[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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/ipx.h

···
 #define IPX_MAX_PPROP_HOPS 8
 
 struct ipxhdr {
-	__u16			ipx_checksum __attribute__ ((packed));
-#define IPX_NO_CHECKSUM	0xFFFF
+	__be16			ipx_checksum __attribute__ ((packed));
+#define IPX_NO_CHECKSUM	__constant_htons(0xFFFF)
 	__be16			ipx_pktsize __attribute__ ((packed));
 	__u8			ipx_tctrl;
 	__u8			ipx_type;

net/ipx/af_ipx.c

···
 /* Note: We assume ipx_tctrl==0 and htons(length)==ipx_pktsize */
 /* This functions should *not* mess with packet contents */
 
-__u16 ipx_cksum(struct ipxhdr *packet, int length) 
+__be16 ipx_cksum(struct ipxhdr *packet, int length)
 {
 	/* 
 	 *	NOTE: sum is a net byte order quantity, which optimizes the 
 	 *	loop. This only works on big and little endian machines. (I
 	 *	don't know of a machine that isn't.)
 	 */
-	/* start at ipx_dest - We skip the checksum field and start with
-	 * ipx_type before the loop, not considering ipx_tctrl in the calc */
-	__u16 *p = (__u16 *)&packet->ipx_dest;
-	__u32 i = (length >> 1) - 1; /* Number of complete words */
-	__u32 sum = packet->ipx_type << sizeof(packet->ipx_tctrl); 
+	/* handle the first 3 words separately; checksum should be skipped
+	 * and ipx_tctrl masked out */
+	__u16 *p = (__u16 *)packet;
+	__u32 sum = p[1] + (p[2] & (__force u16)htons(0x00ff));
+	__u32 i = (length >> 1) - 3; /* Number of remaining complete words */
 
-	/* Loop through all complete words except the checksum field,
-	 * ipx_type (accounted above) and ipx_tctrl (not used in the cksum) */
-	while (--i)
+	/* Loop through them */
+	p += 3;
+	while (i--)
 		sum += *p++;
 
 	/* Add on the last part word if it exists */
 	if (packet->ipx_pktsize & htons(1))
-		sum += ntohs(0xff00) & *p;
+		sum += (__force u16)htons(0xff00) & *p;
 
 	/* Do final fixup */
 	sum = (sum & 0xffff) + (sum >> 16);
···
 	if (sum >= 0x10000)
 		sum++;
 
-	return ~sum;
+	/*
+	 * Leave 0 alone; we don't want 0xffff here.  Note that we can't get
+	 * here with 0x10000, so this check is the same as ((__u16)sum)
+	 */
+	if (sum)
+		sum = ~sum;
+
+	return (__force __be16)sum;
 }
 
 const char *ipx_frame_name(__be16 frame)

net/ipx/ipx_route.c

···
 
 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);
···
 
 	/* 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));