Re: [PATCH net-next] skb: Add documentation for skb_clone_sk

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2014-09-08 at 11:44 -0700, Alexander Duyck wrote:
> On 09/08/2014 10:11 AM, Eric Dumazet wrote:
> > On Mon, 2014-09-08 at 12:18 -0400, Alexander Duyck wrote:
> >> This change adds some documentation to the call skb_clone_sk.  This is
> >> meant to help clarify the purpose of the function for other developers.
> >>
> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> >> ---
> >>  net/core/skbuff.c |   11 +++++++++++
> >>  1 file changed, 11 insertions(+)
> >>
> >> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> >> index a18dfb0..3f83a8a 100644
> >> --- a/net/core/skbuff.c
> >> +++ b/net/core/skbuff.c
> >> @@ -3511,6 +3511,17 @@ struct sk_buff *sock_dequeue_err_skb(struct sock *sk)
> >>  }
> >>  EXPORT_SYMBOL(sock_dequeue_err_skb);
> >>  
> >> +/**
> >> + * skb_clone_sk - create clone of skb, and take reference to socket
> >> + * @skb: the skb to clone
> >> + *
> >> + * For functions such as timestamping it is necessary to clone an skb and
> >> + * hold a reference to the socket for it until the hardware delivers the
> >> + * actual timestamp or the timestamp is timed out.  As as such this
> >> + * function is useful for creating a clone to later be handed off to
> >> + * skb_complete_tx_timestamp or kfree_skb to take care of cleaning up
> >> + * the reference handling for the socket.
> >> + */
> >>  struct sk_buff *skb_clone_sk(struct sk_buff *skb)
> >>  {
> >>  	struct sock *sk = skb->sk;
> > 
> > Note that I have serious doubts about the atomic_inc_not_zero() here.
> > 
> > At this point, we need to have consistent refcounting on the socket.
> > 
> > If we decide the reference is against sk_refcnt, then current sk_refcnt
> > cannot be 0 at this point.
> 
> Isn't that what is guaranteed by using the atomic_inc_not_zero?  If it
> is zero we abort and just return NULL.
> 

Point is : the skb we clone here must have a reference on the socket.

How sk_refcnt could be 0 here ?

If it was 0, then something was broken before skb_clone_sk() call.



> > This might hide a very serious bug.
> > 
> > In TCP tx path for example, we do not take reference on sk_refcnt for
> > each packet, but a reference on sk_wmem_alloc
> > 
> > If skb destructor is sock_wfree() or tcp_wfree(), then we should take an
> > extra reference on sk_wmem_alloc instead of sk_refcnt.
> 
> My concern then would be what I should do about skb_tx_complete as I am
> currently using sock_hold/sock_put to prevent the socket from being
> freed due to the skb_orphan call in sock_queue_err_skb.
> 
> Would I need to change the logic there as well in order to prevent us
> from using the wrong reference to keep the socket valid?

We certainly have to think again and clean this.