linux-kernel - Re: help needed with EXPORT

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Message-ID: <AANLkTimtNOyJCbLxAW8Ebsftb58EWoNFW7Xkevqb6oGo@mail.gmail.com>
Date:	Mon, 23 Aug 2010 07:48:06 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	aijazbaig1@...il.com
Cc:	netfilter-devel@...r.kernel.org, jengelh@...ozas.de,
	linux-kernel@...r.kernel.org
Subject: Re: help needed with EXPORT_SYMBOL

On Mon, Aug 23, 2010 at 1:14 AM, Aijaz Baig <aijazbaig1@...il.com> wrote:
> Hello,
>
> Yeah I do understand that part, but there has to be a way to do what I
> am trying to do here isn't it?
>
> And can anyone tell me can kallsyms_lookup() be used in such a
> situation? If yes than how do I use it?
>
> Regards,
> Aijaz
>
> On Sat, Aug 21, 2010 at 2:53 PM, Jan  Engelhardt <jengelh@...ozas.de>
> wrote:
> On Saturday 2010-08-21 08:57, Aijaz Baig wrote:
>
>>So why is it that when my netfilter module is exporting something to
>>the kernel and im tryin to use it from within a core kernel file, the
>>compiler flags a 'undefined reference to' error?
>
> There, you answered yourself. Exports only go one way. And that is
> core -> module -> module.
>
> On Sat, 2010-08-21 at 12:27 +0530, Aijaz Baig wrote:
>> Hello,
>>
>> Ive been trying to EXPORT a SYMBOL from a netfilter module (actually a
>> hook) to the KERNEL. Various modules seem to export various symbols to
>> the kernel (or so it seems), an example being x_tables which exports a
>> whole lot of functions like xt_register_target and so on and all other
>> netfilter modules which use these functions dont seem to have any
>> problems.
>>
>> So why is it that when my netfilter module is exporting something to the
>> kernel and im tryin to use it from within a core kernel file, the
>> compiler flags a 'undefined reference to' error?
>>
>> I googled and came to the conclusion from the kernelnewbies mailing list
>> that using kallsyms_lookup() seems to be the answer or is it?
>> So, if I do intend to use kallsyms_lookup how do I use it? There aren't
>> so many instances of it being used it seems.
>>
>> Ive seen it being used in dev/core.c like so:
>> symname = kallsyms_lookup((unsigned long)sym, &symsize,&offset,&modname,
>> namebuf);
>>
>> I am basically trying to call a function that MY module EXPORTs inside
>> one of the core linux routines. This would also mean that my netfilter
>> module must be compiled (and linked also maybe?..not so sound with link
>> time stuff) before the address becomes visible so that kallsyms_lookup
>> can find it. So do I really shd be using it and if so, how?
>>
>> Any input is appreciated.
>>
>> Regards,
>> Aijaz Baig.
>>
>>

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.

--
Brian Gerst
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