Re: [PATCH] netlink: Skip loopback interface while looking for a template

[Paul Holzinger <pholzing@redhat.com>]

On 23/01/2025 12:53, Stefano Brivio wrote:
> On Thu, 23 Jan 2025 12:49:06 +0100
> Paul Holzinger <pholzing@redhat.com> wrote:
>
>> On 23/01/2025 09:05, Stefano Brivio wrote:
>>> There might be reasons to have routes on the loopback interface, for
>>> example Any-IP/AnyIP routes as implemented by Linux kernel commit
>>> ab79ad14a2d5 ("ipv6: Implement Any-IP support for IPv6.").
>>>
>>> If we use the loopback interface as a template, though, we'll pick
>>> 'lo' (typically) as interface name for our tap interface, but we'll
>>> already have an interface called 'lo' in the target namespace, and as
>>> we TUNSETIFF on it, we'll fail with EINVAL, because it's not a tap
>>> interface.
>>>
>>> Skip the loopback interface while looking for a template interface or,
>>> more accurately, skip the interface with index 1.
>>>
>>> Strictly speaking, we should fetch interface flags via RTM_GETLINK
>>> instead, and check for IFF_LOOPBACK, but interleaving that request
>>> while we're iterating over routes is unnecessarily complicated.
>> I think hard coding 1 is fine but I think there is also the IFF_LOOPBACK
>> flag that could be used instead.
>>
>>   From strace:
>> ifi_index=if_nametoindex("lo"),
>> ifi_flags=IFF_UP|IFF_LOOPBACK|IFF_RUNNING|IFF_LOWER_UP
> Yeah but that's in the link flags, right? Am I missing something? Here
> we're looking at routes.
>
> Note that 'ip route show' will also fetch link attributes with
> RTM_GETLINK, which is the second netlink query I wanted to avoid here.

Ah yes totally overlooked the fact the ip route does the RTM_GETLINK 
call there.

-- 
Paul Holzinger