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[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 <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Gerrit Renker 2006-12-20 10:25:55 -08:00 committed by David S. Miller
parent 1f8a5fb80e
commit 9a036b9c33
1 changed files with 2 additions and 7 deletions
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9
include/net/tcp.h
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@ -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)