[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, 19 insertions, 12 deletions

diff --git a/net/ipx/af_ipx.c b/net/ipx/af_ipx.c
index c272a38af325..76c661566dfd 100644
--- a/net/ipx/af_ipx.c
+++ b/net/ipx/af_ipx.c
@@ -1234,27 +1234,27 @@ static int ipxitf_ioctl(unsigned int cmd, void __user *arg)
 /* 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); 
-
-        /* Loop through all complete words except the checksum field,
-         * ipx_type (accounted above) and ipx_tctrl (not used in the cksum) */
-        while (--i)
+        /* 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 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);
@@ -1263,7 +1263,14 @@ __u16 ipx_cksum(struct ipxhdr *packet, int length)
         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)