Re: [PATCH 01/13 RFC net-next] net: ipv4: introduce CONFIG_IPV4 to decouple the IPv4 stack

[Fernando Fernandez Mancera <fmancera@xxxxxxx>]

On 7/12/26 1:01 PM, Arnd Bergmann wrote:
> On Sun, Jul 12, 2026, at 03:38, Fernando Fernandez Mancera wrote:
> > Historically, the IPv4 protocol has been linked to the core INET
> > subsystem. Because shared infrastructure like the TCP/UDP engine,
> > routing or INET hashtables live inside net/ipv4/, it has been impossible
> > to compile a kernel with only IPv6 support.
> >
> > This patch introduces the CONFIG_IPV4 Kconfig symbol, which is set to
> > 'def_bool y' for now. This does not allow to completely disable the
> > IPv4 stack yet but it lays the necessary build-system work for that
> > goal.
> I expect this will cause additional (trivial) build regression in the
> next step when randconfig builds run into obscure corner cases, either
> with INET=y IPV4=n IPV6=y or with INET=y IPV4=n IPV6=n.
>
> I can probably give your patch (with IPV4 visible or disabled) an
> early go on the randconfig tree to find these more quickly.
> If I run into regressions, should I just add more 'depends on IPV4',
> or do you have other plans?
Yes, I have a job running randconfig and verifying nothing breaks. If 
something breaks and it isn't core networking stack I would just make 
the Kconfig symbol depend on IPv4.
Then later we will have more time to write a dedicate patch so it does 
not depend on IPv4.
> Should we have some logic to ensure that at least one of IPV4 or
> IPV6 is enabled? I think this would work
>
> config IPV4
>        bool "The IPv4 protocol" if IPV6
>        default INET
>
> which only allows turning IPV4 off if IPV6 has enabled.
I do wonder, should we? I mean, I didn't try it off but I don't see why 
we should not allow a pure L2 system..
Thanks,
Fernando.