: Safe usage of before48/after48 in the DCCP code

Here is as promised the patch to take care of the two ambiguous cases stated in http://www.mail-archive.com/[email protected]/msg01295.html

I have compile-tested it and below is a detailed justification why the use of the (newer variant of) dccp_delta_seqno is equivalent to the old condition.

(NB: It might be possible to use a macro instead.)

--------------------> Patch <------------------------------------------------- [DCCP]: Remove ambiguity in the way before48 is used

This removes two ambiguities in employing the new definition of before48, following the analysis on http://www.mail-archive.com/[email protected]/msg01295.html

(1) Updating GSR when P.seqno >= S.SWL
With the old definition we did not update when P.seqno and S.SWL are 2^47 apart. To ensure the same behaviour as with the old definition, this is replaced with the equivalent condition dccp_delta_seqno(S.SWL, P.seqno) >= 0

(2) Sending SYNC when P.seqno >= S.OSR
Here it is debatable whether the new definition causes an ambiguity: the case is similar to (1); and to have consistency with the case (1), we use the equivalent condition dccp_delta_seqno(S.OSR, P.seqno) >= 0

Detailed Justification [not necessarily commit message]:
--------------------------------------------------------
dccp_delta_seqno(a, b) returns a value >= 0 if either a is `before' b or a = b,
using the new definition: a `before' b <=> 1 <= (b - a) mod 2^48 <= 2^47 - 1.

The old and new definition of `before' are identical as long as the modulo-2^48 difference between a and b is not equal to 2^47. In both cases, a test of the form !before(x, y) has been replaced by dccp_delta_seqno(y, x) >= 0. For all values excluding y = (x + 2^47) % 2^48, the equivalence is clear to see. When y = (x + 2^47) % 2^48 then !before(x, y) returned false in the new definition. However, 'dccp_delta_seqno(x, (x+2^47) % 2^48) >= 0' also returns false; hence we have full equivalence.

---
 net/dccp/input.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

@@ -86,7 +86,8 @@ static int dccp_check_seqno(struct sock 
 	    dh->dccph_type = DCCP_PKT_SYNCACK) {
 		if (between48(DCCP_SKB_CB(skb)->dccpd_ack_seq,
 			      dp->dccps_awl, dp->dccps_awh) &&
-		    !before48(DCCP_SKB_CB(skb)->dccpd_seq, dp->dccps_swl))
+		    dccp_delta_seqno(dp->dccps_swl,
+				     DCCP_SKB_CB(skb)->dccpd_seq) >= 0)
 			dccp_update_gsr(sk, DCCP_SKB_CB(skb)->dccpd_seq);
 		else
 			return -1;

@@ -203,7 +204,8 @@ static int __dccp_rcv_established(struct
 		if (dp->dccps_role != DCCP_ROLE_CLIENT)
 			goto send_sync;
 check_seq:
-		if (!before48(DCCP_SKB_CB(skb)->dccpd_seq, dp->dccps_osr)) {
+		if (dccp_delta_seqno(dp->dccps_osr,
+				     DCCP_SKB_CB(skb)->dccpd_seq) >=  0) {
 send_sync:
 			dccp_send_sync(sk, DCCP_SKB_CB(skb)->dccpd_seq,
 				       DCCP_PKT_SYNC);