Build of several selftests fail if separate output directory is
specified by the following methods:
1) make -C tools/testing/selftests O=<build_dir>
2) export KBUILD_OUTPUT="build_dir"; make -C tools/testing/selftests

Build fails because of several reasons:
1) The kernel headers aren't found.
2) The path of output objects is wrong and hence unaccessible.

These problems can be solved by:
1) Including the correct path of uapi header files
2) By setting the BUILD variable correctly inside Makefile

Following different build scnerios have been tested after making these
changes:
make -C tools/testing/selftests
make -C tools/testing/selftests O=build
make -C tools/testing/selftests o=/opt/build
export KBUILD_OUTPUT="/opt/build"; make -C tools/testing/selftests
export KBUILD_OUTPUT="build"; make -C tools/testing/selftests
cd <any_dir>; make -C <src_path>/tools/testing/selftests
cd <any_dir>; make -C <src_path>/tools/testing/selftests O=build

Muhammad Usama Anjum (10):
  selftests: set the BUILD variable to absolute path
  selftests: Add and export a kernel uapi headers path
  selftests: Correct the headers install path
  selftests: futex: Add the uapi headers include variable
  selftests: kvm: Add the uapi headers include variable
  selftests: landlock: Add the uapi headers include variable
  selftests: net: Add the uapi headers include variable
  selftests: mptcp: Add the uapi headers include variable
  selftests: vm: Add the uapi headers include variable
  selftests: vm: remove dependecy from internal kernel macros

 tools/testing/selftests/Makefile              | 32 +++++++++++++------
 .../selftests/futex/functional/Makefile       |  5 ++-
 tools/testing/selftests/kvm/Makefile          |  6 ++--
 tools/testing/selftests/landlock/Makefile     | 11 ++-----
 tools/testing/selftests/net/Makefile          |  2 +-
 tools/testing/selftests/net/mptcp/Makefile    |  3 +-
 tools/testing/selftests/vm/Makefile           |  2 +-
 tools/testing/selftests/vm/userfaultfd.c      |  3 ++
 8 files changed, 35 insertions(+), 29 deletions(-)

-- 
2.30.2