Re: [PATCH v3 2/8] util, flow, pif: Simplify sock_l4_sa() interface

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 19, 2025 at 12:42:04PM +0100, Stefano Brivio wrote:
> On Tue, 18 Nov 2025 14:34:58 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Tue, Nov 18, 2025 at 01:19:21AM +0100, Stefano Brivio wrote:
> > > On Fri, 14 Nov 2025 10:21:46 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Thu, Nov 13, 2025 at 07:33:13AM +0100, Stefano Brivio wrote:  
> > > > > On Wed, 29 Oct 2025 17:26:22 +1100
> > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > >     
> > > > > > sock_l4_sa() has a somewhat confusing 'v6only' option controlling whether
> > > > > > to set the IPV6_V6ONLY socket option.  Usually it's set when the given
> > > > > > address is IPv6, but not when we want to create a dual stack listening
> > > > > > socket.  The latter only makes sense when the address is :: however.
> > > > > > 
> > > > > > Clarify this by only keeping the v6only option in an internal helper
> > > > > > sock_l4_().  External users will call either sock_l4() which always creates
> > > > > > a socket bound to a specific IP version, or sock_l4_dualstack() which
> > > > > > creates a dual stack socket, but takes only a port not an address.    
> > > > > 
> > > > > I'm not sure if we'll ever need anything different, but I guess that
> > > > > this is not the only obvious semantic of sock_l4_dualstack(), as it
> > > > > could take a sockaddr_inany eventually, and bind() IPv6 address and its
> > > > > v4-mapped equivalent (...does that even work?).    
> > > > 
> > > > Do you mean that if we have a v4-mapped address, then using an IPv6
> > > > "dual stack" socket will listen both for IPv4 traffic and for IPv6
> > > > traffic actually using that v4-mapped address on the wire (presumably
> > > > as a result of a router translating to a local IPv6-only network)?  I
> > > > think that will work, though I haven't tested.  
> > > 
> > > Yes, that's what I meant.
> > >   
> > > > In that case we can determine that we need IPV6_V6ONLY from the
> > > > address.  The only case that doesn't cover is if we want to listen for
> > > > v4-mapped traffic already translated by a router but *not* native IPv4
> > > > traffic.  I don't see a lot of reason to ever do that, so it's in the
> > > > "refactor if we ever discover we need it" pile.  
> > > 
> > > I thought that we might want to listen on both IP versions for whatever
> > > reason, on a single socket, with a specific address (say, that v4-mapped
> > > address and the equivalent untranslated address...?).  
> > 
> > I'm not really sure what you mean by an "equivalent untranslated
> > address".  AFAIK, the only non-wildcard case that will actually listen
> > on both IP versions is a v4-mapped address.
> 
> I mean 192.0.2.1 (untranslated, IPv4) and ::ffff:192.0.2.1 (v4-mapped).
> Will we ever want to listen to both?

Maybe one day, but not soon, I think.

> I don't think we have to care about that right now, though.

Agreed.

> > So, yes we probably should explicitly set IPV6_V6ONLY==0 for v4-mapped
> > addresses as well.
> > 
> > > I know it can't be done now anyway, I'm just saying that
> > > sock_l4_dualstack() forcing wildcard addresses isn't something we should
> > > imply as part of "dualstack".  
> > 
> > Hm, ok.  What if I renamed it to sock_l4_dualwild()?
> 
> Short-hands for "wildcard" aren't necessarily obvious. I would have
> gone with "dual_any" or "dualstack_any" or "v4v6_any" or "inany_any".

'dualstack_any' is definitely a better idea.  Unfortunately, I forget
to change this in my last spin.

> But actually it can also stay like that, I guess, especially as this
> looks refactor-prone for https://bugs.passt.top/show_bug.cgi?id=140.

True.

> I
> just wanted to raise the fact it's not obvious that "dualstack" implies
> :: and 0.0.0.0. It doesn't need to be addressed in code or comments now,
> or ever.

Ok.  Sounds like it's definitely worth a respin.  Perhaps I'll add a
rename patch into some future series.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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