[Bug 275741] sys/modules: Fix processing of WITHOUT_MODULES

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275741

            Bug ID: 275741
           Summary: sys/modules: Fix processing of WITHOUT_MODULES
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: freebsd@kumba.dev

The handling of WITHOUT_MODULES in the FreeBSD kernel build system is currently
suboptimal.  At present, it is only possible to exclude kmods at the top-level
of /usr/src/sys/modules from being built with this variable.  If a kmod is in a
subdirectory, then it is built regardless if it is listed in WITHOUT_MODULES. 
I have tried several forms of specifying subdirectory kmods in this variable to
avoid building them, but none of them work.

The crux of the issue is because the only processing of this variable is done
in /usr/src/sys/modules/Makefile, and it is a simple loop that iterates over
WITHOUT_MODULES and removes any entries from the SUBDIR variable *only* in that
top-level Makefile.  For kmods in subdirectories that have their own Makefiles
that define SUBDIR, no processing of WITHOUT_MODULES takes place, so they will
always be built.

A previous attempt to tackle this issue was found in PR#76225, and the last
comment says this was all fixed back in Jan 2005, however, this fix may have
been undone at some point, as the current Makefile logic is not capable of
excluding kmods in lower-level subdirectories from being built.

The issue was again highlighted by PR#210143, but the reporter did not provide
a recommended fix or any patches, and the reporter eventually self-closed the
PR due to feedback timeout (by developers?).

I have made an effort to fix this in the attached patch by taking several
steps:

1. Move the WITHOUT_MODULES for loop to a new mk file in /usr/src/sys/conf and
call it "kmod.without.mk".  The loop is expanded to process kmod names as
either a top-level name under /usr/src/sys/modules, or as a path relative to
that directory.

2. Include this new mk file in every Makefile that defines a SUBDIR variable in
/usr/src/sys/modules.

3. Update the make.conf(5) manpage to specify the way kmods can be excluded
from building via this variable.

I've made the patch against -CURRENT (git HEAD), but I've only tested against
one of my 14.0-RELEASE systems, as I don't actively run -CURRENT on any machine
at present.  My systems use a custom kernel config that inherits from GENERIC,
and install to /boot/kernel.custom.  After fixing the processing of
WITHOUT_MODULES, I've been able to knock the size of /boot/kernel.custom down
to ~47MB, and I can probably get it smaller once I deep dive whether that
particular system needs any of the i2c or other ancillary bus drivers.

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