RE: Listening on a TCP socket from a Kernel Module

[David Laight <David.Laight@ACULAB.COM>]

From: Neal Cardwell
> Sent: 17 May 2021 02:55
> 
> On Sun, May 16, 2021 at 7:20 PM louisrossberg
> <louisrossberg@protonmail.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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