netdev - Re: [PATCH net-next v2 1/2] Add a helper function to get socket cookie in eBPF

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Date:   Sun, 05 Feb 2017 19:30:24 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Lorenzo Colitti <lorenzo@...gle.com>
Cc:     Chenbo Feng <chenbofeng.kernel@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Chenbo Feng <fengc@...gle.com>
Subject: Re: [PATCH net-next v2 1/2] Add a helper function to get socket
 cookie in eBPF

On Mon, 2017-02-06 at 12:01 +0900, Lorenzo Colitti wrote:
> On Mon, Feb 6, 2017 at 11:17 AM, Chenbo Feng
> <chenbofeng.kernel@...il.com> wrote:
> > +BPF_CALL_1(bpf_get_socket_cookie, struct sk_buff *, skb)
> > +{
> > +       return skb->sk ? sock_gen_cookie(skb->sk) : 0;
> > +}
> > +
> 
> Does this code need to increment the socket refcount, or call
> ACCESS_ONCE to get skb->sk? The socket filter codepath should be safe,
> but if this function is called in xt_ebpf, could it race with
> something that sets skb->sk to null?

I do not see how this could possibly happen.

READ_ONCE() would not prevent the 'old' sk from disappearing anyway.