On Thu, Nov 19, 2020 at 12:50 AM David Gow [off-list ref] wrote:

If BPF_PRELOAD is enabled, and an out-of-tree build is requested with
make O=<path>, compilation seems to fail with:

tools/scripts/Makefile.include:4: *** O=.kunit does not exist.  Stop.
make[4]: *** [../kernel/bpf/preload/Makefile:8: kernel/bpf/preload/libbpf.a] Error 2
make[3]: *** [../scripts/Makefile.build:500: kernel/bpf/preload] Error 2
make[2]: *** [../scripts/Makefile.build:500: kernel/bpf] Error 2
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [.../Makefile:1799: kernel] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:185: __sub-make] Error 2

By the looks of things, this is because the (relative path) O= passed on
the command line is being passed to the libbpf Makefile, which then
can't find the directory. Given OUTPUT= is being passed anyway, we can
work around this by explicitly setting an empty O=, which will be
ignored in favour of OUTPUT= in tools/scripts/Makefile.include.

Signed-off-by: David Gow <redacted>

Seems sensible to me. I have no strong feeling as to whether we just
turn this off on UML or whether we do the fix you proposed here
though. Nevertheless, I would like to see *some* fix go in before
v5.10 is released.

Reviewed-by: Brendan Higgins <redacted>