Re: Question about synchronize_net() in AF_PACKET close()

[Martin Kelly <martin@martingkelly.com>]

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(...);
> 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)