netdev - Re: Question about synchronize_net() in AF

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Date:	Wed, 17 Sep 2014 07:54:50 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Martin Kelly <martin@...tingkelly.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: Question about synchronize_net() in AF_PACKET close()

On Wed, 2014-09-17 at 07:29 -0700, Martin Kelly wrote:
> On 09/10/2014 02:37 PM, Martin Kelly wrote:
> >> The synchronize_net() is also there to protect against the prot hook
> >> which can run asynchronously from the core packet input path on any
> >> cpu.
> >>
> > 
> > Yes, understood. What I'm not clear about is whether it is safe to do
> > the following:
> > 
> > unregister_prot_hook(sk, false);
> > sock_orphan(sk);
> > sock->sk = NULL;
> > call_rcu(...);

Can you describe the ... ?

> > close socket, return to userspace
> > 
> > instead of
> > 
> > unregister_prot_hook(sk, false);
> > synchronize_net();
> > sock_orphan(sk);
> > sock->sk = NULL;
> > close socket, return to userspace
> > 
> > If you don't call synchronize_net() immediately, then other readers
> > could see the protocol hook in the protocol list and try to use it.
> > They could call into prot_hook.func. However, it appears that  such
> > functions ( e.g. packet_rcv() ) touch the socket buffer but not the
> > socket itself, so orphaning the socket before all RCUs have been
> > processed is safe. In addition, no new packets will come in after
> > packet_release() and touch the socket because the socket fd will be
> > removed from the process fd list.



> > 
> > From my testing, I'm not seeing any obvious issues, but I could be
> > missing something. Is orphaning the socket before all RCUs have
> > finished unsafe?
> > 
> 
> (friendly ping)

What problem do you want to solve exactly ?

I believe its not safe, you missed sk_data_ready() call
(sock_def_readable()) 



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