linux-kernel - [PATCH RFC 1/4] pidns: move is-ancestor logic to helper

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Message-Id: <20250721-procfs-pidns-api-v1-1-5cd9007e512d@cyphar.com>
Date: Mon, 21 Jul 2025 18:44:11 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
 Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
 linux-api@...r.kernel.org, linux-doc@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH RFC 1/4] pidns: move is-ancestor logic to helper

This check will be needed in later patches, and there's no point
open-coding it each time.

Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 include/linux/pid_namespace.h |  9 +++++++++
 kernel/pid_namespace.c        | 21 ++++++++++++++-------
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 7c67a5811199..df421a1e3e0b 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -84,6 +84,9 @@ extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
 extern int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd);
 extern void put_pid_ns(struct pid_namespace *ns);
 
+extern bool pidns_is_ancestor(struct pid_namespace *child,
+			      struct pid_namespace *ancestor);
+
 #else /* !CONFIG_PID_NS */
 #include <linux/err.h>
 
@@ -118,6 +121,12 @@ static inline int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
 {
 	return 0;
 }
+
+bool pidns_is_ancestor(struct pid_namespace *child,
+		       struct pid_namespace *ancestor)
+{
+	return false;
+}
 #endif /* CONFIG_PID_NS */
 
 extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 7098ed44e717..5719b1f679ad 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -390,6 +390,19 @@ static void pidns_put(struct ns_common *ns)
 	put_pid_ns(to_pid_ns(ns));
 }
 
+bool pidns_is_ancestor(struct pid_namespace *child,
+		       struct pid_namespace *ancestor)
+{
+	struct pid_namespace *ns;
+
+	if (child->level < ancestor->level)
+		return false;
+	for (ns = child; ns->level > ancestor->level; ns = ns->parent)
+		;
+	return ns == ancestor;
+}
+EXPORT_SYMBOL_GPL(pidns_is_ancestor);
+
 static int pidns_install(struct nsset *nsset, struct ns_common *ns)
 {
 	struct nsproxy *nsproxy = nsset->nsproxy;
@@ -408,13 +421,7 @@ static int pidns_install(struct nsset *nsset, struct ns_common *ns)
 	 * this maintains the property that processes and their
 	 * children can not escape their current pid namespace.
 	 */
-	if (new->level < active->level)
-		return -EINVAL;
-
-	ancestor = new;
-	while (ancestor->level > active->level)
-		ancestor = ancestor->parent;
-	if (ancestor != active)
+	if (!pidns_is_ancestor(new, active))
 		return -EINVAL;
 
 	put_pid_ns(nsproxy->pid_ns_for_children);

-- 
2.50.0