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SubjectRe: [PATCH] l2tp: Restore socket refcount when sendmsg succeeds
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From: Guillaume Nault <g.nault@alphalink.fr>
Date: Tue, 12 Mar 2013 11:36:50 +0100

> On Fri, Mar 01, 2013 at 02:12:52PM -0500, David Miller wrote:
>> Looking at how this code works, it is such a terrible design. This
>> whole reference counting issue exists purely because
>> pppol2tp_sock_to_session() grabs the 'sk' reference.
>>
>> In all but one case, it need not do this.
>>
>> The socket system calls have an implicit reference to 'sk' via
>> socket->sk. If you can get into the system call and socket->sk
>> is non-NULL then 'sk' is NOT going anywhere.
>>
>> And all of these system call handlers have this pattern:
>>
>> session = pppol2tp_sock_to_session(sk);
>> ...
>> sock_put(sk);
>>
>> The only case where the reference count is really needed is that
>> sequence in pppol2tp_release().
>>
>> Long term the right thing to do here is stop having this session
>> grabber function take the 'sk' reference. Then in pppol2tp_release
>> we'll grab a reference explicitly. At all the other call sites we
>> then blast aweay all of the sock_put(sk) paths.
>>
> Could this also apply to l2tp_sock_to_tunnel() (in l2tp_core.h)? As per
> my understanding, none of its callers needs to take a socket reference.
> So sock_hold() could be removed in both pppol2tp_sock_to_session() and
> l2tp_sock_to_tunnel() functions. The corresponding sock_put() calls
> would then be removed from all calling functions but pppol2tp_release().
> If this is correct, I'll send a patch for net-next.

Yes, it could be simplified in this way too. Just make sure that this
interface is only used in system call / user context, where we know
the underlying socket cannot go away on us.


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