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

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Date:	Wed, 22 May 2013 12:12:56 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [ 072/102] ipv6: do not clear pinet6 field

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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