[PATCH] selftests/filesystems: own the idmapped_tmpfile directory by a mapped id

[Danish Khateeb <danishkhateeb03@gmail.com>]

The idmapped_tmpfile fixture creates the test directory as root and makes
it world-writable, but the test mount only maps on-disk ids
[10000, 20000), so the directory's owner has no mapping in it.
inode_permission() refuses MAY_WRITE on such an inode with -EACCES
(HAS_UNMAPPED_ID()), whatever its mode. So:

- mapped_caller_creates_and_links has failed since it was added:

    idmapped_tmpfile.c:145:mapped_caller_creates_and_links:Expected fd (-1) >= 0 (0)

- unmapped_caller_is_refused doesn't test what it describes. Without the
  fsuidgid_has_mapping() check in vfs_tmpfile(), the unmapped caller is
  still refused, with -EACCES from inode_permission(), instead of getting
  a tmpfile owned by (uid_t)-1. The test only fails on the errno.

Give the directory a mapped owner, so that both callers pass the
permission check and only the idmapping decides.

Fixes: d943e68edc5c ("selftests/filesystems: test O_TMPFILE creation on idmapped mounts")
Assisted-by: LLM
Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
---

Notes:
    Tested in QEMU x86_64 (KASAN/lockdep debug config) on next-20260925,
    whose copy of the test is identical to mainline (v7.3-rc4-446):
    
    - as is: unmapped_caller_is_refused passes,
      mapped_caller_creates_and_links fails (openat() -> EACCES)
    - with this patch: both pass
    - with 539dce114465 ("fs: refuse O_TMPFILE creation with an unmapped
      fsuid or fsgid") reverted: the original test fails only on the errno
      (EACCES instead of EOVERFLOW), the patched one fails because the
      unmapped caller gets a tmpfile

 tools/testing/selftests/filesystems/idmapped_tmpfile.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/testing/selftests/filesystems/idmapped_tmpfile.c b/tools/testing/selftests/filesystems/idmapped_tmpfile.c
index bc411ab8281e..5968ac1cd04e 100644
--- a/tools/testing/selftests/filesystems/idmapped_tmpfile.c
+++ b/tools/testing/selftests/filesystems/idmapped_tmpfile.c
@@ -91,6 +91,11 @@ FIXTURE_SETUP(idmapped_tmpfile)
 	ASSERT_EQ(mkdir(self->dir, 0777), 0);
 	/* World-writable so an unmapped caller still passes permission(). */
 	ASSERT_EQ(chmod(self->dir, 0777), 0);
+	/*
+	 * Owned by a mapped id, as inode_permission() refuses writes to an
+	 * inode whose owner has no mapping in the mount.
+	 */
+	ASSERT_EQ(chown(self->dir, MAP_HOST, MAP_HOST), 0);
 }
 
 FIXTURE_TEARDOWN(idmapped_tmpfile)

base-commit: aa98230e410f0ed212b6788c46b1e4d49e0ff7ca
-- 
2.55.0