netdev - Re: [PATCH net-next v3 1/2] netlink: ipv4 igmp join notifications

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Date:   Wed, 26 Sep 2018 10:23:40 -0700
From:   Roopa Prabhu <roopa@...ulusnetworks.com>
To:     Patrick Ruddy <pruddy@...tta.att-mail.com>
Cc:     David Ahern <dsahern@...il.com>, netdev <netdev@...r.kernel.org>,
        Jiří Pírko <jiri@...nulli.us>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Subject: Re: [PATCH net-next v3 1/2] netlink: ipv4 igmp join notifications

On Tue, Sep 25, 2018 at 2:34 AM, Patrick Ruddy
<pruddy@...tta.att-mail.com> wrote:
> On Wed, 2018-09-19 at 21:47 -0700, David Ahern wrote:
>> On 9/18/18 6:12 AM, Patrick Ruddy wrote:
>> >
>> > I've hit a small snag with adding the new groups. The number of defined
>> > groups currently sits at 31 so I can only add one before hitting the
>>
>> I believe you have no more available. RTNLGRP_* has been defined from 0
>> (RTNLGRP_NONE) to 31 (RTNLGRP_IPV6_MROUTE_R) which covers the u32 range.
>>
>> > limit defined by the 32 bit groups bitmask in socakddr_nl. I can use 1
>> > group for both v4 and v6 notifications which seems like the sensible
>> > options since the AF is carried separately, but it breaks the precedent
>> > where there are separate IPV4 and IPV6 groups for IFADDR.
>> >
>> > I have the combined group patches ready and can share them if that's
>> > the preference.
>> >
>> > Has there been any previous discussion about extending the number of
>> > availabel groups?
>> >
>>
>> I have not tried it, but from a prior code review I believe you have you
>> use setsockopt to add groups > 31.
>
> I can certainly join the new groups using setsockopt and
> NETLINK_ADD_MEMBERSHIP.
> I can't see any examples of extending the defined group list within the
> kernel so I assume I just add to the RTNLGRP enum list with a suitable
> comment to indicate that later groups must be joined with the mechanism
> above or am I missing some other way of dynamically adding groups?
>

With a quick look, there are other subsystem specific groups:
xfrm_nlgroups,  nfnetlink_groups ...which i see apps registering using
NETLINK_ADD_MEMBERSHIP.

seems like an overkill to add something like this for your case.

yet another option to consider:
use family: RTNL_FAMILY_IPMR/  RTNL_FAMILY_IP6MR  with RTM_GETADDR/DELADDR
and use the existing groups: RTNLGRP_IPV4_IFADDR /  RTNLGRP_IPV6_IFADDR

(pls check if this will break any existing users)

precedence is ipmr fib rules.