kernel libraries and dead code in MODULAR kernels

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

I've been compiling the same kernel configuration with and without
options MODULAR and noticed that the MODULAR kernels have a not
inconsiderable amount of code that is called neither by the kernel
nor any of the modules.  I've appended a list of the global symbols
from libkern.[ao] that end up being included in amd64 mostly-GENERIC
modular kernels that have no callers and are omitted from non-modular
kernel builds.

The reason for this is that while non-modular kernels build a libkern.a
from which only code that is actually called by something is loaded into
the kernel, modular kernels instead build a libkern.o from which everything
is unconditionally included.  While I understand why this is done (everything
is included in case the separately-compiled modules need it) it causes
a difficult problem if you would like to have a modular kernel without
the dead code.  A few of those functions are in fact no longer used and
could just be eliminated from the build, but the rest seem to end up
there either because their use depends on configuration options which may
or may not be defined, or they are members of a library that is also used
in user space that has a richer API there than the kernel makes use of, or
the use of particular functions (e.g. in the runtime library) is dependent
on the architecture being built.  While it would be possible to go through
these and subset what is built for each kernel by options and architecture
this is a whole lot of work, and a maintenance headache, compared to just
compiling a few extra files and letting the loader figure out what the
kernel it is linking actually wants.

The particular reason I'm concerned about this is that I am now loading
the kernel against additional libraries with APIs the kernel won't make
full use of and don't want to make the dead code issue worse, but I've
found that maintaining the subsets the kernel does use to be quite
painful and annoying, particularly for one library where the subset
depends on the architecture being built.  I wouldn't mind going through
at the end and subsetting the files which are never going to be used out
of the kernel build to save a bit of kernel compile time, but for things
that are option- or architecture-dependent this is way too complex to be
worth the modest savings in build time.  There should be a better way to
keep the kernel free of dead code.

I do notice that for modular kernels the modules are built before the kernel
is.  Is there no way to harvest the undefined global symbols from the modules
and include these to force the kernel link to resolve them?  Then libkern
could go back to being libkern.a, with the bonus that using something in a
module that the kernel doesn't have would cause a complaint when the kernel
is loaded.

Dennis Ferguson

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