Merge patch series "fs: don't warn when a mount is completed from another user namespace"

Merge patch series "fs: don't warn when a mount is completed from another user namespace"

Christian Brauner <brauner@kernel.org> says:

fsopen() records the caller's user namespace in fc->user_ns and hands back
an ordinary file descriptor. The task that calls fsconfig(CMD_CREATE)
doesn't have to be the one that created the context, and mount_capable()
lets it through as long as the caller has CAP_SYS_ADMIN over fc->user_ns,
which anyone in an ancestor namespace does. So fc->user_ns !=
current_user_ns() is something an unprivileged user can arrange.

Both overlayfs and binfmt_misc WARN_ON() that. They're plain WARN_ON()s, so
it can be done in a loop to taint the kernel and flood the log, and it
panics a machine booted with panic_on_warn. Keep refusing the mount, just
stop warning about it. Overlayfs already spells the same check as a plain
error return in ovl_parse_param() for Opt_override_creds.

And add a selftest for both cases.

* patches from https://patch.msgid.link/20260802-work-fill_super-warn-v1-0-4e987911a39a@kernel.org:
  selftests/filesystems: test completing a context from another user namespace
  binfmt_misc: don't warn when the mount is completed from another user namespace
  ovl: don't warn when the mount is completed from another user namespace

Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-0-4e987911a39a@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>