Re: [PATCH net-next v1 3/5] ipv4: enable IFA_IF_NETNSID for RTM_GETADDR

[David Ahern <dsahern@gmail.com>]

On 9/4/18 12:50 AM, Jiri Benc wrote:
> 
> This is a general problem with netlink: unknown attributes are ignored.
> We need a way to detect that certain attribute was understood by the
> kernel or was not. And it needs to work retroactively, i.e. the
> application has to be able to determine the currently running kernel
> does not support the feature (because it's too old).

sure, and that has been discussed before.

> 
> That's why we return back the attribute in responses to a request with
> IFLA_IF_NETNSID present and why we should do the same for
> IFA_IF_NETNSID.
> 
>> See 21fdd092acc7e. I would like to see other filters added for addresses
>> in the same release this gets used. The only one that comes to mind for
>> addresses is to only return addresses for devices with master device
>> index N (same intent as 21fdd092acc7e for neighbors).
> 
> I also question the statement that IFA_F_NETNSID is a filter: my
> understanding of "filter" is something that limits the output to a
> certain subset. I.e., unfiltered results always contain everything that
> is in a filtered result. While with IFA_F_NETNSID, we get a completely
> different set of data. Does that really constitute a filter? Note that
> we can still filter in the target netns.
> 

I'll buy that argument over the 'too coarse' one. Looking at the link
version of this flag, the NLM_F_DUMP_FILTERED flag is not used there so
for consistency the address one should follow suit.