Re: [PATCH net v2] Revert "ipv6: preserve insertion order for same-scope addresses"

On 6/3/26 9:17 AM, Thorsten Leemhuis wrote:
> On 6/3/26 08:53, Íñigo Huguet wrote:
>> Stating that the order is "random" implies that someone will change
>> the default insertion order in the future, as the documentation says
>> that it's not UAPI. So they'll cause the UAPI breakage that we are
>> trying to avoid now. Moreover, if we consider the default order
>> "random" and state that userspace programs must not rely on it, why
>> can't we change it now?
> 
> Because the UAPI is only indirectly important, as it's "something that
> used to work doesn't anymore" is what matters; you in fact can break the
> UAPI all you want, as long as everything continues to work. Or to quote
> Linus from:
> 

I agree with Thorsten here. We must not break the UAPI. Is the current 
order kind of weird? Yes. But software that is running on real systems 
depends on it.

AFAICS, it is not only NetworkManager but systemd-networkd, cloud 
tooling and any other script that uses netlink. I think we must stick to 
the rule of not breaking userspace and the argument of consistency with 
IPv4 is not appealing to me because that isn't a goal of IPv6, plenty is 
different when you compared both protocols.

We should document that the order matters, no that it is "random" 
because it isn't.

> https://www.kernel.org/doc/html/latest/process/handling-regressions.html#on-what-qualifies-as-userspace-interface-abi-api-documented-interfaces-etc
> 
> "I just wanted to point out that the argument about whether it's an ABI
> change or not is irrelevant. If it turns out that some program - not a
> test script, but something with relevance to conscious user expectations
> ~ depended on the old broken behavior, then it needs to be done some
> other way.""
> 
> Yes, he talks about ABIs here, but it's the same for APIs.
> 
> Ciao, Thorsten