Question

I have two kernel modules (say modA and modB). modA exports a symbol with EXPORT_SYMBOL(symA) and modB uses it. I have the header modA.h for modA:

...
extern void symA(int param);
...

and in modB.c:

#include "modA.h"
...
static int __init modB_init(void)
{
    symA(10);
}
...

If i insmod modB all works fine, my modB is correctly linked in the kernel and the function symA is correctly called. However when i build modB the compiler raises a warning: symA is undefined. An LKM is an ELF relocatable so why the compiler raises this warning? How can be this removed?

Answer 1

This issue (and how to compile correctly in this case) is explained in http://www.kernel.org/doc/Documentation/kbuild/modules.txt

Sometimes, an external module uses exported symbols from another
external module. kbuild needs to have full knowledge of all symbols
to avoid spitting out warnings about undefined symbols. Three
solutions exist for this situation.

NOTE: The method with a top-level kbuild file is recommended but may
be impractical in certain situations.

Use a top-level kbuild file If you have two modules, foo.ko and
bar.ko, where foo.ko needs symbols from bar.ko, you can use a
common top-level kbuild file so both modules are compiled in the
same build. Consider the following directory layout:

  ./foo/ <= contains foo.ko       ./bar/ <= contains bar.ko

  The top-level kbuild file would then look like:

  #./Kbuild (or ./Makefile):          obj-y := foo/ bar/

  And executing

      $ make -C $KDIR M=$PWD

  will then do the expected and compile both modules with         full

knowledge of symbols from either module.

Use an extra Module.symvers file When an external module is built,
a Module.symvers file is generated containing all exported symbols
which are not defined in the kernel. To get access to symbols from
bar.ko, copy the Module.symvers file from the compilation of bar.ko
to the directory where foo.ko is built. During the module build,
kbuild will read the Module.symvers file in the directory of the
external module, and when the build is finished, a new
Module.symvers file is created containing the sum of all symbols
defined and not part of the kernel.

Use “make” variable KBUILD_EXTRA_SYMBOLS If it is impractical to
copy Module.symvers from another module, you can assign a space
separated list of files to KBUILD_EXTRA_SYMBOLS in your build file.
These files will be loaded by modpost during the initialization of
its symbol tables.