Question

I’m writing an API as a kernel module that provides device drivers with various functions. I wrote three functions in mycode.c. I then built and loaded the module, then copied mycode.h into < kernel >/include/linux. In a device driver, I have a #include < linux/mycode.h > and call those three functions. But when I build the driver module, I get three linker warnings saying that those functions are undefined.

Notes:

• The functions are declared extern in mycode.h
• The functions are exported using EXPORT_SYMBOL(func_name) in mycode.c
• Running the command nm mycode.ko shows all three functions as being available in the symbol table (capital T next to them, meaning the symbols are found in the text (code) section)
• After loading the module, the command grep func_name /proc/kallsyms shows all three functions as being loaded

So clearly the functions are being exported correctly and the kernel knows what and where they are. So why can’t the driver see their definitions? Any idea what am I missing?

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EDIT: I found some information about this here: 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.

But with all three of these solutions, in order for any driver to use my API, it would have to either create a new Makefile or have direct access to my Module.symvers file? That seems a bit inconvenient. I was hoping they’d just be able to #include my header file and be good to go. Do no other alternatives exist?

Answer 1

From my research, it seems that those are the only three ways to handle this situation, and I’ve gotten each of them to work, so I think I’ll just pick my favorite out of those.