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

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Message-ID: <be02a7af-5a00-fef7-2132-0199fad6ba7a@polimi.it>
Date: Fri, 22 Sep 2023 19:32:55 +0200
From: Farbod Shahinfar <farbod.shahinfar@...imi.it>
To: john.fastabend@...il.com, netdev@...r.kernel.org
Subject: question about BPF sk_skb_stream_verdict redirect and poll/epoll
 events

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.

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.

Sincerely,
Farbod