netdev - Re: question about BPF sk_skb_stream_verdict redirect and poll/epoll events

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Message-ID: <3fd39981aba0e0aa8ced76398c2ea8ad85208f7d.camel@inf.elte.hu>
Date: Sat, 23 Sep 2023 11:36:21 +0200
From: Ferenc Fejes <fejes@....elte.hu>
To: Farbod Shahinfar <farbod.shahinfar@...imi.it>, john.fastabend@...il.com,
  netdev@...r.kernel.org
Subject: Re: question about BPF sk_skb_stream_verdict redirect and
 poll/epoll events

Hi!

On Fri, 2023-09-22 at 19:32 +0200, Farbod Shahinfar wrote:
> Hello,
> 
> I am doing a simple experiment in which I send a message to a TCP
> server 
> and the server echoes the message. I am attaching a BPF sk_skb 
> stream_verdict program to the server socket to redirect the message
> back 
> to the client (redirects the SKB on the same socket but to the TX 
> queue). In my test, I noticed that the user-space server, which is
> using 
> the poll system call, is woken up, and when it reads the socket, 
> receives zero as the number of bytes read.

Do you poll for POLLIN events?

> 
> I first want to ask if this is the intended behavior.
> In case this is the intended behavior, my second question is, what 
> should I do to prevent the user program from waking up? I hope by not
> waking up the user program I could better use the available
> resources.

AFAIK sockets in sockmap are propagates every events to poll (fixme),
but with pollfd::events you can specify the ones makes sense to you
e.g.: POLLERR, POLLHUP, POLLRDHUP, etc.
> 
> Sincerely,
> Farbod

Ferenc