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

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/pid_namespace.h |  9 +++++++++
 kernel/pid_namespace.c        | 23 +++++++++++++++--------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 7c67a5811199..17fdc059f8da 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;
 }
+
+static inline 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..c2783c5fa90b 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -390,11 +390,24 @@ 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;
 	struct pid_namespace *active = task_active_pid_ns(current);
-	struct pid_namespace *ancestor, *new = to_pid_ns(ns);
+	struct pid_namespace *new = to_pid_ns(ns);
 
 	if (!ns_capable(new->user_ns, CAP_SYS_ADMIN) ||
 	    !ns_capable(nsset->cred->user_ns, CAP_SYS_ADMIN))
@@ -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

On Wed, Jul 23, 2025 at 09:18:51AM +1000, Aleksa Sarai wrote:
> This check will be needed in later patches, and there's no point
> open-coding it each time.
> 
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/linux/pid_namespace.h |  9 +++++++++
>  kernel/pid_namespace.c        | 23 +++++++++++++++--------
>  2 files changed, 24 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index 7c67a5811199..17fdc059f8da 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;
>  }
> +
> +static inline 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..c2783c5fa90b 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -390,11 +390,24 @@ 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);

Why do you need to export this? Afaict, this is only used from procfs
and iirc procfs cannot be a module. This could also be a static inline
completely in the header? Otherwise this looks good.