dfilter: Fix "!=" relation to be free of contradictions

Wireshark defines the relation of equality A == B as
A any_eq B <=> An == Bn for at least one An, Bn.
More accurately I think this is (formally) an equivalence
relation, not true equality.

Whichever definition for "==" we choose we must keep the
definition of "!=" as !(A == B), otherwise it will
lead to logical contradictions like (A == B) AND (A != B)
being true.

Fix the '!=' relation to match the definition of equality:
  A != B <=> !(A == B) <=> A all_ne B <=> An != Bn, for
every n.

This has been the recomended way to write "not equal" for a
long time in the documentation, even to the point where != was
deprecated, but it just wasn't implemented consistently in the
language, which has understandably been a persistent source
of confusion. Even a field that is normally well-behaved
with "!=" like "ip.src" or "ip.dst" will produce unexpected
results with encapsulations like IP-over-IP.

The opcode ALL_NE could have been implemented in the compiler
instead using NOT and ANY_EQ but I chose to implement it in
bytecode. It just seemed more elegant and efficient
but the difference was not very significant.

Keep around "~=" for any_ne relation, in case someone depends
on that, and because we don't have an operator for true equality:
  A strict_equal B <=> A all_eq B <=> !(A any_ne B).
If there is only one value then any_ne and all_ne are the same
comparison operation.

Implementing this change did not require fixing any tests so it
is unlikely the relation "~=" (any_ne) will be very useful.

Note that the behaviour of the '<' (less than) comparison relation
is a separate, more subtle issue. In the general case the definition
of '<' that is used is only a partial order.

1 files changed, 1 insertions, 0 deletions

diff --git a/epan/dfilter/dfvm.h b/epan/dfilter/dfvm.h
index a57333c707..2cdc85b189 100644
--- a/epan/dfilter/dfvm.h
+++ b/epan/dfilter/dfvm.h
@@ -53,6 +53,7 @@ typedef enum {
 	PUT_FVALUE,
 	PUT_PCRE,
 	ANY_EQ,
+	ALL_NE,
 	ANY_NE,
 	ANY_GT,
 	ANY_GE,