RTA_ALIGNTO-1) )
-#define RTA_OK(rta,len) ((len) > 0 && (rta)->rta_len >= sizeof(struct rtattr) && \
+#define RTA_OK(rta,len) ((len) >= (int)sizeof(struct rtattr) && \
+			 (rta)->rta_len >= sizeof(struct rtattr) && \
 			 (rta)->rta_len <= (len))
 #define RTA_NEXT(rta,attrlen)	((attrlen) -= RTA_ALIGN((rta)->rta_len), \
 				 (struct rtattr*)(((char*)(rta)) + RTA_ALIGN((rta)->rta_len)))