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

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Date:   Tue, 2 Mar 2021 16:22:17 +0000
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        duanxiongchun@...edance.com, wangdongdong.6@...edance.com,
        jiang.wang@...edance.com, Cong Wang <cong.wang@...edance.com>,
        John Fastabend <john.fastabend@...il.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 Tue, 2 Mar 2021 at 02:37, Cong Wang <xiyou.wangcong@...il.com> wrote:

...

> @@ -350,25 +351,12 @@ static inline void sk_psock_cork_free(struct sk_psock *psock)
>         }
>  }
>
> -static inline void sk_psock_update_proto(struct sock *sk,
> -                                        struct sk_psock *psock,
> -                                        struct proto *ops)
> -{
> -       /* Pairs with lockless read in sk_clone_lock() */
> -       WRITE_ONCE(sk->sk_prot, ops);
> -}
> -
>  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?

> -       if (inet_csk_has_ulp(sk)) {
> -               tcp_update_ulp(sk, psock->sk_proto, psock->saved_write_space);
> -       } else {
> -               sk->sk_write_space = psock->saved_write_space;
> -               /* Pairs with lockless read in sk_clone_lock() */
> -               WRITE_ONCE(sk->sk_prot, psock->sk_proto);
> -       }
> +       if (psock->saved_update_proto)
> +               psock->saved_update_proto(sk, true);
>  }
>
>  static inline void sk_psock_set_state(struct sk_psock *psock,
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 636810ddcd9b..0e8577c917e8 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1184,6 +1184,9 @@ struct proto {
>         void                    (*unhash)(struct sock *sk);
>         void                    (*rehash)(struct sock *sk);
>         int                     (*get_port)(struct sock *sk, unsigned short snum);
> +#ifdef CONFIG_BPF_SYSCALL
> +       int                     (*update_proto)(struct sock *sk, bool restore);

Kind of a nit, but this name suggests that the callback is a lot more
generic than it really is. The only thing you can use it for is to
prep the socket to be sockmap ready since we hardwire sockmap_unhash,
etc. It's also not at all clear that this only works if sk has an
sk_psock associated with it. Calling it without one would crash the
kernel since the update_proto functions don't check for !sk_psock.

Might as well call it install_sockmap_hooks or something and have a
valid sk_psock be passed in to the callback. Additionally, I'd prefer
if the function returned a struct proto * like it does at the moment.
That way we keep sk->sk_prot manipulation confined to the sockmap code
and don't have to copy paste it into every proto.

> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 3bddd9dd2da2..13d2af5bb81c 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -184,26 +184,10 @@ static void sock_map_unref(struct sock *sk, void *link_raw)
>
>  static int sock_map_init_proto(struct sock *sk, struct sk_psock *psock)
>  {
> -       struct proto *prot;
> -
> -       switch (sk->sk_type) {
> -       case SOCK_STREAM:
> -               prot = tcp_bpf_get_proto(sk, psock);
> -               break;
> -
> -       case SOCK_DGRAM:
> -               prot = udp_bpf_get_proto(sk, psock);
> -               break;
> -
> -       default:
> +       if (!sk->sk_prot->update_proto)
>                 return -EINVAL;
> -       }
> -
> -       if (IS_ERR(prot))
> -               return PTR_ERR(prot);
> -
> -       sk_psock_update_proto(sk, psock, prot);
> -       return 0;
> +       psock->saved_update_proto = sk->sk_prot->update_proto;
> +       return sk->sk_prot->update_proto(sk, false);

I think reads / writes from sk_prot need READ_ONCE / WRITE_ONCE. We've
not been diligent about this so far, but I think it makes sense to be
careful in new code.

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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