This follow-up patch completes centralization of kselftest.h and
ksefltest_harness.h includes in remaining seltests files, replacing all
relative paths with a non-relative paths using shared -I include path in
lib.mk

Tested with gcc-13.3 and clang-18.1, and cross-compiled successfully on
riscv, arm64, x86_64 and powerpc arch.

[reddybalavignesh9979@gmail.com: add selftests include path for kselftest.h]
  Link: https://lkml.kernel.org/r/20251017090201.317521-1-reddybalavignesh9979@gmail.com
Link: https://lkml.kernel.org/r/20251016104409.68985-1-reddybalavignesh9979@gmail.com
Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/lkml/20250820143954.33d95635e504e94df01930d0@linux-foundation.org/
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Günther Noack <gnoack@google.com>
Cc: Jakub Kacinski <kuba@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mickael Salaun <mic@digikod.net>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Simon Horman <horms@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

This is not an overlayfs specific header.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/20250509133240.529330-2-amir73il@gmail.com
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

  CC       mount_setattr_test
In file included from mount_setattr_test.c:24:
mount_setattr_test.c: In function ‘mount_setattr_mount_detached_mount_on_detached_mount_and_attach’:
mount_setattr_test.c:1850:60: error: ‘STATX_MNT_ID_UNIQUE’ undeclared (first use in this function); did you mean ‘STATX_MNT_ID’?
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |                                                            ^~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:757:20: note: in definition of macro ‘__EXPECT’
  757 |         __typeof__(_expected) __exp = (_expected); \
      |                    ^~~~~~~~~
mount_setattr_test.c:1850:9: note: in expansion of macro ‘ASSERT_EQ’
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |         ^~~~~~~~~
mount_setattr_test.c:1850:60: note: each undeclared identifier is reported only once for each function it appears in
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |                                                            ^~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:757:20: note: in definition of macro ‘__EXPECT’
  757 |         __typeof__(_expected) __exp = (_expected); \
      |                    ^~~~~~~~~
mount_setattr_test.c:1850:9: note: in expansion of macro ‘ASSERT_EQ’
 1850 |         ASSERT_EQ(statx(fd_tree_subdir, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &stx), 0);
      |         ^~~~~~~~~

Signed-off-by: Christian Brauner <brauner@kernel.org>

Pull vfs overlayfs updates from Christian Brauner:
 "Currently overlayfs uses the mounter's credentials for its
  override_creds() calls. That provides a consistent permission model.

  This patches allows a caller to instruct overlayfs to use its
  credentials instead. The caller must be located in the same user
  namespace hierarchy as the user namespace the overlayfs instance will
  be mounted in. This provides a consistent and simple security model.

  With this it is possible to e.g., mount an overlayfs instance where
  the mounter must have CAP_SYS_ADMIN but the credentials used for
  override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage
  of custom fs{g,u}id different from the callers and other tweaks"

* tag 'vfs-6.15-rc1.overlayfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  selftests/ovl: add third selftest for "override_creds"
  selftests/ovl: add second selftest for "override_creds"
  selftests/filesystems: add utils.{c,h}
  selftests/ovl: add first selftest for "override_creds"
  ovl: allow to specify override credentials

Add a simple test to verify that the new "override_creds" option works.

Link: https://lore.kernel.org/r/20250219-work-overlayfs-v3-4-46af55e4ceda@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Add a simple test to verify that the new "override_creds" option works.

Link: https://lore.kernel.org/r/20250219-work-overlayfs-v3-3-46af55e4ceda@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Add a new set of helpers that will be used in follow-up patches.

Signed-off-by: Christian Brauner <brauner@kernel.org>

Add a simple test to verify that the new "override_creds" option works.

Link: https://lore.kernel.org/r/20250219-work-overlayfs-v3-2-46af55e4ceda@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Test that it is possible to use detached mounts as overlayfs layers.

Link: https://lore.kernel.org/r/20250123-erstbesteigung-angeeignet-1d30e64b7df2@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

Verify that userspace can specify layers via O_PATH file descriptors.

Link: https://lore.kernel.org/r/20250210-work-overlayfs-v2-2-ed2a949b674b@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>