Re: [PATCH net] net: fib_rules: add protocol check in rule_find

On Wed, Jun 27, 2018 at 6:27 PM, Roopa Prabhu [off-list ref] wrote:

From: Roopa Prabhu <redacted>

After commit f9d4b0c1e969 ("fib_rules: move common handling of newrule
delrule msgs into fib_nl2rule"), rule_find is strict about checking
for an existing rule. rule_find must check against all
user given attributes, else it may match against a subset
of attributes and return an existing rule.

In the below case, without support for protocol match, rule_find
will match only against 'table main' and return an existing rule.

$ip -4 rule add table main protocol boot
RTNETLINK answers: File exists

This patch adds protocol support to rule_find, forcing it to
check protocol match if given by the user.

Fixes: f9d4b0c1e969 ("fib_rules: move common handling of newrule delrule msgs into fib_nl2rule")
Signed-off-by: Roopa Prabhu <redacted>
---
I spent some time looking at all match keys today and protocol
was the only missing one (protocol is not in a released kernel yet).
The only way this could be avoided is to move back to the old loose
rule_find. I am worried about this new strict checking surprising users,
but going back to the previous loose checking does not seem right either.
If there is a reason to believe that users did rely on the previous
behaviour, I will be happy to revert. Here is another example of old and
new behaviour.

old rule_find behaviour:
$ip -4 rule add table main protocol boot
$ip -4 rule add table main protocol boot
$ip -4 rule add table main protocol boot
$ip rule show
0:      from all lookup local
32763:  from all lookup main  proto boot
32764:  from all lookup main  proto boot
32765:  from all lookup main  proto boot
32766:  from all lookup main
32767:  from all lookup default

new rule_find behaviour (after this patch):
$ip -4 rule add table main protocol boot
$ip -4 rule add table main protocol boot
RTNETLINK answers: File exists

I found the case where the new rule_find breaks for add.
$ip -4 rule add table main tos 10 fwmark 1
$ip -4 rule add table main tos 10
RTNETLINK answers: File exists

The key masks in the new and old rule need to be compared .
And it cannot be easily compared today without an elaborate if-else block.
Its best to introduce key masks for easier and accurate rule comparison.
But this is best done in net-next. I will submit an incremental patch
tomorrow to
restore previous rule_exists for the add case and the rest should be good.

The current patch in context is needed regardless.

Thanks (and sorry about the iterations).