Re: [PATCH 2/3] tcp, udp, conf: Don't silently ignore listens on unsupported IP versions

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

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On Sun, Jan 11, 2026 at 12:33:28AM +0100, Stefano Brivio wrote:
> On Mon,  5 Jan 2026 19:28:49 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Currently, it's possible to explicitly ask for forwarding from an IPv4
> > address, while disabling IPv4:
> >     $ pasta -t 192.0.2.1/12345 -6
> > or vice versa:
> >     $ pasta -t 2001:db8::1/12345 -4
> > 
> > Currently, the impossible to implement forwarding option will be silently
> > ignored.  That's potentially confusing since in a complex setup, it might
> > not be obvious why the requested forward isn't taking effect.
> > 
> > Specifically, it's ignored at a fairly low level: tcp_listen() and
> > udp_listen() ignore it and return 0.  Those run kind of late to give a
> > good error message.  Change the low-level functions to return -EACCES
> > (chosen because that's what the kernel will return if you request IPv6
> > when it's disabled by sysctl).
> 
> I couldn't quite find out in which case EACCES is returned by the
> kernel. If I set /proc/sys/net/ipv6/conf/all/disable_ipv6 to 1 and then
> bind() an IPv6 address, after setting IPV6_FREEBIND, I get 0.

Huh.  EAFNOSUPPORT seems like it makes more sense, but oddly didn't
spot it.  I was looking at:

    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/addrconf.c#n1098
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/addrconf.c#n2565
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/route.c#n3664

Happy enough to change it to EAFNOSUPPORT if you'd prefer.

> If I disable IPv6 via command line (ipv6.disable=1) I get EAFNOSUPPORT
> on bind(), and EOPNOTSUPP on setting addresses and routes. EACCES, I
> couldn't quite spot it yet.

Huh.  Kind of weird it only fails on bind(), not on socket().

> > Most callers of {tcp,udp}_listen() ignore
> > the return code, so this is a no-op for them.  In the remaining caller,
> > conf_ports_range_except() check for the case explicitly, and provide a
> > meaningful error message.
> > 
> > Of itself, this bug is insignificant, but this is a roadblock to having
> > {tcp,udp}_listen() return socket fds, which in turn is a roadblock to my
> > flexible forwarding work.  So, might as well fix it.
> > 
> > Link: https://bugs.passt.top/show_bug.cgi?id=186
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  conf.c | 10 ++++++++++
> >  tcp.c  |  6 ++----
> >  udp.c  |  6 ++----
> >  3 files changed, 14 insertions(+), 8 deletions(-)
> > 
> > diff --git a/conf.c b/conf.c
> > index 70ea168c..cc3c20a9 100644
> > --- a/conf.c
> > +++ b/conf.c
> > @@ -162,6 +162,16 @@ static void conf_ports_range_except(const struct ctx *c, char optname,
> >  		    optname, optarg);
> >  	}
> >  
> > +	if (addr) {
> > +		if (!c->ifi4 && inany_v4(addr)) {
> > +			die("IPv4 is disabled, can't use -%c %s",
> > +			    optname, optarg);
> > +		} else if (!c->ifi6 && !inany_v4(addr)) {
> > +			die("IPv6 is disabled, can't use -%c %s",
> > +			    optname, optarg);
> > +		}
> > +	}
> > +
> >  	for (i = first; i <= last; i++) {
> >  		if (bitmap_isset(exclude, i))
> >  			continue;
> > diff --git a/tcp.c b/tcp.c
> > index e7fa85f3..67007c05 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -2700,16 +2700,14 @@ int tcp_listen(const struct ctx *c, uint8_t pif,
> >  			/* Restrict to v6 only */
> >  			addr = &inany_any6;
> >  		else if (inany_v4(addr))
> > -			/* Nothing to do */
> > -			return 0;
> > +			return -EACCES;
> >  	}
> >  	if (!c->ifi6) {
> >  		if (!addr)
> >  			/* Restrict to v4 only */
> >  			addr = &inany_any4;
> >  		else if (!inany_v4(addr))
> > -			/* Nothing to do */
> > -			return 0;
> > +			return -EACCES;
> >  	}
> >  
> >  	if (pif == PIF_HOST) {
> > diff --git a/udp.c b/udp.c
> > index eda55c39..8cfa1e1f 100644
> > --- a/udp.c
> > +++ b/udp.c
> > @@ -1162,16 +1162,14 @@ int udp_listen(const struct ctx *c, uint8_t pif,
> >  			/* Restrict to v6 only */
> >  			addr = &inany_any6;
> >  		else if (inany_v4(addr))
> > -			/* Nothing to do */
> > -			return 0;
> > +			return -EACCES;
> >  	}
> >  	if (!c->ifi6) {
> >  		if (!addr)
> >  			/* Restrict to v4 only */
> >  			addr = &inany_any4;
> >  		else if (!inany_v4(addr))
> > -			/* Nothing to do */
> > -			return 0;
> > +			return -EACCES;
> >  	}
> >  
> >  	s = pif_sock_l4(c, EPOLL_TYPE_UDP_LISTEN, pif,
> 
> The rest looks good to me.
> 
> -- 
> Stefano
> 

-- 
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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