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SubjectRe: Fwd: help needed with EXPORT_SYMBOL
Hi everyone

I am trying to understand if its possible to add functions dynamically
to the kernel source based on the presence of certain modules in the
running kernel image.

I did try what brian suggested with the function pointer and yeah it
does work. But I could not understand what peter was trying to say about
modular users since I suppose he mentioned one module (B in this case)
using a function pointer defined in (or by) module A. In my case, since
it is the kernel that is gonna use the function, I need to make sure
that the module doesn't get unloaded while we are using the function.

By the way Jan I couldnt find bridge.c although there is an object file
called bridge.o within /net/bridge. Nonetheless, I did check
nf_nat_ftp.c as an example to see if something similar is being done
there too. What I did find out is that nf_nat_ftp_hook is rcu assigned
in nf_nat_ftp.c however this function pointer is being used with
nf_contrack_ftp.c (im talkin here). In this case, it still is
module1 <- module2 and not kernel <- module. Could you please elaborate
a little how it affects the latter case. Or perhaps u were commenting on
the former case since peter brought that to our notice.

thanks again for your replies and keen to hear your input
Aijaz Baig

> ---------- Forwarded message ----------
> From: Brian Gerst <>
> Date: Tue, Aug 24, 2010 at 4:46 PM
> Subject: Re: help needed with EXPORT_SYMBOL
> To:
> On Tue, Aug 24, 2010 at 12:44 AM, Aijaz Baig <>
> wrote:
> > Hello everyone,
> >
> > thanks for sending in the replies with so many options. I am still a
> > relative newbie so please do bear with me guys if I sound a bit
> naive at
> > times.
> >
> > I'll first start with brian's suggestion. He said:
> > "Use an exported function pointer in the main kernel as a hook that
> the
> > module sets when it is loaded. Note, you must use module_get and
> > module_put around the call to the module to prevent it from
> unloading
> > while in use."
> >
> > I have never used such a technique and do not understand what you
> mean.
> > Could you please elaborate a little. Do you mean I should NF_HOOK
> within
> > the kernel code and then make the function that I would like to
> execute
> > as the hookfn? Well..if this is what u meant then wouldn't it also
> cause
> > all hooks at that particular position to be executed as well?
> >
> > And then peter and finally jan came up with, perhaps better
> suggestions
> > (i really cannot judge the quality of any of these suggestions..m
> too
> > much of a noob to do that). I got confused in the XY explanation by
> jan
> > since there both X and Y are modules it seems and I was talking
> about a
> > module exporting a certain symbol to be used by the kernel. Peter
> did
> > suggest notification chains. Is there a simple example that I can
> look
> > at? Will it allow the kernel to see symbols defined within the
> module?
> >
> > Lastly, will I be able to use kallsyms_lookup() to do what I can? I
> > notice that it replaced EXPORT_SYMTAB sometime back. So if yes I can
> use
> > it, does anyone know how to fill in the very first parameter of this
> > function which says address (of what exactly and how am I supposed
> to
> > know this address?)
> >
> > Keen to hear from you and once again, thank you for your inputs,
> You still haven't explained why you want to lookup a symbol in a
> module from the main kernel. There is probably a better way of doing
> it.
> --
> Brian Gerst

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