netdev - Re: [PATCH V4 net-next 5/5] icmp: add response to RFC 8335 PROBE messages

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Date:   Sat, 20 Mar 2021 11:01:20 -0500
From:   Andreas Roeseler <andreas.a.roeseler@...il.com>
To:     David Ahern <dsahern@...il.com>, David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, yoshfuji@...ux-ipv6.org, kuba@...nel.org,
        dsahern@...nel.org
Subject: Re: [PATCH V4 net-next 5/5] icmp: add response to RFC 8335 PROBE
 messages

On Wed, 2021-03-17 at 21:24 -0600, David Ahern wrote:
> On 3/17/21 9:19 PM, David Miller wrote:
> > From: Andreas Roeseler <andreas.a.roeseler@...il.com>
> > Date: Wed, 17 Mar 2021 22:11:47 -0500
> > 
> > > On Mon, 2021-03-15 at 04:35 +0800, kernel test robot wrote:
> > > Is there something that I'm not understanding about compiling
> > > kernel
> > > components modularly? How do I avoid this error?
> > 
> > > 
> > You cannot reference module exported symbols from statically linked
> > code.
> > y
> > 
> 
> Look at ipv6_stub to see how it exports IPv6 functions for v4 code.
> There are a few examples under net/ipv4.

Thanks for the advice. I've been able to make some progress but I still
have some questions that I have been unable to find online.

What steps are required to include a function into the ipv6_stub
struct? I've added the declaration of the function to the struct, but
when I attempt to call it using <ipv6_stub->ipv6_dev_find()> the kernel
locks up. Additionally, a typo in the declaration isn't flagged during
compilation. Are there other places where I need to edit the ipv6_stub
struct or include various headers? The examples I have looked at are
<fib_semantics.c>, <nexthop.c>, and <udp.c> in the <net/ipv4> folder
and they don't seem to do anything on the caller side of ipv6_stub, so
I think I am not adding the function to ipv6_stub properly. I have been
able to call other functions that currently exist in ipv6_stub, but not
the one I  am attempting to add, so am I missing a step?

I've noticed that some functions such as <ipv6_route_input> aren't
exported using EXPORT_SYMBOL when it is defined in
<net/ipv6/af_inet6.c>, but it is still loaded into ipv6_stub. How can
this be? Is there a different way to include symbols into ipv6_stub
based on whether or not they are explicitly exported using
EXPORT_SYMBOL?