diff options
author | Gerrit Renker <gerrit@erg.abdn.ac.uk> | 2006-12-20 13:25:55 -0500 |
---|---|---|
committer | David S. Miller <davem@sunset.davemloft.net> | 2006-12-22 14:12:01 -0500 |
commit | 9a036b9c33f74c989c4c8ac0abe05e0ed88f1f25 (patch) | |
tree | 7eadf26579e996975e35a5f34298937f7bd114e8 /include/net | |
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 'include/net')
-rw-r--r-- | 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 c99774f15eb..b7d8317f22a 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h | |||
@@ -242,14 +242,9 @@ extern int tcp_memory_pressure; | |||
242 | 242 | ||
243 | static inline int before(__u32 seq1, __u32 seq2) | 243 | static inline int before(__u32 seq1, __u32 seq2) |
244 | { | 244 | { |
245 | return (__s32)(seq1-seq2) < 0; | 245 | return (__s32)(seq2-seq1) > 0; |
246 | } | 246 | } |
247 | 247 | #define after(seq2, seq1) before(seq1, seq2) | |
248 | static inline int after(__u32 seq1, __u32 seq2) | ||
249 | { | ||
250 | return (__s32)(seq2-seq1) < 0; | ||
251 | } | ||
252 | |||
253 | 248 | ||
254 | /* is s2<=s1<=s3 ? */ | 249 | /* is s2<=s1<=s3 ? */ |
255 | static inline int between(__u32 seq1, __u32 seq2, __u32 seq3) | 250 | static inline int between(__u32 seq1, __u32 seq2, __u32 seq3) |