[PATCH 2/2] selftests/filesystems/openat2: make /tmp a mountpoint inside private namespace

[Disha Goel <disgoel@linux.ibm.com>]

In resolve_test.c, the test unshares the mount namespace and attempts to
make /tmp private via:
    mount("", "/tmp", "", MS_PRIVATE, "")

This fails with EINVAL on environments where /tmp is not already a
separate mount point (e.g. if /tmp is just a standard directory on the
root filesystem /). Furthermore, the resolve_no_xdev test relies on
crossing into /tmp to verify RESOLVE_NO_XDEV and trigger an -EXDEV
error. If /tmp is not a mount point, resolving /tmp never crosses a
mount boundary, leading to a test failure.

Fix this by:
1. Making the entire root directory recursively private (MS_PRIVATE |
   MS_REC) after unsharing the mount namespace.
2. Performing a self-bind-mount of /tmp on /tmp inside the isolated
   private mount namespace, which guarantees that /tmp is a separate
   mount point and that resolving /tmp will cross a mount boundary as
   expected by the test.

Signed-off-by: Disha Goel <disgoel@linux.ibm.com>
---
 tools/testing/selftests/filesystems/openat2/resolve_test.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/filesystems/openat2/resolve_test.c b/tools/testing/selftests/filesystems/openat2/resolve_test.c
index eacde59ce158..6216a8546388 100644
--- a/tools/testing/selftests/filesystems/openat2/resolve_test.c
+++ b/tools/testing/selftests/filesystems/openat2/resolve_test.c
@@ -140,9 +140,12 @@ FIXTURE_SETUP(openat2_resolve)
 	if (!openat2_supported)
 		SKIP(return, "openat2(2) not supported");
 
-	/* Unshare and make /tmp a new directory. */
+	/* Unshare and make the mount tree private. */
 	ASSERT_EQ(unshare(CLONE_NEWNS), 0);
-	ASSERT_EQ(mount("", "/tmp", "", MS_PRIVATE, ""), 0);
+	ASSERT_EQ(mount("", "/", "", MS_PRIVATE | MS_REC, ""), 0);
+
+	/* Ensure /tmp is a mountpoint for RESOLVE_NO_XDEV test crossing into /tmp. */
+	ASSERT_EQ(mount("/tmp", "/tmp", NULL, MS_BIND, NULL), 0);
 
 	/* Make the top-level directory. */
 	ASSERT_NE(mkdtemp(dirname), NULL);
-- 
2.45.1