Hello,

the kernel build system (kbuild) allows to compile modules whose sources
live outside the kernel source tree by setting SUBDIRS as follows (from
the module sources's directory):

  $ make -C /lib/modules/$(uname -r)/build modules SUBDIRS=$(pwd)

people use this mechanism to compile kernel modules that are missing in
their kernel sources, or have been updated/fixed in the meantime.

Currently there are a number of deficiencies for building such modules:

  (1) symbol versioning (MODVERSIONS) is not properly supported, and

  (2) only the `modules' make target is supported.

Problem (1) did not exist in the 2.4 kernel series. The current
situation has lead authors of several external modules to implement some
of the kbuild functionality in their own makefiles, which can only lead
to problems later on.

During the kernel compile, a list of all files that export symbols is
created in .tmp_versions/ of the kernel source tree. After everything is
compiled, the module versions of all these object files are resolved
with modpost. When an external module is compiled, it ends up being the
only file in .tmp_versions/, and so symbols in other modules would not
be found.

An additional problem is that resolving symbol versions requires all
module files plus vmlinux to be in the kernel source tree, so modules
cannot be build against a clean kernel source tree.

I propose the attached patch: It adds symbol version dump/load
functionality to modpost: When compiling the kernel, the module versions
are dumped to a file. When compiling external modules, the dumped
symbols are first loaded. Also, to allow a read-only kernel source tree
and not interfere with the kernel sources, the .tmp_versions/ directory
is placed in the external module's directory.

Furthermore, the patch adds clean/distclean/mrproper make targets with
reasonable semantics for external modules, and also adds a modules_add
target that installs the external modules into
/lib/modules/$(KERNELRELEASE). (The modules_install target could be
used, but it has different semantics, so I have used a different name
instead.)


There also is one fix for using tempfiles instead of writing into the
kernel source tree. A few problems with compiling against a read-only
kernel source tree seem to remain.


Regards,
-- 
Andreas Gruenbacher <agruen@suse.de>
SUSE Labs, SUSE LINUX AG