From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gerrit Renker Subject: [PATCH][RFC] tcp: fix ambiguity in the `before' relation Date: Thu, 14 Dec 2006 15:07:06 +0000 Message-ID: <200612141507.06888@strip-the-willow> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org Return-path: Received: from dee.erg.abdn.ac.uk ([139.133.204.82]:53117 "EHLO erg.abdn.ac.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932801AbWLNPWU (ORCPT ); Thu, 14 Dec 2006 10:22:20 -0500 To: David Miller Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org While looking at DCCP sequence numbers, I stumbled over a problem with the following definition of before in tcp.h: static inline int before(__u32 seq1, __u32 seq2) { return (__s32)(seq1-seq2) < 0; } Problem: This definition suffers from an an ambiguity, i.e. always before(a, (a + 2^31) % 2^32)) = 1 before((a + 2^31) % 2^32), a) = 1 In text: when the difference between a and b amounts to 2^31, a is always considered `before' b, the function can not decide. The reason is that implicitly 0 is `before' 1 ... 2^31-1 ... 2^31 Solution: There is a simple fix, by defining before in such a way that 0 is no longer `before' 2^31, i.e. 0 `before' 1 ... 2^31-1 By not using the middle between 0 and 2^32, before can be made unambiguous. This is achieved by testing whether seq2-seq1 > 0 (using signed 32-bit arithmetic). I attach a patch to codify this. Also the `after' relation is basically a redefinition of `before', it is now defined as a macro after before. Signed-off-by: Gerrit Renker --- tcp.h | 9 ++------- 1 file changed, 2 insertions(+), 7 deletions(-) diff --git a/include/net/tcp.h b/include/net/tcp.h index c99774f..b7d8317 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -242,14 +242,9 @@ extern int tcp_memory_pressure; static inline int before(__u32 seq1, __u32 seq2) { - return (__s32)(seq1-seq2) < 0; + return (__s32)(seq2-seq1) > 0; } - -static inline int after(__u32 seq1, __u32 seq2) -{ - return (__s32)(seq2-seq1) < 0; -} - +#define after(seq2, seq1) before(seq1, seq2) /* is s2<=s1<=s3 ? */ static inline int between(__u32 seq1, __u32 seq2, __u32 seq3)