Compile external module against linux source and use functionality in linux source

Compile external module against linux source and use functionality in linux source

[Arjun Pandey]

Hi

I am trying to two things here.
1. Compile my module and export a function.
2. Use this exported function in modified kernel src and compile it.


Step 1 goes through fine. The module gets compiled fine.
However now when i try to compile the kernel i get linker error of
undefined reference to the function exported from the kernel module.

I can't find any documentation on this usecase.


Thanks
Arjun

Compile external module against linux source and use functionality in linux source

[Paul Bolle]

On Thu, 2014-09-25 at 10:49 +0530, Arjun Pandey wrote:
> However now when i try to compile the kernel i get linker error of
> undefined reference to the function exported from the kernel module.

I'm guessing you're compiling that module as, well, a module (ie,
CONFIG_EXAMPLE=m). What happens when you make it built-in
(CONFIG_EXAMPLE=y)?


Paul Bolle

Compile external module against linux source and use functionality in linux source

[Arjun Pandey]

No i am just compiling the external module and using this
functionality in existing linux source code. I am not adding any
additional modules within the tree which use this function.

In my case i am trying to use the function directly in udp_rcv
function of net/ipv4/udp.c

Regards
Arjun

On Thu, Sep 25, 2014 at 12:06 PM, Paul Bolle <pebolle@tiscali.nl> wrote:
> On Thu, 2014-09-25 at 10:49 +0530, Arjun Pandey wrote:
>> However now when i try to compile the kernel i get linker error of
>> undefined reference to the function exported from the kernel module.
>
> I'm guessing you're compiling that module as, well, a module (ie,
> CONFIG_EXAMPLE=m). What happens when you make it built-in
> (CONFIG_EXAMPLE=y)?
>
>
> Paul Bolle
>

Compile external module against linux source and use functionality in linux source

[Paul Bolle]

[Manually corrected top-posting.]

On Thu, 2014-09-25 at 12:11 +0530, Arjun Pandey wrote:
> On Thu, Sep 25, 2014 at 12:06 PM, Paul Bolle <pebolle@tiscali.nl> wrote:
> > I'm guessing you're compiling that module as, well, a module (ie,
> > CONFIG_EXAMPLE=m). What happens when you make it built-in
> > (CONFIG_EXAMPLE=y)?
> >
> No i am just compiling the external module and using this
> functionality in existing linux source code. I am not adding any
> additional modules within the tree which use this function.

So, does the linker error disappear when you build your kernel with your
new module built-in (ie, the new module is part of the kernel image)?
 
> In my case i am trying to use the function directly in udp_rcv
> function of net/ipv4/udp.c


Paul Bolle

Compile external module against linux source and use functionality in linux source

[Arjun Pandey]

My bad. I just hit the default reply button :)

On Thu, Sep 25, 2014 at 12:28 PM, Paul Bolle <pebolle@tiscali.nl> wrote:
> On Thu, 2014-09-25 at 12:24 +0530, Arjun Pandey wrote:
>> I tried moving the module to kernel tree as well. Ideally this will be
>> out of tree module. That still didn't work.
>> I have included a header file with the extern function declaration in
>> the source file that i modified.
>> That didn't help either.
>
> You excluded kernelnewbies. Is that non purpose?
>
>
> Paul Bolle
>

Compile external module against linux source and use functionality in linux source

[Paul Bolle]

On Thu, 2014-09-25 at 12:30 +0530, Arjun Pandey wrote:
> On Thu, Sep 25, 2014 at 12:28 PM, Paul Bolle <pebolle@tiscali.nl> wrote:
> > On Thu, 2014-09-25 at 12:24 +0530, Arjun Pandey wrote:
> >> I tried moving the module to kernel tree as well. Ideally this will be
> >> out of tree module. That still didn't work.
> >> I have included a header file with the extern function declaration in
> >> the source file that i modified.
> >> That didn't help either.

Building a module built-in requires a bit more: changes to a kernel tree
Kconfig file and a Makefile. There must be a lot of documentation on
this. But I would do something like
    git grep -w tristate $(git ls-files "net/*Kconfig*")

and pick a few Kconfig symbols (ie, the things with lines reading
"config EXAMPLE"). The grep the tree again for their macros:
    git grep -w "CONFIG_EXAMPLE" net/

and figure out how things all fit together.

(My guess, of course, is that your linker error disappears when you
manege to build the module built-in. Once we know that, we can look
whether it's actually possible to achieve what you want to do with an
external module.)

Hope this helps.


Paul Bolle

Compile external module against linux source and use functionality in linux source

[Arjun Pandey]

Thanks Paul. Let me try this out.

Regards
Arjun

On Thu, Sep 25, 2014 at 12:59 PM, Paul Bolle <pebolle@tiscali.nl> wrote:
> On Thu, 2014-09-25 at 12:30 +0530, Arjun Pandey wrote:
>> On Thu, Sep 25, 2014 at 12:28 PM, Paul Bolle <pebolle@tiscali.nl> wrote:
>> > On Thu, 2014-09-25 at 12:24 +0530, Arjun Pandey wrote:
>> >> I tried moving the module to kernel tree as well. Ideally this will be
>> >> out of tree module. That still didn't work.
>> >> I have included a header file with the extern function declaration in
>> >> the source file that i modified.
>> >> That didn't help either.
>
> Building a module built-in requires a bit more: changes to a kernel tree
> Kconfig file and a Makefile. There must be a lot of documentation on
> this. But I would do something like
>     git grep -w tristate $(git ls-files "net/*Kconfig*")
>
> and pick a few Kconfig symbols (ie, the things with lines reading
> "config EXAMPLE"). The grep the tree again for their macros:
>     git grep -w "CONFIG_EXAMPLE" net/
>
> and figure out how things all fit together.
>
> (My guess, of course, is that your linker error disappears when you
> manege to build the module built-in. Once we know that, we can look
> whether it's actually possible to achieve what you want to do with an
> external module.)
>
> Hope this helps.
>
>
> Paul Bolle
>

Compile external module against linux source and use functionality in linux source

[Valdis.Kletnieks at vt.edu]

On Thu, 25 Sep 2014 10:49:33 +0530, Arjun Pandey said:

> Step 1 goes through fine. The module gets compiled fine.
> However now when i try to compile the kernel i get linker error of
> undefined reference to the function exported from the kernel module.
>
> I can't find any documentation on this usecase.

In general, things that are built-into the kernel with CONFIG_FOO=y
are *not* allowed or able to reference symbols that are in a module.

This is because a module can potentially not be loaded (consider system
boot before you're up far enough to do a modprobe, as an important
special case), at which point trying to call the non-existent module
gets you an oops or panic.

So you can't find any doc on it because it's not supported.
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Compile external module against linux source and use functionality in linux source

[Arjun Pandey]

Hi Valdis

That makes sense :)
Moving the module to the kernel makes it work.

Thanks
Arjun

On Thu, Sep 25, 2014 at 5:57 PM,  <Valdis.Kletnieks@vt.edu> wrote:
> On Thu, 25 Sep 2014 10:49:33 +0530, Arjun Pandey said:
>
>> Step 1 goes through fine. The module gets compiled fine.
>> However now when i try to compile the kernel i get linker error of
>> undefined reference to the function exported from the kernel module.
>>
>> I can't find any documentation on this usecase.
>
> In general, things that are built-into the kernel with CONFIG_FOO=y
> are *not* allowed or able to reference symbols that are in a module.
>
> This is because a module can potentially not be loaded (consider system
> boot before you're up far enough to do a modprobe, as an important
> special case), at which point trying to call the non-existent module
> gets you an oops or panic.
>
> So you can't find any doc on it because it's not supported.