Subject: Re: [PATCH bpf v7 2/5] bpf: fix wrong copied_seq calculation

From: Jakub Sitnicki <[email-masked]>
Date: Sat, 18 Jan 2025 15:50:22 +0100

On Thu, Jan 16, 2025 at 10:05 PM +08, Jiayuan Chen wrote:
> 'sk->copied_seq' was updated in the tcp_eat_skb() function when the
> action of a BPF program was SK_REDIRECT. For other actions, like SK_PASS,
> the update logic for 'sk->copied_seq' was moved to
> tcp_bpf_recvmsg_parser() to ensure the accuracy of the 'fionread' feature.
>
> It works for a single stream_verdict scenario, as it also modified
> 'sk_data_ready->sk_psock_verdict_data_ready->tcp_read_skb'
> to remove updating 'sk->copied_seq'.
>
> However, for programs where both stream_parser and stream_verdict are
> active(strparser purpose), tcp_read_sock() was used instead of
> tcp_read_skb() (sk_data_ready->strp_data_ready->tcp_read_sock)
> tcp_read_sock() now still update 'sk->copied_seq', leading to duplicated
> updates.
>
> In summary, for strparser + SK_PASS, copied_seq is redundantly calculated
> in both tcp_read_sock() and tcp_bpf_recvmsg_parser().
>
> The issue causes incorrect copied_seq calculations, which prevent
> correct data reads from the recv() interface in user-land.
>
> We do not want to add new proto_ops to implement a new version of
> tcp_read_sock, as this would introduce code complexity [1].
>
> We add new callback for strparser for customized read operation, also as
> a wrapper function it provides abstraction use psock.
>
> [1]: https://lore.kernel.org/bpf/[email-masked]
> Fixes: e5c6de5fa025 ("bpf, sockmap: Incorrectly handling copied_seq")
> Suggested-by: Jakub Sitnicki <[email-masked]>
> Signed-off-by: Jiayuan Chen <[email-masked]>
> ---

[...]

> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index 47f65b1b70ca..6dcde3506a9b 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -646,6 +646,47 @@ static int tcp_bpf_assert_proto_ops(struct proto *ops)
>  	       ops->sendmsg  == tcp_sendmsg ? 0 : -ENOTSUPP;
>  }
>  
> +#if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
> +static int tcp_bpf_strp_read_sock(struct sock *sk, read_descriptor_t *desc,
> +				  sk_read_actor_t recv_actor)
> +{
> +	struct sk_psock *psock;
> +	struct tcp_sock *tp;
> +	int copied = 0;
> +
> +	tp = tcp_sk(sk);
> +	rcu_read_lock();
> +	psock = sk_psock(sk);
> +	if (WARN_ON(!psock)) {
> +		desc->error = -EINVAL;
> +		goto out;
> +	}
> +
> +	psock->ingress_bytes = 0;
> +	/* We could easily add copied_seq and noack into desc then call
> +	 * ops->read_sock without calling symbol directly. But unfortunately
> +	 * most descriptors used by other modules are not inited with zero.
> +	 * Also it not work by replacing ops->read_sock without introducing
> +	 * new ops as ops itself is located in rodata segment.
> +	 */
> +	copied = tcp_read_sock_noack(sk, desc, recv_actor, true,
> +				     &psock->copied_seq);
> +	if (copied < 0)
> +		goto out;
> +	/* recv_actor may redirect skb to another socket(SK_REDIRECT) or
> +	 * just put skb into ingress queue of current socket(SK_PASS).
> +	 * For SK_REDIRECT, we need 'ack' the frame immediately but for
> +	 * SK_PASS, the 'ack' was delay to tcp_bpf_recvmsg_parser()
> +	 */
> +	tp->copied_seq = psock->copied_seq - psock->ingress_bytes;
> +	tcp_rcv_space_adjust(sk);
> +	__tcp_cleanup_rbuf(sk, copied - psock->ingress_bytes);
> +out:
> +	rcu_read_unlock();
> +	return copied;
> +}
> +#endif /* CONFIG_BPF_STREAM_PARSER */
> +
>  int tcp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
>  {
>  	int family = sk->sk_family == AF_INET6 ? TCP_BPF_IPV6 : TCP_BPF_IPV4;
> @@ -681,6 +722,12 @@ int tcp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
>  
>  	/* Pairs with lockless read in sk_clone_lock() */
>  	sock_replace_proto(sk, &tcp_bpf_prots[family][config]);
> +#if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
> +	if (psock->progs.stream_parser && psock->progs.stream_verdict) {
> +		psock->copied_seq = tcp_sk(sk)->copied_seq;
> +		psock->read_sock = tcp_bpf_strp_read_sock;

Just directly set psock->strp.cb.read_sock to tcp_bpf_strp_read_sock.
Then we don't need this intermediate psock->read_sock callback, which
doesn't do anything useful.

> +	}
> +#endif
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(tcp_bpf_update_proto);