Re: [RFC bpf-next 0/7] Programming socket lookup with BPF

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

On 6/21/19 5:51 AM, Florian Westphal wrote:
> Jakub Sitnicki <jakub@cloudflare.com> wrote:
>>> So, at least for this part I don't see a technical reason why this
>>> has to grab a reference for listener socket.
>>
>> That's helpful, thanks! We rely on TPROXY, so I would like to help with
>> that. Let me see if I can get time to work on it.
> 
> AFAICS so far this would be enough:
> 
> 1. remove the BUG_ON() in skb_orphan, letting it clear skb->sk instead
> 2. in nf_queue_entry_get_refs(), if skb->sk and no destructor:
>    call nf_tproxy_assign_sock() so a reference gets taken.
> 3. change skb_steal_sock:
>    static inline struct sock *skb_steal_sock(struct sk_buff *skb, bool *refcounted)
>     [..]
>     *refcounted = skb->destructor != NULL;
> 4. make tproxy sk assign elide the destructor assigment in case of
>    a listening sk.
> 

Okay, but how do we make sure the skb->sk association does not leak from rcu section ?

Note we have the noref/refcounted magic for skb_dst(), we might try to use something similar
for skb->sk

> This should work because TPROXY target is restricted to PRE_ROUTING, and
> __netif_receive_skb_core runs with rcu readlock already held.
> 
> On a side note, it would also be interesting to see what breaks if the
> nf_tproxy_sk_is_transparent() check in the tprox eval function is
> removed -- if we need the transparent:1 marker only for output, i think
> it would be ok to raise the bit transparently in the kernel in case
> we assign skb->sk = found_sk; i.e.
>  if (unlikely(!sk_is_transparent(sk))
> 	 make_sk_transparent(sk);
> 
> I don't see a reason why we need the explicit setsockopt(IP_TRANSPARENT)
> from userspace.
>