Re: [PATCH] net: inet_diag: always export IPV6_V6ONLY sockopt

[Phil Sutter <phil@nwl.cc>]

On Thu, Jul 09, 2015 at 02:57:57PM +0200, Eric Dumazet wrote:
> On Thu, 2015-07-09 at 10:42 +0200, Phil Sutter wrote:
> > Reconsidering my commit 20462155 "net: inet_diag: export IPV6_V6ONLY
> > sockopt", I am not happy with the limitations it causes for socket
> > analysing code in userspace. Exporting the value only if it is set makes
> > it hard for userspace to decide whether the option is not set or the
> > kernel does not support exporting the option at all.
> > 
> > From an auditor's perspective, the interesting question for AF_INET6
> > sockets is: "Does it NOT have IPV6_V6ONLY set?" Because it is the
> > unexpected case. This patch allows to answer this question reliably.
> > 
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > Cc: Eric Dumazet <edumazet@google.com>
> > ---
> >  net/ipv4/inet_diag.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/net/ipv4/inet_diag.c b/net/ipv4/inet_diag.c
> > index 9bc2667..e622d2e 100644
> > --- a/net/ipv4/inet_diag.c
> > +++ b/net/ipv4/inet_diag.c
> > @@ -152,8 +152,7 @@ int inet_sk_diag_fill(struct sock *sk, struct inet_connection_sock *icsk,
> >  				       inet6_sk(sk)->tclass) < 0)
> >  				goto errout;
> >  
> > -		if (ipv6_only_sock(sk) &&
> > -		    nla_put_u8(skb, INET_DIAG_SKV6ONLY, 1))
> > +		if (nla_put_u8(skb, INET_DIAG_SKV6ONLY, ipv6_only_sock(sk)))
> >  			goto errout;
> >  	}
> >  #endif
> 
> Okay, I guess you don't dump millions of sockets.

I hope not, although I don't know what tools other than ss use
inet_diag.

> Can you remind me why this attribute is useful for non listening
> sockets ?

Indeed, it isn't.

> It looks you are interested to know if a _listener_ is v6 only or not.

Is it possible to select that in a generic way? If I didn't misread the
code, 'ss -l' filters them out locally. It looks like sk->sk_state can
be used to determine that, but it seems to be used for TCP states only?
Or is the value 10 (TCP_LISTEN) used for e.g. UDP as well?

Thanks, Phil