[PATCH RFC] setns: return 0 directly if try to reassociate with current namespace

[Chen Hanxiao]

We could use setns to join the current ns,
which did a lot of unnecessary work.
This patch will check this senario and
return 0 directly.

Signed-off-by: Chen Hanxiao <chenhanxiao@xxxxxxxxxxxxxx>
---
 kernel/nsproxy.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index ef42d0a..66eea63 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -236,6 +236,34 @@ SYSCALL_DEFINE2(setns, int, fd, int, nstype)
 	ops = ei->ns_ops;
 	if (nstype && (ops->type != nstype))
 		goto out;
+	switch (ops->type) {
+	case CLONE_NEWIPC:
+		if (ei->ns == tsk->nsproxy->ipc_ns)
+			return 0;
+		break;
+	case CLONE_NEWNET:
+		if (ei->ns == tsk->nsproxy->net_ns)
+			return 0;
+		break;
+	case CLONE_NEWNS:
+		if (ei->ns == tsk->nsproxy->mnt_ns)
+			return 0;
+		break;
+	case CLONE_NEWPID:
+		if (ei->ns == tsk->nsproxy->pid_ns_for_children)
+			return 0;
+		break;
+	case CLONE_NEWUSER:
+		if (ei->ns == current_user_ns())
+			return 0;
+		break;
+	case CLONE_NEWUTS:
+		if (ei->ns == tsk->nsproxy->uts_ns)
+			return 0;
+		break;
+	default:
+		goto out;
+	}
 
 	new_nsproxy = create_new_namespaces(0, tsk, current_user_ns(), tsk->fs);
 	if (IS_ERR(new_nsproxy)) {
-- 
1.9.0

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