selftests/filesystems/openat2: make /tmp a mountpoint inside private namespace

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>
Link: https://patch.msgid.link/20260917095707.16986-2-disgoel@linux.ibm.com
Tested-by: andy_gao <andy23320@yahoo.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions