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

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Message-ID: <20190621125155.2sdw7pugepj3ityx@breakpoint.cc>
Date:   Fri, 21 Jun 2019 14:51:55 +0200
From:   Florian Westphal <fw@...len.de>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [RFC bpf-next 0/7] Programming socket lookup with BPF

Jakub Sitnicki <jakub@...udflare.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.

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.