Subject: Re: [PATCH net] net: fib_rules: Fix fib_rules_ops->compare implementations to support exact match

From: Shmulik Ladkani <[email-masked]>
Date: Wed, 4 Oct 2017 08:34:34 +0300

Hi David,

On Tue, 03 Oct 2017 14:54:18 -0700 (PDT) David Miller <[email-masked]> wrote:

> I don't see any inconsistency.  When you insert using NLM_F_EXCL the
> insertion fails if any existing rule matches or overlaps in any way
> with the keys in the new rule.

Please note that current situation is as follows:

A: Generic (non /0), followed by specific that overlaps, ALLOWED

# ip ru add from 0.0.0.0/1 iif eth2 pref 33 table 33
# ip ru add from 10.0.0.0/8 iif eth2 pref 33 table 33

A Reversed: Specific, followed by generic (non /0) that overlaps, ALLOWED

# ip ru add from 10.0.0.0/8 iif eth2 pref 33 table 33
# ip ru add from 0.0.0.0/1 iif eth2 pref 33 table 33

B: 0.0.0.0/0, followed by specific that overlaps, ALLOWED

# ip ru add from 0.0.0.0/0 iif eth2 pref 33 table 33
# ip ru add from 10.0.0.0/8 iif eth2 pref 33 table 33

B Reversed: Specific, followed by 0.0.0.0/0, FAILS

# ip ru add from 10.0.0.0/8 iif eth2 pref 33 table 33
# ip ru add from 0.0.0.0/0 iif eth2 pref 33 table 33
(File exists)

Is there any reason why 0.0.0.0/0 should be treated differently, meaning,
insertion of 0.0.0.0/0 is order dependant (where other overlapping
rules are allowed REGARDLESS order of insertion)?

Please do note there is absolutely NO "overlapping" detection logic in
'fib4_rule_compare' whatsoever; just strict comparison of the FRA_SRC
addresses.

The only exception is if the new FRA_SRC address is 0.0.0.0/0 - which is
considered "colliding" with ANY existing rule.

The "treat /0 as a collision" existed way prior NLM_F_EXCL enforcement
was introduced, as the single usecase of ->compare() was for DELRULE
which had wildcard semantics.
Alas for NEWRULE+NLM_F_EXCL it exposes the above anomaly.

Best,
Shmulik