linux-kernel - [PATCH 16/23] proc_ns_operations: Add can

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Date:   Thu, 30 Jul 2020 15:00:46 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     viro@...iv.linux.org.uk, adobriyan@...il.com, davem@...emloft.net,
        ebiederm@...ssion.com, akpm@...ux-foundation.org,
        christian.brauner@...ntu.com, areber@...hat.com, serge@...lyn.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        ktkhai@...tuozzo.com
Subject: [PATCH 16/23] proc_ns_operations: Add can_get method

This is a new method to prohibit some namespaces in intermediate state.
Currently, it's used to prohibit pid namespace, whose child reaper is not
created yet (similar to we have in /proc/[pid]/pid_for_children).

Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
---
 fs/proc/namespaces.c    |    5 +++++
 include/linux/proc_ns.h |    1 +
 kernel/pid_namespace.c  |    1 +
 3 files changed, 7 insertions(+)

diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index ab47e1555619..70fc23295315 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -149,6 +149,11 @@ static const char *proc_namespaces_getlink(struct dentry *dentry,
 		ns = get_namespace_by_dentry(pid_ns, dentry);
 		if (!ns)
 			goto out;
+		ret = -ESRCH;
+		if (ns->ops->can_get && !ns->ops->can_get(ns)) {
+			ns->ops->put(ns);
+			goto out;
+		}
 
 		ret = __ns_get_path(&path, ns);
 		if (ret == -EAGAIN)
diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index 906e6ebb43e4..e44ec466711a 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h
@@ -19,6 +19,7 @@ struct proc_ns_operations {
 	int type;
 	struct ns_common *(*get)(struct task_struct *task);
 	void (*put)(struct ns_common *ns);
+	bool (*can_get)(struct ns_common *ns);
 	int (*install)(struct nsset *nsset, struct ns_common *ns);
 	struct user_namespace *(*owner)(struct ns_common *ns);
 	struct ns_common *(*get_parent)(struct ns_common *ns);
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 4a01328e8763..da8490390f51 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -452,6 +452,7 @@ const struct proc_ns_operations pidns_for_children_operations = {
 	.real_ns_name	= "pid",
 	.type		= CLONE_NEWPID,
 	.get		= pidns_for_children_get,
+	.can_get	= pidns_can_get,
 	.put		= pidns_put,
 	.install	= pidns_install,
 	.owner		= pidns_owner,