: Make dccp_delta_seqno return signed numbers

[DCCP]: Make dccp_delta_seqno return signed numbers

Problem:
--------
Using dccp_delta_seqno(a, b) produces unusable results when -- by accident or coincidence -- sequence number b precedes a. If e.g. a and b are merely reordered and have a distance 1, their delta_seqno is 2^48-1, which would indicate a loss of 2^48-2 packets.

Fix:
----
The fix is by using signed 48-bit arithmetic for dccp_delta_seqno, so that dccp_delta_seqno(a, b) returns:
* > 0 if a is `before' b OR
* 0 if a = b OR
* < 0 and > -2^47 if b is `before' a OR
* -2^47 if neither a before' b nor b before' a

This implements http://www.mail-archive.com/[email protected]/msg01153.html

---
 net/dccp/dccp.h |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

@@ -113,9 +113,12 @@ static inline void dccp_inc_seqno(u64 *s
 	*seqno = ADD48(*seqno, 1);
 }
 
-static inline u64 dccp_delta_seqno(u64 seqno1, u64 seqno2)
+/* return > 0 if seqno1 is `before' seqno2, <= 0 otherwise */
+static inline s64 dccp_delta_seqno(const u64 seqno1, const u64 seqno2)
 {
-	return ((seqno2 << 16) - (seqno1 << 16)) >> 16;
+	u64 delta = SUB48(seqno2, seqno1);
+
+	return TO_SIGNED48(delta);
 }
 
 /* is seq1 < seq2 ? */