netdev - Re: [Patch bpf-next v2 2/9] sock: introduce sk_prot->update

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Message-ID: <CAM_iQpUr7cvuXXdtYN9_MQPYy_Tfi88fBGSo3c8RRpMFBr55Og@mail.gmail.com>
Date:   Fri, 5 Mar 2021 16:57:49 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Lorenz Bauer <lmb@...udflare.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [Patch bpf-next v2 2/9] sock: introduce sk_prot->update_proto()

On Fri, Mar 5, 2021 at 4:27 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > On Tue, Mar 2, 2021 at 10:23 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > >
> > > On Tue, Mar 2, 2021 at 8:22 AM Lorenz Bauer <lmb@...udflare.com> wrote:
> > > >
> > > > On Tue, 2 Mar 2021 at 02:37, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > > >
> > > > ...
> > > > >  static inline void sk_psock_restore_proto(struct sock *sk,
> > > > >                                           struct sk_psock *psock)
> > > > >  {
> > > > >         sk->sk_prot->unhash = psock->saved_unhash;
> > > >
> > > > Not related to your patch set, but why do an extra restore of
> > > > sk_prot->unhash here? At this point sk->sk_prot is one of our tcp_bpf
> > > > / udp_bpf protos, so overwriting that seems wrong?
>
> "extra"? restore_proto should only be called when the psock ref count
> is zero and we need to transition back to the original socks proto
> handlers. To trigger this we can simply delete a sock from the map.
> In the case where we are deleting the psock overwriting the tcp_bpf
> protos is exactly what we want.?

Why do you want to overwrite tcp_bpf_prots->unhash? Overwriting
tcp_bpf_prots is correct, but overwriting tcp_bpf_prots->unhash is not.
Because once you overwrite it, the next time you use it to replace
sk->sk_prot, it would be a different one rather than sock_map_unhash():

// tcp_bpf_prots->unhash == sock_map_unhash
sk_psock_restore_proto();
// Now  tcp_bpf_prots->unhash is inet_unhash
...
sk_psock_update_proto();
// sk->sk_proto is now tcp_bpf_prots again,
// so its ->unhash now is inet_unhash
// but it should be sock_map_unhash here

Thanks.