[PATCH 3/9] pid: Implement ns_of_pid.

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

A current problem with the pid namespace is that it is
easy to do pid related work after exit_task_namespaces which
drops the nsproxy pointer.

However if we are doing pid namespace related work we are
always operating on some struct pid which retains the pid_namespace
pointer of the pid namespace it was allocated in.

So provide ns_of_pid which allows us to find the pid
namespace a pid was allocated in.

Using this we have the needed infrastructure to do pid
namespace related work at anytime we have a struct pid,
removing the chance of accidentally having a NULL
pointer dereference when accessing current->nsproxy.

Signed-off-by: Eric W. Biederman <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
 include/linux/pid.h |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index f84d532..c4b56c0 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -123,6 +123,17 @@ int next_pidmap(struct pid_namespace *pid_ns, int last);
 extern struct pid *alloc_pid(struct pid_namespace *ns);
 extern void FASTCALL(free_pid(struct pid *pid));
 
+/* ns_of_pid returns the pid namespace in which the specified
+ * pid was allocated.
+ */
+static inline struct pid_namespace *ns_of_pid(struct pid *pid)
+{
+	struct pid_namespace *ns = NULL;
+	if (pid)
+		ns = pid->numbers[pid->level].ns;
+	return ns;
+}
+
 /*
  * the helpers to get the pid's id seen from different namespaces
  *
-- 
1.5.3.rc6.17.g1911