linux-kernel - RE: Listening on a TCP socket from a Kernel Module

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Date:   Mon, 17 May 2021 12:50:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Neal Cardwell' <ncardwell@...gle.com>,
        louisrossberg <louisrossberg@...tonmail.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: RE: Listening on a TCP socket from a Kernel Module

From: Neal Cardwell
> Sent: 17 May 2021 02:55
> 
> On Sun, May 16, 2021 at 7:20 PM louisrossberg
> <louisrossberg@...tonmail.com> wrote:
> >
> > Hello, can somebody point me in the right direction for
> > what I would use to listen on a TCP socket from the
> > kernel? I am working on a kernel module and have spent the
> > past day looking through include/net and include/linux for
> > something that would allow me to do so. I know TCP
> > listening is typically done in userspace, but it should be
> > possible at the kernel level right?  tcp_diag looks
> > promising, but it seems like that is mainly for monitoring
> > sockets, and I'm not sure if I would be able to provide
> > responses from it.
> >
> > Louis Rossberg,
> > Warped Technologies
> 
> Perhaps kernel_listen() and related functions (kernel_bind(),
> kernel_accept(), etc.) in net/socket.c might do the trick for your use
> case? Looking at how the callers of these functions structure their
> code might give you enough to go on.

They should work.
There are a couple of issues though:
- There is no getsockopt() support in current kernels.
- You may need to use __sock_create() rather than sock_create_kern()
  in order to hold a reference to the network namespace.
It may be possible to use the wakeup callbacks that select/poll use.
But it is probably safer to use blocking operations from a separate
kernel thread.

	David

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