# [TCP]: Fix ambiguity in the `before' relation.

```Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=9a036b9c33f74c989c4c8ac0abe05e0ed88f1f25
Commit:     9a036b9c33f74c989c4c8ac0abe05e0ed88f1f25
Parent:     1f8a5fb80e63aab63de81169ab749d73e7509e3f
Author:     Gerrit Renker <[EMAIL PROTECTED]>
AuthorDate: Wed Dec 20 10:25:55 2006 -0800
Committer:  David S. Miller <[EMAIL PROTECTED]>
CommitDate: Fri Dec 22 11:12:01 2006 -0800```
```
[TCP]: Fix ambiguity in the `before' relation.

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 <[EMAIL PROTECTED]>
Signed-off-by: David S. Miller <[EMAIL PROTECTED]>
---
include/net/tcp.h |    9 ++-------
1 files 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)
-
To unsubscribe from this list: send the line "unsubscribe git-commits-head" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
```