RE: [Patch bpf 2/3] net: poll psock queues too for sockmap sockets

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

Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> Yucong noticed we can't poll() sockets in sockmap even
> when they are the destination sockets of redirections.
> This is because we never poll any psock queues in ->poll().
> We can not overwrite ->poll() as it is in struct proto_ops,
> not in struct proto.
> 
> So introduce sk_msg_poll() to poll psock ingress_msg queue
> and let sockets which support sockmap invoke it directly.
> 
> Reported-by: Yucong Sun <sunyucong@gmail.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Jakub Sitnicki <jakub@cloudflare.com>
> Cc: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> ---
>  include/linux/skmsg.h |  6 ++++++
>  net/core/skmsg.c      | 15 +++++++++++++++
>  net/ipv4/tcp.c        |  2 ++
>  net/ipv4/udp.c        |  2 ++
>  net/unix/af_unix.c    |  5 +++++
>  5 files changed, 30 insertions(+)
> 

[...]
  						  struct sk_buff *skb)
>  {
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index e8b48df73c85..2eb1a87ba056 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -280,6 +280,7 @@
>  #include <linux/uaccess.h>
>  #include <asm/ioctls.h>
>  #include <net/busy_poll.h>
> +#include <linux/skmsg.h>
>  
>  /* Track pending CMSGs. */
>  enum {
> @@ -563,6 +564,7 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
>  
>  		if (tcp_stream_is_readable(sk, target))
>  			mask |= EPOLLIN | EPOLLRDNORM;
> +		mask |= sk_msg_poll(sk);
>  
>  		if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {
>  			if (__sk_stream_is_writeable(sk, 1)) {


For TCP we implement the stream_memory_read() hook which we implement in
tcp_bpf.c with tcp_bpf_stream_read. This just checks psock->ingress_msg
list which should cover any redirect from skmsg into the ingress side
of another socket.

And the tcp_poll logic is using tcp_stream_is_readable() which is 
checking for sk->sk_prot->stream_memory_read() and then calling it.

The straight receive path, e.g. not redirected from a sender should
be covered by the normal tcp_epollin_ready() checks because this
would be after TCP does the normal updates to rcv_nxt, copied_seq,
etc.

So above is not in the TCP case by my reading. Did I miss a
case? We also have done tests with Envoy which I thought were polling
so I'll check on that as well.

Thanks,
John