diff options
author | Gerrit Renker <gerrit@erg.abdn.ac.uk> | 2006-12-20 10:25:55 -0800 |
---|---|---|
committer | David S. Miller <davem@sunset.davemloft.net> | 2006-12-22 11:12:01 -0800 |
commit | 9a036b9c33f74c989c4c8ac0abe05e0ed88f1f25 (patch) | |
tree | 7eadf26579e996975e35a5f34298937f7bd114e8 /Documentation/video4linux/README.pvrusb2 | |
parent | 1f8a5fb80e63aab63de81169ab749d73e7509e3f (diff) |
[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>
Diffstat (limited to 'Documentation/video4linux/README.pvrusb2')
0 files changed, 0 insertions, 0 deletions