RE: [Patch bpf-next v3 1/4] tcp: introduce tcp_read_skb()

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> This patch inroduces tcp_read_skb() based on tcp_read_sock(),
> a preparation for the next patch which actually introduces
> a new sock ops.
> 
> TCP is special here, because it has tcp_read_sock() which is
> mainly used by splice(). tcp_read_sock() supports partial read
> and arbitrary offset, neither of them is needed for sockmap.
> 
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Jakub Sitnicki <jakub@cloudflare.com>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> ---
>  include/net/tcp.h |  2 ++
>  net/ipv4/tcp.c    | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 1e99f5c61f84..878544d0f8f9 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -669,6 +669,8 @@ void tcp_get_info(struct sock *, struct tcp_info *);
>  /* Read 'sendfile()'-style from a TCP socket */
>  int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
>  		  sk_read_actor_t recv_actor);
> +int tcp_read_skb(struct sock *sk, read_descriptor_t *desc,
> +		 sk_read_actor_t recv_actor);
>  
>  void tcp_initialize_rcv_mss(struct sock *sk);
>  
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 9984d23a7f3e..a18e9ababf54 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1709,6 +1709,53 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
>  }
>  EXPORT_SYMBOL(tcp_read_sock);
>  
> +int tcp_read_skb(struct sock *sk, read_descriptor_t *desc,
> +		 sk_read_actor_t recv_actor)
> +{
> +	struct tcp_sock *tp = tcp_sk(sk);
> +	u32 seq = tp->copied_seq;
> +	struct sk_buff *skb;
> +	int copied = 0;
> +	u32 offset;
> +
> +	if (sk->sk_state == TCP_LISTEN)
> +		return -ENOTCONN;
> +
> +	while ((skb = tcp_recv_skb(sk, seq, &offset)) != NULL) {
> +		int used;
> +
> +		__skb_unlink(skb, &sk->sk_receive_queue);
> +		used = recv_actor(desc, skb, 0, skb->len);
> +		if (used <= 0) {
> +			if (!copied)
> +				copied = used;
> +			break;
> +		}
> +		seq += used;
> +		copied += used;
> +
> +		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
> +			kfree_skb(skb);

Hi Cong, can you elaborate here from v2 comment.

"Hm, it is tricky here, we use the skb refcount after this patchset, so
it could be a real drop from another kfree_skb() in net/core/skmsg.c
which initiates the drop."

The tcp_read_sock() hook is using tcp_eat_recv_skb(). Are we going
to kick tracing infra even on good cases with kfree_skb()? In
sk_psock_verdict_recv() we do an skb_clone() there.

.John