[Buildroot] [Bug 16075] New: Cannot use linux kernel extensions from external tree (building zfs into kernel)

[bugzilla@busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=16075

            Bug ID: 16075
           Summary: Cannot use linux kernel extensions from external tree
                    (building zfs into kernel)
           Product: buildroot
           Version: 2024.02.1
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Other
          Assignee: unassigned@buildroot.uclibc.org
          Reporter: mscdex@mscdex.net
                CC: buildroot@uclibc.org
  Target Milestone: ---

I'm wanting to build zfs into the kernel, so after reading through the
documentation it seemed like the best way to go about it would be through
Buildroot's Linux kernel extension mechanism.

I already have an existing, working external tree, so according to the
documentation I added a `linux/Config.ext.in` and
`linux/linux-ext-zfs-inkernel.mk` to my external tree.

Re-running `make menuconfig` and navigating to the Linux Extensions section
shows both the built-in Buildroot extensions and my new extension. However,
after I select my extension and save the Buildroot configuration, just about
every make target except `menuconfig` fails with:

linux/linux.mk:646: *** Package 'linux' defined a second time in 'linux/';
previous definition was in '/home/foo/buildroot-external/linux/'.  Stop.
make: *** [Makefile:23: _all] Error 2

I tried explicitly adding a reference to the Config.ext.in in my external
tree's Config.in and adding a reference to the linux-ext-zfs-inkernel.mk in my
external tree's external.mk, but that did not make any difference.

For what it's worth here are the contents of the two files (there may be issues
with them as I obviously haven't been able to test them yet):

linux/Config.ext.in:
  config BR2_LINUX_KERNEL_EXT_ZFS_INKERNEL
      bool "In-kernel ZFS"
      help
        In-kernel ZFS module

linux/linux-ext-zfs-inkernel.mk:
  ZFS_INKERNEL_VERSION = 2.2.2
  ZFS_INKERNEL_SOURCE =
https://github.com/openzfs/zfs/releases/download/zfs-$(ZFS_INKERNEL_VERSION)
  ZFS_INKERNEL_INSTALL_STAGING = NO
  ZFS_INKERNEL_INSTALL_TARGET = NO
  define ZFS_INKERNEL_COPY_BUILTIN
    cd $(ZFS_INKERNEL_BUILDDIR) && ./copy-builtin $(LINUX_DIR)
  endef
  ZFS_INKERNEL_POST_CONFIGURE_HOOKS += ZFS_INKERNEL_COPY_BUILTIN
  LINUX_EXTENSIONS += zfs-inkernel
  ZFS_INKERNEL_CONF_OPTS = \
      --with-linux=$(LINUX_DIR) \
      --enable-linux-builtin
  ZFS_INKERNEL_MAKE = true
  $(eval $(autotools-package))

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