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

[Fernando Fernandez Mancera <fmancera@xxxxxxx>]

On 7/13/26 4:22 PM, Arnd Bergmann wrote:
> On Mon, Jul 13, 2026, at 16:00, Fernando Fernandez Mancera wrote:
> > 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.
> Ok
>
> > > 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..
> I expected a pure L2 system to be CONFIG_ETHERNET=y CONFIG_INET=n.
>
> Which user-visible parts of CONFIG_INET would you want keep working
> when both v4 and v6 are disabled?
Yes, you are right. So I think introducing something as you proposed 
makes sense. Right now I cannot think about something needed from INET 
for pure L2.
Thank you!
Fernando.