Re: [ 072/102] ipv6: do not clear pinet6 field

[Roman Gushchin]

On 22.05.2013 01:47, Eric Dumazet wrote:
> On Tue, 2013-05-21 at 15:44 +0400, Roman Gushchin wrote:
> > Hi, all!
> >
> > I think, it's good, but not enough.
> >
> > We still can't rely on the sk->sk_family field by dereferencing the
> > inet_sk(sk)->pinet6 field, because we can set the sk_family field to
> > the PF_INET6 value before setting pinet6 to an appropriate value
> > (assuming it is NULL just because it was not a PF_INET6 socket in a
> > previous life).
> >
> > net/ipv6/af_inet6.c:
> > static int inet6_create(struct net *net, struct socket *sock, int
> > protocol, int kern)
> > {
> > 	<...>
> > 	err = -ENOBUFS;
> > 	sk = sk_alloc(net, PF_INET6, GFP_KERNEL, answer_prot);
> > 	if (sk == NULL)
> > 		goto out;
> > 	<...>
> > 	sk->sk_destruct		= inet_sock_destruct;
> > 	sk->sk_family		= PF_INET6;
> > 	sk->sk_protocol		= protocol;
> >
> > 	sk->sk_backlog_rcv	= answer->prot->backlog_rcv;
> >
> > 	inet_sk(sk)->pinet6 = np = inet6_sk_generic(sk);
> > 	<...>
> > }
> >
> > net/core/sock.c:
> > struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
> > 		      struct proto *prot)
> > {
> > 	struct sock *sk;
> >
> > 	sk = sk_prot_alloc(prot, priority | __GFP_ZERO, family);
> > 	if (sk) {
> > 		sk->sk_family = family;
> > 	<...>
> > }
> >
> >
> > So, we need to care about setting sk_family to PF_INET6 _strictly_ after
> > setting the pinet6 field to a valid value (using rcu_assign_pointer(),
> > for instance).
>
> This can never happen.
>
> A socket cannot be find in a hash chain while pinet6 is not set.
>
> For a given socket pointer sk (say TCP or UDP), pinet6 is a constant and
> cannot change. (This is a property of SLAB_DESTROY_BY_RCU : slab cannot
> be merged, so all objects are of the same type)

I missed that.
> So the order of writing sk_family / pinet6 is irrelevant.

Agree.

Thank you for clarification!

Regards,
Roman
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