[IPX]: Annotate and fix IPX checksum

Calculation of IPX checksum got buggered about 2.4.0.  The old variant
mangled the packet; that got fixed, but calculation itself got buggered.
Restored the correct logics, fixed a subtle breakage we used to have even
back then: if the sum is 0 mod 0xffff, we want to return 0, not 0xffff.
The latter has special meaning for IPX (cheksum disabled).  Observation
(and obvious fix) nicked from history of FreeBSD ipx_cksum.c...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/ipx/ipx_route.c b/net/ipx/ipx_route.c
index 8bfaefaf8841..68560ee0d797 100644
--- a/net/ipx/ipx_route.c
+++ b/net/ipx/ipx_route.c
@@ -19,7 +19,7 @@ DEFINE_RWLOCK(ipx_routes_lock);
 
 extern struct ipx_interface *ipx_internal_net;
 
-extern __u16 ipx_cksum(struct ipxhdr *packet, int length);
+extern __be16 ipx_cksum(struct ipxhdr *packet, int length);
 extern struct ipx_interface *ipxitf_find_using_net(__be32 net);
 extern int ipxitf_demux_socket(struct ipx_interface *intrfc,
                                struct sk_buff *skb, int copy);
@@ -238,7 +238,7 @@ int ipxrtr_route_packet(struct sock *sk, struct sockaddr_ipx *usipx,
 
         /* Apply checksum. Not allowed on 802.3 links. */
         if (sk->sk_no_check || intrfc->if_dlink_type == htons(IPX_FRAME_8023))
-                ipx->ipx_checksum = 0xFFFF;
+                ipx->ipx_checksum = htons(0xFFFF);
         else
                 ipx->ipx_checksum = ipx_cksum(ipx, len + sizeof(struct ipxhdr));