kernel: add a helper to get an owning user namespace for a namespace

Return -EPERM if an owning user namespace is outside of a process
current user namespace.

v2: In a first version ns_get_owner returned ENOENT for init_user_ns.
    This special cases was removed from this version. There is nothing
    outside of init_user_ns, so we can return EPERM.
v3: rename ns->get_owner() to ns->owner(). get_* usually means that it
grabs a reference.

Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/ipc/namespace.c b/ipc/namespace.c
index d87e6baa1323..578d93be619d 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -165,10 +165,16 @@ static int ipcns_install(struct nsproxy *nsproxy, struct ns_common *new)
 	return 0;
 }
 
+static struct user_namespace *ipcns_owner(struct ns_common *ns)
+{
+	return to_ipc_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations ipcns_operations = {
 	.name		= "ipc",
 	.type		= CLONE_NEWIPC,
 	.get		= ipcns_get,
 	.put		= ipcns_put,
 	.install	= ipcns_install,
+	.owner		= ipcns_owner,
 };